Monday, April 25, 2011

The Molten Salt Reactor Family: Fuel

I intend to offer a series of posts designed to explain the sometimes bewildering complexity of Molten Salt Reactor Technology. This first post explains two nuclear fuel breeding cycles.

Rather than offering a single potential reactor design, the Molten Salt Reactor (MSR) idea offers a large number of design options, each of which would require a significant amount of research, before a prototype reactor could be built. The Molten Salt Reactor designer is faced with a bewildering number of elective choices, each offering a set of advantages and disadvantages. Each choice that the designer makes will dictate a number of design features some of which require further choices.

Lets start with nuclear fuel. My father first demonstrated that not only U-235 but also Pu-239 could be used as a reactor fuel in MSRs. During the ORNL Molten Salt Reactor experiment Oak Ridge scientists tested the use of the three fissionable materials that can be used as nuclear fuels, Plutonium-239 (Pu-239), Uranium-235 (U-235) and Uranium-233 (U-233). Once during the operation of the Molten Salt Reactor Experiment (MSRE) they used all three potential fuels in the reactor at the same time.

Of the three potential fuels, U-233 had some significant advantages. Neither U-235 nor Pu-239 produced enough neutrons per neutron hit, to support breeding more nuclear fuel at a slow (thermal) neutron speed range. U-233, produced by breeding thorium did produce enough neutrons to breed thorium at a slow temperature range. We will see that this offers a very large advantage. U-235 is not efficiently produced by breeding, while Pu-239 can only be produced in the breeding range with fast neutrons.

Breeding means that for every fuel atom used in the nuclear process, at least one new fuel atom is produced. Thus in a plutonium fast breeder, if a neutron strikes a plutonium atom, it is very likely to fission into two smaller atoms, almost always with three neutrons left over. Those neutrons will be moving fast and will contain a lot of energy. Fast neutrons are more likely to produce fission in plutonium atoms than slow neutrons. Neither U-235 nor Pu-239 produce enough neutrons to maintain breeding if they encounter a slow (also called thermal) neutron. Thus Plutonium can only be produced as a nuclear fuel in so called fast reactors. There are, as we shall see, some major disadvantages to fast reactors.

Fast reactors are often thought of as having liquid sodium as their coolant, although liquid lead, and a liquid lead-bismuth mixture have also been used as a coolant in fast reactors. In addition it is possible to build fast Molten Salt Reactors. The stability of Molten Salt Reactor operations in enhanced by Xenon-135 removal. Xenon-135 is a radioactive gas that is a byproduct of nuclear fission and has a very large neutron cross section. Because it is very likely to capture neutrons, Xenon-135 can adversely effect a chain reactor in a reactor. Thus it would be highly desirable to get Xenon-135 out of a reactor core quickly after it is produced. That is impossible in a solid core reactor, but it is not difficult to do in a Molten Salt Reactor. The presence of Xenon-135 adversely effects to the ability of reactors to breed nuclear fuel, so any MSR that is designed as a thorium breeder would have a system for moving Xenon-135 out of its core.

There are decided advantages for fuel reprocessing with MSRs. Compare the fuel reprocessing technique for a Molten Salt Reactor with the fuel reprocessing technique proposed for the Integral Fast Reactor (IFR) a LMFBR. In two fluid MSR, the blanket salt flows out of the blanket, and protactinium and U-233 are withdrawn from it by chemical processes. Once they are processed out of the carrier salt, the U-233 is re-fluoridated and returned to the core. The protactinium is set aside until it undergoes a nuclear transformation to U-233, and then that U-233 is returned to the core. In a IFR, the spent fuel is fished out of the reactor core, and once recovered, dumped into a molten salt bath, in which it dissolves. Then by use of electroplating, various material from the old fuel, for example plutonium, are separated out of the bath, and deposited on electrodes. Eventually the separated metal, is recovered, melted and mixed into an alloy, which is then cooled enough to serve as fuel elements, and then returned into the reactor. The MSR fuel reprocessing technology is much simpler than the fuel reprocessing technology designed for the IFR.

In addition fast reactors require 10 times as much nuclear fuel to produce a chain reaction as thermal breeder reactors. It does not really matter if the fast reactor is cooled by liquid metal of liquid salts, a fast breeder reactor just needs a who lot more fuel in order to operate than a thermal breeder reactor does. This makes fast reactors poor candidates to replace fossil fuels like coal with nuclear power, because many reactors will have to be built quickly, and fueling enough fast reactors quickly will be a big challenge.

