Thermal-hydraulic–iodine chemistry coupling: Insights gained from the SARNET benchmark on the THAI experiments Iod-11 and Iod-12

Abstract

In the SARNET2 WP8.3 THAI Benchmark the capability of current accident codes to simulate the iodinetransport and behavior in sub-divided containments has been assessed. In THAI test Iod-11 and Iod-12,made available for the benchmark, the distribution of molecular iodine (I2) in the five compartments ofthe 60 m3vessel under stratified and well mixed conditions was measured. The main processes addressedare the I2transport with the atmospheric flows and the interaction of I2with the steel surface. Duringtest Iod-11 the surfaces in contact with the containment atmosphere were dry. In Iod-12, steam wasreleased, which condensed on the walls.Nine post-test calculations were conducted for Iod-11 and eight for Iod-12 by seven organizationsusing four different codes: ASTEC-IODE (CIEMAT, GRS and TUS), COCOSYS-AIM (AREVA, FZ-Jülich andGRS), ECART (Pisa University) and MELCOR (Pisa University and VTT). Different nodalizations of the THAIvessel with 20–65 zones were applied.Generally, for both tests the analytical thermal-hydraulic results are in a fairly good agreement withthe measurements. Only the calculated local relative humidity deviates significantly from the measuredvalues in all calculations. The results in Iod-11 for the local I2concentration in the gaseous phase arequite diverse. Three calculations show only minor deviations from the measurement, whereas the othersare substantially different from the measured I2concentrations. For Iod-12, no calculation delivers asatisfactory evolution of the I2concentration in all five compartments of the vessel. There are threemediocre results standing out in the Iod-11 exercise which are from the same user–code combinations.The discrepancies derive from various reasons which are discussed in the paper.In the benchmark a significant user effect was detected, i.e. results achieved with the same code differedconsiderably.This work highlights the need of a detailed iodine adsorption/desorption model and precise thermal-hydraulic modeling for an accurate simulation of I2transport in a sub-divided containment, as well asexperienced users or straight forward user guidelines.