5 ms·

A “fast breeder” version of a molten salt reactor has fast neutrons in the reactor core. These neutrons easily interact with the actinides, transmuting them int

by uvdiv 14y ago

A “fast breeder” version of a molten salt reactor has fast neutrons in the reactor core. These neutrons easily interact with the actinides, transmuting them into fissile isotopes and then fissioning them to produce energy. In other words, molten salt reactors burn actinides. The same is not true for traditional uranium reactors because they have thermal (slow-moving) neutrons in the reactor core. Neutrons at these slower speeds don’t interact with the actinides, so you can’t burn actinides in a traditional uranium reactor. This is no different from a panoply of fast breeder reactors which are NOT molten salt reactors, particularly liquid-sodium cooled (solid-fuel) reactors. The difference is that liquid sodium reactors have dozens of commercial-scale demonstrations, and hundreds of billions of $$$ of R&D investment -- whereas the molten salt fast breeders (chloride salt reactors) are nothing more than paper models to date. As a side note, it looks like using thorium as a fuel is not actually critical. Thorium can be used as fuel in molten salt fast breeder reactor, which is a benefit for long-term sustainability, but thorium has little relation to the cost of constructing a new reactor today. There's a plausible suggestion that liquid-fuelled reactors could be cheaper than LWRs. E.g.: they are far more compact (smaller), and the nuclear components apparently have less complexity. (Speculative) But the point is an important one. The "big" selling points of thorium -- fuel efficiency, and spent fuel -- are very long-term issues. It's safe to argue they can be deferred. ("Thorium is premature optimization") As of this past month, China now has a $350m institute with 140 PhDs plugging away on molten salt fast breeder reactors. Actually they are thermal reactors (not fast breeders), and the focus is on solid-fuel reactors with molten salt as a coolant (although they are also considering molten-fuel reactors, as a lower priority).

pkrein 14y ago
thanks for the correction on china, will update!
- uvdiv 14y ago
  Here's a source to back that -- a talk from one of the researchers involved: http://www.youtube.com/watch?v=5UT2yYs5YJs http://www.youtube.com/watch?v=5UT2yYs5YJs Relevant stuff (liquid-fuel vs. liquid-coolant) somewhere around @23:30.
thingification 14y ago
What you say about U/Pu fast breeders is true, but there are other pertinent differences between those and LFTR: 1. LFTR safety is based on passive cooling. Pu breeders do not share this property. Fukushima-style accidents involving loss of active cooling power for decay products are not a failure mode of LFTR. 2. MSR reactors don't suffer from the safety problems introduced by the use of liquid sodium. 3. MSR reactors don't suffer from the control complications introduced by Xe poisoning in solid fuels. 4. MSRs involve a continuous liquid process. Essentially all industrial chemical processes work that way, and that's because liquids are very convenient to work with. There are good reasons to think this will pay off in multiple cost-saving ways. Another way of looking at the billions spent on Pu breeders is that it might be an indication that other technologies are worth a bet. Orders of magnitude less have been spent on MSR than on U/Pu breeders. I'd say the big selling points of Th are not fuel efficiency and spent fuel, for exactly the reasons you note. The big selling points are the promise of lower cost and public acceptance (both of these promises come to a large extent from the real safety advantages of MSR over conventional reactors). The hard one, public acceptance, will only happen if we can figure out how to sell it, but luckily, making that effort is something that those of us who think we need nuclear power have a duty to choose to support, not something that's in the lap of the gods. Re China: it's possible my memory is playing tricks on me, but I believe that one of the guys at the top of that program, when he came to a North American conference to speak about it, said that MS as coolant was the initial focus simply because that is a natural early step, but MS fuel was very much a long term goal (it's on youtube if somebody wants to check). Edit: I just checked (somebody already linked to it), and my memory was playing tricks: he said that they're developing both in parallel, with the priority initially on MS as coolant, because that is "more technically ready". It still seems that MS as fuel is where they want and intend to go eventually.
- uvdiv 14y ago
  Oh, thank you. I intended only a very narrow point about chloride reactors being untested (not MSRs: just fast-spectrum chloride MSRs). I didn't mean to provoke a debate between molten-salt and liquid metal reactors. I claim neutrality. :) LFTR safety is based on passive cooling. Pu breeders do not share this property. I'm unfamiliar with the details, but I read that liquid-sodium reactors are capable of fully-passive decay heat removal, e.g. [0] (slide 6). Certainly sodium has a much higher boiling point than water. By the way, fast-spectrum MSRs are really Pu breeders -- that's the point of actinide burning! 3. MSR reactors don't suffer from the control complications introduced by Xe poisoning in solid fuels. This is not a relevant issue with fast-spectrum reactors, e.g. [1] (c.f. [2]): "The xenon-135 and samarium-149 mechanisms are dependent on their very large thermal neutron cross sections and only affect thermal reactor systems. In fast reactors, neither these nor any other fission products have a major poisoning influence." 4. MSRs involve a continuous liquid process. Essentially all industrial chemical processes work that way, Pretty sure it's a batch process? [0] http://rpd.ans.org/presentations/DesignFeatures_Economics.pdf http://rpd.ans.org/presentations/DesignFeatures_Economics.pd... [1] http://www.hss.doe.gov/nuclearsafety/techstds/docs/handbook/h1019v2.pdf http://www.hss.doe.gov/nuclearsafety/techstds/docs/handbook/... [2] Xe-135 absorption: http://www.nndc.bnl.gov/sigma/getPlot.jsp?evalid=15174&mf=3&mt=102&nsub=10 http://www.nndc.bnl.gov/sigma/getPlot.jsp?evalid=15174&m...

