Experimental and Modeling Approach of Irradiation Defects Recovery in Zirconium Alloys: Impact of an Applied Stress

Abstract

During neutron irradiation, both interstitial and vacancy loops are formed in high concentration in zirconium alloys. Due to this high density of loops, the material is considerably hardened, but the recovery of the radiation damage during a heat treatment leads to a progressive softening of the irradiated material. The recovery of the radiation induced hardening has been investigated using microhardness tests. Transmission electron microscopy (TEM) observations performed on irradiated foils have also shown that the loop density falls while the loop size increases during the thermal annealing. Furthermore, the TEM analysis has revealed that only vacancy loops are present in the material after long term annealing, the interstitial loops having entirely disappeared. A numerical cluster dynamic modeling has also been used in order to reproduce the material recovery for various annealing conditions. The microstructural evolution during mechanical testing with various loading conditions has also been studied. It has been shown that during a creep test with low applied stress (130 MPa) and high temperature (450°C), the microstructure evolution can essentially be explained by the thermal recovery of the loops leading to glide of dislocations as found for an non-irradiated material. At intermediate temperature (400°C), it is shown that for low stress level (130 MPa) the microstructure evolution can also be explained by the thermal recovery of loops, whereas for higher stress (250 MPa), sweeping of loops by gliding dislocations can also occur. In addition, for an applied stress of 130 MPa and a temperature of 400°C, dislocation density is higher in the irradiated material than in the non-irradiated material deformed in the same conditions. It is also shown that secondary slip systems are more activated in the irradiated material than in the non-irradiated material. From this detailed analysis, the mechanical behavior during creep is interpreted in terms of microscopic deformation mechanisms.

Experimental study and numerical modelling of the irradiation damage recovery in zirconium alloys

“…The main risk of rupture at high temperature is due to cladding creep under the internal pressure of fission gases and the temperature due to residual power of the fuel [1]. During post-irradiation creep, the cladding is subjected to heat treatments combined with an applied stress [2] that can affect the irradiation induced microstructure as well as the mechanical properties.…”

“…However, because of complex coupling that can arise between thermal recovery and deformation, the understanding of the post-irradiation creep behaviour requires also the study of the creep deformation mechanisms as it has been done elsewhere [2].…”

Thermomechanical loading applied on the cladding tube during the pellet cladding mechanical interaction phase of a rapid reactivity initiated accident

“…Sugiyama et al [3] results indicated that 60% and 85% of the cladding section was at a temperature lower than 350°C and 480°C respectively at cladding fracture. In laboratory tests that aimed at studying RIA conditions should therefore take into account the temperature level as it affects the material mechanical and thermal behavior [7] and the fracture process [10][11][12], the quantity of dissolved hydrides [13] and the extent of the irradiation recovery [14]. The temperature gradient in the cladding wall is likely one of the thermomechanical loading conditions that is most difficult to reproduce experimentally.…”

The recovery of irradiation damage for Zircaloy-2 and Zircaloy-4 following low dose neutron irradiation at nominally 358 °C

“…The increase in flow stress due to hai loop formation by irradiation generally results in the activation of slip on the basal ({0 0 0 1}) and pyramidal (f1 0 1 1g or f1 0 1 2g) planes [1][2][3][13][14][15]. The other type of loop formed from irradiation damage of zirconium alloys is a vacancy hci loop with a Burgers vector b hci ¼ 1 2 h0 0 0 1i that are formed on the basal planes ({0 0 0 1}) [1,2,6,[9][10][11][12][13][34][35][36][37][38][39][40].…”

“…Since the processes of loop nucleation and change in precipitate structure are believed to be interrelated and are primarily driven by solid-state diffusion, there is a strong dependence of irradiation temperature and flux on these processes that strongly influence the irradiation hardening of Zircaloy [1][2][3][4][5][6][34][35][36][37][38][39][40][41][42][43][44][45]. Irradiation of Zircaloy at higher temperatures (326-450°C) have generally been reported to result in nominally 10-60% less irradiation hardening (Dr) than for irradiation at lower temperatures of 260-326°C [5,6,44,45].…”