B. Montagnini

Spectrum shaping of accelerator-based neutron beams for BNCT

We describe Monte Carlo simulations of three facilities for the production of epithermal neutrons for Boron Neutron Capture Therapy (BNCT) and examine general aspects and problems of designing the spectrum-shaping assemblies to be used with these neutron sources. The first facility is based on an accelerator-driven low-power subcritical reactor, operating as a neutron amplifier. The other two facilities have no amplifier and rely entirely on their primary sources, a D-T fusion reaction device and a conventional 2.5 MeV proton accelerator with a Li target, respectively.

Simplified PN and AN methods in neutron transport

Abstract The paper deals with general properties of the odd-order SP N equations. The equivalence between these equations and the so-called A N equations is demonstrated. The A N equations have the simple structure of a system of multigroup diffusion equations and are easily transformed into a set of boundary integral equations, which allows to apply a boundary element-response matrix solution technique and obtain a proper setting for the proofs of the boundary and interface conditions which are to be used. Some sharper results for the special class of the transport problems in which the total cross section is constant are discussed.

A coarse-mesh method for 1-D reactor kinetics

Abstract A 1-D coarse-mesh reactor-kinetics method is presented. The nodal flux is approximated by means of a cubic expansion formula and the so called time-integrated algorithm is used for time integration. Application to numerical problems has shown that this method is, in addition to being fairly accurate, very fast and therefore well-suited for implementation into some of the current codes for LWR accident analysis.

On the Relation Between Spherical Harmonics and Simplified Spherical Harmonics Methods

The purpose of the paper is, first, to recall the proof that the AN method and, therefore, the SP2N−1 method (of which AN was shown to be a variant) are equivalent to the odd order P2N−1, at least for a particular class of multi-region problems; namely the problems for which the total cross section has the same value for all the regions and the scattering is supposed to be isotropic. By virtue of the introduction of quadrature formulas representing first collision probabilities, this class is then enlarged in order to encompass the systems in which the regions may have different total cross sections. Some examples are reported to numerically validate the procedure.

The quasi-diffusive approximation in transport theory: Local solutions

The one velocity, plane geometry integral neutron transport equation is transformed into a system of two equations, one of them being the equation of continuity and the other a generalized Fick`s law, in which the usual diffusion coefficient is replaced by a self-adjoint integral operator. As the kernel of this operator is very close to the Green function of a diffusion equation, an approximate inversion by means of a second order differential operator allows to transform these equations into a purely differential system which is shown to be equivalent, in the simplest case, to a diffusion-like equation. The method, the principles of which have been exposed in a previous paper, is here extended and applied to a variety of problems. If the inversion is properly performed, the quasi-diffusive solutions turn out to be quite accurate, even in the vicinity of the interface between different material regions, where elementary diffusion theory usually fails. 16 refs., 3 tabs.