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The criticality problem
Safe nuclear reactors have been built since the 1960s, and the obstacle to new ones is a regulatory environment that has been restrictive since the anti-nuclear push of the 1980s. Part of the difficulty is that a conventional reactor balances on a knife edge called criticality, where each generation of neutrons must cause exactly one more reaction. If there are too many, the reaction runs away. If there are too few, it shuts off, so the reactor has to hold at exactly 1.0, and guaranteeing that under every possible circumstance is hard.
The energy amplifier
An alternative called the energy amplifier has been around since the late 1980s and early 1990s, pushed by Carlo Rubbia, a former CERN director and Nobel laureate in physics. It always operates below the 1.0 threshold. Subcritical, a company co-founded by Fred Turner, who also runs the health insurer Curative, and his wife, designs for 0.97. At that level the reaction always fizzles out, so a powerful particle accelerator aimed at the fuel supplies the extra neutrons that drive it forward.
If the accelerator is turned off, all of the energy output stops, which makes it a big on-off switch for fission. Even ten times the accelerator's power can't make it run away, so nothing can push it into a criticality accident. Each plant would be about 300 megawatts and cost about $1 billion to build. The only comparable system under active construction is in China, based on a US design from the 2010s that the US stopped working on after Fukushima.
How does an accelerator make neutrons?
In an accelerator-driven system, a beam of high-energy protons strikes a heavy metal target such as lead. Each proton knocks many neutrons out of the nuclei it hits, a process called spallation. A single proton can release dozens of neutrons. Those neutrons travel into the surrounding fuel and drive fission there.
Why does a reactor at 0.97 fizzle out?
The number 0.97 is the effective multiplication factor, the average number of new fissions each fission causes. Below 1.0, every generation of neutrons is smaller than the last, so the chain reaction dies without help. With an outside neutron source, the total settles at the source rate multiplied by 1 divided by 1 minus k. At 0.97 that multiplier is about 33.
Why did Rubbia propose thorium fuel?
Carlo Rubbia shared the 1984 Nobel Prize in Physics for the discovery of the W and Z particles, and he later proposed the energy amplifier in the 1990s. His design used thorium, which is fertile: when thorium-232 absorbs a neutron, it turns into uranium-233, which can fission. Thorium is more abundant in Earth's crust than uranium. Proponents argued that the cycle leaves less long-lived waste than a conventional uranium cycle.
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