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Fusion at Our Service

Fusion at Our Service
(L to R) Mike Kotschenreuther, Swadesh Mahajan, Prashant Valanju and Erich Schneider. Photo: Marsha Miller

(L to R) Mike Kotschenreuther, Swadesh Mahajan, Prashant Valanju and Erich Schneider

Fusion has long been pursued as a pure and virtually inexhaustible source of energy. It is fusion reactions, after all, that power the stars.

When fusion—the joining of two similar atomic nuclei to form a heavier nucleus—occurs in an uncontrolled chain reaction, it can result in a powerful thermonuclear explosion. But controlled reactions can also release tremendous energy and large numbers of neutrons.

While the day is not yet here when we can harness the energy from fusion reactions, three entrepreneurial physicists in the Institute for Fusion Studies have an idea that would use the high energy neutrons produced by fusion to make nuclear power derived from fission safer and cleaner.

They hope their invention will lead to the construction of a new kind of hybrid fusion-fission reactor in the near future, which, if all works as planned “would allow nuclear power—a low carbon source of energy—to take its place in helping us combat global warming,” says Mike Kotschenreuther, a senior research scientist in the Institute for Fusion Studies (IFS).

Kotschenreuther, Swadesh Mahajan and Prashant Valanju, reported the ideas behind their fusion-fission hybrid in the January 2009 issue of Fusion Engineering and Design. They coauthored the study with mechanical engineer Erich Schneider.

Their system, at its most basic level, would use neutrons from fusion reactions to augment the burning of transuranic waste as fuel, producing energy in the process. This could eliminate almost all (about 99 percent) of the toxic, long-lived waste that is generated from nuclear power plants and it would drastically reduce the need for any additional or expanded geological repositories for storing nuclear waste.

“Most people cite nuclear waste as the main reason they oppose nuclear fission as a source of power,” says Mahajan, senior research scientist. (The waste is not currently reprocessed in the U.S., and the amount of waste to be stored in Yucca Mountain in Nevada is already expected to exceed the limits set by Congress before its scheduled opening in 2020.)

The centerpiece of the fusion-fission hybrid would be a high power Compact Fusion Neutron Source (CFNS). The CFNS design is based on a tokamak, which is a machine that is highly successful in confining fusion plasmas at extreme temperatures exceeding 100 million degrees Celsius for sufficiently long times.

The small size of the CFNS would be made possible by a crucial invention called the Super X Divertor, which is designed to handle the enormous heat and particle fluxes peculiar to fusion devices. If it works as proposed, the Super X Divertor would enable the CFNS to safely produce large amounts of neutrons without destroying the system.  According to the CFNS design, these neutrons would be provided to a surrounding blanket of spent fission fuel and would allow the use of this transuranic waste as nuclear fuel.

“The intense heat generated in a nuclear fusion device can literally destroy the walls of the machine,” says research scientist Valanju, “and that is the thing that has been holding back a highly compact source of nuclear fusion.”

The larger fuel cycle would work in two major steps. First, the waste generated by nuclear reactors would be destroyed in other standard nuclear reactors. This step uses about 75 percent of the waste and produces energy, but it does not destroy the more radiotoxic, transuranic, long-lived waste, what the scientists call “sludge.”

In the second step, the sludge would be destroyed in the fusion-fission hybrid. “To burn this really hard-to-burn sludge, you really need to hit it with a sledgehammer, and that’s what we have invented here,” says Kotschenreuther.

The scientists say that a great advantage of the CFNS is that it would be no larger than a small room, and only a few such devices would be needed. One hybrid would be needed to destroy the waste produced by 10 to 15 nuclear reactors (there are 100 such reactors in the U.S.).

Combined with the substantial decrease in the need for geological storage, the CFNS-enabled waste-destruction system would be much cheaper and faster than other routes, they say.

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

Several groups are considering implementing the Super X Divertor on their machines, including the MAST tokamak in the United Kingdom, and the DIIID (General Atomics) and NSTX (Princeton University) in the U.S. Next steps include performing extended simulations, transforming the concept into an engineering project, and seeking funding for building a prototype.

This article also appeared in the Spring 2009 issue of Focus magazine.

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Thursday, 23 September 2021

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