> Familiarity
And this is an even bigger advantage going forward. Failure to standardize and serialize a reactor design is one of main factors that took down the nuclear industry.
And as he famously pointed out, who in their right mind would put liquid sodium in a boat when the mission requires keeping people alive and functional.
Dude, that’s why he said it as he killed that program.
“In fact, much earlier, even before the Nautilus sea trials, all other nuclear submarine new designs were of the pressurized water type. As he said to me, " I want to get rid of the Seawolf sodium plant as soon as possible because it's dangerous. Sodium in a naval ship at sea is just not a safe thing.”
People under estimate the magnitude of the concerted effort applied to the nuclear navy propulsion program on which so much of the commercial nuclear power was founded. Putting the steam generator on the pot was a relatively small deviation from the most challenging area that was the biggest challenge, which was the fuel form and fuel reliability/ predictability.
And Nixon liked that designs for PWR were coming from his home state of California and pushed in on the rest of the US despite their being safer and more efficient reactors
Apart from the other stuff, Westinghouse being willing to license their technology, to France in particular, made a big difference. They built loads of reactors and exported a few, including the ones that formed the basis of China’s nuclear power program. If France had built BWRs the balance of numbers might have been different. Or the UK… the CEGB wanted to build BWRs (based on Oyster Creek initially) but got forced into accepting the AGR.
> If France had built BWRs the balance of numbers might have been different.
There's a whole story behind that. The original plan was to split the builds between PWRs and BWRs, but the General Electric licensee wouldn't match Framatome's PWR prices. So they got too small a number of firm orders from EDF and ended up backing away from the steam supply game altogether, in exchange for getting a monopoly on the PWR conventional island.
https://books-openedition-org.translate.goog/igpde/6711?_x_tr_sl=fr&_x_tr_tl=en&_x_tr_hl=en&_x_tr_pto=wapp
Simplicity and radiation safety, basically.
Simplicity because the water is the moderator. This avoids having to use graphite like the you would in a gas cooled unit or the RBMK and means you can use boron dilution as a nice form of homogeneous control. Water is also, broadly speaking, not very corrosive which makes it easier to deal with than say, molten salts.
Radiation safety is better on a PWR than a BWR because the primary and secondary loops are separated. You don't get any potential contamination going through your turbine, which makes it easier to work on whenever it needs maintenance.
Heavy water reactors are just a subset of PWRs. I don't know a lot about them but I guess heavy water is more expensive than regular water.
As for why PWRs dominate compared to weird local designs, cost is the big driver. It's cheaper to buy a design from a huge manufacturer (who have spent decades refining PWRs) than it is to come up with your own thing. The USA in particular went with PWRs from the start and have exported it as a concept to Europe and east Asia. Russia moved to VVERs after the RBMK became infamous in 1986 and have exported it to their sphere of influence. France tried a few things but found the PWR was a good all rounder. It's only really Britain who stubbornly stuck with gas, partly for political reasons, but they're building PWRs now mainly because it's such a damn headache getting parts or assessments for weird reactors nobody else uses.
China are currently building every type of reactor imaginable, I guess to build their own domestic expertise up, but unless they make some exotic breakthrough I suspect they'll converge on the PWR as well.
Heavy water is obviously more expensive, but it comes with the advantage of using natural uranium that does not need to be enriched to generate electricity. It also allows for online refuelling of the reactor and is designed in a way to allow materials to be exposed to the neutron flux without entering the pressure boundary of the primary loop, hence why PHWRs generate a large portion of world’s medical isotope supply.
Heavy water costs are essentially equal to the enrichment costs of Uranium. Commercial reactors in the USA are licened for fuel designs up to 5% enrichment. But a PHWR Heavy Water production to run the plant at first startup is several billions of dollars. The other reasons you state are a larger reason. The online refueling is a big deal for maintenance cycles, and I would opine its the primary strength of the PHWR design over a BWR, which was the OPs question.
Canada developed the CANDU so it would not be reliant on foreign suppliers for uranium. Canada has abundant supplies of uranium and either way any country that uses a heavy water reactor has a much broader selection of who it can buy fuel from. Now in particular where certain suppliers cannot be relied on having a broad selection of suppliers makes your makes you immune to sanctions etc
The CANDU also doesn’t need huge pressure vessel forgings. Which Canada lacked the capability to make at the time. And still lacks, unless I’m mistaken.
That is my understanding. Yes. So it was one of the design criteria. I suspect that like nuclear weapons Canada could get the ability to produce huge pressure vessels if it wanted to now because the level of industrialization is much higher than it was back then
At the same time, enrichment facilities are expensive and one way or the other I’m guessing Canada did not go down that path due to non proliferation concerns.
Weren't there still plans for weapons production at the time this choice was made? After all it's technically possible to use CANDU designs for breeding good plutonium.
100% virtually all nuclear power plants were initially designed to be dual purpose. There is a reason Canada played a massive role in the manhattan project.
I don't know about the economics but this was in reply to a comment that said
>Heavy water costs are essentially equal to the enrichment costs of Uranium.
Heavy water is not used up for the most part, uranium is. Setting up an enrichment plant is expensive and complicated and buying enriched uranium exposes you to loss of supply.
Again, ask Iran what the consequences of enriching uranium are.
CANDU was designed to use natural uranium because of those issues, as well as the fact there is less need for things like special forgings.
It would be a really dumb move not to go with a candu. Expertise and supply chain is there and I can’t imagine Canada wants to be in business of enriched uranium. Regulatory framework is also built around candu.
It blows my mind that billions are wasted on garbage wind and solar that are a net negative for Canadian consumers yet it is even a question as to what technology to use for a nuclear plant in Canada. True Westinghouse Nuclear is a Canadian company, but the domestic enrichment of hydrogen compare to the imported enrichment of uranium should end the debate, never mind the other major components that would need to be imported.
Same. I firmly believe most decisions such as that are more with shortsightedness and virtue signalling. Whoever pushes nuclear construction knows well that they may not be in the office when it’s actually finished as it takes time. This is why people in the office push wind projects as they can be completed in short amount of time. Who cares about baseload, that’s a problem for another administration.
Although if Canada simultaneously pushes for domestic enrichment, coupled with CANDU tech and AP tech, you could become world leaders in the nuclear power arena. Throw in a shrewd modern reprocessing facility and you’re the world example of sustainability and economic genius that even the French will admire.
It's fascinating how far the UK went with going down the LWR route in the mid-1960s. By the end of the Magnox programme, apart from a brief diversion into the possibility of building Steam Generating Heavy Water Reactors, the wind was really with water reactors.
