Friday, August 9, 2013

LFTRs by Baby Steps

Some of my readers may wonder why I do not mention the LFTR more often even though Nuclear Green Revolution is committed to the LFTR movement. The answer is, that I believe the development of the LFTR will be incremental and that several and perhaps numerous steps will precede the stage in which the LFTR is under intense development. Those steps will produce Molten Salt Reactors that are not Thorium breeders even though Thorium may be used in the reactor's cores or blankets. 

 The development of a 200 MW Molten Salt Reactor prototype would be the first stage in the development of the LFTR whether or not Thorium went into the core or a core blanket. Since the LFTR is intended to be a highly scalable reactor, the initial test reactor would operate in a thermal neutron range and be moderated with graphite. A 200 MW prototype could be built largely by up scaling ORNL technology tested in the Molten Salt Reactor experiment. The prototype, itself, could be a commercial prototype as a electricity generating reactor, it could produce 80 - 100 MW of electricity twenty-four hours a day. Or, it could be a peak generator that produces electricity sixteen hours a day and stores heat in a molten salt vault eight hours a day. The stored heat could then be used with hypercritical CO2 powered turbines. The CO2 could be heated by extracting heat from the stored molten salts. Once the CO2 passes through the turbine, it is recaptured and reheated. 

 Smaller Molten Salt Reactors could be used to provide heat for industrial processes as well as electricity. Waste heat from the operation of Molten Salt Reactors could be used to recover fresh water from sea water. In addition, small Molten Salt Reactors would serve as small distributive electricity generators for isolated communities and, if the price of molten salt generated electricity is low enough, for communities that are connected to the grid. The development of molten salt nuclear technology requires the development of a product that is a viable source of energy at a low cost. U-235 Molten Salt Reactors would be the first product that could meet the demand for energy in a competitive fashion. 

The road to the LFTR begins with the design and construction of viable Uranium fueled Molten Salt Reactors. These reactors must be operated in the thermal neutron range because fuel would be far too expensive in a fast reactor. The development of a Uranium fueled Molten Salt Reactor would open the door to the use of Thorium as part of the carrier salt, moderator salt, coolant salt mix. It should be noted that David LeBlanc proposes a Denatured Molten Salt Reactor which includes a large amount of Thorium in the core salts, but which is not intended to breed U233 from Thorium. U-233 is produced in the Denatured Molten Salt Reactor by conversion. The DMSR constitutes a step toward the LFTR. Probably the most viable step toward the LFTR that will be available in the near future. The DMSR allows experimentation with technologies that could be used in LFTRs.

Fast Molten Salt Reactors are, of course, possible. The interesting thing about Thorium, is that it can be used to breed at any neutron speed. In addition to Thorium, U238-Pu239 breeding is possible in a Molten Salt Reactor. The big advantage to Molten Salt Pu breeding is that the coolant moderator is not dangerous while sodium reactor coolant is dangerous. Proponents of liquid metal fast reactors might argue that a great deal has already been invested in sodium cooled fast reactors and therefore more money should be invested in them. Some people who advocate Molten Salt Fast Reactors comment that investing money in sodium cooled fast reactors is throwing good money after bad. However, it could turn out that Fast Molten Salt Reactors might prove to be a quicker and lower cost path. The Fast Molten Salt Reactor could serve as an excellent Pu burner if it's operation was intended to eliminate trans Uranium elements found in nuclear fuels. There is little doubt that the fast Uranium fueled Molten Salt Reactor would be cheaper to build and safer to operate than the sodium cooled fast reactor. 

A major disadvantage of fast Molten Salt Reactors is their limited scalability. Fast reactors require a great deal of fuel compared to thermal reactors. The fuel required to start ten thermal breeders equal the amount of fuel required to start one fast breeder. Since Plutonium is proposed to be recovered from conventional nuclear fuel, the cost of recovering Plutonium that would be used in a fast breeder would be quite large. In order to start a large number of fast breeders, a huge amount of fissionable material, Plutonium239, U235, and U233 would be required. This makes it very difficult to use fast breeders in any scheme to build nuclear powered societies by 2050. Moderated thermal reactors using Thorium fuel would be far more scalable. 

The Indians have designed a heavy water moderated reactor that can breed Thorium; although it would be more expensive than a Molten Salt Reactor.  In the Indian scheme of things, U233 used to fuel their Thorium breeders comes from hybrid fast breeders when they start operation. Both thermal and fast breeder Molten Salt Reactors can be used to breed Thorium. Thorium can be breed alongside U-238 in fast reactors if we should choose to do so. The latter option would add a measure of proliferation resistance to our fast Molten Salt Reactor design, but the proliferation resistance would be largely unnecessary. 

Both one and two fluid Molten Salt Reactors can be used to breed Thorium. The one fluid option was favored by ORNL, but in fact both one and two fluid options have technical problems as well as advantages. At any rate, there seems to be some preference to one fluid designs at present, especially among French Molten Salt Reactor designers. ORNL, itself, vasilated between the one and two fluid approaches. The two fluid approach may have some costs and technical advantages that need to be carefully examined before discarding it is considered.

The LFTR, that is, the Thorium Molten Salt Breeder Reactor is more complex than a simple Molten Salt Reactor. That complexity may not add greatly to the end cost of LFTRs, but it will have some effect on the time required to develop the LFTR. Since the LFTR project will produce the technology required to build electrical generating molten salt non breeders before the LFTR is ready for production, I hold that the Molten Salt Reactor should come before the LFTR. Not because I think that in the long run it is the better choice, but because it will be available when we need it and we need it now. It is not clear how long it will be before the LFTR is available.

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