EnergyUnlimited wrote:Dezakin wrote:EnergyUnlimited wrote:There are still plenty of issues related to molten salt reactor left to resolve and perhaps the most important one is related to removal of lanthanide fission products of high neutron crossection from salt melt.
Failing that our reactor will work for few months at best and remaining mix will become to be just intractable mess.
That simply isn't true. If that were the case you would have to have refueling times in light water reactors themselves on the order of months rather than ever year or two. The lanthanide fission products aren't significant neutron poisons compared to xenon (which is continuously removed) and certainly can be overcome with higher fissile load and harder neutron spectra.
Lanthanides (notably Sm149; 74500 barns and Gd 157; 200 000 barns) are still a major trouble. They are an order of magnitude less problematic than Xe 135 is but they are accumulating in the mixture (Sm 149 is not radioactive) and with progress of time they can wipe out enough neutrons to prevent further fission.
They form equilibrium concentration because they form at low concentration. They're at worst annoying.
http://energyfromthorium.com/2010/06/20 ... n-poisons/Harder neutron spectrum in LFTR may not be achievable.
Fluorine itself has some moderator properties as well (what is often overlooked in on-line discussions).
Perhaps chlorine is better here (in chloride salt melt) but there are other troublesome issues with it.
It certainly isn't overlooked. When we say harder neutron spectrum, we mean epithermal. While Chlorine is fascinating, its far less mature and requires enrichment.
Because of uranium's particularly high affinity to fluorine, fluoridation can remove most of the neutron poisons. Removing these neutron poisons in a design is an obvious goal, but not necessary as you're implying.
How so?
These poisons will form nonvolatile fluorides.
Nothing will change.
You can via brute force remove everything but the uranium using fluoride volatility of UF6 to my knowledge. I humbly bow to those more experienced in molten salt chemistry, but wasn't aware of any real showstopping problems with molten salt reprocessing. Things we'd like to do better sure.
Salt distillation process might come to a rescue (it is possible to run it at ~1500*C because LiF and BeF2 are volatile at these temperatures and fission products fluorides are not volatile) but halide induced corrosion (pitting) of vessel construction materials and also its thermal stress related deterioration are still not resolved.
That's news to me. There never was much question as to what to do for the vessels for reprocessing, there's a whole range of materials that would be appropriate for running high temperature molten salts, from graphite to certain types of Hastelloy. That's easy stuff, as aluminum refining deals with these sorts of conditions regularly.
Hastelloy (variete designed to resist high temperatures) has a an upper range of working temperature at ~1200*C.
http://www.hpalloy.com/alloys/descripti ... LOY_X.html.
Lithium fluoride boils at 1680*C (OK, under vacuum you might chip off 200-300*C) but it will still boil well above maximum working temperature of Hastelloy.
So Hastelloy is still unsuitable for this process.
On another hand AfF3 boils at 1270*C, so under vacuum it may go ~1000*C - well within range of Hastelloy.
Second material mentioned (graphite) is a more or less porous, also quite a brittle one and It may not be suitable (due to safety reasons) for use as construction material for distillation vessels dealing with multi tonnage quantities of highly radioactive stock.[/quote]
I know that the vessel material for vacuum distillation was never a real concern. The problem with the reactor is then you have to find a material that has compatibility with the molten salt along with the appropriate neutronic properties, which is why Hastelloy is desired for barrier materials of two fluid liquid fluoride breeder regimes.
I'll forward your concern to the LFTR community forum relating to vacuum distillation material, as there are several chemists and nuclear engineers there with far more experience than I. I read something about tungsten being an appropriate tubing material
For sure there were good reasons for its abandonment in the past.
It may prove successful but it may also fail like commercial FBR did - due to intractable engineering difficulties.
It was abandoned because of a political battle between Milt Shaw and Alvin Weinberg.
There is a world beyond America.
Weird but true.
There wasn't anyone pursuing liquid halide cooled reactors at this time besides the US, so there was no program to abandon. Much of the blame for abandoning this program lay at the feet of one Milt Shaw.
I'm not unrealistic. If we started doing large scale prototypes (larger than the research reactor at ORNL) we might not have a commercial power reactor for several decades given the timescale of engineering cycles. But if we don't pursue it, it will certainly take longer to develop.
I don't dismiss this technology.
There is a good chance that working commercial installations can be made.
I am just more cautious than you are.
There are still unresolved technological issues there.
Yes, but most of these issues are which engineering compromises we must make. Can we devise a barrier material that lasts the lifetime of the plant or must we replace it every several years. What enrichment are we forced to work with for the start charge? How much plutonium solubility can we allow in a reactor, and must we remove neptunium to prevent solubility issues?
We aren't facing the question on weather we can build a reactor that provides power, we can do that. We don't know how cost effective we can make it yet and I believe we differ on how serious some of the engineering challenges are.
I honestly invite you to ask some of your questions and raise your concerns on Kirk's forum, there are plenty of professionals there who would be better able to answer than me.
There are a whole host of different designs. Graphite moderated thermal, salt only moderation epithermal, D2O moderation extremely thermal, liquid chloride fast reactors. All different types of geometries, from pan in pan, tube in tube, single fluid, two fluid, a 'one and half' fluid design, all with their different advantages and challenges. Different salt prefrences from FLiBe, FLiNaK, chlorides, etcetera. We have many options.
True.
But up to date we commercialized none.
Well, yeah. We haven't even explored on a prototype level more than the single salt graphite moderated FLiBe reactor on a very small scale. I'm pessimistic because developing this technology will require billions of dollars of investment to design and prototype the reactor, the reprocessing system, and to make sure that the concerns (such as the vessel material for vacuum distillation is compatible with the salt) are addressed before scaling up to a full commercial design. I think we will eventually, but the problem is that for decades this has been out of the mindshare. The only breeders anyone really knew about were liquid metal fast breeder reactors.
Sure, this reactor is gaining awareness, but I think its going to take a large government investment to develop it, and that is at least a decade away.
As per my taste FLiBe is most promissing.
Chloride based designs come next.
Sodium & Potassium based designs are even more difficult to reprocess by salt distillation and less promising by the same.
Reactor grade chlorine needs isotopic separation (tedious, expensive).
Yes, really there only are two salt families that have any attention: Fluorides and Chlorides. Fluorides for thermal and epithermal regimes, and chlorides for fast reactor regimes. FLiBe is my preference for thermal reactors as well, for the reasonable moderation ratio, the low neutron absorption and minor neutron multiplication effect of the Beryllium n->2n reaction, and compatibility with many different vessel materials. Thermal regimes are excellent for the thorium fuel cycle.
But chloride reactors are the most awesome. Look, I'm not going to get ahead of myself here because we've not really prototyped any sort of operational chloride salt fast reactor, but the initial estimates give a chloride fuel fast reactor one of the hardest neutron spectra with the highest neutron budget of any reactor design. They can be excellent for incinerating actinides and then doing whatever you want with left over neutrons from deactivating long lived fission products to breeding specialty isotopes. Sure they're a political nightmare because they can breed obscene quantities of Pu239 without much effort, but given they're a good decade or two away after the development of a fluoride reactor regime, they still are fun to think about.
Of course the chlorine enrichment problem makes them a bit expensive, but this is decades away anyways so its still a fun idea to think about. In the meantime, I'd place priority on the liquid fluoride thorium breeder using FLiBe in the two fluid design with online processing of the fuel such that it outcompetes LWRs. Given they have low pressures and high power densities, they've already got some potential cost advantages over LWRs with their requirement for massive pressure vessels.