The interaction of neutrons with non-fissile materials in an assembly is of importance for a number of phenomena and must be calculated by neutron physics codes as well. The interactions fall into two categories, namely absorption and scattering, each with their associated effects.

Neutron absorption in non-fissile materials is mostly undesirable since it removes neutrons from the multiplication chain and has to be compensated by more fissile material. The exception is the case of burnable absorbers, for example
gadolinium in the fuel or zirconium diboride as a coating on the pellet surface. The latter was developed by Westinghouse and also called IFBA for Integral Fuel Burnable Absorber (Secker and Brown, 2010). The purpose of the burnable neutron absorbers is to remove excess core reactivity at the beginning of an operation cycle when the core contains some fresh fuel, and to optimise assembly power distribution. While serving this purpose, these absorbers also introduce disadvantages, for example lower thermal conductivity of gadolinium bearing fuel and an increase in the gas pressure in the rod from helium produced in the IFBA coating.

The evolution of the radial power distribution in fuels with burnable poison is a complicated function of neutron fluence and spectrum. This is illustrated in Fig. 9.7, which shows the radial power distribution for various burn-ups of fuel with gadolinium. After some time, the absorbing isotopes are converted to less absorbing ones which, however, remain in the fuel matrix chemically as gadolinium with an influence on fuel properties.

When fast neutrons collide with and are scattered by atoms in assembly material, the atoms can be knocked out of their position leaving a vacancy defect in the matrix, and they can end up in a new, interstitial position deforming the matrix. During the lifetime of an assembly, every atom is displaced 20 times or more (a fast neutron (E > 1 MeV) fluence of 6 x 1020/cm2 will typically cause 1 displacement per atom or 1 dpa). The implications of this interaction are changing material properties as well as enhanced material creep and growth (Adamson, 2000).

Significant safety and performance issues arising from these effects are:

BWR channel bow. This can occur in a neutron fluence gradient causing uneven growth of the channel box. The deformation may lead to control blade insertion problems, which can be exacerbated by channel box bulging in a pressure gradient, differential hydriding and so-called shadow corrosion. A geometry change of the channel box will also influence the local power because of changes in the fuel-to- moderator ratio. A countermeasure is beta-quenching at the final production stage of the strip from which the channel is to be formed. This treatment reduces irradiation growth (at least to moderate fluences) and consequently channel bow.

PWR guide tube bow. S- or C-shaped bowing has been observed, which can be caused by creep in response to axial hold-down forces from the top nozzle springs and lateral forces from cross flow as well as by uneven irradiation-induced growth in a neutron flux gradient. The resulting geometry will impede control rod insertion and can lead to differential power in the quadrants of a core (Andersson et al. , 2004). The problem can be alleviated by using zirconium alloys, which exhibit less growth, for example Zirlo and M5, and by optimising the hold-down spring and guide tube strength.

Spacer spring relaxation. The springs, which are a part of the intermediate grids, will relax and thus lose their ability to keep the long fuel rods firmly in place. The resulting slack can lead to vibration, grid-to-rod fretting and fuel failure, in particular in PWRs. Karoutas et al. (2004) reported an interesting countermeasure, which exploits the fact that cold-worked Zircaloy has higher irradiation growth than recrystallised-annealed Zircaloy. While the grid strips are fully annealed, the springs retain the cold-work of the final forming operation. As they try to grow longitudinally more than the strip material surrounding them, they bend towards the fuel rod instead and counteract the spring relaxation.

Fast neutrons also enhance the creep-down and creep-out of the fuel rod cladding.