Long Straight Closed Two-Phase Thermosiphons - Experiments of IKE and Validation of ATHLET Models

Abstract

In recent years, there has been a global focus on passive residual heat removal using thermosiphons or heat pipes for spent fuel storage pool and wet storage cooling. In the event of a station blackout with loss of active cooling modes, a passive heat removal system based on two-phase closed thermosiphons (TPCT) can effectively maintain the cooling of spent fuel pools due to their high heat transport capacity and simple design.

GRS develops and validates computer codes like AC² to simulate all relevant nuclear power plant phenom-ena during normal operation, incidents, accidents, and severe accidents. In 2016, the University of Stuttgart and GRS initiated an R&D cooperation to enhance the thermal-hydraulic code system ATHLET (Analysis of Thermalhydraulics of Leaks and Transients), enabling it to model thermosiphon and heat pipe operations. After addressing key model gaps, such as properties of water/steam in near-vacuum conditions and intro-ducing new heat transfer correlations for boiling and condensation under such conditions, in 2020, the re-search focus expanded to study complex thermosiphon configurations. Experiments in four different TPCT test facilities with one 3/10-meter-long straight TPCTs, with one/nine straight/bended TPCTs were and are performed. For two selected experiments (3-meter-long straight TPCT and nine 10-meter-long straight TPCTs) post-test calculations with improved German ATHLET program (as part of coupled code system AC² of the main modules ATHLET, ATHLET-CD and COCOSYS of GRS) are presented and the results are compared to experimental measurements. The correlations taken from literature fit the general operation behaviour quite well but doesn’t fit for 3-meter-long to 10-meter-long TPCTs in all details. Therefore further model improvements based on semi-empirical correlations of the new experiments will be done.