NuFuel & MMSNF 2015

First Workshop on Research into Nuclear Fuel in Europe
and Materials Modeling and Simulation for Nuclear Fuels Workshop
Karlsruhe, Germany, November 16th to 18th, 2015

Updated: Tue 08 Dec 2015, 14:27

Talk 3.9: Fission products chemisorption mechanism following severe accidents: a separate effect study on CsOH reaction with stainless steel

Fidelma Giulia Di Lemma, Yamashita Shinichiro, Nakajima Kunihisa, Takada Juntarou, Osaka Masaiko, Nagase Fumihisa.
  • Development Group for LWR Advanced Technology, LWR Key Technology Development Division, Nuclear Science and Engineering Center, Japan Atomic Energy Agency (JAEA)

Abstract

Fission products chemisorption on reactor structural materials is an important phenomenon to be considered following an accident, as it can lead to retention of fission products in the containment, affecting the radioactive release to the environment and reactor decommissioning. The knowledge acquired from this study could help improve the models for cesium chemisorption and could be applied to evaluate the distribution and properties of Cs deposits in the Fukushima-Daiichi Nuclear Power Plant (1F-NPP), providing guidance for its decommissioning.

These separate effect studies were performed vaporizing CsOH over stainless steel samples, varying experimental parameters, such as temperature (800–1000ºC) and atmosphere (H2/Ar, H2/H2O/Ar). Sample surfaces were analyzed by SEM/EDX, XRD to evaluate the deposit microstructure and chemical composition. Cross sectional analyses were also applied to investigate the corrosion mechanism. Finally leaching tests were performed to evaluate the behavior of such deposit, showing that Cs is retained more effectively at low temperature and in reducing conditions. Our studies demonstrate a congruent distribution between Cs and Si, in agreement with the existing models[1] and the performed thermodynamics calculations, which predict the formation of cesium silicates under the experimental conditions. The XRD patterns permitted the chemical identification of the formed Cs compound as CsFeSiO4. These results show the importance of the presence of impurities in stainless steel, as they can affect the formation of stable cesium deposits following a severe accident. More tests are however needed to extend our observations to a possible model (such as tests at 600ºC, with high oxygen potentials, and using pre-oxidized samples). Finally it would be interesting to evaluate the effect of other chemicals on Cs adsorption, such as boron, which has been shown to be important during accident progression in a BWR.

Reference:
  1. M. Elrick, R. A. Sallach, A.L. Ouellette, S.C. Douglas. Tech Rep. NUREG/CR–3197 (1984).