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PostPosted: Apr 25, 2011 2:43 pm 
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Why was there never much interest in marrying the fuel channel heavy water moderated CANDU with Fermi's and ORNL's aqueous homogenous reactor technology? Many of the problems of the homogeneous reactor could be solved by CANDU design and a uranyl nitrate or uranyl fluoride heavy water solution would offer neutron economy, zero fuel fabrication cost, avoiding refuelling machines, and increased power output advantages.

You could run on natural uranium without the bigger fuel fabrication economic penalty. There would also be interesting options of removing fission gas as well as possibly interesting electromagnet removal of lanthanide fission products, plus the advantage of operating the entire circuit in chemically reducing state to avoid corrosion issues.

Of course the entire primary loop will be radioactive, and the steam generators are going to take some neutron flux in addition to increased gamma flux. Though a pool type design would probably solve most of these issues.


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PostPosted: Apr 25, 2011 4:21 pm 
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Cyril R wrote:
Why was there never much interest in marrying the fuel channel heavy water moderated CANDU with Fermi's and ORNL's aqueous homogenous reactor technology? Many of the problems of the homogeneous reactor could be solved by CANDU design and a uranyl nitrate or uranyl fluoride heavy water solution would offer neutron economy, zero fuel fabrication cost, avoiding refuelling machines, and increased power output advantages.

Interesting question.
I don't have an answer, just a guess....
The philosophy in solid fuel NPP design is to contain the fission products in the fuel bundles, so that if there is a leak in the PHT loop, your whole reactor building won't get contaminated.
Of course with a complex system using pressurised fuel channels and robotic fuelling machines (just to mention the main items), there will always be small leaks somewhere.
With "clean" D2O coolant you basically just get a bit of tritiated water vapour contamination in the building, which can be managed by high-efficiency driers (I think they dry to a dew point of something like -70C).
Things could be much worse with FPs leaking out.
Of course the system would likely be far more leak-tight, since there would be no fuelling ports & robots.
But its still pressurised, so there is the potential.
With fluoride salt, one can take that potential away – possibly even operate under slight negative gauge pressure.
Problem is the traditional zirconium fuel channel tubes are incompatible with halide salts, and most other high-temperature alloys absorb too many neutrons.

The other thing is that I’m not sure you could get enough uranyl nitrate or uranyl fluoride in the heavy water solution to get a fuel concentration comparable to solid UO2 fuel pellet-loaded bundles with coolant circulating around them…
For sure the aqueous homogenous fuel would seriously increase non-fission neutron absorbtion in uranium while in the epithermal range, as well as kill off any fast fission factor.
It's a pretty safe bet that such a reactor could NOT go critical on NU -- which is what Canada was after in the first place, as we don't have U enrichment technology.....


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PostPosted: Apr 25, 2011 7:28 pm 
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But now that LEU is a commercial commodity......


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PostPosted: Apr 26, 2011 8:05 am 
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There will be fission products in the coolant all the time, because of fuel failures. These are kept at low level by water cleanup systems. I’m wondering if we could simply use such a system with much higher capacity, so fission products are rapidly removed from the coolant, in order to keep the radioactivity low, but use such cleanup so as to keep the actinides in the loop.

The uranium concentration is an issue with CANDU of course; most of the reactor volume is actually in the calandria. This will probably force the use of a slurry reactor, as ORNL determined that such reactors could be run at up to 4 grams UO3 per gram water. This is a roughly similar heavy metal density as the standard solid fuelled CANDU, so natural uranium can be used. Its important to consider the favorable fission product removal & elimination of fuel cladding effect on neutronics.

Such a reactor would have high pumping power requirements as a disadvantage, but much better neutronics and no fuel fabrication costs which are important advantages for natural uranium fuelled reactors.

The molten salt version still has the big advantage of low pressure operation.


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PostPosted: Apr 26, 2011 8:46 am 
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Cyril R wrote:
There will be fission products in the coolant all the time, because of fuel failures. These are kept at low level by water cleanup systems.

