In order to avoid fuel failure, mechanical stresses must be limited. Such stresses are generated by the expanding pellets pressing against the cladding radially and pulling it axially, also called pellet-clad mechanical interaction (PCMI). The total strain induced in the cladding tube is usually limited to 1% elastic + plastic hoop strain or 2.5% equivalent plastic strain.

Fuel pellet dimensional changes have three main components: thermal expansion as an instantaneous response to temperature increase, fuel densification (in-core shrinkage) during the first MWd/kg of burn-up, and fuel swelling due to fission products. The development of swelling and densification is illustrated with Halden Reactor in-core measurements in Fig. 9.11.

Fuel swelling is a slow process where the volume change is typically in the order of 0.5-0.8% AV/V per 10 MWd/kgU burn-up, and the fuel and cladding accommodate to each other during steady-state operation. Failure because of mechanical overstraining is then mostly related to rapid power changes and the resulting thermal expansion of the fuel pellets. In some cases with high power and temperature, expansion due to swelling caused by gaseous fission products will also play a role. Fuel vendors prescribe the rate of permissible power change for their products and subject them to ‘ramp testing’, i. e., a rapid power increase, in

test reactors. An example of such ramp testing is shown in Fig. 9.12. The cladding failed in this case as indicated (in the figure) by the decrease in clad strain and the increase in detectable gamma activity. The failure limit is a function of many parameters: the burn-up of the fuel, the conditioning level before the start of the power increase, the power increment, the ramp rate and the final power level all play a role. Vendors therefore qualify their respective products with extensive ramping campaigns.

The release of fission products from the pellets to the rod’s free volume has two performance limiting consequences. One is the effect of the fission product iodine, which plays a role in so-called stress corrosion cracking (SCC) and is available in sufficient quantity in a fuel rod for the SCC mechanism to work. Liner or barrier fuel, as described in Section 9.4.2, is the remedy adopted for mitigating the problem in BWRs and CANDU, and power change restrictions are applied in PWRs and BWRs.

Another limitation stems from the release of krypton and xenon. These noble gases are generated with about 28 cm3 (stp) per MWd from fission of U-235 in a thermal neutron spectrum. If the fuel temperature is high enough, they diffuse out of the fuel matrix to the rod’s free volume and cause a pressure increase. In Pu-MOX fuel, the generation and release of helium contributes in addition. When the rod pressure exceeds the outside system pressure, the cladding will creep outwards, and a gap may open between the cladding and the fuel pellets if creep — out is faster than fuel swelling. As a consequence, the fuel temperature will increase, which in turn will lead to more fission gas release, even higher pressure and more creep-out. Consequently, safety regulations set a limit on rod pressure

(NEA, 2003). Fission gas release and rod pressure build-up is a life-limiting factor, but it was found that considerable overpressure can be sustained before the fuel temperature responds with an increase, see Fig. 9.13 (Wiesenack et al., 2006). A remedy is to use large grain fuel (50-100 pm), which has a longer average diffusion path to the grain boundaries and thus retains the fission gases better than standard grain fuel (8-10 pm).