January 27, 2009

University of Texas at Austin Proposes Compact Tokomak Fusion/Fission Hybrid to Burn Nuclear Waste

U of T at Austin scientists propose destroying nuclear waste from other nuclear fission reactors using a fusion-fission hybrid reactor, the centerpiece of which is a high power Compact Fusion Neutron Source (CFNS) made possible by a crucial invention, the Super X Divertor. This has a different fusion system than the Lawrence Livermore fusion fission hybrid follow up to the National Laser Ignition Facility Project. Other alternatives are advanced fission nuclear reactors is liquid flouride fission reactors or accelerator driven fission reactors.

UPDATE:A new article on this site estimating how long it will take to make a Compact Fusion Neutron source.

This site had previously looked at non-direct electric uses for nuclear fusion and transmutation was one of them. Transmutation is over three times easier to do than fusion for electricity. It does not have to be positive energy generating for the nuclear fusion part. The electricity is supplied and the fusion device is viewed as an "energy using neutron generator". The uranium is converted by the neutrons back to an isotope or into plutonium that the nuclear fission reactor can use as fuel. The fusion neutron generator only has to be available about half the time.

The CFNS would provide abundant neutrons through fusion to a surrounding fission blanket that uses transuranic waste as nuclear fuel. The fusion-produced neutrons augment the fission reaction, imparting efficiency and stability to the waste incineration process. One hybrid would be needed to destroy the waste produced by 10 to 15 LWRs (light water reactors). So Seven or eleven hybrids would be need to match up with the existing 104 nuclear reactors in the United States. Thirty to forty-five would be need to match up with the worlds existing nuclear fission reactors. The process would ultimately reduce the transuranic waste from the original fission reactors by up to 99 percent.

A fusion-assisted transmutation system for the destruction of transuranic nuclear waste is developed by combining a subcritical fusion–fission hybrid assembly uniquely equipped to burn the worst thermal nonfissile transuranic isotopes with a new fuel cycle that uses cheaper light water reactors for most of the transmutation. The center piece of this fuel cycle, the high power density compact fusion neutron source (100 MW, outer radius <3 m), is made possible by a new divertor with a heat-handling capacity five times that of the standard alternative. The number of hybrids needed to destroy a given amount of waste is an order of magnitude below the corresponding number of critical fast-spectrum reactors (FR) as the latter cannot fully exploit the new fuel cycle. Also, the time needed for 99% transuranic waste destruction reduces from centuries (with FR) to decades.

The subcritical FFTS (Fusion Fission Transmutation Scheme) acquires a definite advantage over the critical FR (Fast reacotor) approach because of its ability to support an innovative fuel cycle that makes the cheaper LWR do the bulk (75%) of the transuranic transmutation via deep burn in an inert matrix fuel form. This cycle is not accessible to the FR approach because the remaining marginally fissionable long‐term radiotoxic and biohazardous transuranics cannot be stably and safely burned in critical reactors. The fission part of the Hybrid consists of standard FR components; a sodium‐cooled metal fueled lattice featuring geometry similar to that of the Generation‐IV Sodium Fast Reactor (SFR) is proposed. The critical milestone in the development of the Hybrid lies in the realization of the CFNS as a relatively inexpensive, high source density fusion neutron source.

Super X Divertor
The CFNS is based on a tokamak, which is a machine with a "magnetic bottle" that is highly successful in confining high temperature (more than 100 million degrees Celsius) fusion plasmas for sufficiently long times.

The crucial invention that would pave the way for a CFNS is called the Super X Divertor. The Super X Divertor is designed to handle the enormous heat and particle fluxes peculiar to compact devices; it would enable the CFNS to safely produce large amounts of neutrons without destroying the system. The SuperX‐Divertor (SXD) is a new magnetic configuration that allows the system to safely exhaust large heat and particle fluxes peculiar to CFNS‐like devices.

"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 scientists say their Super X Divertor invention has already gained acceptance in the fusion community. Several groups are considering implemented 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 will include performing extended simulations, transforming the concept into an engineering project, and seeking funding for building a prototype.

Waste Destruction System

The scientists' waste destruction system would work in two major steps.

First, 75 percent of the original reactor waste is destroyed in standard, relatively inexpensive LWRs. This step produces energy, but it does not destroy highly radiotoxic, transuranic, long-lived waste, what the scientists call "sludge."

In the second step, the sludge would be destroyed in a CFNS-based fusion-fission hybrid. The hybrid's potential lies in its ability to burn this hazardous sludge, which cannot be stably burnt in conventional systems.

"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.

One hybrid would be needed to destroy the waste produced by 10 to 15 LWRs.

The process would ultimately reduce the transuranic waste from the original fission reactors by up to 99 percent. Burning that waste also produces energy.

The CFNS is designed to be no larger than a small room, and much fewer of the devices would be needed compared to other schemes that are being investigated for similar processes. In combination 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, say the scientists.

A seven page paper on the Super X Divertor (SXD) and High Power Density Experiment by the University of Texas at Austin team

A new magnetic geometry, the Super X divertor (SXD), is invented to solve severe heat exhaust problems in high power density fusion plasmas. SXD divertor plates are moved to the largest major radii inside the TF coils, increasing the wetted area by 2-3 and line length by 3-5. 2D simulations show a several fold decrease in divertor heat flux and plasma temperature atthe plate. A small high power density device using SXD is proposed, for either 1) useful fusion applications using conservative physics, such as a Component Test Facility or 2) to develop more advanced physics modes for a pure fusion reactor in an integrated fusion environment.

A highly technical presentation on the Super X Divertor (SXD)

The heat flux is up to 5 times less with the SXD versus a regular divertor.
Without the lower heat there would need to be super-materials from the EU or Japan in a few decades because the USA is not developing advanced fusion materials anymore. With the SXD current materials appear to be good enough.

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