10.29. The use of burnable absorber rods or lumped burnable poison (LBP) plays an important role in PWR core management. In Westinghouse cores, these may take the form of aluminum oxide-boron carbide rods placed in those positions in a fuel assembly normally left open for the insertion of rod cluster control elements (§5.185). This means that LBP rods may only be used in assemblies not assigned to rod cluster control positions.

10.30. Since the first core initially contains no fission-product poisons, it is necessary to use burnable absorber rods in addition to soluble boron to compensate for the excess reactivity. It is not possible to use additional soluble boron as an alternative since the need to maintain a negative mod­erator temperature coefficient limits the boron concentration in the cool­ant. However, since first core use is limited to new reactors, most LBP use is for tailoring the individual assembly reactivity to avoid local power peaking in designing reload core patterns (§10.46). However, it is empha­sized that neutrons are lost unproductively in LBP rods. Therefore, only sufficient rods should be used to control the power distribution so that it is within licensing peaking factor limits. These peaking factors are normally determined by loss-of-coolant accident criteria (§12.90).

10.31. During the burnup cycle, burnable poison rods are depleted, fission product poisons are formed, and the fissile atom concentration is reduced. Although this generally results in a loss of fuel assembly reactivity, attention must be given to the relative reaction rates so that the fixed absorber is not depleted so rapidly that an unacceptable power peak would appear in midcycle. Another concern with some absorbers is the residual poisoning effect remaining after the initial burnup cycle. This leads to a reactivity penalty when the fuel assembly is reinserted in subsequent cycles (see Fig. 13.3). Therefore, a general design objective is to minimize the use of lumped burnable poison rods.