As irradiation proceeds the fuel swelling and clad creepdown (the latter only in LWRs and AGRs) lead to closure of the gap between fuel pellets or bars and the cladding. In AGRs, however, the gap is always small because the cladding is pressurised onto the fuel pellets during manufacture. Fuel creep due to the weight of the fuel stack can enhance gap closure in (high temperature) fast reactor oxide fuel (Bailly et al., 1999).

Once the gap is closed, a significant tensile stress is exerted on the cladding by the fuel pellets/bar, and a corresponding significant compressive stress is exerted on the fuel pellets or bar by the cladding. The stresses in the fuel and/or cladding are relieved by elasticity and creep, the latter causing permanent deformation of the fuel pellets/bar and cladding. In the case of ceramic fuel pellets, the wheatsheaf shape of the pellets means that clad stress is concentrated at the pellet ends, and the cladding therefore deforms to a greater extent over pellet-pellet interfaces. The result is ‘ridging’ or ‘bambooing’ of the cladding. If the stresses on the pellet are high, axial extrusion of fuel in the hot central region and/or hot pressing (removal of porosity under stress and temperature) are also possible. Axial extrusion in turn leads to partial, or even total, filling of any dishes in the pellet end faces. The mechanical effects of pellet and cladding contact are collectively known as ‘pellet-cladding mechanical interaction’, or PCMI. This is distinct from ‘pellet-cladding interaction’, or PCI, which also includes the chemical effects (see 14.2.12).

During steady-state conditions the stresses induced by fuel-cladding contact are small, since the stress accumulation is slow and there is ample time for stress relaxation due to fuel and/or cladding creep. The result is that there is little mechanical effect, other than a slow increase in clad circumferential strain due to the inexorable swelling of the fuel. This is not, however, true during fast transient increases in pin power, when fuel thermal expansion (including the resulting opening of radial fuel cracks in oxide fuel) and gaseous swelling can impose significant stresses on the cladding. If the stresses on the cladding are high enough, the cladding can fail by either stress-corrosion cracking (see 14.2.12), or ductility exhaustion (in the case of the latter, the high stresses are relieved by clad creep, generating large creep strains in the process).