Reactivity Behavior of a Reactor Core Loaded with Gadolinium-Poisoned Fuel Assemblies

To obtain a longer power cycle length, the beginning-of-cycle excess reactivity of the reactor core must be increased. In the case of the Dodewaard reactor, this was done by raising the ²³⁵U enrichment of the fuel from 2.5 to 2.8%. To meet the shutdown margin criterion, the burnable poison, gadolinium, was added to 2 out of 36 fuel pins of each fresh fuel assembly. When a large number of these types of fuel assemblies are used to reload a reactor core, we must know the reactivity behavior of the hot operating core, as well as the variation in shutdown margin of the cold core as a function of burnup. By using a relatively simple analytical model, a reasonable prediction of these two phenomena could be made as shown by comparison with actual measurements and results of calculations with the FLARE computer code.