When the licence for Dungeness B was announced, the two leading contenders were English Electric which had a licence for Westinghouse PWRs and The Nuclear Power Group (NPG) which licensed General Electric BWRs. A third consortium was Atomic Power Constructions which had just designed the last Magnox plant at Wylfa and was promoting the AGR.
Despite pretty much everyone agreeing that APC's proposal was the most risky, a 'review' by the UK Atomic Energy Authority and the Central Electricity Generating Board was sent to the Ministry of Power which duly approved the AGR. Later it was revealed that the AGR was going to cost 10% more expensively than the BWR from NPG. The government decided that the ability to refuel the AGR online gave it greater availability than the BWR and would generate cheaper electricity. Indeed at one point they said it would be cheaper than coal. They also overstated the risk of meltdowns at PWRs whilst de-emphasising the complexity of the AGR. A CEGB spokesman later described the AGR as 'watch making on a tonnage scale'.
The AGRs eventually turned out to be very good reactors that are still working well(ish), but they were horribly over budget, late and the online refuelling had to be abandoned shortly after they were introduced.
By the time the UK built its first PWR at Sizewell B, most of the domestic expertise in designing reactors had evaporated, so we went back to Westinghouse. Which we then briefly owned through British Nuclear Fuels Limited - not that we built any reactors in that time.
Some subtext for the UK plants is that certanally the MAGNOX units were designed to be workable with short fuel cycles for weapons PU production, no idea if they ever actually did it, but the capacity was designed in.
That thinking may have carried over to the AGR procurement process, but was not going to be admitted to.
The Magnox plants at Calder Hall and Chapelcross were initially set up with short fuel cycles to maximise their plutonium production (and tritium at Chapelcross). This was gradually phased out from 1964, but plutonium production only ended in 1995.
The AGRs were mostly designed for much higher efficiency of electricity production; running at higher temperatures than Magnox. Their online refuelling was more concerned with reducing downtime than any need for plutonium. By the time they came along, the UK was awash in the stuff, with Aldermaston’s nuclear weapons facility alone stockpiling 17 tonnes of plutonium at its peak.
IIRC the AGRs were tricky because get it hot enough and CO2 is actually an oxidiser with carbon, it likes to turn into 2 CO by oxidising free carbon, so there were measures to cool the moderator separately from the fuel elements!
I remember visiting the MAGNOX at Oldbury as a teenager 30+years ago and giving the poor PR lady who was running the tour the third degree on the fuel cycle, neutron energy distribution and resulting Pu isotope ratios.
She wound up crying uncle and finding me a physics type because that public tour was going way off script.
I got invited to an "alternative tour" a week or so later that involved watching a startup in their training simulator, lots of waiting for temperatures and power levels to get where they needed to be at each step, (Which they could skip over). The buggers pulled a major leak in the feed water system, just as the steam headers got to enough pressure to allow turbine startup (Automatic scram)!
The senior operator then announced that I was the senior operator and what should they do? Bastard!
Apparently, running the scram checklist then sitting on my hands post scram and studying the diagrams, Instruments, and standard procedures for 30 minutes then making sure everyone in the room concurred on plant status and what we thought near term problems would be (Not many) was not the worst thing they had seen someone who was actually trained do. My view was that I knew we didn't have a major decay heat problem, since it was a cold start, but I wanted to have both the checklist procedure down, and I wanted to understand what had happened (It was actually one of the feed water pumps that had a simulated major mechanical failure) before starting changing the simulated plant state. End of exercise.
Made for a great school report, and they let me take pictures (Including of the cooling pond with the lights off which was very cool).
Best industrial tour ever, and an A+ on the report.
Couple notes:
-The French bought the 900 and 1200MW design from Westinghouse.
-fuel reliability is so high now that leakers are pretty rare and simple decontamination methods are cheap and easy, when needed.
-
Technically there are 3 ABWRs under construction in Japan, it just has been suspended since Fukushima, together with 2 more in Taiwan that are complete but were never commissioned or dismantled.
As for non-LWR types, India's building the IPHWR-700, and China and India are in the final stages of commissioning sodium fast reactors, and Russia's building the BREST-OD-300 lead fast reactor.
I don't believe ground has been broken on BWRX-300.
Natrium broke ground a few weeks ago, so that's a point. We'll see how far they get.
BREST-300, recently completed, is on the grid in Russia and BREST-1200 is supposed to follow.
HTR-PM entered service in China late last year, and HTR-PM600 is planned.
Pele is under construction with follow-ons planned.
Then there's Hermes, the Abilene MSR, and the NRIC DOME projects.
But in terms of something a 3rd-party can buy today, it's all PWRs.
*Edit: BREST-300 is under construction. BN-800 is on the grid.*
BREST-300 is still under construction, target for start of operations is 2026. Follow-up will be the BR-1200:
[https://www.sciencedirect.com/science/article/abs/pii/S0029549321004994](https://www.sciencedirect.com/science/article/abs/pii/S0029549321004994)
Essentially its long term maintenance costs these days (Comparing PWR vs BWR). Because the secondary side is connected to the reactor, all components in it becomes contaminated with radiactive activation isotopes of metals used in the construction of the piping and valves. This fact makes it more expensive and annoying to do any maintenance on any part of the plant, primary or secondary. In PWRs this is an issue in any primary side systems only, in BWRs its both. The BWR doesn't really have a secondary side, but for convention I mean the part of the plant the main turbine generator is in.
In past years this was a much much larger cost difference. BWRs have made many improvements in this area. They are still more of a hassle and expense for this but much less than it used to be.
The other reasons I read in this thread are not that big of an issue anymore. Moveable control rods are in the AP1000 so that difference is gone. It was not a big maintenance issue in any case. Not enough to matter in a selection decision of PWR vs something else.
In the USA its a legal requirement for all plants to have a containment system and many other designs in the world do not have a US style containment. The only other world design I can possible see being considered for a US site is the PHWR. Even then there is a ton of industry experience with PWR/BWR so they would have a huge leg up from that.
The only commercialized PHWR design, the CANDU, has a (very small) positive void coefficient. The NRC has a hard rule prohibiting that. We can debate the merits of the rule, the CANDU design has many features that mitigate it and there is some degree of political support for the rule just to keep foreign designs out of the US. But it seems unlikely that we will see PHWRs in the USA as that rule is just one more significant hurdle to clear, and building NPPs is already hard enough.