True, but fuel failures are rare: If you’ve ever visited a CANDU station, you may have noticed the great difference in size between the regular spent fuel pool and the failed fuel pool – which is usually empty, or close-to.

Cyril R wrote:
This will probably force the use of a slurry reactor, as ORNL determined that such reactors could be run at up to 4 grams UO3 per gram water. This is a roughly similar heavy metal density as the standard solid fuelled CANDU, so natural uranium can be used. Its important to consider the favorable fission product removal & elimination of fuel cladding effect on neutronics.

Would a slurry of UO3 be as easy to process as a uranyl solution ? ….wouldn’t there be some extra steps ?

Also, having “a roughly similar heavy metal density as the standard solid fuelled CANDU” doesn’t automatically guarantee the same neutronic performance: with micro-particles in a slurry, you still basically have a homogeneous mix of fuel & moderator, so I still maintain that the aqueous homogenous fuel would seriously increase non-fission neutron absorbtion in uranium while in the epithermal range, as well as kill off any fast fission factor, and that such a reactor could NOT go critical on NU.

Although the heterogeneous CANDU lattice doesn’t quite take maximum advantage of fuel-moderator segregation – due to heat transfer requirements for the solid fuel – the “partial heterogeneity” certainly has a strong positive impact on neutronic performance.
With an MSR version, where the fuel is its own heat transfer medium, where light nuclides like hydrogen and deuterium are kept out, and where fuel channel diameter can be increased without adversely impacting local fuel bundle heat transfer distribution, then the desirable effect of a heterogeneous lattice can be brought closer to optimum -- perhaps good enough for an isobreeder running on NU & equilibrium TRUs alone.....


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PostPosted: Apr 26, 2011 9:07 am 
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I was thinking about removing the fission products in solution and letting the actinide fuel particles flow through/back to the reactor. Plus sparge out gasses & perhaps magnetic removal of some noble metals (?).

Most moderation is done in the calandria, no? The D2O in the fuel tubes has a quite high mean free path so I was thinking that with 10 cm tubes (fairly standard CANDU size) you'd still get some fast fission.

As for the resonance losses in particular, I wouldn't be too bothered about using slightly enriched material, in the range of 1-2 percent enrichment, if necessary, to outcompete for increased resonance U238 absorptions. These days everything less than LWR enrichment is okay.

I’m not too worried about isobreeding, if you can do CR of 0.9 that’s going to be much better than CANDU. There are serious diminishing returns for trying harder to get to isobreeding, and as you’ve argued, the more important thing is to get any MSR started. I was just wondering about the aqeous homogeneous designs married to CANDU.


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PostPosted: Apr 30, 2011 12:45 am 
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If you are interested mainly in a fluid fuel for easy removal of fission products, you may as well have a higher enrichment and light water. Uranyl phosphate solution will have two useful characteristics:-
1. Easy to sparge out Kr and Xe gases.
2. Most of fission products phosphates will be insoluble and get precipitated. A small amount from bottom could be filtered and returned to the core. Once a year, part of the fuel can be processed and returned to original state.
What neutrons you lose to light water can be more than compensated by removal of fission products poisons.


Last edited by jagdish on May 01, 2011 7:58 am, edited 1 time in total.

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PostPosted: Apr 30, 2011 9:06 am 
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Sounds promising, Jagdish! Any references for the behaviour of uranyl phosphate solutions, solubilities and FP behaviour?


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PostPosted: May 01, 2011 8:11 am 
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Uranyl salts are generally soluble and the phosphate has been mentioned in Aqueous Homogeneous Reactors.
http://www.energyfromthorium.com/pdf/FFR_chap03.pdf
Otherwise, the Phosphates hardly figure in the solubility table.
http://en.wikipedia.org/wiki/Solubility_table
That is why I thought of phosphate in preference to sulphate.


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