Thermal stratification and mixing induced by steam injection into a pressure suppression pool

QC 2025-02-18

Abstract

Boiling Water Reactors (BWRs) employ the Pressure Suppression Pool (PSP) to prevent containment overpressure. Steam released from the primary coolant system is injected into the pool and condensed rapidly upon direct contact with subcooled water. However, steam injection can lead to the development of thermal stratification in the pool. The increased surface temperature of the stratified pool also increases containment pressure compared to a mixed pool. This process can become of safety significance, as it was observed in the Fukushima Daichi accident. 

The primary objective of PSP safety analysis is to verify that pool temperature remains within acceptable limits. The state-of-the-art approach has to rely on assumptions about the fraction of the pool acting as a heat sink, i.e. elevation of the thermocline. There is a need for more mechanistic approaches that can adequately resolve the interactions among various phenomena, safety systems, and operational procedures that can clarify the degree of conservatism in the currently employed approaches to safety analysis. For this purpose, a computationally efficient tool known as the Effective Heat Source (EHS) and Effective Momentum Source (EMS) models have been proposed. The models are used to simulate the integral effect of steam injection on the large-scale pool without explicit modeling of the dynamics of the interface between steam and water.

This thesis aims to make a significant step toward the development and validation of the predictive capabilities of the EHS/EMS models for the assessment of the PSP performance during steam injection through spargers. To achieve this goal, a synergic framework that integrates both experimental and numerical campaigns has been developed.

Conceptual design and conditions for a series of Integral Effect Tests (IETs) of steam injection through a sparger into a large-scale pool in the PANDA facility have been proposed. A set of Separate Effect Tests (SETs) of steam injection in the SEF-POOL facility is proposed to develop new correlations for the EMS. A Bubble-based Particle Tracking Velocimetry (Bub-PTV) technique is developed and implemented in SEF-POOL to measure the streamwise velocity profiles induced by steam injection.

Modeling guidelines for CFD simulations using EHS/EMS models are developed for the prediction of the thermal behavior of the pool. A turbulence source to represent the effects of steam condensation is proposed. The applicability and validity of the modeling approaches are assessed by comparing them with the measurements obtained in IETs. Also, an approach of scaling based models is proposed to predict the erosion velocity of the thermocline.

The developed modeling approaches are applied to analyze the PSP performance of a Nordic BWR during various realistic scenarios. The possibility of thermal stratification and the effects of activation of different systems on the pool behavior are investigated.

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