Burnable poison (burnable absorber) assemblies are constructed of boron — added absorber (borosilicate glass tube or Al2O3-B4C pellet) and corrosion — resistant cladding (stainless steel or zircaloy-4), as shown in Fig. 3.46.

Burnable poison rods are used to maintain a negative moderator temper­ature coefficient during reactor operation. As mentioned in the list [1](1) of Sect. 3.3.4, moderator temperature coefficients become less negative as soluble boron concentration increases. For a long cycle length, therefore, a high concentration of soluble boron to compensate for the excess reactivity may lead to a positive moderator temperature coefficient. Such an increase in soluble boron concentration should be restrained by using burnable poisons. From the viewpoint of reactivity control, thus, burnable poisons are used to compensate for a part of the excess reactivity corresponding to operating cycle length. The compensating reactivity amount depends on the operating cycle length and varies from 0 to 8 % Ak/k in the current operation. For a long cycle length, PWRs also adopt gadolinia-added fuel and the use of conventional burnable poisons is decreasing. Burnable poisons also play a role in flattening core radial power distribution by adjusting their location in the core.

D • Thimble Plug

: Burnable Poison Rod

Fig. 3.46 Burnable poison (burnable absorber) assembly of PWR (17 x 17 Type) [ ] (Copyright Mitsubishi Heavy Industries, Ltd., 2014 all rights reserved)