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PostPosted: Sep 14, 2010 11:06 am 
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Cyril R wrote:
Thanks Jaro. The pro-LWR way to think of it ; ) would be to say it makes sense to use mostly thorium as fertile rather than U238 in for a BWR or PWR. :lol:

Agree !

The other thing I should point out regarding DMSR etc., is that graphite is a much poorer moderator than light nuclei such as hydrogen or deuterium (in H2O or D2O) -- which means that the neutron flux spends much more time bumping around in the resonance region....

Incidentally, 1eV on the spectrum chart is equivalent to a temperature of 11,605 Kelvin -- so the evetual end state of neutrons that don't get absorbed is definitely well inside the region where absorbtion is much lower for U238 than Th, for both LWRs and DMSRs.
Its just that in reactors with inefficient moderator or a tight fuel rod/channel lattice pitch, most of the neutrons never make it there.....


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PostPosted: Sep 14, 2010 11:50 am 
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Possibly sowing more confusion, but data from JAEA on absorbtion cross sections (n,g)
Code:
Nuclide      0.0253eV        Maxwell         Res Int
  U-238        2.683         2.690            275.6
  Th-232       7.338         7.300             84.29

'Maxwell' is a 300K spectrum, so CANDU-like. The resonance integral is calculated over 0.5 eV - 14 MeV, but they don't state what spectrum is assumed, or anything about self-shielding which makes tall, narrow peaks less effective than short, wide ones. I don't think this question is answerable by back-of-envelope methods, there are too many factors.


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PostPosted: Sep 14, 2010 2:02 pm 
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Not too relevant to PWRs or Nuscale but if you want more evidence that U238 is quite often far more absorbing than Th there is the data table 4.12 of the Ph.d thesis of Alexis Nuttin (Grenoble group). This early reference case was graphite moderated but a bit harder spectrum than the original MSBR design. The average or effective cross section of Th is only 1.50 barns and for U238 it is 8.9 (5.9 times higher). As I mentioned in another thread, for the DMSR design and a softer spectrum the ratio was U238 being 1.89 times higher. I don't recall the MSBR work ever showing the U238 absorption rates since there was so little of it but I suspect the ratio was somewhere in between.

From the LightBridge data for a seed and blanket PWR, their core seems to have roughly the same ratio of U235 to fertile (U238+Th) as a regular PWR has U235 to U238. I don't know if their metal to water ratio is any different but if not then I suspect that in a PWR spectrum the average or effective cross section of U238 and Thorium are quite similar.

David LeBlanc


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PostPosted: Sep 14, 2010 2:08 pm 
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Thanks Luke.
Luke wrote:
The resonance integral is calculated over 0.5 eV - 14 MeV, but they don't state what spectrum is assumed, or anything about self-shielding which makes tall, narrow peaks less effective than short, wide ones.

Similar numbers can be found in Basic Nuclear Engineering by A.R. Foster and R.L. Wright (p.242, 1977 ed.)
Quote:
The effective resonance integral for U238 runs from 9.25 b for pure metal to an upper limit of 240 b for an infinitely dilute mixture of U238 in moderator. For thorium the values run between 11.1 b for pure metal and 69.8 b for the infinitely dilute mixture.

I suspect that LWRs, with their UO2 ceramic fuel and tight lattice, would be closer to the "pure metal" case, as far as resonance absorbtion.
But its probably very close between Th and U238 in this case: both reactors can go critical on 3% to 5% fissile enrichment.
By contrast, the thorium equivalent of a natural uranium HW reactor will NOT go critical -- on just 0.71% U235.
One would probably need a minimum of 2% U235 (2.8 times higher).


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PostPosted: Sep 14, 2010 2:57 pm 
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jaro wrote:
Thanks Luke.
Luke wrote:
The resonance integral is calculated over 0.5 eV - 14 MeV, but they don't state what spectrum is assumed, or anything about self-shielding which makes tall, narrow peaks less effective than short, wide ones.

Similar numbers can be found in Basic Nuclear Engineering by A.R. Foster and R.L. Wright (p.242, 1977 ed.)
Quote:
The effective resonance integral for U238 runs from 9.25 b for pure metal to an upper limit of 240 b for an infinitely dilute mixture of U238 in moderator. For thorium the values run between 11.1 b for pure metal and 69.8 b for the infinitely dilute mixture.

I suspect that LWRs, with their UO2 ceramic fuel and tight lattice, would be closer to the "pure metal" case, as far as resonance absorbtion.
But its probably very close between Th and U238 in this case: both reactors can go critical on 3% to 5% fissile enrichment.
By contrast, the thorium equivalent of a natural uranium HW reactor will NOT go critical -- on just 0.71% U235.
One would probably need a minimum of 2% U235 (2.8 times higher).


So, if you want to run on LEU and maximise thorium, that means 90% Th232, 8% U238, and 2% U235. A lot more thorium than I would have thought was possible. LWRs such as NuScale would need about twice that, which means twice as much U238, and less thorium...

At the risk of going off-topic again, does all this mean metallic fuel thorium-uranium alloy in CANDU is ideal?


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PostPosted: Sep 14, 2010 3:21 pm 
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Modern LWR's burn down to under 1% U-235. The enrichment of the fresh fuel is as low as 3%, so you don't need nearly 3% enrichment to be critical, just to have an optimum burn-up with a refueling interval of 18 months.

The crucial factor in keeping critical with natural uranium fissile abundance is using graphite or heavy water moderation to avoid losing too many neutrons to light water absorption. With this type of design, especially with a low power density to reduce losses from xenon and proactinium, thorium would be a far superior choice for fertile.


