Initial core of nuclear power plant

An initial core for a nuclear reactor constituted by a plurality of fuel assemblies having different average enrichments including first fuel assemblies having a lowest average enrichment, second fuel assemblies having a highest average enrichment and third fuel assemblies having an average enrichment higher than the first fuel assemblies and lower than the second fuel assemblies, and which comprises:

• a plurality of square shaped cells each constituted by four first fuel assemblies;
• a plurality of L-shaped cells each constituted by three second fuel assemblies, at least one L-shaped cell being arranged at a corner of at least one square shaped cell so that the at least one L-shaped cell adjoins the at least one square shaped cell, a ratio of the number of fuel assemblies constituting the L-shaped cell to the total number of the fuel assemblies in the initial core being at least 10%;
• and
• the third fuel assemblies being arranged at adjacent positions with respect to at least selected ones of the second fuel assemblies.