The in-vessel retention (IVR) strategy is an important measure for mitigating severe reactor accidents. It has been successfully applied to the severe-accident management of advanced pressurized water reactors such as the AP1000, HPR1000, and CAP1400. In the implementation of the IVR strategy, the lower head is deformed by the heat load of the high-temperature melt. This affects the heat-removal capacity of the pressure-vessel external cooling and the successful implementation of the IVR strategy. It is necessary to examine the stress, failure, and deformation of the lower head.

This study aims to develop a large deformation model for the pressure-vessel lower head and analysis of its application in the FOREVER experiment.

A mechanistic model called the lower-head large-deformation model was developed to address the limitations of the simplified film stress model of the Integrated Severe Accident Analysis (ISAA) program Lower Head Thermal Creep Module (LHTCM), which is very simple, and the absence of a deformation calculation module in the LHTCM model. This model was based on Timoshenko plate and shell theory, the Norton creep law, and large-deformation plasticity theory. Then, the model was integrated into the ISAA program to calculate the FOREVER-EC2 experiment.

The overall deformation result predicted by the large-deformation model exhibits the characteristic egg-like shape, with maximum displacement occurring at the bottom position of the lower head. The failure time predicted by the large-deformation model is 394.33 min, with an error of only 1.9% relative to the experiment. The predicted bottom elongation is consistent with the experimentally measured value. Additionally, the predicted location of the breach is consistent with the experiment, occurring between 75o and 85°.

The lower-head large-deformation model of this study can accurately predict the stress, failure time, overall deformation, and location of the breach of the lower head in a severe core melting accident.