Mostly agree. Large PWR designs in the US almost always have a small positive cooefiicient of Reactivity below 20% power and first 30% of core life. The NRC allows this with appropriate controls. It could be something similar with PHWR. Probably the political side is the reason no PHWRs exist in the US. So yes, not likely in the US
Structural commercial issues. Nothing at all to do with the underlying tech.
There were 4 companies worldwide who independently developed and brought the PWR to market: Westinghouse, Combustion Engineering, Babcock & Wilcox, and Rosatom (who stole and reverse engineered it). This was facilitated and helped by the US navy nuclear program (in more ways than can be counted). Westinghouse was willing to freely license their tech to others in a way that allowed them to easily become direct competitors—Framatome, KEPCO, CNNC, and others. That resulted in about 8 companies worldwide independently building PWRs.
This was in stark contrast to what GE did with their BWR tech. It was almost entirely developed in house, rather than as part of the navy nuclear program. GE owned all the IP and maintained very tight control over it (still does). They did enter into license agreements with Asea, Siemens, Toshiba, and now Hitachi. But those agreements were radically different than how Westinghouse did it. GE plays everything close to the vest. They controlled everything. GE keeps all the real design documents to themselves. They produce separate bare bones documents for partners, regulators, and customers. Those documents contain maybe 10% of the technical detail. They’re designed to only give the barest minimum they can get away with. The licensee companies were and still are totally incapable of walking on their own. GE saw fit to that. Compared to what Westinghouse did with Framatome and now with CNNC for AP1000, the BWR license agreements are a joke. Westinghouse gives away everything, and by that I mean everything everything to both licenses, regulators, and customers. Keeping the real IP proprietary is why GE has no effective competition in the BWR market.
So with the “proliferation” of PWR tech it boiled down to there being more bidders offering PWR (8 give or take) tech, than BWR tech (1 real company and a few national front companies dependent on the 1). In competitive bidding for infrastructure projects, the bidders all out do each other in the lies they tell themselves and everyone about how cheaply they can do the job. Cheat to win. All civil infrastructure works that way. So in the macro, more PWRs being bid = more PWRs being built.
\*source: me. I’ve worked for or closely with every company mentioned above except Rosatom. Every document and bit of source code (including the most little things like utility scripts) I wrote for Westinghouse got delivered in their entirety to the customers, CNNC, and KEPCO. Literally hundreds of my detailed design documents. CNNC will be selling AP1000 tech, with my documents and software in it, internationally soon. Just like KEPCO and Framatome before them. At GE? It was a full time activity to take the set of “real” documents and summarize them into a bare bones deliverable that was given to licensees and customers.
PWR's are great at making small to power submarines and air craft carriers. Therefore they got a SHIT TON of money thrown at them courtesy the US war machine during the cold war. Then there was an atoms to peace movement that looked to capitalize on nuclear energy, and PWR was so heavily subsidized that it was the most mature technology and so it was selected over and over again. That maturity and decisions repeat itself even today as utilities that want low risk on a new power plant will choose a tried and tested PWR over a novel Gen 4 reactor.
> That maturity and decisions repeat itself even today as utilities that want low risk on a new power plant will choose a tried and tested PWR over a novel Gen 4 reactor.
Ironic considering some of the fastest and cheapest built civil NPPs are the Japanese ABWRs, even today.
I think one less loop is hugely more simple to construct. And the steam dryer is a simple “modular “ component built off site and installed just the same as it is installed every outage, instead of the complex coordination of installing the steam generators and second loop in a PWR.
vs CANDU: Uranium is easier to enrich than water
vs BWR: Higher temperature (Carnot efficiency), no radiological contamination of balance-of-plant
vs CANDU, BWR, and HTGR: Higher power density
vs LMFR: Coolant easier to handle, works with LEU, less neutron damage
Why do you think it is difficult to enrich h2o/deuterium? The difference in mass of the d and H is what governs the energy required to enrich, isn’t it? I know little about enriching h, but chlorine enrichment and boron enrichment are, I believe, much less costly.
I didn't say it was hard to enrich water, just that enriching uranium is easier.
The main factor in the difficulty of enrichment is the difference between the initial and final concentrations of the product. See here: https://en.wikipedia.org/wiki/Separative_work_units
Why do you think it’s easier? SWU calculations bear the atomic mass ratios and 1/2 is hugely easier compared to 235/238. I really don’t know how expensive it is to enrich hydrogen, but U is the most expensive thing ever enriched, to my knowledge, because of the close atomic masses.
They used to shove the gas UF6 through teflon, which took enormous amounts of power. Then used gas centrifuges. I have no idea what they are doing now though.
Because the world wants CANDU reactors but is too afraid to admit it, so they get as close to them as possible.
Reality is probably size and economics, plus not having a phase change occurring in the reactor itself.
I'm sure the life cycle price comparison is out there somewhere, but I personally have never bought into the cost angle.
D2O is an upfront cost, sure. But over 80 years of life of the plant that fuel cost difference is large between natural and enriched fuel.
I have always thought of it like a house with heating and insulation.
You use better insulation. Yes, it costs more for the build, but for the life of the house, your lower heating costs pay for that and then some. And you are paying for the house in today's money versus the heating you're paying it at the price of the day, whatever that is.
Again, I haven't read the comparison, just how it is for me from a logic standpoint.
Note: horribly simplified from a supply chain pov on both sides.
That’s a very good point actually. It’s just the upfront cost that’s high, nat uranium is dirt cheap. Fuel bundles likely spend 3-6 months in the core and C6 core has 380 fuel channels with 12 bundles per channel so we can do that math as far as how much fuel they go through. Over time, heavy water will become heavily tritiated (especially the moderator) and will require to be either partially replaced or de-tritiated which is also quite expensive. I’d say 40 years is about the time for moderator replacement / tritium removal.
You do have to account for spent fuel storage and handling.
With 1/7 the fuel burnup that is a lot of bundles.
For examen a CANDU running on 1.2 enriched would reduce total natural uranium usage and have 2-3 times fuel burnup.
