Iodine Behaviour in Phébus FPT2, comparison with Previous Phébus Tests
JACQUEMAIN Didier, ZABIEGO Maxime (1), CORNU Bernard, GARNIER Yves (2). 5th PHÉBUS FP Technical Seminar Aix-en-Provence, June 24-26, 2003
The Phébus F.P. program is a wide international effort to investigate, through a series of in-pile integral experiments, PWR severe accident phenomena, in particular bundle degradation and the subsequent release and transport of radioactive materials through the primary circuit towards the containment vessel. Three tests simulating a low pressure cold leg break have already been successfully performed: FPT0 with trace irradiated fuel and under a steam-rich environment in 1993, FPT1 with re-irradiated BR3 fuel (23 GWd/tU) and a steam-rich environment in 1996 and FPT2 with re-irradiated BR3 fuel (32 GWd/tU) and a steam-poor environment in 2000. In all three tests, a Ag-In-Cd control rod was inserted in the fuel bundle. This test series has provided experimental data of high interest, particularly concerning iodine release from the fuel bundle, transport through the reactor coolant system (RCS) and behaviour in the containment for different conditions of the containment sump (pH = 5.0, T = 90°C in FPT0 and FPT1 and pH = 9.0, T = 120°C in FPT2).
The analysis of the FPT2 data presented in this paper shows that in the hot leg of the RCS CsI and CsOH are not the predominant Cs and I vapour species, contradicting the usually admitted speciation for steam/hydrogen or pure hydrogen gas stream at temperatures close to 700°C. The gaseous iodine fraction measured in the containment during and after the main Zircaloy-oxidation phase – under steam-starved conditions - is significantly lower than in the previous Phébus tests FPT0 and FPT1 : relative to containment inventory, gaseous iodine fractions represented 33 % in FPT0 and 4 % in FPT1 after the main Zr-oxidation phase to be compared with less than 0.7 % during the steam starved phase in FPT2. Highest gaseous iodine fractions are measured in the containment following the secondary Zr-oxidation phase associated with some hot material re-location in a zone where degraded material from the Ag-In-Cd control rod accumulated, suggesting Ag-In-Cd material play a significant role on iodine chemistry in the circuit hot leg. At that time, the measured fraction, 0.7 % of the iodine containment inventory, is comparable to that obtained in FPT1. On the long term chemistry phase, the gaseous iodine fraction in the containment is strongly reduced to reach a value of about 0.01 % in presence of the warm (T = 120°C) basic sump (pH = 9.0) lower than gaseous iodine fractions measured in previous Phébus tests by about an order of magnitude.
Implications of this series of Phébus tests results for PWR iodine source term studies are summarised and future integral and analytical programmes which appear necessary to reduce remaining uncertainties in the field of iodine source term studies are briefly presented.
(2) : CEA