Effects of elevated temperature steam oxidation and subsequent quenching on light water reactor clad

摘要

Following a postulated severe accident, the temperature of fuel pins of light water reactor escalates because of decay heat and devoid of coolant. The presence of steam inside the core causes exothermic oxidation of the cladding surface at elevated temperatures. The in-vessel injection system of severe accident management guidelines injects the cold water inside the hot uncovered core to arrest the escalation of temperature rise and terminate the accident progression. However, the heated clad may be subjected to thermal shock during the quenching process, which inevitably assists in the failure and fragmentation of the severely oxidised fuel pins column. In this paper, the effect of thermal quenching shock on the severely oxidised fuel pins column during the reflooding of the hot uncovered core is studied experimentally. A 2×2 square lattice arrangement of zircaloy-4 clads is used for experiments. First, the clads are oxidised in the presence of steam in the temperature range of 1100-1250℃, and then the test-section is flooded with water at ambient temperature from its bottom. The violent heat transfer between the clad and cold water has been observed during the reflooding. Furthermore, the clad has failed with a circumferential crack at multiple locations along the clad length during this period; however, longitudinal cracks are not observed on the clad surface. The scanning electron micrographs of the clad samples show a significant oxide layer over the clad surface. At the same time, energy dispersive x-ray spec-troscopy reveals significant oxygen diffusion inside the zirconium metal matrix, particularly near the outer tube surface. Additional experiments are performed to study the clad failure mechanism during the reflooding of the severely oxidised clad. The set of experiments reveals that the clad fails under the thermal quench shock (＞7.41 ℃/s) for severely oxidised cladding (ECR＞25) at an average temperature above 1100 ℃. The failed clad is likely to create debris. The average quench front velocity is 0.6 cm/s while the reflood velocity is 1.13 cm/ s. Quench velocity below the reflood velocity indicates excessive water evaporation and dispersed type flow ahead of the quench front. The total analytical H_2 generated before reflooding is 3.75 g.