A neutron booster for spallation sources—application to accelerator driven systems and isotope production

One can design a critical system with fissile material in the form of a thin layer on the inner surface of a cylindrical neutron moderator such as graphite or beryllium. Recently, we have investigated the properties of critical and near critical systems based on the use of thin actinide layers of uranium, plutonium and americium. The thickness of the required fissile layer depends on the type of fissile material, its concentration in the layer and on the geometrical arrangement, but is typically in the μm–mm range. The resulting total mass of fissile material can be as low as 100 g. Thin fissile layers have a variety of applications in nuclear technology—for example in the design neutron amplifiers for medical applications and “fast” islands in thermal reactors for waste incineration. In the present paper, we investigate the properties of a neutron booster unit for spallation sources and isotope production. In those applications a layer of fissile material surrounds the spallation source. Such a module could be developed for spallation targets foreseen in the MYRRHA (L. Van Den Durpel, H. Aı̈t Abderrahim, P. D’hondt, G. Minsart, J.L. Bellefontaine, S. Bodart, B. Ponsard, F. Vermeersch, W. Wacquier. A prototype accelerator driven system in Belgium: the Myrrha project, Technical Committee Meeting on Feasibility and Motivation for Hybrid concepts for Nuclear Energy generation and Transmutation, Madrid, Spain, September 17–19, 1997 [1]). or MEGAPIE (M. Salvatores, G.S. Bauer, G. Heusener. The MEGAPIE initiative: executive outline and status as per November 1999, MPO-1-GB-6/0_GB, 1999 [2]) projects. With a neutron multiplication factor of the booster unit in the range 10–20 (i.e. with a keff of 0.9–0.95), considerably less powerful accelerators would be required to obtain the desired neutron flux. Instead of the powerful accelerators with proton energies of 1 GeV and currents of 10 mA foreseen for accelerator driven systems, similar neutron fluxes can be obtained with a medical cyclotron with proton energies of 350 MeV and currents of 2 mA. A basic limitation of such booster units is of course the limited lifetime of the thin fissile layer due to burn up and fission product poisoning. Nevertheless, for initial investigations of full power ADS systems, such units would allow the generation of the full neutron flux without the use of ADS class accelerators. The properties of the neutron booster have been intensively studied through systematic investigations of the different parameters (thickness, Hohlraum, reflector, etc.) of the concept. The calculations were performed with deterministic codes solving the neutron transport equation, and those results have been partially verified with the probabilistic code MCNP (Monte Carlo N-Particle Transport Code System). Layer of pure 235U and 242mAm fuel have been initially investigated, but non-proliferation concerns led to the use of more conventional fuel layer. A specific geometry of a booster spallation source is presented. The effects of layer-burn up and lifetime are discussed.