Energy From Thorium Discussion Forum

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PostPosted: Feb 27, 2010 6:30 pm 
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Suppose you have a 1 1/2 fluid system with say 30M^3 fuel salt and 60M^3 blanket blanket.

Daily remove 1000 liters from the fuel salt (30 day cycle)
Vacuum distill to remove U (transfer to fuel salt)
send remaining salt to the blanket (fission products, Pa, and all)

Daily move 900 liters from blanket salt to fuel salt

Daily remove 100 liters from the blanket salt (600 day cycle)
Since the blanket salt has been out of fissioning for an extended time the decay heat will be markedly less
(fresh fuel has 7% less offgas & noble metal decay heat this would have 0.08% less offgas decay & noble metal heat).
Vacuum distill to transfer UF4, LiF, BeF2 (and ZrF4, CsF) to fuel salt.
Wait a year or so to let Pa233 decay and heat reduce (now at 0.06% less offgas & noble metals)
add fluorine to move UF3 to UF4
Remove U and Th to fuel salt
Keep remainder on-site for a long time (several decades)
Send to central repository for extracting Pu (and Am, Cm if this reactor burned much pu).

Advantages:
A large blanket serves to recoup more of the neutrons, keeping them from hitting the outside wall.
Fission products in the blanket keep it radioactive making it less of a prolif concern. In fact, much less than typical seed and blanket solid fuel jobs.
Placing the extracted core salt into the blanket means we productively recoup most of the fission product decay heat, without paying extra.
95% of the Pa is generated in the core, if we place the extracted core salt into the blanket where the flux is much smaller and the volume can be larger we lose fewer neutrons to Pa capture but without the complexity of Pa extraction.
If we can guarantee that the blanket and fuel salt have the same density and the level in the blanket and fuel salt are the same then there is no pressure on the wall between them - which would mean no strength is required and if the salts are the same except for U then there is no safety issue if the salts mix. So we could have a doggie door going from the fuel salt to the blanket and vice versa to physically guarantee that the pressures on the two sides of the wall is the same. Instead of a 5 cm thick first wall maybe we could do with a 1 cm thick wall - it only needs to be strong enough to hold up the empty reactor not the massive weight of the fuel salt.


Disadvantages:
We have to buy more salt - but we reduce the fission product storage heat load by a factor of six, so we will save on the cooling system for fission products.


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PostPosted: Feb 28, 2010 6:26 pm 
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Location: Teesside, UK
The only confounding factor I can think of at the moment is the possible suppression of UF4 vapour pressure in fluoride salt mixtures. The ideal (as in gas) case is vapour pressure(X) = (vapour pressure of pure X).(mole fraction of X in mix). For ZrF4 in LiF or particularly NaF salts, the Zr vapour pressure is far less than the ideal case, due to formation of various xLiF.yZrF4 compunds. UF4 may be similar, but I haven't found anything yet on the vapour composition above FLiBe-UF4. This old paper suggests there are some LiF-UF4 compounds in FLiBe-UF4, but gives no vapour data. Any difficulties in separating UF4 from the carrier salt don't much matter for ordinary reprocessing, as you always want to put all the U and as much carrier salt as possible straight back into the core together. For your suggestion here, some further method, such as fluorination or an electrochemical process, may be needed to improve the split of U from carrier or you end up with too much carrier in the core and not enough in the blanket.


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PostPosted: Feb 28, 2010 7:04 pm 
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Joined: Jul 28, 2008 10:44 pm
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Here I'm transferring 1000 liters per day from core to blanket after ideally removing all U and nothing else.
Second, I'm transferring 100 liters per day of cleaned salt (everything but Pu and fission products) to the fuel salt.

Going from core to blanket:
If the U is stubborn, then I end up removing some Zr, and Cs too. This would result in the core salt being richer in Zr, and Cs than the blanket salt.
If it is very stubborn, then I end up also removing some LiF and BeF2. This would result in the core salt being richer in Li and Be than the blanket.

Going from blanket (and storage) to core:
We are trying to send everything and certainly the uranium will be all gone before we get much of the Th out so this isn't an issue. Likewise for Li, and Be. It is probable that we do not all of the thorium extracted and send some of it to decades storage. This will require some makeup Th.

So it is possible that the core salt ends up being richer in Zr, Cs, Li, and Be compared to the blanket.
I'd like to keep the processing to simple vacuum distilling if possible.
We may be able to compensate if we split the vacuum distilling for 100 liters/day from blanket to fuel into two steps.
First U goes to the core, second, we extract Zr, Cs, Li, and Be and split core/blanket as it needed to keep the proper balance. With this we should be able to keep the salts balanced.


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