Cyril R wrote: Re Nickel: Ni58 neutron, alpha barely shows on the nndc databases. Less than 0.001 barn @ 2 MeV and downhill from there.
Yes the Cx is 0.001 barns but the boron concentration is only 0.01% so there are more than 10^4 more atoms of nickel than boron in the wall. At 2MeV we will 95% of the generated helium is from 58Ni(n,alpha). The French had problems with wall lifetime due to 58Ni(n,alpha) and their spectrum is considerably slower than a chloride salt reactor. They plan on a mixed approach where the top and bottom are reflectors (they use thick Hastalloy as the reflector) and they plan on periodically replacing the reflector. Theirs is a 1.5 fluid design and is 2.5meters in diameter.
Re the wall. It is big because of single fluid design being a big reactor (roughly similar total core power density as PWR).
What is the diameter you are thinking of?
What is the fuel concentration? How much fissile are you planning on?
Re neutron leakage effect. My thinking is that leakage affects burnup in this reactor because more leakage means the once through cycle has to stop earlier as the conversion ratio will drop sooner and we don’t want to add fissile. So indirectly it affects fuel evolution with less pu being burned out. That makes sense to you?
Yes that makes some sense but it is more complex to think about if the reactor fuel is never steady state. Allow me to think out loud.
The idea is to start with clean salt and an initial fissile load of lots of SNF/plutonium and LEU20 with enough thorium added to make the reactor just critical. As the initial fissile burns you generate both u233 and pu239 and consume th232 and u238. You keep adding thorium to keep the machine just critical. Initially, I think this means you add more thorium than is consumed due to the high eta of plutonium and the lack of fission product absorptions. The hope is that fuel evolves to consume almost all of the plutonium and u238 so that the machine is running on 232Th and 233U. All this while fission products are building up and reducing the reactivity of the machine so that later in life we have to add less thorium than what is required for replacement to keep it critical. Eventually the machine stalls out under the fission product absorption load.
The trick is going to be the balance of plutonium and LEU20. As I understand it, you don't want to add fissile once the reactor is started and you don't want to have fancy chemical processing (specifically the ability to remove thorium from the reactor w/o removing plutonium). As the fertile (232Th and 238U) transmutes your reactivity will go up, as the fission products build up it will go down.
Seems like the higher the initial concentration of fissile the longer your reactor will run. Also higher plutonium content relative to LEU20 in your startup fissile will result in higher eta and thus more u233 produced initially allowing more thorium to be added and again resulting in a longer life reactor.
It sounds possible for a single fluid reactor. I suspect that the wall and economics remain a challenge but if we can put absorbers in front of the wall then it may well work. Maybe I'm too conservative but I'm still worried about getting sufficient R&D funds over enough time to make this happen. It seems hard enough to get there starting from MSRE and a chloride reactor would start considerably further behind.