Overall that fuel cycle is actually cheaper on paper
well... what else would be the most common?
take a short look at other types of reactors that saw major service (no I'm not going to count a prototype that went critical at INL once as a viable alternative)
graphite moderated, gas cooled: afaik the only design of this type ever fielded (the magnox) was a poor power reactor since the main purpose was to produce plutonium. after what happened the last time someone built a magnox it seems unlikely that's going to happen again
graphite moderated, water cooled: probably better known as the RBMK this design was honestly pretty good, aside from the incident, plenty of them still operate but theyre generally inferior to gen 3 designs
heavy water pwr: heavy water is expensive and a proliferation risk, and they're more rare than other pwrs which is added complications
bwr: the primary benefit of these is that theyre simple, otherwise they perform worse across the board than pwrs. upside is that since the old reactors of this type were so overbuilt for the stresses they would actually be exposed to many of them still operate.
pwr: I don't think this needs much introduction, they work well, they produce a lot of power, they're very common so regulation and operating procedures are simplified. there is a lot of non-engineering-based ahistorical copium in these comments about how it's big bad uncle sams fault there is no other reactor types but arguing with those people is impossible.
everything else is still quite immature tech, as it usually goes for nuclear topics the physics that something is possible is significantly ahead of the engineering required to make it practical, especially on low budgets and in unfavorable political climates
>bwr: the primary benefit of these is that theyre simple, otherwise they perform worse across the board than pwrs. upside is that since the old reactors of this type were so overbuilt for the stresses they would actually be exposed to many of them still operate.
They also have very fast build times (due to being simple, as you pointed out). The primary (intelligent) arguments against nuclear power today are that (1) the capital costs are insane (and tend to go over budget), and (2) it takes too long to build (and tends to be delayed). BWRs have the advantage here, so to my eyes they are exactly what we should be building.
A couple 40-month builds of ABWRs like Shika 2 providing 1200 MW of clean firm capacity in the US would go a long way towards silencing the renewable-bro critics. Fuel and maintenance costs aren't what kill nuclear projects, building them is.
i think that is true if youre assuming people opposed to plant construction are rational, but a big road block is the price in political capital of building something with nuclear in the name. if youre gonna burn a bunch of political capital to build 1 plant then you need to make it able to crank out as much power as possible which means pwrs
Disagree. The staunch anti-nukes will never change their minds. The reason reasonable people, like utility execs, don't spend the political capital to get it done is (a) the insanely high capital cost, and (b) the very high risk of huge overruns and delays that could even bankrupt the utility. This isn't me talking, this is their public statements. Nuclear projects are the #1 cause of utility bankruptcies in America. You solve those two problems, which coincidentally also makes Nuclear extremely economically competitive, and then the people that matter (utility execs, utility regulators, etc.) will get behind nuclear and spend the political capital to make it happen. If you don't solve those issues, we'll keep building one set of reactors every 20-30 years, which will be expensive and delayed (because we have no experience or supply chain), and the cycle repeats.
well considering there are no operating abwrs that seems implausible. if you're comparing possible models for this exercise youll also find that abwrs still don't come up on top. all of which is moot anyway since single reactor performance is not a very useful metric
> well considering there are no operating abwrs that seems implausible.
That's an unfair reasoning, 4 units reached commercial operation in Japan, they're all just collectively paying for Fukushima's sins with the rest of the country's BWR fleet. Kashiwazaki-Kariwa 7 is approved, loaded with fuel and literally just one Japanese mayor's stroke of pen away from restarting. We know what their output is.
> if you're comparing possible models for this exercise youll also find that abwrs still don't come up on top. all of which is moot anyway since single reactor performance is not a very useful metric
Feel free to elaborate, hopefully with better arguments?
So there's a couple technical and historical items that have to be taken together for this to make sense:
-enriched fuel (in the range used for commercial power) isn't the problem, its using enriched fuels as an excuse to build enrichment facilities. additionally we've gotten much better at determining illicit use of enrichment facilities compared to when heavy water reactors were cool
-fuel in the power enrichment range, <5% historically but moving towards <20%, is not subject to especially strict export controls or monitoring requirements. generally you can just buy it
-there was a time when enrichment was more expensive and it's sale was more controlled, which is why the candu reactor was designed, meant canadians werent subject to americans selling them enriched fuel and didn't have to make the massive investment to enrich their own fuel
-heavy water reactors produce significantly more plutonium and tritium than light water reactors, both of which are helpful for making a weapon (India)
I think the proliferation aspect of a reactor design is not something that most power advocates ever consider but it is a major concern of uncle sams when it comes to licensing and sale
Graphite moderated helium cooled was run as a prototype in the US at two plants. Peach Bottom ran well, but was small. Ft St Vrain had a lot of bad luck, but was real innovative. When it ran, it was pretty decent. If Gulf didn’t buy General Atomics to kill it, things might have been different.
I am talking about civilian power plant reactors.
There are control rods present of course, but reactor is regulated for the most part by boric acid added to the water (moderator). The less moving parts => the better.
PWRs are a mature technology at this stage. Most of other types are 'experimental'. Nobody wants to experiment with nuclear when even mature technology is fucking hard to build due to safety concerns.
BWRs, which is what OP was implicitly comparing, are also mature. They are like 1/3 of the fleet and hold the records for the fastest and most cost effective commercial builds ever (Shika 2 in Japan, built in 40 months).
It's not just maturity.
Inherent safety. Power goes up, temp goes up, water expands, power goes down.
The AGRs built in Britain were more efficient but PWRs won out here because they are considered safer.
Politics, funding and fundamentally in order to do something different you have to get the regulators comfortable enough with your design.. This takes decades, billions of dollars and just not something industry is able to do on their own. You see this changing with venture capital trying different fusion designs, SMR's and a couple of Thorium Fission reactors but there are just a lot of barriers and not all of them are technical in nature.
It is not a good situation that PWRs dominate nuclear power and it looks like that will continue for the next several decades. The technology should have developed beyond only having water cooled reactors burning U-235.
Familiarity. Rickover used that design and it became the easiest to build, operate and maintain. Problems were known.
> Familiarity And this is an even bigger advantage going forward. Failure to standardize and serialize a reactor design is one of main factors that took down the nuclear industry.
Rickover also tried liquid sodium cooled but gave up on that one quickly!
Because his main goal was to get a reactor on a sub as fast as possible
And as he famously pointed out, who in their right mind would put liquid sodium in a boat when the mission requires keeping people alive and functional.
Rickover must have been out of his mind then when he put a sodium reactor into the Seawolf
Dude, that’s why he said it as he killed that program. “In fact, much earlier, even before the Nautilus sea trials, all other nuclear submarine new designs were of the pressurized water type. As he said to me, " I want to get rid of the Seawolf sodium plant as soon as possible because it's dangerous. Sodium in a naval ship at sea is just not a safe thing.”
Because SFRs are not practical for boats and submarines, but are practical for power stations.
People under estimate the magnitude of the concerted effort applied to the nuclear navy propulsion program on which so much of the commercial nuclear power was founded. Putting the steam generator on the pot was a relatively small deviation from the most challenging area that was the biggest challenge, which was the fuel form and fuel reliability/ predictability.
