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PostPosted: Feb 16, 2014 1:15 pm 
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Japanese researchers proposed a LWR-breeder: the reduced moderation water reactor (RMWR), about ten years ago, which is basically a re-engineered ABWR, with "tight lattice" fuel assemblies and less water in the core (see attachment from IAEA).

How interesting would it be to use Th-Pu fuel in this type of reactor ? Are there specific safety issues ("reduced moderation") with the RMWR ?


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PostPosted: Feb 16, 2014 4:35 pm 
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This is roughly similar to the Shippingport thorium core. Which was PWR not BWR but the idea is similar, get a tight pitch for faster spectrum, reduce loss to Pa and get more neutrons from the Pu-TRU chain. Starting up with Pu/TRU fissile (rather than Shippingports HEU) will make the Pu/TRU advantage from faster spectrum much stronger.

No unsolvable reactivity safety issues, in fact it is a highly undermoderated reactor. Core flooding after a LOCA is an issue, but if you borate the flooder water it is not a problem. The safety issues have more to do with thermal management - sufficient cooling of the cladding. Tighter pitch means less coolant and worse natural circulation after loss of pumps...


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PostPosted: Apr 02, 2014 6:39 am 
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There have been some interesting studies on the RMWR in the context of things like Uranium Nitride fuel to allow for increased lattice pitch and thus ameliorating some of the safety issues.

But remember if the reactor is flooded core damage is essentially impossible even if natural circulation breaks down because the cooling at the bottom of the reactor will boil and that will force mixing.


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PostPosted: Apr 02, 2014 11:47 am 
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In another context and thread, I have suggested a reduced moderation MSR.
It really simplifies matters by using a water tube boiler with water acting as coolant and moderator/moderated neutron source. The liquid fuel stays in the fuel space and the heat transferred in boiler tubes, as in any boiler. It will have to be a pressurized reactor keeping water moderator in liquid phase. The fuel can be in a mixed liquid/solid phase to maintain the temperature at melting point. A low cost waste burner/DMSR fuel can be the initial design and thorium/U233 introduced later.


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PostPosted: Apr 02, 2014 2:39 pm 
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jagdish wrote:
In another context and thread, I have suggested a reduced moderation MSR.

With fluid fuel you can do a LOT better than that.

Think of "reduced moderation" as something that allows you to make the fuel channels much wider: With the fuel circulating for heat transfer, the only limitation on fuel channel diameter is the surface area available for thermal neutrons to keep the reactivity at critical, while the inner part performs the breeding function, in a fast spectrum. That's the essence of a bi-modal reactor.

To be sure, the surface area required for thermal neutrons to keep the reactivity at critical is MINIMIZED by having the lowest energy neutron spectrum external to the large fuel channels.
Minimizing the surface area of fuel channels means, of course, having a smaller number of wider ones - as opposed to a large number of thin ones.
This is somewhat similar to the RMWR idea, but without the problem of maintaining adequate cooling of solid fuel rods in a very tight lattice.
Also, the overall layout of the RNWR - which is very homogeneous - results in increased fissile loading requirements, because there is no significant thermal neutron population.


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PostPosted: Apr 02, 2014 5:46 pm 
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There are no free lunches. As just one example, bigger fuel channels means bigger fuel inventory, which becomes a problem even for NU fuel if you're thinking of rapid fuel processing.

There are also thermal hydraulic issues with heating up just the sides of a fat fuel channel. The central fuel will be subcooled, but that's where most of the mass flow occurs, so you end up with serious issues such as not heating up most of your coolant flow. This is a power reactor.


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PostPosted: Apr 02, 2014 8:29 pm 
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I was thinking of reversed arrangement-a water tube boiler. The moderator-coolant water in the tubes provides limited moderation to the liquid fuel near them,- a moderated neutron source substitute for accelerator- and the bi-modal fission away from it.
With fuel coming from reprocessing of used fuel, high inventory is really a plus point. It could be promoted and licensed as a waste burner. There is so much used fuel and depleted uranium around that fissile feed is the only limitation. This requirement is reduced by reduced moderation and bi-modal burning. It is, I think, an improvement on the transatomic idea.


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PostPosted: Apr 03, 2014 5:52 am 
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Cyril R wrote:
There are no free lunches. As just one example, bigger fuel channels means bigger fuel inventory, which becomes a problem even for NU fuel if you're thinking of rapid fuel processing.

There are also thermal hydraulic issues with heating up just the sides of a fat fuel channel. The central fuel will be subcooled, but that's where most of the mass flow occurs, so you end up with serious issues such as not heating up most of your coolant flow. This is a power reactor.


That is also a problem that occurs in LWRs (albeit at a much smaller scale, I admit), and the solution is somehow similar : add flow-shaping structures (like swirl vanes on spacer grids of fuel assemblies) to get a nice turbulent mixing. Of course there is a limitation to that, but surely you can find a compromise.


