In trying to answer the persistent question about LFTRs: “why wasn’t this done before?” I’ve obtained a report from 1962 made to President Kennedy where future development options for nuclear power were laid before him. Alvin Weinberg specifically references this report in his 1994 memoir (“First Nuclear Era”) and goes on to say that it recommends both the plutonium fast-breeder reactor and the liquid-fluoride thorium reactor as technologies that should be developed. Here’s what Weinberg had to say in his book:
“Until then I had never quite appreciated the full significance of the breeder. But now I became obsessed with the idea that humankind’s whole future depended on the breeder. For society generally to achieve and maintain a living standard of today’s developed countries depends on the availability of a relatively cheap, inexhaustible source of energy. (As I write these words, I realize that until recently I tended to dismiss solar energy as too expensive, and fusion as probably infeasible. I really don’t know whether this will always be the case.)
“The breeder became central in my thinking about nuclear-energy development. And, with Glenn Seaborg’s becoming the chair of AEC in 1960, the breeder acquired ever-increasing status with AEC—especially recognition as an essentially inexhaustible source of energy.
“In 1962, the AEC issued a report to the president on civilian nuclear power. Lee Haworth, a superbly responsible physicist-administrator, was in charge of drafting the report. He projected a nuclear deployment by 2000 of about 700 gigawatts (compared with the actual deployment in 1993 of 102 gigawatts), which seemed at the time quite reasonable. Both the fast breeder based on the 239Pu-238U cycle and the thermal breeder based on the 233U-232Th cycle figured prominently in the report. Indeed, the report implied that both systems should be pursued seriously, including large-scale reactor experimentation. It particularly favored molten uranium salts for the thermal breeder. But the molten-salt system was never given a real chance. Although the AEC established an office labeled “Fast Breeder,” no corresponding office labeled “Thermal Breeder” was established. As a result, the center of gravity of breeder development moved strongly to the fast breeder; the thermal breeder, as represented by the molten-salt project, was left to dwindle and eventually to die.”
At any rate, I have obtained a copy of this report and scanned it and made it available as a PDF. I think it is worthy of our study in an attempt to figure out why decisions were made that led us to the current situation.
Here are some interesting passages from the report:
In the thorium-uranium-233 cycle, the situation is quite different. U-233 emits more neutrons in thermal fission than does U-235; on the other hand, it is only slightly better in fast fission than in slow. Hence, thermal breeders offer greatest promise, minimizing as they do the power density and fuel endurability requirements. However, thermal breeders have a different complication in that fission products act as strong absorbers of slow neutrons, requiring that these products not accumulate too much. Among the most promising solutions of this difficulty is to use the fuel in fluid form, thus permitting continuous extraction and reprocessing to remove the fission products. Various fluid fuels have been studied for this purpose. The currently most promising approach is the use of fused uranium salts which can be circulated, both for reprocessing purposes and for heat transport. This technology is, however, in a fairly early stage.
Even when breeder reactors become economic and begin to be installed there will be a complication regarding fuel supplies. At least for some time to come, economic breeders will have breeding gains so low that they will produce not more than 3% or 4% of their fuel inventory each year. Hence, since the annual growth in energy consumption is about 6%, it will be necessary, if nuclear power increases its fractional share of the total load, to fuel some portion of the installations with fissionable uranium-235.
This leads to no great problem in the thorium-uranium thermal breeders. The fuel demand can be fulfilled simply by charging some of them, initially at least, with U-235, though at some sacrifice in economics and in the amount of U-233 that they produce.
On the other hand the “fast” reactors required to breed an excess of plutonium are economically attractive only when plutonium rather than U-235 is used to fuel them. Hence the most promising arrangement for incorporating them in a rapidly expanding nuclear power economy would undoubtedly be to use thermal converters to help provide the plutonium needed for added installations. This combination would continue until increases in the relative “yield” of plutonium from the breeders, together with a lower relative rate of growth of electrical energy consumption enabled the breeders to catch up and produce enough plutonium by themselves.
We get somewhat of an insight into the thinking of the Atomic Energy Commission with regards to breeder reactors. If they were to use uranium-plutonium, then plutonium supplies were crucial due to the fact that each fast breeder needed 10 to 15 times as much fissile material to generate a unit of power as a thermal reactor did. The light-water reactors at the time were producing plutonium as a byproduct. The fast-breeder needed and wanted that plutonium. Reactors like LFTR needed a tiny fraction of the fissile inventory as the fast breeder did and could be started on HEU.
Here’s an image of how the AEC envisioned light-water reactors running on enriched uranium and producing plutonium, and fast-breeder reactors needing that plutonium, working together.
The picture begins to become clearer, especially when we consider how Weinberg described what the AEC did with this report, establishing a “Fast Breeder” office but no “Thermal Breeder” office.
More thoughts on this later…