Influence of Water Density Change on the Criticality in Special Fuel Lattices

Changes in the sign of the reactivity coefficient due to changes in the water density are very important in all water-moderated lattices. This sign is determined by the lattice parameters, such as the fuel enrichment and the lattice pitch. It is, of course, negative in undermoderated lattices. However, in special lattices, for example, in spent-fuel storage pools, this reactivity coefficient could be positive, even though one would predict a negative value from the lattice parameters. An example of this effect is presented, and the unexpected sign is explained.