Immobilisation process for contaminated zeolitic ion exchangers from Fukushima
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Immobilisation process for contaminated zeolitic ion exchangers from Fukushima D. Pletser1, R. K. Chinnam1, M. Kamoshida2, W.E. Lee1 1 Centre for Nuclear Engineering, Imperial College London, SW7 2AZ London,United Kingdom 2 Center for Technology Innovation – Energy, Research & Development Group, 7-2-1 Omikacho, Hitachi-shi, Japan Abstract The clean-up of the Fukushima Daiichi site, after the March 2011 earthquake and tsunami, continues to generate large amounts of spent adsorbents. These adsorbents need to be disposed of permanently in a low temperature immobilisation process to avoid volatilising radioactive Cs and Sr species. To this end an immobilisation process with a maximum temperature of 600 ºC was developed by sintering model waste with glass frit to form a dense Glass Composite Material (GCM) wasteform. A zeolitic model wasteform, chabazite, was sintered with a lead borosilicate glass composition at a maximum temperature of 600 ºC. The sintering process was optimised with various thermal treatment steps to ensure that volatile species, aqueous or otherwise, were released before full sintering to yield a dense final wasteform. With this process dense wasteforms of up to 40 wt. % chabazite have been achieved. Introduction The Tohoku earthquake and subsequent tsunami of March 2011 which led to the accident at Fukushima Daiichi has caused a large amount of devastation on the site. Reactors 1-4 were severely affected by the two loss-of-coolant accidents and subsequent hydrogen explosions. While cold shut down was achieved in all reactors in December 2011, large volumes of cooling water are still injected to cool the residual decay heat from the remaining cores[1]. Multiple water treatment systems are currently in place to accommodate this, such as the Hitachi High Performance Advanced Liquid Processing System (HP-ALPS). However, the treatment of approximately 700 m3/day of contaminated water continues to generate large volumes of spent adsorbents, which can be highly radioactive. These spent adsorbents are currently stored on-site, but will require immobilisation in a suitable wasteform before being disposed of permanently. Currently the majority of radionuclide species present in the effluent cooling water consist of Cs and Sr species with short half-lives, the longest of the key targeted nuclides is137Cs with a 30.08 year half-life, which means that the final waste form will need to have different performance standards than normal HLW (High Level Waste). The final wasteform will be ILW (Intermediate Level Waste) with a repository lifetime of 300 years and will likely be stored at a surface storage facility. Another aspect to consider in any process used to immobilise the adsorbents is Cs and Sr volatilisation. The system has a high chloride content due to the use of seawater as an emergency coolant and the ingress of groundwater. The high level of chlorides could lead to volatile CsCl and SrCl species. Both the chloride species of Sr and Cs are volatile at higher temperatures[2], so to ensure minimal volatilis
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