Sandia’s In-Zinerator Concept For Transmuting Unburned Nuclear Fuel

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Fusion Transmutation of Waste: Design and Analysis of the In-Zinerator Concept (2006, Sandia report, 102 pages)

The separation and transmutation of transuranics is part of a solution to decreasing the volume and heat load of nuclear waste significantly to increase the repository capacity. A fusion neutron source can be used for transmutation as an alternative to fast reactor systems. Sandia National Laboratories is investigating the use of a Z-Pinch fusion driver for this application. This report summarizes the initial design and engineering issues of this “In-Zinerator” concept. Relatively modest fusion requirements on the order of 20 MW can be used to drive a sub-critical, actinide-bearing, fluid blanket. The fluid fuel eliminates the need for expensive fuel fabrication and allows for continuous refueling and removal of fission products. This reactor has the capability of burning up 1,280 kg of actinides per year while at the same time producing 3,000 MWth. The report discusses the baseline design, engineering issues, modeling results, safety issues, and fuel cycle impact.

A scoping level analysis of a transmutation reactor driven by Z-Pinch fusion has been initiated. The “In-Zinerator” concept burns up long-lived actinides from light water reactor waste in a subcritical blanket driven by high energy fusion neutrons. Significant power is produced while at the same time providing a repository benefit by transmuting actinides into shorter-lived fission products that produce much less radioactive heat.

A D-T fusion target yield of 200 MJ fired once every ten seconds will be adequate to design a reactor capable of transmuting 1,280 kg of actinides per year while at the same time producing 3,000 MWth. This defines the requirements of a Z-Pinch pulsed power fusion source for use as a transmutation reactor. It also provides guidance to the Z-Pinch experimental program at Sandia National Laboratories as to what extrapolations from current capabilities will be required to enable this mission application. This research has focused on the design of the transmutation blanket surrounding a future Z-Pinch fusion source. The fusion source initiates a burn and energy multiplication in the blanket which contains actinides in a fluid fuel form.

The In-Zinerator effectively converts actinides into fission products. Figure 1 shows the effectiveness of transmuting actinides for 50 years. The blue line shows the total heat production from 64 metric tons of actinides. The green line shows the heat production from the sum of the all the fission products produced as a result of fissioning the 64 metric tons of actinides. After 10 years of cooling the heat load is decreased by a factor of 10, and after 50 years the heat load is reduced by a factor of 500.

The integration of the In-Zinerator design in the fuel cycle was also of interest in this work. The In-Zinerator support ratio in the fuel cycle is 1:5, meaning that one In-Zinerator will be required for every 5 light water reactors in order to burn up the transuranic actinides as fast as the light water reactor fleet produces them. The current fleet of light water reactors would then require about 20 In-Zinerators, each producing 1,000 MWe to stabilize transuranic levels. Although it is too early to estimate the cost of the In-Zinerator, an economic analysis was performed to set the goals in comparison to the cost of transmuting actinides using a FR (fast reactor) fleet.

Due to the better support ratio offered by the In-Zinerator, this concept can cost up to 25% more than a FR and still be competitive. Whether FRs or In-Zinerators are used, reprocessing and transmutation are likely to add at least 2.0 mil/kWh to the cost of nuclear power across the entire fleet.

UREX Reprocessing

UREX reprocessing is used for about 10% of existing nuclear waste. This process is used in France, Japan, Russia and the UK.

Tritium Breeding

One of the constraints on the choice of the actinide mixture was to
contain Li for breeding of tritium. It has long been a goal of fusion reactor designs to use the intense neutron flux to breed tritium from the 6Li(n,t)4He reaction. With the proper design, it is easy to provide enough 6Li to generate sufficient tritium to sustain the fuel supply for the fusion targets. Natural Li contains 7.5% 6Li and 92.5% 7Li, but it can be enriched or depleted to get to a desired tritium breeding ratio (ratio of bred tritium to burned up tritium). The goal for this work was to design a blanket with a tritium breeding ratio (TBR) of 1.2 to allow for some loss in processing. However, it should be noted that a much smaller TBR will be required for an actual plant to minimize tritium leakage to the environment.

Unburned Fuel from Existing Light Water Reactors


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