Thursday, March 04, 2010

Fusion : Fission Hybrid Reactor Advantages

A wide range of newer, cheaper, safer, and more reliable nuclear reactors are vying for approval and licensing with the US Nuclear Regulatory Commission. But one approach that may be better than all the Gen IV or Gen V fission reactors is the hybrid fusion and fission reactor.
Hybrid nuclear fusion combines the two forms of nuclear power, fission and fusion, in a single reactor. This has several advantages over fission alone: it minimises the environmental impact, reduces risks, enlarges reserves of nuclear fuel and is more flexible to operate.

Fission, the process behind conventional nuclear power, harnesses energy from the radioactive decay of uranium and other fissile materials. Fusion, meanwhile, is an experimental technology that extracts energy from processes similar to those occurring inside the sun, where hydrogen atoms are fused together to form helium.

"Pure" fusion is often touted as the solution to all our energy problems, and it has undeniable advantages over fission. It produces no long-lived nuclear waste and needs no fuel other than water. But it could take another 50 years to make fusion technically and economically viable - if it can ever be made to work at all.

One problem with fusion is the size of the reactor core. To make a fusion reaction self-sustaining requires a plasma volume of about 3300 cubic metres, more than three times the proposed volume of ITER, the world's most advanced fusion project now under construction in France.

Another unsolved issue is how to construct a reactor wall, or "blanket", capable of withstanding intense bombardment from high-energy neutrons generated by the plasma. Materials that can do this do not yet exist.

Hybrid nuclear power potentially solves both these problems. First, the blanket is itself a fission reactor that burns nuclear fuels and generates neutrons. In the process it absorbs high-energy neutrons from the plasma, reducing the energy flux reaching the outer wall by a factor of 50, meaning that existing materials could be used.

Second, a hybrid reactor's plasma ball can be much smaller than in a pure fusion reactor - about the same size as ITER's, in fact - because energy generated by fission can be fed back into the plasma to keep it burning.

Hybrid reactors have other advantages too. One is that the fission reaction can burn a range of fuels, including the long-lived high-level nuclear waste produced in conventional fission reactors. It "transmutates" these waste products into isotopes that decay over a hundred years rather than tens of thousands. Not only does this eliminate some of the nuclear industry's waste problems, it also potentially helps to rid the world of plutonium and other weapons-grade materials.

Hybrid reactors also sidestep looming shortages of the high-grade uranium required to fuel conventional reactors, as they can run on non-enriched uranium and thorium. Low-grade uranium and thorium are plentiful in most parts the world. And because the fissile material produced in the blanket remains at well below critical mass, hybrid reactors have a much lower risk of suffering an accident than conventional reactors, as runaway reactions and consequent meltdown are impossible.

Finally, the power output of a hybrid reactor can be easily varied. That would allow nuclear power to be combined with renewables, which are inherently unpredictable, to provide baseload power. _NS
The last mentioned advantage -- the ability to vary the output of the hybrid reactor to compensate for the unreliable power output of wind and solar -- is a bit of a stretch, a sop thrown to the few green deadheads with the attention span to read that far. It is a pity that science and technology have been reduced to that level, but there it is.

When a more reality-based politics comes to planet Earth, we will be able to look at scientific theories and technological applications for their own merits, without the "green affirmative action" misdirection.

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