Core optimisation issues of MOX fueled ALLEGRO reactor

ALLEGRO is a gas cooled prototype reactor concept of fast neutron spectrum. It will be the demonstrator reactor of Gen4 Gas Cooled Fast Reactors (GFR) and it will be used to examine the viability of the GFR system and to test a new type of carbide fuel, which would be applied as the standard fuel of GFRs. As ALLEGRO is in its initial design phase, several safety issues need to be solved. In this paper two main neutronics related safety problems were examined. One of them is the outstandingly high reactivity worth of control and shutdown rods, thus modifications in the core design were needed. The second examined safety issue was the fulfillment of the shutdown criteria for the modified core design. The calculations were performed by the MCNP Monte Carlo based code. As a first approach, Pu content in central fuel assemblies was changed, thus the radial power distribution became flatter but the reactivity worth of control rods did not reduce significantly. Thus, further changes in the core design were needed. According to the results, modifications to the number and geometry of the absorber assemblies are suggested. The new modified core fulfills the shutdown requirements, with the application of control rods of significantly reduced worth in central positions and three new shutdown devices in outer positions.