Characterisation and Decontamination of Concrete and Plastic Infrastructure from the Hunterston A Spent Nuclear Fuel Storage Pond

Abstract

Nuclear power plants can become heavily contaminated with radionuclides during their operational lifetime. These contaminated materials give rise to high doses of radiation and heat, increasing risk to workers and the surrounding environment. Rapid, in-situ characterisation is vital to determine the extent of radionuclide contamination and develop appropriate decontamination methods to help reduce the volume of intermediate level waste (ILW) produced during decommissioning and minimise the costs of post operational clean out (POCO) procedures. Complex environments such as spent nuclear fuel (SNF) storage ponds require extensive research into the physical and chemical composition of the pond structure. The long-term storage of spent Magnox fuel in wet storage ponds has resulted in the leakage of SNF and contamination of the surrounding infrastructure creating a huge, complex decommissioning challenge. Of particular concern are the fission products, Sr-90 and Cs-137, and corrosion product, Co-60, due to their abundance and mobility. This project focuses on the interactions of these radionuclides with concrete and plastic materials relevant to the nuclear industry at conditions representative of the alkaline SNF ponds. In this project, sorption experiments simulating the alkaline conditions of SNF storage pond were conducted. Surface and solution measurements were used to determine the uptake of stable Sr, Cs and Co onto concrete and HDPE coupons representative of the radionuclides and materials found in the pond environments. Laser-induced breakdown spectroscopy (LIBS) was used to analyse the coupons to assess its ability for use as a rapid, in-situ characterisation tool in the nuclear industry. Characteristic Sr, Cs and Co emission lines were resolved from the concrete and HDPE matrices with LIBS. Limits of detection (LOD) for each contaminant were determined or each matrix and all contaminants were identified > 0.01 mg/cm2 under alkaline conditions at a working distance of 8 cm. In addition, LIBS was used to analyse model waste discharge pipeline samples coated with biofilm. LIBS was able to resolve characteristic Sr, Cs, Co, Ru and Eu emission lines from the coupons with and without a biofilm present and identified variations in emission intensities depending on the presence of the biofilm. To build on model sorption experiments, authentic samples were obtained from the Hunterston A SNF storage pond. A concrete core was obtained from the middle of the pond wall and plastic discs were obtained from temporary pontoons used during decommissioning operations. Radiometric, chemical and microscopy techniques were used to characterise the distribution of radionuclide contamination on the sample surfaces and in the bulk. Sr-90, Cs-137, Am-241 and Pu-238-241 were identified as the predominant radionuclide contaminants. Autoradiography of the plastic disc surfaces showed the distribution of radioactivity was associated with areas of increased damage and the presence of metal oxide precipitates, characterised with Raman and LIBS. Autoradiography of the concrete core determined contamination was heterogeneously distributed across the surface. Analysis of the core cross-section determined radionuclide contamination was predominantly isolated within the protective layers on the core surface, but some activity had penetrated 5-10 mm into the bulk concrete. Sequential extraction showed Cs-137 was strongly bound to silicates and aggregate phases in the concrete bulk and could not be leached under natural conditions. Sr was found to be predominantly isolated within the cement, likely in calcium silicate hydrate phases. Finally, polymer based hydrogels were found to simultaneously remove fission product and actinide contamination from the plastic discs and painted surface of the concrete core. Increasing the contact time and repeat applications resulted in increased removal of activity for all material surfaces. Hydrogels were able to take up 2

Date of Award	1 Aug 2023
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Dirk Engelberg (Supervisor) & Scott Heath (Supervisor)