Some time ago I wrote an essay on LFTR/Molten Salt Reactor safety from the prospective of a system of barriers to radiation release. My agenda was to argue that LFTR safety could be achieved through a system of barriers to the release of radioactive materials. This argument assumed that a fuel spill was the over riding safety issue. However, the classic texts on MSR safety (Gat and Dodds) do not examine MSR safety primarily in terms of a system of barriers. Gat and Dodds believed that
The Ultimate Safe Reactor (USR) is a special concept of a molten-salt reactor with prime and complete emphasis on safety. The USR uses a processing frequency, yet to be developed, that is about an order of magnitude higher from that contemplated for the molten salt breeder reactor (MSBR). The MSBR had a ten-day inventory turn around in the fuel processing. The USR uses a one day or less of turnaround of the fuel inventory. This rather fast turnaround reduces the build up of all fission products with half-lives of a few days or longer. The reactor is an epithermal spectrum reactor and uses no moderator per se in the core. The clean core consists solely of a low-pressure vessel. Freeze valves are used throughout. The prime circulating pump is sized to assure no critical cold slug accident can occur. Furthermore, the USR uses the Th-U fuel cycle with a breeding ratio of exactly one. Thus, the USR has all the safety benefits that are passive, inherent and non-tamperable and, in addition, has proliferation-resistant attributes and simplified waste that is free of fissile material, which can be transported in any arbitrary size or quantity from the processing part of the plant.
Beyond the ultimate safe reactor Gat and Dodd argued that there could be an absolute and ultimate safe reactor:
The absolute and ultimate safe reactor (A+USR) is a special concept of the USR which utilizes natural convection to transfer the heat from the core to the heat exchanger. The A+USR has no safety-related mechanical operating parts nor any externally-actuated controls, it becomes the ultimate in PINT-safety. The reactor responds internally and inherently to a change in power demand via its temperature response.
Frequent processing of the fuel increases the fuel inventory in the processing part and puts high demand on the performance of the processing units. The removal of the fission products from the fuel stream occurs at low concentrations, which requires precision and sophistication. In an actual plant, an optimization between performance, inventory and safety is needed.Thus Gat and Dodd saw MSR (and LFTR) safety in terms of reactor design features, that prevented accidents from happening, and prevented bad things from happening in the rare event of an accident. Gar and Dodds, argue, in effect that absolute and ultimate safety can be manufactured into Molten Salt Reactors, and can be implemented through low cost mass production manufacturing methods.
As a consequence of the Gat and Dodds argument is that an elaborate and costly system of barriers is not required. to assure absolute and ultimate nuclear safety. Mass produced, factory manufactured features can in most cases be low priced. Thus from the Gat and Dodds perspective LFTRs can be more safe at trivial costs than LWRs can be with the massive expenditure of money on safety features. This leads us to consider drastic, cost lowering changes in the way reactors are built.
Even the worst sort of reactor disaster, say an aircraft attack on a reactor, would not cause a massive release of radioisotopes, because the nuclear fuel would be continuously cleaned of radioisotopes. Since an attack on a reactor no longer poses great danger for a civilian population, the reactor holds little value as a target for terrorist.
Secondly, LFTRs can be air cooled. Meaning that they do not have to be sited next to water, and water shortages posed no difficulties for LFTRs.
i recently observed on Narry Brook's blog, Brave New Climate:
David LeBlanc has designed a very simple, low material LFTR that could easily mass produced. David tells me:
My work on the tube within tube will take very little material but I don`t have a number off the top of my head. Cost figures would be pretty much guesswork at this point but seems obvious that a simple tube should not cost very much. As for output levels, we could have a 1000 MWe tube within tube but I typically look around 200 MWe as a good size and this is about 1 meter wide (inner tube) and 6 meters long. This is surrounded by 60 to 100 cm of blanket salt and then an outer Hastelloy vessel. The tube material might be Hastelloy or Molybdenum (or many other things). David adds, “The heat exchangers will be a bigger user of metals like Hastelloy and that will be the same for just about any design.” in addition the LFTR would meed a couple of closed cycle gas turbine generators.David has discussed lowering reactor costs by building them with stainless steel. Using CO2 instead of helium we could get about 175 MWe from each. You could easily mass produce 4 per day, 400 if you wanted too. LFTRs are very safe, and all you need is a steel shed with prefabricated concrete radiation containment barriers and a cement floor to house the things. Thus not only would the mass manufacture of LFTRs allow for the timely deployment of huge ammounts of post carbon energy sources, but mass manufacture is entirely consistent with greatly enhanced nuclear safety, while lowering nuclear manufacturing costs. That safety in turn would allow for great cost savings in the construction of nuclear housing facilities.
Update 10/22/09: David LeBlanc disagrees with my assessment of the potential of low cost LFTR technology. I eat crow and go back to the drawing boards. Yesterday David wrote me: “There are a few options for cheap salts without tritium and still below Melting point 525. One is RbF-NaF-27%(Th,U)F4 (I think its 27, might be 22%) but that salt isn`t an option for a fuel salt of a Two Fluid (too much Th+U). The other is old fashioned NaF-ZrF4 which you can break even (with a bigger fissile load) and you can`t really get the melting point down much to use stainless steel.
I wouldn`t want to think of not using a containment building. All we need is something that is air tight and safe against aircraft crashes. It needs to be air tight for any gaseous leaks like Xenon. It doesn`t need to hold pressure or be a big volume so that makes it far cheaper than for LWRs.”
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