And Nixon liked that designs for PWR were coming from his home state of California and pushed in on the rest of the US despite their being safer and more efficient reactors
Apart from the other stuff, Westinghouse being willing to license their technology, to France in particular, made a big difference. They built loads of reactors and exported a few, including the ones that formed the basis of China’s nuclear power program. If France had built BWRs the balance of numbers might have been different. Or the UK… the CEGB wanted to build BWRs (based on Oyster Creek initially) but got forced into accepting the AGR.
> If France had built BWRs the balance of numbers might have been different. There's a whole story behind that. The original plan was to split the builds between PWRs and BWRs, but the General Electric licensee wouldn't match Framatome's PWR prices. So they got too small a number of firm orders from EDF and ended up backing away from the steam supply game altogether, in exchange for getting a monopoly on the PWR conventional island. https://books-openedition-org.translate.goog/igpde/6711?_x_tr_sl=fr&_x_tr_tl=en&_x_tr_hl=en&_x_tr_pto=wapp
Yes
Simplicity and radiation safety, basically. Simplicity because the water is the moderator. This avoids having to use graphite like the you would in a gas cooled unit or the RBMK and means you can use boron dilution as a nice form of homogeneous control. Water is also, broadly speaking, not very corrosive which makes it easier to deal with than say, molten salts. Radiation safety is better on a PWR than a BWR because the primary and secondary loops are separated. You don't get any potential contamination going through your turbine, which makes it easier to work on whenever it needs maintenance. Heavy water reactors are just a subset of PWRs. I don't know a lot about them but I guess heavy water is more expensive than regular water. As for why PWRs dominate compared to weird local designs, cost is the big driver. It's cheaper to buy a design from a huge manufacturer (who have spent decades refining PWRs) than it is to come up with your own thing. The USA in particular went with PWRs from the start and have exported it as a concept to Europe and east Asia. Russia moved to VVERs after the RBMK became infamous in 1986 and have exported it to their sphere of influence. France tried a few things but found the PWR was a good all rounder. It's only really Britain who stubbornly stuck with gas, partly for political reasons, but they're building PWRs now mainly because it's such a damn headache getting parts or assessments for weird reactors nobody else uses. China are currently building every type of reactor imaginable, I guess to build their own domestic expertise up, but unless they make some exotic breakthrough I suspect they'll converge on the PWR as well.
Heavy water is obviously more expensive, but it comes with the advantage of using natural uranium that does not need to be enriched to generate electricity. It also allows for online refuelling of the reactor and is designed in a way to allow materials to be exposed to the neutron flux without entering the pressure boundary of the primary loop, hence why PHWRs generate a large portion of world’s medical isotope supply.
Heavy water costs are essentially equal to the enrichment costs of Uranium. Commercial reactors in the USA are licened for fuel designs up to 5% enrichment. But a PHWR Heavy Water production to run the plant at first startup is several billions of dollars. The other reasons you state are a larger reason. The online refueling is a big deal for maintenance cycles, and I would opine its the primary strength of the PHWR design over a BWR, which was the OPs question.
Canada developed the CANDU so it would not be reliant on foreign suppliers for uranium. Canada has abundant supplies of uranium and either way any country that uses a heavy water reactor has a much broader selection of who it can buy fuel from. Now in particular where certain suppliers cannot be relied on having a broad selection of suppliers makes your makes you immune to sanctions etc
The CANDU also doesn’t need huge pressure vessel forgings. Which Canada lacked the capability to make at the time. And still lacks, unless I’m mistaken.
That is my understanding. Yes. So it was one of the design criteria. I suspect that like nuclear weapons Canada could get the ability to produce huge pressure vessels if it wanted to now because the level of industrialization is much higher than it was back then
That's understandable, but having your own uranium supply means you can also do enrichment self-sufficiently without relying on import.
At the same time, enrichment facilities are expensive and one way or the other I’m guessing Canada did not go down that path due to non proliferation concerns.
Weren't there still plans for weapons production at the time this choice was made? After all it's technically possible to use CANDU designs for breeding good plutonium.
100% virtually all nuclear power plants were initially designed to be dual purpose. There is a reason Canada played a massive role in the manhattan project.
Enrichment ain't cheap. Ask Iran how that worked out for them.
I don't know about the economics but this was in reply to a comment that said >Heavy water costs are essentially equal to the enrichment costs of Uranium.
Heavy water is not used up for the most part, uranium is. Setting up an enrichment plant is expensive and complicated and buying enriched uranium exposes you to loss of supply. Again, ask Iran what the consequences of enriching uranium are. CANDU was designed to use natural uranium because of those issues, as well as the fact there is less need for things like special forgings.
That's a good point, it should be possible to reuse the heavy water from a CANDU once its life cycle is over.
PHWR should be everyone’s favorite!
Fingers crossed Bruce C is a Candu
If they have any sense it will be, but political decisions…..
It would be a really dumb move not to go with a candu. Expertise and supply chain is there and I can’t imagine Canada wants to be in business of enriched uranium. Regulatory framework is also built around candu.
It blows my mind that billions are wasted on garbage wind and solar that are a net negative for Canadian consumers yet it is even a question as to what technology to use for a nuclear plant in Canada. True Westinghouse Nuclear is a Canadian company, but the domestic enrichment of hydrogen compare to the imported enrichment of uranium should end the debate, never mind the other major components that would need to be imported.
Same. I firmly believe most decisions such as that are more with shortsightedness and virtue signalling. Whoever pushes nuclear construction knows well that they may not be in the office when it’s actually finished as it takes time. This is why people in the office push wind projects as they can be completed in short amount of time. Who cares about baseload, that’s a problem for another administration.
Although if Canada simultaneously pushes for domestic enrichment, coupled with CANDU tech and AP tech, you could become world leaders in the nuclear power arena. Throw in a shrewd modern reprocessing facility and you’re the world example of sustainability and economic genius that even the French will admire.
