Increasing complexity in the modelling of BR2 irradiations

The paper describes the calculation methods applied at SCK.CEN for the neutronic and gamma-heating characterization of irradiation devices in BR2. From the first years of reactor operation till now, increasingly complex calculations have been carried out, following the development of the particle transport codes and the extending capacity and speed of the computers. From the 1960s till the late 1980s, the main irradiations in BR2 were carried out in the framework of the fast reactor development programmes. The geometry (central part of BR2) generally lent itself quite easily to a 1-D (R-geometry) modelling. As most irradiation rigs contained a cadmium thermal-neutron-absorbing screen, strong spectral variations occurred at the interface of the BR2 core and the rig. Multigroup 1-D neutron transport calculations were therefore used, reaching practically the maximum capacity of the computers then available. In the late 1980s, when the fast reactor programmes regressed and a stronger demand for thermal reactor fuel irradiations occurred, most irradiations were carried out away from the BR2 reactor centre, often in the core/reflector transition zone. 1-D modelling was no longer sufficient and therefore 2-D neutron transport S_N codes were used, in (R,Θ) or (X,Y) geometry. In the meanwhile increased computer capacities allowed the computations within reasonable delays. Nowadays, as very fine neutron flux and/or fission density distributions are to be calculated in various devices irradiated simultaneously in BR2 requiring different geometrical representations in a deterministic treatment (e.g. one device lending itself best to a (R,Θ) representation, another to an (X,Y) representation), Monte Carlo codes are being applied. In addition, a third (mostly the Z) dimension can be introduced.