24 February 2009

Fission-fusion hybrid mops up nuclear waste

By
Cosmos Online
Scientists have designed a new system that they say could rid the world of radioactive waste produced by nuclear power, opening the door to faster, cleaner and more efficient energy production.
fusion-fission hybrid

Two-in-one: The fusion-fission hybrid can use fusion reactions to burn nuclear waste as fuel (people are shown for scale). It could produce energy and would be used to help destroy the most toxic, long-lived waste from nuclear power. Credit: Angela Wong

SYDNEY: U.S. scientists have designed a new system that they say could rid the world of radioactive waste produced by nuclear power stations, opening the door to faster, cleaner and more efficient energy production.

“Many people will be willing to give nuclear [power] a chance if the waste could be destroyed,” said Prashant Valanju, a nuclear physicist from the University of Texas in Austin.

Valanju is part of a team whose “waste deconstruction” scheme, reported last month in the journal Fusion Engineering and Design, proposes the use of nuclear fusion to break down the waste from existing fission power plants.

Holy Grail

Fission-based nuclear power stations generate electricity from the energy created when large atoms such as uranium are broken apart. This process creates large amounts of energy but also radioactive ‘transuranic’ waste, which is heavier than uranium and includes plutonium, which can be used to create nuclear weapons.

Nuclear fusion is the opposite process to fission and is the fusion of atoms of hydrogen to form helium, in the same process that powers the Sun. Functional nuclear fusion power plants are one of the Holy Grails of modern physics, as these would not require uranium or create radioactive waste, but the technology is still decades away (see Fusion 2.0, Cosmos Online).

“Existing nuclear fission reactors [can be] used to relatively cheaply destroy 75 per cent of the waste by using a known method that does not create any new transuranic waste,” explained Valanju. However, the nasty stuff that remains, nicknamed ‘nuclear sludge’, is nearly impossible to destroy with fission-only reactors and must be stored.

These waste products continue to be highly radioactive as they break down in a process that can take millions of years, so storing the sludge safely is a major headache.

Hybrid solution

The researcher’s new hybrid solution to destroying the waste combines the processes of fission and fusion in the one vessel. Though fusion can’t yet be used to generate power (as it eats more power than it produces) it can be used as a powerful source of neutrons, which are fired off when atoms of hydrogen smash together to form helium.

These neutrons would then be used to break apart the heavy radioactive atoms. “The hybrid can destroy the vast majority of this sludge, along with its long-lived biohazardous components,” said Valanju.

In the new design, a compact fusion reactor, known as a tokamak, is positioned next to the waste material in a doughnut formation. When combined with a fission reaction, enough neutrons can be produced to break down the nuclear sludge in a constant and steady process.

It’s not yet clear if the process would produce a net level of energy, or if the fusion reaction would use up all the power produced in the fission process. However, even if it doesn’t produce power, it could be an effective solution for dealing with the most stubborn nuclear waste.

Swadesh Mahajan a University of Texas fusion scientist and another co-author of the study said the hybrid design is a way of utilising fusion technology to reduce fission waste until fusion technology becomes more widespread.

“The hybrid we designed should be viewed as a bridge technology,” he said. “Through the hybrid, we can bring fusion via neutrons to the service of the energy sector today.”

“Environmental Achilles heel”

Matthew Hole, a physicist from Australian National University in Canberra said the hybrid scheme was a “different spin on an old idea” and while it is interesting in theory he questioned if it would work in practice.

When spent uranium is involved “reaction dynamics can be complicated and messy,” said Hole, who argued that combining fission and fusion, in the one vessel could damage the structure.

Valanju said an absolute cost for the scheme has not yet been calculated, but that ultimately the system would be cheaper than the combined methods of breaking waste down and storing the remainder.

“By providing a technical solution to the environmental Achilles heel of nuclear power, this research can greatly boost our ability to combat global warming,” he said.

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