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PostPosted: Sep 14, 2010 4:58 pm 
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Cyril R wrote:
So, if you want to run on LEU and maximise thorium, that means 90% Th232, 8% U238, and 2% U235. A lot more thorium than I would have thought was possible.

Yeah -- but that's JUST to go critical.... With solid fuel, you'd likely be out of business as soon as some Xenon builds up - on the first day :lol:

Remember that CANDU only gets about 7500 MWd/t burnup -- very low compared to LWRs (~45 GWd/t these days ?).
But its still economical, because of the absolutely minimal processing required to make NU fuel -- versus ANY sort of enriched stuff.

Even with an MSR, the isotopics will change over time.... an equilibrium derived from 2% enriched start-up may not be sustainable (at least not without very agressive FP removal, etc.)

Cyril R wrote:
At the risk of going off-topic again, does all this mean metallic fuel thorium-uranium alloy in CANDU is ideal?

I would say "yes".
Metallic fuels were tested in the early development days, but obviously abandoned in favour of ceramic oxide pellets....
I suspect the reason may be that about the only metallic alloy that was found to be acceptable for power reactor service was Zr-U, which is fiendishly expensive, so only makes sense if you are definitely going to be doing reprocessing..... Reprocessing makes little economic sense with spent NU fuel, so throwing away so much Zr is simply unthinkable.....


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PostPosted: Sep 14, 2010 5:19 pm 
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Moliterno wrote:
Modern LWR's burn down to under 1% U-235.

Possibly -- but don't forget that you also have about 1% Pu by that time 8)

CANDU reactors can burn down to 0.21% U235 (plus 0.40% plutonium).

But that doesn't mean that a core full of fuel with this much burnup could remain critical -- far from it !!
What happens is that a fraction of the core load is (close to) SNF, while the rest is fresh fuel, or intermediate burnup -- an equillibrium core mix which, by the way, sealed small modular reactors can't benefit from, if they're intended to run unrefuelled for 20 or more years.....


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PostPosted: Jan 10, 2013 7:39 am 
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I wonder about some of the safety claims. The concept seems very safe but there's one specific scenario I can imagine that has not been described by NuScale.

What happens if the recirc valves fail to open for whatever reason and the SGs are unvailable? Seems to me the core would boil dry, condensing its steam onto the inner containment wall. Then it gets interesting. Is the small size of the core sufficient for radiative cooling of most of the fuel assemblies? Or will it largely melt down and collect at the bottom, to be cooled there through the vessel wall (which has the condensed coolant to cool it on the other side for invessel retention). I wonder if they've designed for hydrogen pressure and inhibiting of condensation heat transfer. It seems to me they haven't, in stead are counting on not getting any core damage so no large amount of hydrogen either. But they still have radiolysis hydrogen, which adds up to a lot over a permanent station blackout which they claim this design can take.


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PostPosted: Jan 12, 2013 8:21 pm 
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Regarding metal fuel:-
Thorium has a melting point close to that of Zirconium, besides better thermal conductivity. It makes more sense to use metallic thorium. A layer of ThO2 can be passive enough to provide the nessary cladding.
Quote:
Cyril R wrote:
So, if you want to run on LEU and maximise thorium, that means 90% Th232, 8% U238, and 2% U235. A lot more thorium than I would have thought was possible.

Yeah -- but that's JUST to go critical.... With solid fuel, you'd likely be out of business as soon as some Xenon builds up - on the first day

Just make it 4% U-235 for necessary burn up till U-233 is built up. Check with AHWR LEU.
Quote:
Moliterno wrote:
Modern LWR's burn down to under 1% U-235.

Possibly -- but don't forget that you also have about 1% Pu by that time

In case of PWR's, why don't they fill a few with Heavy Water to burn the spent fuel further?


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PostPosted: Nov 26, 2013 4:29 am 
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Here is information on pressure drops for nuscale:

http://www.iaea.org/NuclearPower/Downlo ... BOWSER.pdf

Core: 8 kPa
Riser cone: 4.5 kPa
SG: < 0.1 kPa (????!!!).

So basically it barely works with NuScale's numbers, but note the other simulations/groups get very different results. If those are correct the design simply won't work at the advertised power level of NuScale. The SG pressure drop graph is clearly incorrectly labeled, but it seems to suggest 0.1 kPa which isn't plausible at all (even with very short SG tubing).

Sounds like a pretty big financial risk. If those 45 MWe modules turn out to be 20 MWe modules, NuScale is financially dead.


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PostPosted: Jun 25, 2014 3:01 pm 
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ASME video presentation by Jesse Reyes (Nuscale CTO) with design details and update on progress toward NRC certification

https://www.asme.org/engineering-topics/media/nuclear-power/video-small-modular-reactors-nuclears-big

Haven't watched the whole thing (runtime over an hour) .. first part = Dr. Reyes talking about NuScale SMR, second part = Paul Murphy presentation "The Development and Financing of SMR Projects"

My apologies if posting at the end of an old thread is improper


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PostPosted: Jul 02, 2014 7:26 am 
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200MW Canadian/Indian PHWR is the small reactor currently in use. Its Calandria, loading machines, heat exchangers and generators can be factory built and carried to site.
It is a versatile machine and can be used with various fuels, liquid moderator and coolants.
LWR's fuels are being developed with increasing enrichment for high burn up.
20% LWR with thorium as additional fertile and in lieu of consumable poison for re-activity stabilization can be developed as for the version of Indian AHWR
https://docs.google.com/file/d/0B6OcA2a ... y=CL7u17QB
PHWR can also be used with thorium fuel
http://dae.nic.in/writereaddata/.pdf_38
Very high burn up will ensure economy of operation, besides low capital cost of reactor per MW.


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