Design of Epithermal Neutron Beams Using Spent Fuel Elements at the Musashi Reactor

Abstract

The Musashi reactor is a TRIGA-II type reactor. With a thermal power of only 100 kW, a thermal neutron beam with satisfactory characteristics was produced for use in BNCT¹. Brain tumors of 99 patients and malignant melanoma of 9 patients were treated at the Musashi reactor^2,3. Unfortunately, the Musashi reactor was shut down in 1990 because of a water leak trouble in the reactor tank. An expanded purpose to utilize the reactor to treat deep-seated tumors with an epithermal neutron beam would help justify installing a new tank and restarting the reactor. In addition to extensive experiences by thermal neutron capture therapy at the Musashi Institute of Technology, we now have a chance to design an epithermal neutron beam with wider applicability. Recently, a method of generating an epithermal neutron beam using a ²³⁵U fission plate in the thermal column of a TRIGA reactor was proposed and this looks promising⁴. This idea has been adapted to upgrade the present epithermal neutron beam at the Brookhaven Medical Research Reactor (BMRR), and to produce a very good beam in terms of intensity and quality’. In this paper, we present the evaluation of using spent fuel elements of TRIGA fuel (UZrH 8.4/90/1.6 w%, 20% enrichment ²³⁵U) as a converter assembly coupled to the 100 kW Musashi reactor to produce an epithermal neutron beam. To predict neutron fluxes and doses in this study, Monte Carlo Neutron Photon (MCNP) code has been used. Our design goals in this study are: a) an epithermal neutron flux >0.5×10⁹ ncm⁻²s⁻¹; b) the dose from fast neutrons < 5×10⁻¹¹ cGycm²n⁻¹; c) the dose from gamma rays < 3×10⁻¹¹ cGycm²n⁻¹; d) the multiplication factor (K _eff) in the spent fuel elements < 0.95. The first goal was selected to have a full-dose treatment of time in about two hours. A peak (2–3 cm deep) thermal neutron fluence of 5×10¹² ncm ⁻² could be delivered. The second and third are deduced from the experiences by thermal neutron capture therapy at the Musashi reactor. The fourth is generally recognized as a safety value in Japan.