Revisiting the QUENCH-11 integral reflood test with a new thermal–hydraulic model: Existence of a minimum injection rate

Abstract

During a severe accident in a pressurized water reactor (PWR), one of the most important accident management measure is the injection of water to stop, or at least to slow down, the core degradation and avoid the reactor vessel rupture. Nevertheless, the success of a reflood is not guaranteed because of possible negative effects. The oxidation of the Zirconium (contained in the cladding materials) is exothermal and produces hydrogen. The quenching of a degraded core at high temperature may result in high hydrogen generation in very short time as seen in the LOFT-LP-FP2 experiment, in the CORA and QUENCH test series, and as deduced for the TMI-2 accident from the pressure history. The relevant parameters to determine the efficiency of reflood are the core state (geometry and temperature), the system pressure and the water injection rate. In this paper, the effects of the injection rate and the maximum temperature at reflooding onset on the hydrogen source term are investigated using the QUENCH-11 test as a base case. The calculations and studies have been done with the ASTEC/CATHARE V2 code developed by the French Safety Institute (IRSN). First, the calculation of QUENCH-11 test is presented. The models of ASTEC/CATHARE V2 code permit to describe all transient phases: boil-off, heat-up due to Zircaloy oxidation, temperature escalation at reflooding onset, fast hydrogen production, cooling, material melting. The predictions of hydrogen production have been significantly improved (about 20%) using a new reflooding model. Second, alternative scenarii of QUENCH-11 test have been calculated in order to study the effect of the injection flow rate and the initial temperature at reflooding onset. The results of the calculations showed that the hydrogen production was promoted by high temperatures and low reflooding rates. Moreover, they exhibits a threshold phenomena. The hydrogen production was dramatically enhanced for injection rate lower than 1g/rods and initial temperature greater than 1800K.