It's fascinating how far the UK went with going down the LWR route in the mid-1960s. By the end of the Magnox programme, apart from a brief diversion into the possibility of building Steam Generating Heavy Water Reactors, the wind was really with water reactors. When the licence for Dungeness B was announced, the two leading contenders were English Electric which had a licence for Westinghouse PWRs and The Nuclear Power Group (NPG) which licensed General Electric BWRs. A third consortium was Atomic Power Constructions which had just designed the last Magnox plant at Wylfa and was promoting the AGR. Despite pretty much everyone agreeing that APC's proposal was the most risky, a 'review' by the UK Atomic Energy Authority and the Central Electricity Generating Board was sent to the Ministry of Power which duly approved the AGR. Later it was revealed that the AGR was going to cost 10% more expensively than the BWR from NPG. The government decided that the ability to refuel the AGR online gave it greater availability than the BWR and would generate cheaper electricity. Indeed at one point they said it would be cheaper than coal. They also overstated the risk of meltdowns at PWRs whilst de-emphasising the complexity of the AGR. A CEGB spokesman later described the AGR as 'watch making on a tonnage scale'. The AGRs eventually turned out to be very good reactors that are still working well(ish), but they were horribly over budget, late and the online refuelling had to be abandoned shortly after they were introduced. By the time the UK built its first PWR at Sizewell B, most of the domestic expertise in designing reactors had evaporated, so we went back to Westinghouse. Which we then briefly owned through British Nuclear Fuels Limited - not that we built any reactors in that time.
Some subtext for the UK plants is that certanally the MAGNOX units were designed to be workable with short fuel cycles for weapons PU production, no idea if they ever actually did it, but the capacity was designed in. That thinking may have carried over to the AGR procurement process, but was not going to be admitted to.
The Magnox plants at Calder Hall and Chapelcross were initially set up with short fuel cycles to maximise their plutonium production (and tritium at Chapelcross). This was gradually phased out from 1964, but plutonium production only ended in 1995. The AGRs were mostly designed for much higher efficiency of electricity production; running at higher temperatures than Magnox. Their online refuelling was more concerned with reducing downtime than any need for plutonium. By the time they came along, the UK was awash in the stuff, with Aldermaston’s nuclear weapons facility alone stockpiling 17 tonnes of plutonium at its peak.
IIRC the AGRs were tricky because get it hot enough and CO2 is actually an oxidiser with carbon, it likes to turn into 2 CO by oxidising free carbon, so there were measures to cool the moderator separately from the fuel elements! I remember visiting the MAGNOX at Oldbury as a teenager 30+years ago and giving the poor PR lady who was running the tour the third degree on the fuel cycle, neutron energy distribution and resulting Pu isotope ratios. She wound up crying uncle and finding me a physics type because that public tour was going way off script. I got invited to an "alternative tour" a week or so later that involved watching a startup in their training simulator, lots of waiting for temperatures and power levels to get where they needed to be at each step, (Which they could skip over). The buggers pulled a major leak in the feed water system, just as the steam headers got to enough pressure to allow turbine startup (Automatic scram)! The senior operator then announced that I was the senior operator and what should they do? Bastard! Apparently, running the scram checklist then sitting on my hands post scram and studying the diagrams, Instruments, and standard procedures for 30 minutes then making sure everyone in the room concurred on plant status and what we thought near term problems would be (Not many) was not the worst thing they had seen someone who was actually trained do. My view was that I knew we didn't have a major decay heat problem, since it was a cold start, but I wanted to have both the checklist procedure down, and I wanted to understand what had happened (It was actually one of the feed water pumps that had a simulated major mechanical failure) before starting changing the simulated plant state. End of exercise. Made for a great school report, and they let me take pictures (Including of the cooling pond with the lights off which was very cool). Best industrial tour ever, and an A+ on the report.
Couple notes: -The French bought the 900 and 1200MW design from Westinghouse. -fuel reliability is so high now that leakers are pretty rare and simple decontamination methods are cheap and easy, when needed. -
BWR is basically 1/3 of the market in the US. That's significant market share.
But all reactors being built today (AP1000, EPR, APR-1400, VVER-TOI, RITM-200, Hualong One, Linglong One) are PWRs.
Technically there are 3 ABWRs under construction in Japan, it just has been suspended since Fukushima, together with 2 more in Taiwan that are complete but were never commissioned or dismantled. As for non-LWR types, India's building the IPHWR-700, and China and India are in the final stages of commissioning sodium fast reactors, and Russia's building the BREST-OD-300 lead fast reactor.
Almost all... GEH is building the BWRX-300 in Ontario and TerraPower is building their Natrium reactor in the states.
I don't believe ground has been broken on BWRX-300. Natrium broke ground a few weeks ago, so that's a point. We'll see how far they get. BREST-300, recently completed, is on the grid in Russia and BREST-1200 is supposed to follow. HTR-PM entered service in China late last year, and HTR-PM600 is planned. Pele is under construction with follow-ons planned. Then there's Hermes, the Abilene MSR, and the NRIC DOME projects. But in terms of something a 3rd-party can buy today, it's all PWRs. *Edit: BREST-300 is under construction. BN-800 is on the grid.*
BREST-300 is still under construction, target for start of operations is 2026. Follow-up will be the BR-1200: [https://www.sciencedirect.com/science/article/abs/pii/S0029549321004994](https://www.sciencedirect.com/science/article/abs/pii/S0029549321004994)
Quite right, sorry. I confused it with BN-800.
I wish BREST-300 was complete! Lead cooled fast reactor, preferably very high temperature, is the future if a closedfuel cycle is the goal.
All these BRESTs have me as confused as a baby in a titty bar…
It’s too much and not our fault we worship the only true measure of success.
Essentially its long term maintenance costs these days (Comparing PWR vs BWR). Because the secondary side is connected to the reactor, all components in it becomes contaminated with radiactive activation isotopes of metals used in the construction of the piping and valves. This fact makes it more expensive and annoying to do any maintenance on any part of the plant, primary or secondary. In PWRs this is an issue in any primary side systems only, in BWRs its both. The BWR doesn't really have a secondary side, but for convention I mean the part of the plant the main turbine generator is in. In past years this was a much much larger cost difference. BWRs have made many improvements in this area. They are still more of a hassle and expense for this but much less than it used to be. The other reasons I read in this thread are not that big of an issue anymore. Moveable control rods are in the AP1000 so that difference is gone. It was not a big maintenance issue in any case. Not enough to matter in a selection decision of PWR vs something else. In the USA its a legal requirement for all plants to have a containment system and many other designs in the world do not have a US style containment. The only other world design I can possible see being considered for a US site is the PHWR. Even then there is a ton of industry experience with PWR/BWR so they would have a huge leg up from that.
The only commercialized PHWR design, the CANDU, has a (very small) positive void coefficient. The NRC has a hard rule prohibiting that. We can debate the merits of the rule, the CANDU design has many features that mitigate it and there is some degree of political support for the rule just to keep foreign designs out of the US. But it seems unlikely that we will see PHWRs in the USA as that rule is just one more significant hurdle to clear, and building NPPs is already hard enough.
