Study of the Ageing Effects on the Lower Head Failure in a PWR Reactor

Abstract

The paper studies influence the ageing effects on the failure of a Reactor Pressure Vessel (RPV) during a severe accident with a core meltdown in a Nuclear Power Plant (NPP). The studied plant is a generic high-power Generation III Pressurized Water Reactor (PWR) developed in the frame of the EU NARSIS project. A Total Station Blackout (SBO) accident was simulated with MELCOR 2.2 severe accident integral computer code. Results of the analysis, temperatures in the lower head and pressures in the lower plenum were used as initial and boundary conditions for the Finite Element Method (FEM) simulations. Two FEM models were developed, a simple two-dimensional axis-symmetric model of the lower head to study fundamental phenomena and complex 3D model to include interactions with the RPV and reactor internals. Ageing effects of a lower head were incorporated into the FEM models to investigate its influence onto lower head response. The ageing phenomena are modelled in terms of degraded mechanical material properties as σ(T), E(T). The primary outcome of the study is the quantitative estimation of the influence of ageing process onto the timing of reactor vessel failure. Presented novel methodology and results can have an impact on future consideration about Long-Term Operation (LTO) of NPPs.

Preliminary Analysis of an Aged RPV Subjected to Station Blackout

“…This study investigated the influence and impact of ageing and creep phenomena on the behaviour of a 4500 MWth NPP RPV [4]. An SBO event and its main thermo-mechan- The RPV displacement (Figure 14b) is observed to occur in two stages: the first is characterised by a small and linear displacement rate, and the second is characterised by a large and accelerating displacement rate.…”

“…This study investigated the influence and impact of ageing and creep phenomena on the behaviour of a 4500 MWth NPP RPV [4]. An SBO event and its main thermo-mechanical consequences were analysed preliminarily by coupling MELCOR and MARC codes: results from former were the inputs of the latter.…”

“…Figure 3 illustrates the logical sequence of the methodology we developed to analyse the effects of ageing on the load bearing capacity of the RPV subjected to SA. In this study, we investigated the performance of aged RPV of the 4500 MWth PWR (reference system of the EU NARSIS project [21]), described in [4], and subjected to severe accident conditions. This is not an easily replaceable component, due to geometric constraints and radiological aspects (neutron irradiation), and the overall plant lifetime depends on how its integrity is maintained and managed.…”

“…As described in [4], the Reactor Coolant System (RCS) model of a generic high-power Generation III PWR [21] under total Station Black Out (SBO) accident with a core meltdown was implemented by MELCOR 2.2 9541.…”

“…As indicated in the IAEA SSG-2, 2019 [4], the several degradation mechanisms an equipment may experience in operation are mainly: Since many of them occur simultaneously, deterministic analysis must be performed in a conservative manner so as to evaluate any possible synergy between them. Such analysis is particularly important for class 1 components such as the Reactor Pressure Vessel (RPV), which is characterized by a high geometric complexity and by types of alteration and degradation depending on the environment and operation conditions.…”