Plutonium Assembly in the Core of the BR3/Vulcain Core

The main phase in the VULCAIN Research and Development Programme has consisted of runs at power and at a high burn-up of a core with a high power density. During this experiment, carried out in the BR3 reactor at Mol, it was decided to introduce into the BR3/VULCAIN core a peripheral assembly consisting of 19 vibro-compacted rods and 18 pellet-type rods containing plutonium-enriched natural uranium oxide. From power distribution calculations with five neutron groups (three rapid and two superposed thermal) using a two-dimensional diffusion code for a triangular lattice, the plutonium enrichment was fixed at 6%. This value was determined on the basis of criteria for the burn-out margins, the local maximum burn-up, the temperature at the centre of the rods and the evolution of fission gases. It was found that the theoretical power distribution predictions were in good agreement with the values measured in the VENUS facility. In view of the non-uniform distribution of plutonium in the vibro-compacted rods which contain plutonium only in the fine powder fraction, an appropriate technique was developed for the neutron and thermal calculations. From an analysis of the axial and radial distribution of the gamma rays emitted by the non-irradiated rods, the authors devised a model for the plutonium distribution in the rods in question. The power distribution over the transverse cross-section of a rod was then calculated by means of a transport code; the results obtained made it possible to determine the corresponding heat flux and temperature distributions. The burn-out margins were estimated by means of Tong correlations, and then served to determine the arrangement of the vibro-compacted and pellet-type rods within the assembly. For the chosen configuration, the calculated burn-up figures showed local maxima of 37 000 MWd/t and approximately 30 000 MWd/t for pellet-type and vibro-compacted rods respectively. The plutonium assembly was introduced into the BR3/VULCAIN core at the end of September 1966. The reactor reached full power on 6 December 1966, since when it has worked practically uninterruptedly at full load. (author)