Method of operating a nuclear reactor by neutrons spectral shift using water displacement rod spiders

摘要

1. A process of operating a ligth water moderated and cooled reactor comprising a core constituted by fuel assemblies provided with guide tubes and immersed in the light water, part at least of said assemblies being replaced at predetermined time intervals corresponding to operating cycles of the reactor, wherein bars for modifying the neutron energy spectrum in the core are introduced within some at least of the guide tubes, said bars containing predominantly fertile neutron-absorbing material, during a first phase of the cycle, for decreasing the ratio between the moderator volume and the fissile material volume in the core and shifting the neutron energy spectrum toward higher energies and wherein said spectrum variation bars are out of said guide tubes during a second phase of the cycle, characterized in that said fertile material in the spectrum variation bars comprises a mixture of thorium and depleted uranium, the content of uranium with respect to the amount of uranium and thorium being of from 5 to 20% by weight.