Technical noteNeutron physics analyses of accelerator-driven subcritical assemblies
Introduction
The use of accelerator-driven subcritical reactors for burning radioactive waste and plutonium and for energy generation is being studied by many research groups (e.g. in the USA, in Japan, and in a number of European countries).
One important reason for this interest is the possibility to generate fission energy and, at the same time, burn radioactive waste with these reactor systems. A decisive advantage of accelerator-driven subcritical reactors is expected to be the absence of energetic reactivity accidents (of the type occurring, e.g. in Chernobyl) provided that sufficient subcriticality is ensured.
Work in many laboratories has resulted in a variety of concepts of such hybrid systems, and the development of new technical variants is not yet over.
Thus, a technology was elaborated within the ADTT (Accelerator Driven Transmutation Technology) project in Los Alamos, USA under the leadership of C.D. Bowman (Bowman et al., 1992) in which radioactive waste is dissolved in a molten salt solution. The molten salt solution is piped through a moderator (e.g. graphite) arranged around the neutron-producing target (spallation target) and, in turn, surrounded by a reactor pressure vessel. The neutrons, which are thermalized by scattering in the moderator, reach a very high neutron flux density and hit the actinides and fission products of the radioactive waste, which are transmutated (burned) by fission or neutron capture (Jameson et al., 1995, Bowman and Newman, 1996). As a consequence of the high thermal neutron flux density and the high cross sections for thermal neutrons, incineration is achieved in a relatively short period of time (within hours or a few days).
In Europe, C. Rubbia and co-workers published two studies in late 1995 (Rubbia et al., 1995a, Rubbia et al., 1995b), in which accelerator-driven subcritical reactors with solid fuel and a fast neutron spectrum are examined. The fuel of these assemblies is based on thorium. Thorium instead of uranium as a basic fuel offers the advantage of the burning process generating very little plutonium and higher actinides (Am, Cm…).
The power of such a hybrid system is regulated via the proton current intensity of the accelerator. This must be the higher, the more reactor power is to be achieved. Moreover, the required proton current intensity is the higher, the further the reactor deviates from criticality (keff=1). The power, L, of the reactor is approximately directly proportional to the proton current intensity, I, and inversely proportional to (1−keff): L∼I/(1−keff). Studies by Rubbia and co-workers have indicated that the hybrid systems they examined (Rubbia et al., 1995a, Rubbia et al., 1995b) require for a reactor power of 1500 MWth an accelerator which is able to accelerate protons to 1 GeV (1000 MeV) and generate a proton current intensity of 10–20 mA. Even higher proton current intensities are needed in the designs of other groups.
Fig. 1, which was taken from (Rubbia et al., 1995a), shows the basic design of such a subcritical system driven by an accelerator. The high-energy protons hit the spallation target (which may be made of liquid lead) and generate some 20–40 neutrons per proton. The neutrons lose part of their kinetic energy in the target and in the subsequent ‘buffer zone’, which may also consist of lead, before they enter the fuel and blanket material region where they cause nuclear fissions and, hence initiate the production of more neutrons.
In the former Institute for Neutron Physics and Reactor Technologies (INR)1 code systems were developed in order to assess the neutron physics of existing concepts and to perform neutron physics calculations. These codes describe the neutron processes in a large energy range, from the generation of the neutron source from high-energy protons till the burn-up characteristics of the reactor.
Section snippets
Calculation methods for accelerator-driven subcritical reactors
Neutron physics calculations for critical reactors, and also for subcritical reactors with external neutron sources, have been carried out at INR for a long time. This is true for reactors both with fast and with thermal neutron spectra. However, when starting the work on accelerator-driven subcritical reactors, external codes had to be adopted for the calculation of the spallation neutron source.
Applications
ADS system studies were started as soon as reliable qualified calculation tools were available. After some preliminary studies of the capabilities of ADS for the incineration of plutonium, minor actinides and long lived fission products, see (Broeders et al., 1996), the participation to the IAEA ADS benchmark resulted in insights of large interest. Especially the power distribution towards the central neutron source proved to be unsatisfactory with radial form factors varying from about 2.5 to
Summary and outlook
Accelerator driven subcritical reactor systems (ADS) have been proposed both for energy production and for the incineration of the long-lived isotopes of the back-end of the nuclear fuel cycle. The option to operate at sufficiently low criticality levels to avoid accidents with super-criticality improves the over-all safety of a reactor system. On the other hand the coupling of powerful proton accelerators with large power-generating nuclear reactor systems strongly increases the complexity of
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