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PostPosted: Apr 03, 2014 12:35 pm 
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This concept looks very similar to the SCWFR reactor. A lot of work on this reactor type was performed in Japan. A German university in Stuttgart (Magnus Mori) participated.
The difference is that the SCWFR (supercritical water fast reactor)works with supercritical water as coolant and moderator. The challenge of this reactor type is the big difference of the thermodynamic and nuclear properties of the water at the inlet (280°C) and the outlet (520°C) at 25MPa. It results in an uneven axial performance and burn-up.

If this concept keeps the temperature below 370°C this challenge is avoided and it might be a very interesting option.

Holger


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PostPosted: Apr 03, 2014 1:36 pm 
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Boris H wrote:
Cyril R wrote:
There are no free lunches. As just one example, bigger fuel channels means bigger fuel inventory, which becomes a problem even for NU fuel if you're thinking of rapid fuel processing.

There are also thermal hydraulic issues with heating up just the sides of a fat fuel channel. The central fuel will be subcooled, but that's where most of the mass flow occurs, so you end up with serious issues such as not heating up most of your coolant flow. This is a power reactor.


That is also a problem that occurs in LWRs (albeit at a much smaller scale, I admit), and the solution is somehow similar : add flow-shaping structures (like swirl vanes on spacer grids of fuel assemblies) to get a nice turbulent mixing. Of course there is a limitation to that, but surely you can find a compromise.


It is a very minor problem in LWRs. Check how tight the fuel tubes are and how small they are.

But yes, turbulent tricks can be used. Keep in mind though salt is viscous. It likes to be laminar. In any case Jaro's 10 cm fuel tubes would stretch whats possible here. Likely it would have to be a bundle of smaller tubes coiling (like a helix) inside the 10 cm tube. It means a lot more pipe where we don't want it. I've no idea how to adequately mix flow in a 10 cm tube without really complicating core design.


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PostPosted: Apr 05, 2014 3:31 am 
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HolgerNarrog wrote:
This concept looks very similar to the SCWFR reactor. A lot of work on this reactor type was performed in Japan. A German university in Stuttgart (Magnus Mori) participated.
The difference is that the SCWFR (supercritical water fast reactor)works with supercritical water as coolant and moderator. The challenge of this reactor type is the big difference of the thermodynamic and nuclear properties of the water at the inlet (280°C) and the outlet (520°C) at 25MPa. It results in an uneven axial performance and burn-up.
If this concept keeps the temperature below 370°C this challenge is avoided and it might be a very interesting option.
Holger

A dual mode may be preferable to SCW Fast Reactor. Higher temperature of Super Cooled Water could give a higher choice of salts but all choices may remain open. There may be advantages in keeping the temperature below triple point.
Dual mode is an uneven burning but so long as the fuel is in same connected space, it would keep on mixing by convection.
Fur start of MSR for used fuel burning, Keep It Simple. Treat power as an additional benefit.


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PostPosted: Apr 08, 2014 8:57 am 
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camiel wrote:
Japanese researchers proposed a LWR-breeder: the reduced moderation water reactor (RMWR), about ten years ago, which is basically a re-engineered ABWR, with "tight lattice" fuel assemblies and less water in the core (see attachment from IAEA).

How interesting would it be to use Th-Pu fuel in this type of reactor ? Are there specific safety issues ("reduced moderation") with the RMWR ?

We tend to classify reactors as MSR or the more common water cooled reactors.
A water cooled and moderated MSR could be a reduced moderation reactor with many of the advantages of all three. For a change, it could be a PWR instead of the BWR.
It could be as simple as a boiler with radiation protection of a reactor. Will not require any air intake or exhaust.
P.S. We could start it as a waste burner and evolve to DMSR and a thorium fueled reactor.


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PostPosted: Apr 10, 2014 2:07 pm 
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There were plenty of initiatives to run LWR and Candu reactors with a thorium share (mix some ThO2 to the UO2 tablets in the fuel rods) in the fuel. The burn-up of the fuel could be increased due to the higher breeding ratio of the 232Th - 233U fuel. The build-up of higher actinides is reduced.

The main reason that thorium is not used yet in commercial reactors is the creation of 232U by a n -> 2n reaction from 233U. In the decay chain of the 232 U very hard gamma radiation is emitted.

As a consequence the handling of radiated thorium fuel is challenging. The actual nuclear infrastructure is not suitable to handle thorium fuel.

This might change in the future when processes become more and more remote.


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PostPosted: Apr 10, 2014 4:28 pm 
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HolgerNarrog wrote:
The main reason that thorium is not used yet in commercial reactors is the creation of 232U by a n -> 2n reaction from 233U. In the decay chain of the 232 U very hard gamma radiation is emitted.

Looking at the x-sections for (n,2n) reactions, it appears that the main problem is Thorium: You get Th231, which quickly decays to Pa231 (25h), which absorbs a neutron to become Pa232, which decays to U232 (1.3d).

HolgerNarrog wrote:
As a consequence the handling of radiated thorium fuel is challenging. The actual nuclear infrastructure is not suitable to handle thorium fuel.

This might change in the future when processes become more and more remote.

Yes. Agree.


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PostPosted: Apr 11, 2014 2:47 am 
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That's for really fast neutrons. Even RMWR has very few 5-10 MeV neutrons and no >10 MeV neutrons.


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