All the load bearing components in the core of the reactor, namely clad tubes, guide tubes (GT), GT assemblies and BWR channels undergo irra­diation creep, albeit at different rates. The clad tube is a crucial boundary which has to withstand steep temperature and pressure gradient across its thickness. Steady-state creep dominates the service life of the clad and only in very rare cases the material may enter tertiary creep range. The creep rate of clad material under an irradiation environment is many times higher (depending on the material chemistry and the flux) than that under out-of­pile conditions. Further, the dimensional changes in clad tube (an aniso­tropic material) happen in a preferential direction which gives rise to other unwanted problems. The irradiation creep is not just the thermal creep imposed with high defect density. In the former the interstitial and vacancy loops that form during irradiation play a major role in the creep mechanism; in the latter the creep rate increases with temperature. The irradiation creep is weakly dependent on irradiation temperature (‘athermal’) and in-reactor thermal creep controls the deformation above ~400°C.

Two mechanisms are proposed to explain the irradiation creep phenom­enon: (a) stress-induced preferential absorption (SIPA), where extra planes of atoms accumulate on crystal planes so as to produce creep strain in the direction of the applied stress and (b) stress-induced preferential nucle — ation (SIPN), which assumes that nucleation of loops is preferred on planes with a high resolved normal stress. Both of these mechanisms assume that the growth or formation of loops occur at a favorable orientation with respect to applied stress and causes macroscopic strain. Neutron irradia­tion produces large quantities of point defects — vacancies and self intersti­tial atoms (SIAs). These defects migrate to different sinks like dislocations and grain boundaries, in a preferential manner due to the anisotropy of the zirconium crystal lattice, in order to reduce the energy of the system. Because of the diffusional anisotropy, interstitial atoms tend to migrate to dislocations lying on prism planes and to grain boundaries oriented paral­lel to prism planes, while vacancies drift preferentially to dislocations lying on basal planes and to boundaries parallel to basal planes. This gives rise to elongation in one direction and contraction in the other.136 The creep rate is controlled by dislocation glide and this in turn can be controlled by suitable alloying elements and by choosing an appropriate texture of zir­conium matrix.

The total strain measured in an irradiation creep consists of the strain due to thermal creep (eth), irradiation creep (eirr) and irradiation growth (eg) and is assumed to be additive:

£ = % + £irr + £ [366]

The creep rate ((e)) is given by the empirical relation

£ = f (<pmone-Q/RT, f,d, p, a) [3.67]

where A is a constant, 9 the flux, a the stress, f the texture parameter, d the grain size, m (~0.4-0.7) and n (~0.8-2) are constants, and the others have their usual meaning.

As the dislocation density in a CWSR material is high and, as glide but not climb is the rate controlling mechanism under reactor conditions, the creep rate of CWSR is higher than that of recrystallized material as shown in the figures. The creep rate (a) increases with increase in the flux, (b) increases with increase in temperature (contribution by thermal creep predominates above 400°C), (c) is higher along the rolling direction than the transverse direction, (d) decreases with fluence (as radiation hardening sets in) and (e) depends upon the type of alloy (Nb and Sn content increases creep resistance). The irradiation creep rates of cold-worked Zr-2.5wt.%Nb alloy are about one-third of those of cold-worked Zircaloy-2137 at comparable temperature, stress and fast neutron flux while the creep down of HANA (High Performance Alloy for Nuclear Applications) alloy (after a dose of 12 MWd/Kg U) is half that for Zircaloy-4.138