Burnable absorber arrangement for fuel bundle

In a boiling water reactor core whose operation is characterized by a substantial proportion of steam voids with concomitantly reduced moderation toward the top of said core when said reactor is in its hot operating condition, said reduced moderation leading to slower burnup and greater conversion ratio in an upper core region so that when said reactor is in its cold shut down condition the resulting relatively increased moderation in said upper core region is accompanied by a reactivity profile that peaks in said upper core region, a fuel assembly comprising:

• a component of fissile material distributed over a substantial axial extent of said fule assembly;
• and
• a component of neutron absorbing material having an axial distribution characterized by an enhancement in an axial zone of said fuel assembly, designated the cold shutdown control zone, corresponding to at least a portion of the axial region of said core when the cold shutdown reactivity peaks, the aggregate amount of neutron absorbing material in said cold shutdown zone of said fuel assembly being greater than the aggregate amount of neutron absorbing material in the axial zones of said fuel assembly immediately above and immediately below said cold shutdown control zone whereby the cold shutdown reactivity peak is reduced relative to the cold shutdown reactivity in said zones immediately above and immediately below said cold shutdown control zone, said cold shutdown zone having an axial extent measured from the bottom of the fuel assembly in the range between 68-88 percent of the height of said fissile material in said fuel assembly.