There are two breeding cycles, the Uranium 238 breeding cucle, and the thorium 232 breeding cycle. Both cycles have some advantage. Plutonium-239 produces more neutrons per fission event than thorium, but fewer fission events per neutron in the thermal spectrum. In fact Pu-239 produces so many fewer fission events in the thermal spectrum than in the fast spectrum, that it is impossible to achieve a positive breeding ratio for the U-238/Pu-239 breeding cycle in a thermal reactor. On the other nand U-233 produces about as many neutrons per fission event in the thermal range as in the fast range, and about as many fission events. That means that the Th-232/U-233 breeding cycle is as effective in the thermal range asin the fast range, and because thorium breeding only requires about 10% of the nuclear fuel in the thermal range as U-238 breeding requires in the fast range, thorium breeding cycle reactors can be deployed far faster.

In addition Liquid fuel reactors have advantages over solid fuel reactors. Once a sollid fuel is inserted into a reactor it almost always stayes there for a year or more, while fission products build up in the fuel. We have already seen that Xenon-135 becomes a reactor control problem, although Xenon-135 eventually reaches an equalibrium because of its short half-life. The presence of Xenon-135 in a nuclear core, can interfear with a reactor's capacity to bread, especially in the thermal breeding range. Thus Thorium fuel cycle breeder reactor are better candidates for rapid deployment than U-238 fuel cycle breeders, and liquid fuel thorium breeding reactors have advantages as solid fuel thorium breeder. Liquid fueled thorium breeders, as we have already noted, have advantages over solid fuel U-238 breeders. Thus the Thorium fuel cycle Molten Salt Reactor (often called the LFTR) would seem to offer several advantages over U-238 fuel cycle liquid metal fast reactor.

In the next post of this series I intend to explain the difference between single fluid and twoi fluid Molten Salt Reactors.


Barry Brook said...

Where are the processed fission products stored, in the short- and medium-term? Are they immobilized?

Charles Barton said...

Barry, there are numerous options for managing waste fission products. This would also be the case for IFR fission products. Each reactor becomes in effect a little mine. Fission products can be processed out. In the case of MSRs three classes of fission products are likely to be quickly processed out, for a variety of reasons. They are radioactive gases, volatile fission products, and nobel metals. Removing the gases and metals improves reactor operation and should be done continuously. Removing the volatile FPs is primarily a safety issue. These classes of products are likely to be removed by different processes, and thus are easy to keep separate. Other fission products can be removed during periodic batch cleaning of the coolant salts. Since more than one process is likely to be used during the batch cleaning, further separation of FPs will probably occur. As individual FPs complete the nuclear decay process, stable daughter products can be marketed for industrial use. Some of these may be quite valuable. During the decay period heat from FP decay can be handled by wet pools or by passive air cooling. FPs can be stored in casks. Some radioactive fission products may have industrial uses. While a few long half lived FPs are best handled by dropping them in a deep hole.

FPs with short and middle term half lives can be kept in secure storage until the daughter products reach stability at which case they can be sold.

Protactinium-233 would be kept in a secure room close to the MSR core, until it undergoes nuclear transformation to U-233. Then it is added to the core salts. Other actinides can be cleaned out of the carrier salt and then either returned to the core, or transfered to a fast reactor for disposal.

Barry Brook said...

Thanks Charles, yes, I understand all of this, and as I've noted before, I'm (hesitantly) excited by the LFTR's prospects. Perhaps I didn't ask my question properly (specifically enough). Note that this is more a devil's advocate critique.

What form is the Pr-233, radioactive gases and volatile FPs stored, during and in the months after the continuous reprocessing? They are *highly* radioactive at this time and also to produce much decay heat, requiring continuous cooling. How are they cooled? How are they secured?

This is obviously relevant to Fukushima, given the problems with the SNF ponds. You briefly mention wet pools, passive air cooling, 'secure storage' and dry casks, but don't give sufficient details to judge whether this on-site stage has been satisfactorily thought through.

Charles Barton said...

Well Barry you have some good questions, and the answers are beyond any claim to expertise that I might be foolish enough to make. So I place a shout out on the Energy from Thorium discussion pages, in the hope that answers will be brought forward. I would also suggest that If the answers are not forth comming, that you ask Kirk Sorensen or David LeBlanc for answers. - Charles

Charles Barton said...

Barry, I have received one suggestion from Jess Gehin so far. This suggestion refers you To ORNL-4541 in Kirk's document repository. The referral is to chapter 7, but chapters 8 and 9 might also be helpful.

Rick Maltese said...

Charles. I think it's great how much you do help bring these subjects to light. But I am confused about Xenon. I can't understand the best way to handle Xenon. Are you saying it's a problem that still needs to be solved or does one of the variants of the MSR fluoride, chloride or otherwise handle the problem well?


Blog Archive

Some neat videos

Nuclear Advocacy Webring
Ring Owner: Nuclear is Our Future Site: Nuclear is Our Future
Free Site Ring from Bravenet Free Site Ring from Bravenet Free Site Ring from Bravenet Free Site Ring from Bravenet Free Site Ring from Bravenet
Get Your Free Web Ring
Dr. Joe Bonometti speaking on thorium/LFTR technology at Georgia Tech David LeBlanc on LFTR/MSR technology Robert Hargraves on AIM High