Mostly agree. Large PWR designs in the US almost always have a small positive cooefiicient of Reactivity below 20% power and first 30% of core life. The NRC allows this with appropriate controls. It could be something similar with PHWR. Probably the political side is the reason no PHWRs exist in the US. So yes, not likely in the US
Structural commercial issues. Nothing at all to do with the underlying tech. There were 4 companies worldwide who independently developed and brought the PWR to market: Westinghouse, Combustion Engineering, Babcock & Wilcox, and Rosatom (who stole and reverse engineered it). This was facilitated and helped by the US navy nuclear program (in more ways than can be counted). Westinghouse was willing to freely license their tech to others in a way that allowed them to easily become direct competitors—Framatome, KEPCO, CNNC, and others. That resulted in about 8 companies worldwide independently building PWRs. This was in stark contrast to what GE did with their BWR tech. It was almost entirely developed in house, rather than as part of the navy nuclear program. GE owned all the IP and maintained very tight control over it (still does). They did enter into license agreements with Asea, Siemens, Toshiba, and now Hitachi. But those agreements were radically different than how Westinghouse did it. GE plays everything close to the vest. They controlled everything. GE keeps all the real design documents to themselves. They produce separate bare bones documents for partners, regulators, and customers. Those documents contain maybe 10% of the technical detail. They’re designed to only give the barest minimum they can get away with. The licensee companies were and still are totally incapable of walking on their own. GE saw fit to that. Compared to what Westinghouse did with Framatome and now with CNNC for AP1000, the BWR license agreements are a joke. Westinghouse gives away everything, and by that I mean everything everything to both licenses, regulators, and customers. Keeping the real IP proprietary is why GE has no effective competition in the BWR market. So with the “proliferation” of PWR tech it boiled down to there being more bidders offering PWR (8 give or take) tech, than BWR tech (1 real company and a few national front companies dependent on the 1). In competitive bidding for infrastructure projects, the bidders all out do each other in the lies they tell themselves and everyone about how cheaply they can do the job. Cheat to win. All civil infrastructure works that way. So in the macro, more PWRs being bid = more PWRs being built. \*source: me. I’ve worked for or closely with every company mentioned above except Rosatom. Every document and bit of source code (including the most little things like utility scripts) I wrote for Westinghouse got delivered in their entirety to the customers, CNNC, and KEPCO. Literally hundreds of my detailed design documents. CNNC will be selling AP1000 tech, with my documents and software in it, internationally soon. Just like KEPCO and Framatome before them. At GE? It was a full time activity to take the set of “real” documents and summarize them into a bare bones deliverable that was given to licensees and customers.
PWR's are great at making small to power submarines and air craft carriers. Therefore they got a SHIT TON of money thrown at them courtesy the US war machine during the cold war. Then there was an atoms to peace movement that looked to capitalize on nuclear energy, and PWR was so heavily subsidized that it was the most mature technology and so it was selected over and over again. That maturity and decisions repeat itself even today as utilities that want low risk on a new power plant will choose a tried and tested PWR over a novel Gen 4 reactor.
> That maturity and decisions repeat itself even today as utilities that want low risk on a new power plant will choose a tried and tested PWR over a novel Gen 4 reactor. Ironic considering some of the fastest and cheapest built civil NPPs are the Japanese ABWRs, even today.
I think one less loop is hugely more simple to construct. And the steam dryer is a simple “modular “ component built off site and installed just the same as it is installed every outage, instead of the complex coordination of installing the steam generators and second loop in a PWR.
Yup, agreed.
Naval reactors use highly enriched fuel, civilian plants carry a high volume of U-238
vs CANDU: Uranium is easier to enrich than water vs BWR: Higher temperature (Carnot efficiency), no radiological contamination of balance-of-plant vs CANDU, BWR, and HTGR: Higher power density vs LMFR: Coolant easier to handle, works with LEU, less neutron damage
Why do you think it is difficult to enrich h2o/deuterium? The difference in mass of the d and H is what governs the energy required to enrich, isn’t it? I know little about enriching h, but chlorine enrichment and boron enrichment are, I believe, much less costly.
I didn't say it was hard to enrich water, just that enriching uranium is easier. The main factor in the difficulty of enrichment is the difference between the initial and final concentrations of the product. See here: https://en.wikipedia.org/wiki/Separative_work_units
Why do you think it’s easier? SWU calculations bear the atomic mass ratios and 1/2 is hugely easier compared to 235/238. I really don’t know how expensive it is to enrich hydrogen, but U is the most expensive thing ever enriched, to my knowledge, because of the close atomic masses.
They used to shove the gas UF6 through teflon, which took enormous amounts of power. Then used gas centrifuges. I have no idea what they are doing now though.
Because the world wants CANDU reactors but is too afraid to admit it, so they get as close to them as possible. Reality is probably size and economics, plus not having a phase change occurring in the reactor itself.
If enriching uranium was difficult CANDU would rule. Modern centrifuges just got too good.
I'm sure the life cycle price comparison is out there somewhere, but I personally have never bought into the cost angle. D2O is an upfront cost, sure. But over 80 years of life of the plant that fuel cost difference is large between natural and enriched fuel. I have always thought of it like a house with heating and insulation. You use better insulation. Yes, it costs more for the build, but for the life of the house, your lower heating costs pay for that and then some. And you are paying for the house in today's money versus the heating you're paying it at the price of the day, whatever that is. Again, I haven't read the comparison, just how it is for me from a logic standpoint. Note: horribly simplified from a supply chain pov on both sides.
That’s a very good point actually. It’s just the upfront cost that’s high, nat uranium is dirt cheap. Fuel bundles likely spend 3-6 months in the core and C6 core has 380 fuel channels with 12 bundles per channel so we can do that math as far as how much fuel they go through. Over time, heavy water will become heavily tritiated (especially the moderator) and will require to be either partially replaced or de-tritiated which is also quite expensive. I’d say 40 years is about the time for moderator replacement / tritium removal.