by uvdiv 14y ago

pkrein 14y ago

thanks for the correction on china, will update!

uvdiv 14y ago
Here's a source to back that -- a talk from one of the researchers involved: http://www.youtube.com/watch?v=5UT2yYs5YJs http://www.youtube.com/watch?v=5UT2yYs5YJs Relevant stuff (liquid-fuel vs. liquid-coolant) somewhere around @23:30.

thingification 14y ago

What you say about U/Pu fast breeders is true, but there are other pertinent differences between those and LFTR: 1. LFTR safety is based on passive cooling. Pu breeders do not share this property. Fukushima-style accidents involving loss of active cooling power for decay products are not a failure mode of LFTR. 2. MSR reactors don't suffer from the safety problems introduced by the use of liquid sodium. 3. MSR reactors don't suffer from the control complications introduced by Xe poisoning in solid fuels. 4. MSRs involve a continuous liquid process. Essentially all industrial chemical processes work that way, and that's because liquids are very convenient to work with. There are good reasons to think this will pay off in multiple cost-saving ways. Another way of looking at the billions spent on Pu breeders is that it might be an indication that other technologies are worth a bet. Orders of magnitude less have been spent on MSR than on U/Pu breeders. I'd say the big selling points of Th are not fuel efficiency and spent fuel, for exactly the reasons you note. The big selling points are the promise of lower cost and public acceptance (both of these promises come to a large extent from the real safety advantages of MSR over conventional reactors). The hard one, public acceptance, will only happen if we can figure out how to sell it, but luckily, making that effort is something that those of us who think we need nuclear power have a duty to choose to support, not something that's in the lap of the gods. Re China: it's possible my memory is playing tricks on me, but I believe that one of the guys at the top of that program, when he came to a North American conference to speak about it, said that MS as coolant was the initial focus simply because that is a natural early step, but MS fuel was very much a long term goal (it's on youtube if somebody wants to check). Edit: I just checked (somebody already linked to it), and my memory was playing tricks: he said that they're developing both in parallel, with the priority initially on MS as coolant, because that is "more technically ready". It still seems that MS as fuel is where they want and intend to go eventually.

uvdiv 14y ago
Oh, thank you. I intended only a very narrow point about chloride reactors being untested (not MSRs: just fast-spectrum chloride MSRs). I didn't mean to provoke a debate between molten-salt and liquid metal reactors. I claim neutrality. :) LFTR safety is based on passive cooling. Pu breeders do not share this property. I'm unfamiliar with the details, but I read that liquid-sodium reactors are capable of fully-passive decay heat removal, e.g. [0] (slide 6). Certainly sodium has a much higher boiling point than water. By the way, fast-spectrum MSRs are really Pu breeders -- that's the point of actinide burning! 3. MSR reactors don't suffer from the control complications introduced by Xe poisoning in solid fuels. This is not a relevant issue with fast-spectrum reactors, e.g. [1] (c.f. [2]): "The xenon-135 and samarium-149 mechanisms are dependent on their very large thermal neutron cross sections and only affect thermal reactor systems. In fast reactors, neither these nor any other fission products have a major poisoning influence." 4. MSRs involve a continuous liquid process. Essentially all industrial chemical processes work that way, Pretty sure it's a batch process? [0] http://rpd.ans.org/presentations/DesignFeatures_Economics.pdf http://rpd.ans.org/presentations/DesignFeatures_Economics.pd... [1] http://www.hss.doe.gov/nuclearsafety/techstds/docs/handbook/h1019v2.pdf http://www.hss.doe.gov/nuclearsafety/techstds/docs/handbook/... [2] Xe-135 absorption: http://www.nndc.bnl.gov/sigma/getPlot.jsp?evalid=15174&mf=3&mt=102&nsub=10 http://www.nndc.bnl.gov/sigma/getPlot.jsp?evalid=15174&m...