MODELLING

Neutron transport in the fission range of heavy metal energies has been studied for many years within the nuclear reactor industry. In ADS, the situation is more complicated than in conventional nuclear reactors. In this case, there is dual transport modelling required, the transport of medium energy charged particles in the energy range 1-3 GeV in the spallation target, and the transport of neutrons down to low — energy range.

A two-step process of spallation and evaporation of the residual nucleus occurs when medium-energy protons collide with a nucleus. If the residual nucleus has high mass and moderately high excitation energy, it might undergo fission in competition with the evaporation reaction.

In regard to presently developed methodologies, the nuclear cascade processes can be calculated by the NMTC (Coleman and Armstrong, 1970) and HETC (Radiation Shielding Information Centre, 1977) codes using two-body collision theory, which is valid until a particle slows down and its wavelength becomes longer than the average distance between the nuclei. In this regime, an optical potential model can be used, based on quantum mechanics. These codes have been developed to calculate high-energy fission, for targets with high atomic number such as uranium and the actinides, by various laboratories including JAERI (NMTC) (Nakahara and Tsutsui, 1982), BNL (NMTC) (Takahashi, 1984), LANL (LAHET) (Prael and Lichtenatein, 1989). Other nuclear cascade codes FLUKA (Ranft et al., 1985) and CASIM (VanGinnekin et al., 1971) have been developed by the international community.

Two areas of microscopic nuclear physics have been studied by OECD/NEA, using data from a thin target benchmark and transport modelling using thick target physics (IAEA-TECDOC-985, 1997a).

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