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PostPosted: Jan 30, 2014 4:28 pm 
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The whiter paper is available on the Transatomic web site:
http://transatomicpower.com/white_papers/TAP_White_Paper.pdf


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PostPosted: Jan 30, 2014 7:12 pm 
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Thanks !
Some details at last.

I especially like this:

Quote:
There are other ways of achieving a neutron spectrum capable of burning SNF. For example, thermal-spectrum CANDU reactors are able to run on spent nuclear fuel because they also employ an efficient moderator (heavy water instead of light water) to reduce neutron capture. However, burnup in CANDUs is limited by the accumulation of fission products that are trapped in the fuel rods. The TAP reactor circumvents this limitation by continuously removing fission products from its liquid fuel.


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PostPosted: Jan 30, 2014 9:54 pm 
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Cladded zirconium hydride as a moderator. What will they clad it with? Hastelloy-N? The thermal neutron flux will rapidly damage the Hastelloy. Other typical cladding materials would be eaten up by the fluorides. The fluoride salt and the hydride are chemically reactive, if the salt gets into the moderator you'll have HF formation and ZrF4 formation.

Seems to me they've traded one set of problems (graphite's poor moderation and swelling) for another set (ZrH reactivity and instability).


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PostPosted: Jan 31, 2014 12:01 am 
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Kirk Sorensen wrote:
if the salt gets into the moderator you'll have HF formation
At 650C, hydrogen will pass through the cladding and into the salt regardless of whether the cladding is intact or not: HF will then accelerate the process. Not cool.


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PostPosted: Jan 31, 2014 1:39 am 
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Just to spell out their claim. Starting on 1.8% LEU is indeed easy, no problem with that claim (graphite can do far lower than that with the same salt). However for their claims of complete burn up while using a makeup feed of 1.8% LEU or SNF, they will need a conversion ratio of at least 0.98. That with the U-Pu cycle in a thermal spectrum using ZrH1.8 as a moderator.


David L.


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PostPosted: Jan 31, 2014 2:37 am 
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Figure 10 shows the actinide concentration with 2x 235U to 239Pu. Since 235U and 239Pu have the same fission cross-section it says they are burning twice as much 235U as 239Pu. Barring significant 238U fission the makeup fuel must then also be around 2/3rds 235U and 1/3 238U (the precursor to 239Pu).

Given the light enrichment almost all the fission of 235U and 239Pu must be happening in the thermal regime.

The only way this could work as a near isobreeder would be if there is a fantastic fission of the 238U. 238U looks very nice for neutrons above 1MeV. Could the bonus be that huge??

Also, I don't see the fluorine inelastic capture effect that I've seen in other fluoride MSR's - even fast fluoride MSRs from France.


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PostPosted: Jan 31, 2014 4:09 am 
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Some really strange stuff is in this paper.

They mention Ponomarev-Stepnoi et al., which have looked at thermal cycling of zirconium hydride. But the temperature is too low and it wasn't in a nuclear reactor radiation flux with neutrons and gammas everywhere. It is not a good reference and even then, several percent hydrogen release is a LOT of hydrogen (some of us are worried about embrittlement of a few hundred grams of tritium).

TRIGA reactors aren't a good reference. They operate cold, very cold in fact. You could barely make bath water with that reactor. Stability is very different at low temperature, and terminal hydrogen solubility in cladding is very low at such temperatures. It is well known that at elevated temperatures, hydrogen badly embrittles most cladding. The ones that aren't embrittled (aluminium etc.) are unsuitable either on high temperature or on fluoride salt compatibility grounds. Has anyone ever tried zirconium hydride in a high temperature reactor?

Hyperion, the company that wanted to use hydride fuel, is down in the gutter.

The 1200C boiling point mentioned in the paper, where does that come from? LiF based salts have very high boiling points. LiF >1600C, UF4 >1400C. No gross boiling would occur with LiF-UF4 eutectic even at 1500C because the LiF keeps volatility down. Before those temperatures really scary things happen with zirconium hydride. They are not very clear on the natural circulation afterheat removal so we can't judge the failure modes of that system.

In all, its a pretty hairy design.


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PostPosted: Jan 31, 2014 4:50 am 
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Then there's this quote:

Quote:
A light-water reactor core, filled with solid fuel pins that are poor heat conductors, requires a cooling period of months to reach a stable cladding temperature. This mismatched timing – hours to overheat versus many months to cool off – is what makes nuclear safety for light-water reactors enormously challenging, and leaves these reactors particularly vulnerable to disasters that were not anticipated at the design stage, known as “beyond design basis” accidents.


That's completely misleading. It does not require months to reach a stable cladding temperature, LWRs can achieve cold shutdown in a matter of hours. The issue is continuous decay heat removal. The TAP has a lot of decay heat stores (offgas tanks, salt seekers tanks, reactor vessel, drain tank, processing lines, etc.). Any reactor with a reliable passive cooling system for this decay heat is safe. LWR has no downside over a TAP here. The TAP appears to have a lot more opportunities for cooling systems to fail since they have more of them, some have high volatility (offgas tanks), others have corrosive fluids (bismuth or other liquid metal extractions), high temperatures... lots of opportunities for "beyond design basis" accidents. A particular accident is the residual decay heat in the reactor vessel after a rapid drain (station blackout). With no fuel salt left there's little thermal capacity amount of zirconium hydride is much smaller than graphite in a graphite version, things heat up rapidly on residual fission gas, entrained salts and such. It will overheat the cladding, reactor vessel/piping, likely release large amounts of hydrogen that will make things worse... ORNL found this a particular problem and they proposed circulating gas with the primary pumps to cool things down... which doesn't work in a station blackout.

