Assessment of Spray and Pool Scrubbing Efficiencies for Means of Mitigation Against Aerosol Dispersion in the Context of Fuel Debris Retrieval at Fukushima Daiichi: Part II

Abstract

The general context of this paper is an evaluation of strategies that can be used to mitigate aerosol dispersion during fuel debris or corium retrieval in damaged Fukushima Daiichi reactors. Knowledge of the aerosol source terms released during fuel debris retrieval operations is one of the key factors for assessing aerosol dispersion leading to the potential dissemination of radionuclides into the environment. Our approach is to couple experimental results from integral tests obtained during laser cutting experiments, well-controlled analytical tests with separated effects performed in a dedicated facility to reproduce two-phase flow such as flows representative of pool scrubbing and spray scrubbing conditions, and numerical simulations. Integral tests provide relevant information on the airborne particle release fraction during laser cutting for underwater conditions at different water depths, such as the particle concentration and particle size distribution. However, the detailed characterization of two-phase flows, such as the size and velocity of gas bubble and water droplets generated by spray systems, is not possible during laser cutting integral tests. Therefore, a more analytical approach is necessary to obtain detailed information on two-phase flow, composed of bubbles in water, inducing pool scrubbing phenomenon, and droplets in gas generated by spray scrubbing systems used for mitigating dust dispersion, which are essential to the physical mechanisms of both processes and enable their respective efficiencies to be evaluated. The main objectives of this work were to develop models and ensure their validation based on experimental approach for predicting the pool scrubbing and spray scrubbing efficiencies in the context of fuel debris removal at Fukushima Daiichi.