Fuel assembly for a nuclear power boiling water reactor

A fuel assembly for a nuclear power boiling water reactor, comprising:

• a plurality of fuel rods, each comprising a cladding tube and nuclear fuel material positioned in the cladding tube;
• and
• a fuel channel surrounding said plurality of fuel rods;
• wherein each one of at least 95% of the plurality of fuel rods comprises nuclear fuel material in the form of Uranium (“U”) enriched in 235U;
• wherein each one of at least 20% of the plurality of fuel rods belongs to a first set of fuel rods;
• wherein each fuel rod in the first set of fuel rods comprises both U enriched in 235U and Thorium (“Th”);
• and
• wherein the first set of fuel rods comprises at least a first and a second subset of fuel rods, wherein the ratio, with regard to weight, between Th and U, in each fuel rod of said first subset, is higher than the ratio, with regard to weight, between Th and U, in each fuel rod of said second subset
• wherein each one of between 20% and 90% of the plurality of fuel rods belongs to a second set of fuel rods, wherein each fuel rod in the second set comprises U enriched in 235U, but does not comprise a substantial amount of Th or a burnable absorber, wherein the enrichment of 235U in the U in the second set is within the range 4.00-6.00%.