Deep-Burn: making nuclear waste transmutation practical
Introduction
Nuclear energy could continue to evolve as a safe, clean, economical, and reliable energy source, free of supply disruption risks, and free of carbon emission problems if a long-term manageable solution to the nuclear waste problem is used. Complementing and improving on the geologic repository solution, methods to reduce the volume and toxicity of the nuclear waste and its weapons-usable contents have been proposed, involving the use of a large fleet of systems based on liquid metal-cooled, fast breeder reactor technology. Such systems were the central elements in a DOE-commissioned “roadmap” report issued in 1999. Critics of the report have argued that significant amounts of new waste would be created in the proposed scheme with ample opportunities for plutonium diversion because of repeated reprocessing, that deployment times would be very long, and that costs would be prohibitively high.
Addressing these concerns, the “Deep Burn” concept is a different and we believe significantly better option for the destruction (transmutation) of spent fuel nuclear waste from light water reactors (LWRs) and other sources.
Deep-Burn is based on the use of gradually thermalized neutrons and high burn-up fuel forms in modular helium reactors (MHRs) (see Fig. 1). These reactors have annular graphite-moderated cores, low power densities, and are passively safe at power levels of 600 MW, such that there is no fuel failure or fission product release under any credible accident. They operate safely at high temperatures, producing electric power at close to 50% efficiency.
Fuel temperatures are below design levels under normal and abnormal conditions, even under postulated loss of coolant accidents when heat removal would be accomplished via heat radiation and conduction to ground.
Section snippets
The nuclear waste issue
Nuclear waste is highly radioactive, and can remain toxic for hundreds of thousands of years. To date, the method that it is being developed to handle this problem is to load the discharged fuel without further processing into specially designed canisters, and store them in geologically stable repositories where the radioactive materials can decay for long times, with minimal release to the outside environment. This is the basis of the “once through” LWR fuel cycle. The risks associated with
Challenges of nuclear waste transmutation
The transmutation of nuclear waste involves treating reactor spent fuel to separate individual waste streams to be irradiated with neutrons in order to transmute fissionable or toxic isotopes into more stable ones, and ensure easier disposition. This seemingly straightforward procedure presents non-trivial risks and challenges, highlighted by the STATS Panel on nuclear waste, a panel that was convened from 1991 to 1994 to evaluate several waste partitioning and transmutation schemes.
Given the
The Deep-Burn Transmutation option
To meet the “reasonable objectives” for waste destruction just outlined and to overcome the problems associated with previously proposed systems, we have developed a transmutation concept based on the following:
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Readily available and useful neutrons.
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Favorable cross sections, and stable reactivity behavior.
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Good fuel form for irradiation, capable of accepting high levels of burnup.
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Final stored waste not usable for weapons.
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Good waste form for final disposition, with strong and effective barriers to
The Deep-Burn Transmutation concept—a summary
As envisioned in the Deep-Burn concept, the transmutation of nuclear waste involves processing spent reactor fuel to extract uranium and fission products, manufacturing the transuranics (Pu and minor actinides) left out from the reprocessing step into fuel, and destroying them by nuclear fission. The initial processing of nuclear waste uses the well-proven UREX (uranium extraction) process, extensively described in previous documents, including the DOE sponsored transmutation roadmap.
In the
Thermal neutron transmutation
A first departure in the Deep-Burn approach from other transmutation concepts is the use of thermalized neutrons. A careful review of the actinide cross sections from a transmutation standpoint shows that thermal neutrons can provide a better alternative to fast neutrons for the destruction of transuranic actinides: The large cross sections of thermal neutrons provide for high specific destruction rates (rates per unit mass) and together with the relatively low radiation damage, they allow for
Control of Deep-Burn Transmuters
Traditionally, reactor operation using non-fertile fuels (such as in systems designed to destroy surplus weapons plutonium) have relied on parasitic burnable poisons (boron, erbium, and others) for burnup and reactivity control roles. In the Deep-Burn Transmuter, the unprecedented availability of non-fissile transuranics coming from the partitioning of nuclear waste allows these roles to be taken non-parasitically by these actinides.
Fig. 7 is a plot of the thermal capture cross sections for
Neutronic requirements for a Deep-Burn Transmuter
Graphite is the ideal moderator for Deep-Burn system. Neutrons slow down gradually in graphite, with much lower loss of energy per collision relative to light–water moderated systems. The large number of slowing down steps that each neutron undergoes during the thermalization process in graphite allows them to interact with resonances to a much larger extent than it is possible in a light water moderated system. The fuel and geometry of graphite moderated cores can be designed to take advantage
The reactor-based Deep-Burn MHR Transmuter
Combining the basic MHR design with the requirements for thermal transmutation (high burn-up fuel, low alpha factor, and utilization of TRU resonances for reactivity control), the Deep-Burn MHR Transmuter is developed. The same 600 MW MHR annular core layout proposed for commercial energy production is employed (see Fig. 12), but with two different TRISO fuels: a driver fuel (DF) containing waste plutonium and neptunium, and a transmutation fuel (TF) containing the thermally non-fissile
Deep-Burn Transmuter performance
Based on the thermal Deep-Burn MHR Transmuter design described above, a burn-up analysis was performed to evaluate its performance. The flow sheet for this process is illustrated in Fig. 15. The driver fuel is the TRU waste plutonium (Pu, including fissile isotopes) plus neptunium (Np), which comes directly from the UREX partitioning process. Pu and Np do not present special problems in the TRISO fuel fabrication process. Np is kept together with Pu to contribute a prompt negative component to
Deep-Burn (TRISO) fuel in a repository
The TRISO coating is an excellent engineered barrier for containing radionuclides in a geologic repository environment. In the early 1980s, ORNL developed coated particle waste forms containing simulated nuclear waste. The simulated waste was coated with pyrocarbon and SiC. Leaching tests were performed on the coated waste particles and more conventional glassified waste forms. For the coated waste particles, radionuclides were not detected in the leachate using sensitive analytical techniques.
Conclusions
Recently, a preliminary assessment of the Deep-Burn concept was developed as part of an on-going evaluation process sponsored by the US DOE. A summary of the conclusions follows.
The development of the Deep-Burn MHR transmutation concept is motivated by the need for a useful and practical nuclear waste management tool that could allow a more efficient, safe and secure use of a geological nuclear waste repository. At the rate waste is produced by the existing fleet of nuclear reactors in the US,