A preliminary coupling test case has been carried out, in which a simple pipe using LBE as cooling fluid is simulated to verify the correct exchange of data between both codes.

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Abstract

The thermal-hydraulic system code ATHLET is developed by the Gesellschaft für Anlagen- und Reaktorsicherheit (GRS) mbH in the last decades for the analysis of the whole spectrum of design basis and beyond design basis accidents for PWRs and BWRs.

Recently, a new version of the ATHLET (Analysis of THermal-hydraulics of LEaks and Transients) code has been released, in which the physical properties as well as heat transfer correlations for innovative coolants like helium, sodium, lead and Lead-Bismuth Eutectic (LBE) are implemented. Such development allows extending the capability of the ATHLET code for the analysis of thermal-hydraulic behavior of the future Generation IV reactor concept. Furthermore, ATHLET was coupled with the commercial 3D Computational Fluid Dynamics (CFD) software package ANSYS CFX to improve the system code simulation capabilities for flows with pronounced 3D phenomena such as flow mixing, thermal stratification, etc..

In the present thesis the new coupling methodology between the system code ATHLET and the ANSYS CFX will be applied to the safety analysis of the spallation loop of the experimental reactor MYRRHA, which is able to demonstrate the Accelerator Driven System (ADS) full concept, allowing the study of efficient transmutation of high-level nuclear waste. In particular, the potentialities of the ANSYS CFX code are used to simulate the most complicated zone of the entire spallation loop from the physical point of view, namely the windowless target. The remaining part of the spallation loop is simulated with the ATHLET code.

A preliminary coupling test case has been carried out, in which a simple pipe using LBE as cooling fluid is simulated to verify the correct exchange of data between both codes.

Subsequently, first coupled 1D-3D calculations for a simplified open configuration of the spallation loop are presented. Difficulties related to the LBE mass conservation in the CFD calculations are noticed, which in turn causes stability issues in the coupled codes analysis.

Therefore, slight modifications in the ATHLET nodalization were necessary before

starting the simulation of a closed loop configuration. Transient analyses have been carried

out, in which the LBE mass flow rate is changed in few seconds caused by a possible

variation of the pump impeller velocity. Although the presence of the two LBE free

surfaces inside the CFD domain has strongly influenced the mass balance during the

transient simulations, first coupled calculations obtained with ATHLET – ANSYS CFX

showed reasonable results.