Energy From Thorium Discussion Forum

BWRs currently operate with relatively low steam conditions (~285C).
Why are they not operated at PWR primary loop pressures and temperatures? (320-340C?)

Surely the materials problems would not be that different considering as corrosion issues at these slightly higher temperatures will already have been dealt with by the PWR design?
A simplistic Carnot analysis would suggest that the electrical output for a certain thermal power by about 10% or so.

It probably has to do with pressure vessel fabrication limits at the time the BWR was invented.

At some 335C the boiling pressure is 2x as high as at 285C. So, the pressure vessel wall thickness is increased by 2x. A little more in fact due to temperature effect. Consider that BWR vessels are bigger than PWRs and this must have looked like an enormous fabrication issue at the time, and even today would be considered quite significant.

Operating at a higher pressure does improve stability of the boiling, for a number of reasons.

Since the BWR there have been few major innovations in LWRs. This is due to the increasing political and social entrenchment and regulatory ratcheting. So the HP-BWR never stood a chance.

There was some master thesis about the HP BWR from a Scandinavian student I think it was.

There seems to be so much room for improvement - even before you go for the crazy things like SCWRs.

Start with ESBWRs and progress to higher and higher steam qualities and simultaneously develop RBWR type cores to enable breeding from either thorium or DU (calculations show both are feasible and the former is simpler).

Higher steam qualities were tried in the early days of BWRs. The so called superheat BWRs. These were dropped because of fuel integrity issues - dry steam is not as good a coolant as boiling water. Eventually the saturated BWR at modest pressure was the path of least resistance.

Doing anything major with the nuclear part is hard. Operating at 335C means your pressure vessel is twice as thick. The bottom head forging, for an ESBWR rated vessel, has to be half a meter thick. That's hard even today, with all those control rod penetrations and other complications. The fuel rod design has to be changed, to something closer to PWR. A lot of testing would be needed to ensure safety in accident and transient performance.

If you want to improve efficiency, that can be done in other ways. The steam separators, steam dryers, steam reheaters, moisture separators, steam turbine blades, feedwater heaters, all can be improved. AREVA has done much already, they are getting 37% net efficiency in the EPR and Kerena (BWR) as well.

edit: wrong thread... moved post.

Pity we can't go for minimum supercritical conditions - ~380C.

That way we could potentially eliminate the steam seperators and so on and shrink the BWR core down again.
Probably still cold enough to allow use of existing claddings.

Has press size levelled out at the current 600t ingots or would they be pushing to larger levels?
This might enable a HP-BWR.

380C is asking a bit much from zirconium alloy. Maybe the most recent ODS zircalloys (Zirlo/M5) would be good enough. But we're talking about 23 MPa operating pressure, min. That's a design pressure of near 29 MPa. The forgings would be enormous, even with Inconel 718 they would be bigger than a 7 MPa ESBWR A508 steel vessel.

But other than materials there is a serious downside with supercritical. There's no recirculation flow inside the vessel. While this simplifies the reactor towards a once-through direct cycle (the simplest possible cycle) it also means you can't be fully passively safe. You always need pumps, even be they steam driven like RCIC.

ESBWR is pretty clever. A good if somewhat conservative compromise between all the requirements and challenges.

It is like Dr. Peterson has said: the ESBWR is the state of the art in LWR technology. The only way to better it is to switch to AHTRs and MSRs.

There has been some conceptualisation of HP-BWRs in Sweden :

http://www.neimagazine.com/features/fea ... st-of-both

Thanks Camiel. Looks like the Swedes have considered this the most.

Not sure I agree with some of the reasoning though. The modern fine motion control rod drive of the ABWR and ESBWR has fault resistant scram actuation and logic, backed up by electric drive insertion of the control rods, backed up by a liquid poison injection system. It is really quite difficult to come up with a scenario that fails all of this, and I doubt that if you can come up with an extreme enough event that it would help to have gravity inserted control rods (because the rods are likely stuck completely).

Control rods inserted from the bottom of the BWR means a higher control/shutdown worth. BWRs have more reactivity in the lower core. Inserting from the top means that advantage is lost plus the control rods become enormously long. This means an enormous drive and rod removal length. This increases the required height of the building, bad. Its too tall already at 60+ meters.

As for best of both worlds, the lower temperature and pressure of the BWR is quite nice on the materials, meaning thinner easier to make pressure vessel and easier on the cladding. Looks like an increase in pressure and temperature is not nice on the cladding at all, especially when you consider the increased void fraction meaning less liquid coolant entering the core. Definately looks worse than PWR on the cladding. In terms of the vessel it means a half meter thick reactor vessel head if made of A508 steel. And 370+ mm thick ring sections for the 20+ meter length of the vessel.

All of this means substantial redesign of nuclear cores and control rods and steam separators and dryers (control rods must now penetrate these). It just isn't worth it in my opinion. I think the way forward is to build out a standardized design like the ESBWR rather than endlessly trying to improve it requiring extensive redesign and relicensing.

Incremental improvements in the steam cycle are a great way to increase efficiency.

The AP1000 is a derivative of the AP600 and there is talk of pushing to a CAP2100 design with additional loops.

Is there any chance of pushing the ESBWR to larger sizes? After all it is just a larger version of the SBWR.

It is possible, in fact the ESBWR has been gradually uprated all the time since SBWR. The last version was 4000 MWt, they then widened the core a bit while keeping the same vessel size, this got them the 4500 MWt it is today. Going much bigger would need a bigger vessel. Certainly possible especially with superalloy vessel. Question is, is this worth it? I think we should stick to the standard version and try to build as many as possible to get cost down. Time to get some economy of production rather than paper economy of reactor scale.

I assume that the RMWR type core designed for the ABWR could be built for the ESBWR as well?
This would seem to make the reactor rather 'future-proof' as if the price of uranium rises drastically you can switch to a closed fuel cycle?

It also means you can forget an independent reactor construction programme and just throw everything at pumping out ESBWRs.

Would annular fuel pellet/fuel rods also help with uprating the BWR? I think that flow and heat transfer would be improved while also keeping the centerline fuel temps down.

Yes, it should be about as good as with PWR. Limits are similar - limits to dryout, critical heat flux, cladding temperature, cladding strain etc.

The cladding is the limit. BWRs operate with peak fuel temperatures below 1200C. The fuel melting point is >2800C so no issues here.

If you provide 50% more surface area, you can get 50% more power with the same cladding margins.

Big question with natural circulation is whether this would still work. Pressure drop rises much faster with power density. So this annular fuel may not be much use for ESBWR. It would be more interesting for ABWR-II and Kerena reactors.

If we are going to change the fuel rods we should make other changes. The fuel assemblies are too many. 1132 for the ESBWR. Making them twice the diameter means the number drops to 283 assemblies. Much faster refuelling, many fewer (albeit bigger and wider) control rods.

But I don't think we should do any of this. I think we should build ESBWRs as they are right now, no changes to the nuclear island, only incremental changes to the non nuclear, turbine and steam systems. Changing the fuel design just delays eveything 15-20 years. Not worth the gain. We needed a thousand ESBWRs decades ago.

Any thoughts on the Lightbridge type helical cruciform metal fuel for BWRs? The surface area would be much greater, and the central temperature lower.