Modelling of Heat and Mass transfer in the Steam Generator of a PWR under accident conditions and validation based on experimental data
Motivation:
During the low power and shutdown operation the decay heat in a Pressurizer Water Reactor (PWR) is removed by the Residual Heat Removal System (RHRS). As part of the reactor safety assessment, it is necessary to account for a wide range of accidents including the Loss of RHRS. The contribution to the reactor core damage frequency of such accident is considerable and thorough evaluation is needed. Up to these days, due to large scale and geometrical complexity, nuclear power plants are evaluated with dedicated system thermal-hydraulic codes (SYS-TH) such as ATHLET, RELAP, MARS etc. Although SYS-TH codes have been developed for multiple decades, they still have difficulties accurately simulating phenomena arising during multiphase flow. This is especially pronounced during the mid-loop operation, where additional complexity is added due to the presence of non-condensable gas.
Objectives:
The project targets the development of reliable thermal-hydraulic models for the primary circuit of a PWR in the Loss of RHRS accident conditions during mid-loop operation. The focus is on the accurate modelling of the condensation process in a steam generator. There, we find complex thermal hydraulic states, such as two-phase flow, reverse flows, counter-current flow limitation, overspilling, fill-and-down, water entrainment, and potentially inert gases.
Methods and results:
For the numerical calculation, we use the best estimate system thermal-hydraulic code ATHLET, and validate the models by comparison against experimental data. Some phenomena of interest will be assessed in a more detailed manner with CFD simulations.
A sub-objective of the project was to gain a deeper understanding of the heat transfer phenomena in steam generators under transient conditions. Several activities contributed to the fulfillment of this goal, such as the comprehensive analysis of accessible literature and internal reports, the analysis of experiments and the execution of a series of simulations providing detailed information on the occurring phenomena.
The development of a combined heat transfer model and its implementation into an existing model of the associated partner with the Modelica system code was carried out during the work at the partner's site FRAMATOME. A new model of a steam generator was created and tested, and it was found that Modelica is currently not suitable to meet the modeling and simulation requirements due to deficiencies in the two-phase numerical calculation. Further model development was therefore carried out using the thermal-hydraulic code of the ATHLET system. A complete model of the PKL with a 3/4 circuit configuration was created and a model validation for the loss of the residual heat removal system in the middle of the circuit was carried out.
Publications:
E. Smigelskis
Post-test thermal-hydraulic analysis of PKL III I3.1 Experiment on loss of RHRS during mid-loop operation with Athlet
FISA2022 & EURADWASTE’22, 30.05.-03.06.2022, Lyon, France