They mention a Burns and Roe (esteemed engineering firm) report on the decay heat removal system. Is it available online?


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PostPosted: Jan 31, 2014 9:05 am 
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This research,

http://www.sciencedirect.com/science/ar ... 1510002370

indicates that zirconium hydride isn't stable at all during a transient or accident. It evolves gaseous hydrogen to a certain equilibrium pressure that is temperature dependent. That means pressurized moderator rods, which will then creep and fail at 700C unless they are relatively thick (which is bad for neutronics).

I wonder about hydrogen becoming HF though. If the salt is kept in a sufficiently reducing state, there would not be sufficient free fluorine left to make 2HF from H2. The H2 could then indeed be removed fairly easily through the offgas system.


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PostPosted: Jan 31, 2014 10:45 am 
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Cyril R wrote:
I wonder about hydrogen becoming HF though. If the salt is kept in a sufficiently reducing state, there would not be sufficient free fluorine left to make 2HF from H2. The H2 could then indeed be removed fairly easily through the offgas system.


You may be right about that. ORNL sparged molten salt all the time with H2-HF mixtures to remove oxide contaminants. Oxide intrusion in the molten salt may be the biggest concern about HF formation.


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PostPosted: Jan 31, 2014 12:42 pm 
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Lars wrote:
The only way this could work as a near isobreeder would be if there is a fantastic fission of the 238U. 238U looks very nice for neutrons above 1MeV. Could the bonus be that huge??
What's so fantastic about that ? ....it works in fast reactors: Just need to make sure that the fast peak in the bi-modal spectrum is big enough, and that the thermal peak is well away (ie. below) from the energy level where eta is poor. The TAP report does say their design minimizes flux in the epithermal range, and this can be seen in their Figure 8.

Lars wrote:
Also, I don't see the fluorine inelastic capture effect that I've seen in other fluoride MSR's - even fast fluoride MSRs from France.
The fluorine inelastic scattering effect occurs mainly below 0.6 MeV. The high eta region, plus U238 fission, are around 2 MeV and higher: No connection between that and fluorine inelastic scattering.


Attachments:
TAP_Fig-8.JPG
TAP_Fig-8.JPG [ 182.22 KiB | Viewed 6522 times ]
Li6,7_C12_F19_(n,TOT)_(n,g)_etc_fast_with_MSFR_spectrum.gif
Li6,7_C12_F19_(n,TOT)_(n,g)_etc_fast_with_MSFR_spectrum.gif [ 465.01 KiB | Viewed 6522 times ]
Pu239_U233_U235_U238_eta_fast.gif
Pu239_U233_U235_U238_eta_fast.gif [ 37.58 KiB | Viewed 6522 times ]
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PostPosted: Jan 31, 2014 1:02 pm 
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Fantastic, as in, mostly a fast spectrum reactor would be needed to get to isobreeding. CANDUs gets lots of fast fission since the lengthwise uranium rod profile has no moderator and D2O has a low slowing down power. With H the slowing down power is so good you can't have as much fast fission as a CANDU. Since they are using F and Li7 in the fuel salt the fast fission bonus there is limited as well.

Since a CANDU at BOL doesn't get a CR close to 1, it follows that it is very unlikely that a TAP can run at isobreeding on SNF (no Th).

This is even more the case when you consider Ni/Cr/Fe cladding losses for the hydride moderator, Zr losses, H losses (over D), Li7 losses and processing actinide losses.

Note that the cladding that Transatomic will use is given as "--". Oh yeah. That makes it plausible.


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PostPosted: Jan 31, 2014 4:11 pm 
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This paper also makes an incorrect claim about U-233 and weapons proliferation, stating its usefulness as fissile material.


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PostPosted: Jan 31, 2014 8:53 pm 
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So Zirconium Hydride would be a useful moderator but for hydride's annoying tendencies to release hydrogen in a wide variety of conditions?


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PostPosted: Jan 31, 2014 9:10 pm 
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E Ireland wrote:
So Zirconium Hydride would be a useful moderator but for hydride's annoying tendencies to release hydrogen in a wide variety of conditions?
"Useful" yes, but far from ideal.
Designers of Candu reactors would love to get rid of natural zirconium -- and substitute either material enriched in Zr-90/94 or C-SiC composite.
Still, it's a lot better than the Iron and Nickel in steel.
Hydrogen also absorbs a lot of neutrons, which is why >99.5% pure heavy water is used in Candu.
Then of course there's all that neutron-absorbing Xenon, that builds up as soon as the reactor starts operating - and which is stuck in the fuel, unless it's a fluid.....


Attachments:
Fe54,56_Ni58,60_Zr90,91,92,94,96_(n,g)_epi.gif
Fe54,56_Ni58,60_Zr90,91,92,94,96_(n,g)_epi.gif [ 54.42 KiB | Viewed 6456 times ]
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