Session: 03-01: Major facilities experience in handling accidents and D/D&D

Paper Number: 109536

109536 - Aerosol Mitigation Means for Fukushima Daiichi Fuel Debris Removal 

One of the important challenges for the decommissioning of the damaged reactors of the Fukushima Daiichi Nuclear Power Plant is the retrieval of the fuel debris.

Due to the specificity of each severe accident scenarios occurred in reactors, the status of each power plant unit is consequently different. The fuel debris are located inside and outside the Reactor Pressure Vessel (RPV) and the molten core concrete interaction (MCCI) is more or less pronounced. It follows that different fuel debris retrieval configurations have to be studied to propose the best strategies for the dismantling which reduce the risk for workers and environment. It is especially primordial to investigate the cutting conditions in air or underwater and also various cutting tools such as thermal and mechanical ones, in considering their specificity, their capabilities regarding the implementation in the 1F Primary Containment Vessel (PCV) and their airborne particles generations. Concerning the thermal cutting tools, the laser technique is well adapted to the cutting of expected material such as fuel debris which exhibits irregular shapes. In addition, comparatively to others cutting techniques (grinder, plasma torch, arc air, arc saw), the laser cutting technique generates the lowest mass of removed material and subsequently, the lowest mass of airborne aerosol particles. Comparisons between aerosols size production emitted during the cutting of stainless steel with different tools show that laser produces particles slightly smaller than other tools, but most of cutting tools produce particles whose size are under 0.5 µm which requires developing mitigation means against particles dispersion upstream the HEPA (High Efficiency Particulate Air) filtration system. In addition to the study of the technological aspects regarding the capability of the cutting tool to be deployed in hostile radiating environment, the risk of the fuel debris retrieval operations has to be evaluated to limit the radionuclides dissemination outside the containment vessel.

The general context of the article is to evaluate strategies that can be used to mitigate aerosols dispersion during the fuel debris or corium retrieval of Fukushima damaged reactors. 

IRSN is involved in several projects led by ONET Technologies along with CEA to provide relevant information to analyze the risk of aerosol resuspension induced by fuel debris retrieval. The knowledge of the aerosol source term emitted during fuel debris retrieval operations is one of the key issues for the assessment of aerosol dispersion that can lead to the release of radionuclides into the environment. Such information are also necessary to define an efficient strategy to mitigate this risk. Various mitigation means could be implemented during the decommissioning of Fukushima Daiichi damaged reactors, depending on the operations such as cutting of fuel debris or metallic structures, investigation in PCV by robots. It is also important to consider accidental scenarios such as example earthquake event, to define countermeasures limiting the consequence in terms of safety and radioprotection. We propose to study various mitigation means as the spray scrubbing technology used to collect airborne particles and therefore to limit their dispersion during the cutting operations. Resuspension of deposited particles may also occur during the decommissioning operations due to various type of stress, such as aeraulic, mechanical, vibrational and also during underwater operations. To address these particle resuspension issues, another mitigation means made by coating of resins is introduced.

Presenting Author: Emmanuel Porcheron Instiute For Radiological Protection and Nuclear Safety (IRSN)

Presenting Author Biography: Emmanuel Porcheron received his PhD in Fluids Mechanics in 1998 and his Habilitation to supervise research in 2014. He has been working for IRSN since 1999 leading research projects for nuclear safety applied to thermal hydraulics, hydrogen risk, instrumentation for multiphase flow, and D&D. He is deputy head of laboratory of Aerosols Physics and metrology and since 2015 in charge of R&D projects for radioactive aerosol issue during fuel debris retrieval at Fukushima Daiichi.