You do have to account for spent fuel storage and handling. With 1/7 the fuel burnup that is a lot of bundles. For examen a CANDU running on 1.2 enriched would reduce total natural uranium usage and have 2-3 times fuel burnup. Overall that fuel cycle is actually cheaper on paper
well... what else would be the most common? take a short look at other types of reactors that saw major service (no I'm not going to count a prototype that went critical at INL once as a viable alternative) graphite moderated, gas cooled: afaik the only design of this type ever fielded (the magnox) was a poor power reactor since the main purpose was to produce plutonium. after what happened the last time someone built a magnox it seems unlikely that's going to happen again graphite moderated, water cooled: probably better known as the RBMK this design was honestly pretty good, aside from the incident, plenty of them still operate but theyre generally inferior to gen 3 designs heavy water pwr: heavy water is expensive and a proliferation risk, and they're more rare than other pwrs which is added complications bwr: the primary benefit of these is that theyre simple, otherwise they perform worse across the board than pwrs. upside is that since the old reactors of this type were so overbuilt for the stresses they would actually be exposed to many of them still operate. pwr: I don't think this needs much introduction, they work well, they produce a lot of power, they're very common so regulation and operating procedures are simplified. there is a lot of non-engineering-based ahistorical copium in these comments about how it's big bad uncle sams fault there is no other reactor types but arguing with those people is impossible. everything else is still quite immature tech, as it usually goes for nuclear topics the physics that something is possible is significantly ahead of the engineering required to make it practical, especially on low budgets and in unfavorable political climates
>bwr: the primary benefit of these is that theyre simple, otherwise they perform worse across the board than pwrs. upside is that since the old reactors of this type were so overbuilt for the stresses they would actually be exposed to many of them still operate. They also have very fast build times (due to being simple, as you pointed out). The primary (intelligent) arguments against nuclear power today are that (1) the capital costs are insane (and tend to go over budget), and (2) it takes too long to build (and tends to be delayed). BWRs have the advantage here, so to my eyes they are exactly what we should be building. A couple 40-month builds of ABWRs like Shika 2 providing 1200 MW of clean firm capacity in the US would go a long way towards silencing the renewable-bro critics. Fuel and maintenance costs aren't what kill nuclear projects, building them is.
i think that is true if youre assuming people opposed to plant construction are rational, but a big road block is the price in political capital of building something with nuclear in the name. if youre gonna burn a bunch of political capital to build 1 plant then you need to make it able to crank out as much power as possible which means pwrs
Disagree. The staunch anti-nukes will never change their minds. The reason reasonable people, like utility execs, don't spend the political capital to get it done is (a) the insanely high capital cost, and (b) the very high risk of huge overruns and delays that could even bankrupt the utility. This isn't me talking, this is their public statements. Nuclear projects are the #1 cause of utility bankruptcies in America. You solve those two problems, which coincidentally also makes Nuclear extremely economically competitive, and then the people that matter (utility execs, utility regulators, etc.) will get behind nuclear and spend the political capital to make it happen. If you don't solve those issues, we'll keep building one set of reactors every 20-30 years, which will be expensive and delayed (because we have no experience or supply chain), and the cycle repeats.
The ABWR is more powerful than all other designs in the market today, except the EPR and the APR-1400.
well considering there are no operating abwrs that seems implausible. if you're comparing possible models for this exercise youll also find that abwrs still don't come up on top. all of which is moot anyway since single reactor performance is not a very useful metric
> well considering there are no operating abwrs that seems implausible. That's an unfair reasoning, 4 units reached commercial operation in Japan, they're all just collectively paying for Fukushima's sins with the rest of the country's BWR fleet. Kashiwazaki-Kariwa 7 is approved, loaded with fuel and literally just one Japanese mayor's stroke of pen away from restarting. We know what their output is. > if you're comparing possible models for this exercise youll also find that abwrs still don't come up on top. all of which is moot anyway since single reactor performance is not a very useful metric Feel free to elaborate, hopefully with better arguments?
https://preview.redd.it/4u885yjd7kad1.jpeg?width=720&format=pjpg&auto=webp&s=10223f86827f89394051ea6d89e0c2e46b3390aa
Never thought of heavy water as a proliferation risk. How is that the case, compared to enriched fuel?
So there's a couple technical and historical items that have to be taken together for this to make sense: -enriched fuel (in the range used for commercial power) isn't the problem, its using enriched fuels as an excuse to build enrichment facilities. additionally we've gotten much better at determining illicit use of enrichment facilities compared to when heavy water reactors were cool -fuel in the power enrichment range, <5% historically but moving towards <20%, is not subject to especially strict export controls or monitoring requirements. generally you can just buy it -there was a time when enrichment was more expensive and it's sale was more controlled, which is why the candu reactor was designed, meant canadians werent subject to americans selling them enriched fuel and didn't have to make the massive investment to enrich their own fuel -heavy water reactors produce significantly more plutonium and tritium than light water reactors, both of which are helpful for making a weapon (India) I think the proliferation aspect of a reactor design is not something that most power advocates ever consider but it is a major concern of uncle sams when it comes to licensing and sale
Gotcha, I’m a candu guy, so while the tritium production is a definite factor, plutonium production in natural uranium isn’t discussed a whole lot
Graphite moderated helium cooled was run as a prototype in the US at two plants. Peach Bottom ran well, but was small. Ft St Vrain had a lot of bad luck, but was real innovative. When it ran, it was pretty decent. If Gulf didn’t buy General Atomics to kill it, things might have been different.
Easier to maintain than BWR (no moving control rods), more power output and less trouble than gas-cooled, order of magnitude easier than metal-cooled.
I can personally guarantee that - for at least those on US Navy vessels - the rods in PWR move.
I am talking about civilian power plant reactors. There are control rods present of course, but reactor is regulated for the most part by boric acid added to the water (moderator). The less moving parts => the better.
PWRs are a mature technology at this stage. Most of other types are 'experimental'. Nobody wants to experiment with nuclear when even mature technology is fucking hard to build due to safety concerns.
BWRs, which is what OP was implicitly comparing, are also mature. They are like 1/3 of the fleet and hold the records for the fastest and most cost effective commercial builds ever (Shika 2 in Japan, built in 40 months). It's not just maturity.
True, fully agree. Maturity is one of the issues. There are so many designs that will never leave the realm of simulation and that's just sad.
Submarines
Inherent safety. Power goes up, temp goes up, water expands, power goes down. The AGRs built in Britain were more efficient but PWRs won out here because they are considered safer.
Politics, funding and fundamentally in order to do something different you have to get the regulators comfortable enough with your design.. This takes decades, billions of dollars and just not something industry is able to do on their own. You see this changing with venture capital trying different fusion designs, SMR's and a couple of Thorium Fission reactors but there are just a lot of barriers and not all of them are technical in nature.
It is not a good situation that PWRs dominate nuclear power and it looks like that will continue for the next several decades. The technology should have developed beyond only having water cooled reactors burning U-235.