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by QuarterReptile
3443 days ago
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So, at the risk of sounding foolish: what's the advantage of a fast reactor? If I'm remembering correctly, U-235 has a much better thermal than fast fission cross section. As to the rest, I'm sure there are risks to pressurizers and natural circulation, but they seem a lot more comfortable than trying to avoid sodium leaks. Or is there so little corrosion in sodium systems that it works out pretty well? |
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Physical realities of note:
1. The only fissile nuclide (ie one that readily splits) that existed on Earth in 1938 is the minority uranium isotope, U-235, at a concentration of 0.7% compared to U-238. More exist now (such as Pu239, U233), but we had to synthesize them.
2. U-235 is 1000s of times more likely to split if it is hit with a slow (aka "thermal") neutron than a fast one.
3. If the (very plentiful) U-238 nuclide absorbs a neutron, it converts a few neutrons into protons until it becomes Plutonium-239, now a fissile nuclide(!). Same can be said about Thorium-232 and fissile U-233.
4. Every fissile nuclide releases substantially more secondary neutrons per neutron absorbed if hit with a fast neutron instead of a slow one.
So the implications of these is as follows. To get a critical chain reaction working in the first place, Facts 1 and 2 necessitated the slowing-down of neutrons in an arrangement of natural (unenriched) uranium. This was originally done with very pure graphite in Chicago in a squash court by Enrico Fermi and co. in December, 1942. This led to natural-uranium fueled, thermal neutron plutonium-production reactors in Washington state using Fact 3 for the Manhattan Project. These reactors essentially converted diluted fissile material (natural uranium) into concentrated fissile material (chemically-separable Plutonium) for use in nuclear bombs.
With Plutonium and enriched uranium available in the 1950s, it was now possible to start a chain reaction without slowing the neutrons down (getting around Fact 2). It was thought that Uranium was exceedingly scarce worldwide (turned out to be not entirely true), so rapidly converting lots of U-238 into Pu-239 fuel was thought essential to scale nuclear as a power source. To do this (without burning quickly through all the world's U-235), you need lots and lots of excess neutrons. Enter Fact 4.
Facts 3 and 4 led to the development of fast breeder reactors, which can produce world-scale clean energy for thousands upon thousands of years using known resources. So really the sustainability of breeding is the key capability of fast reactors, and the roughly 100x improvement in safety (measured by core damage frequency) is a bonus. You can also burn nuclear waste if you do a fully-closed fuel cycle (because fast neutrons can split even non-fissile nuclides like Pu240, Np237, Am241), emitting only fission products that decay to stability in hundreds of years instead of hundreds of thousands of years.