Retrieval and conditioning of the STE2 sludges The STE2 station served to collect the effluents from the UP2‑400 plant, to treat them and to store the precipitation sludges resulting from the treatment. The STE2 sludges are precipitates that fix the radiological activity contained in the effluents and they are stored in seven silos. A portion of the sludges has been encapsulated in bitumen and packaged in stainless steel drums in the STE3 facility. Following ASN’s banning of bituminisation in 2008, Orano studied other conditioning methods for the non-packaged or stored sludges. The scenario for the retrieval and conditioning of the STE2 sludges presented in 2010 was broken down into three steps: • retrieval of the sludges stored in silos in STE2 (BNI 38); • transfer and treatment, initially envisaged by drying and compaction, in STE3 (BNI 118); • conditioning of the resulting pellets into “C5” packages for deep geological disposal. ASN authorised the first phase of the work to retrieve the sludges from STE2 in 2015. The Creation Authorisation Decree (DAC) for STE3 was modified by the Decree of 29 January 2016 to allow the installation of the STE2 sludges treatment process. At the end of 2017 however, Orano informed ASN that the process chosen for treating the sludges in STE3 could lead to difficulties in equipment operation and maintenance. Orano proposed an alternative scenario using centrifugation and in August 2019 it submitted a Safety Options Dossier (DOS), which is however based on as yet insufficiently substantiated hypotheses. An inspection conducted at the end of 2019 confirmed that the project was not sufficiently mature for ASN to be able to give an opinion on this DOS. In 2022, during the technical discussions held between Orano, ASN and IRSN, Orano committed itself to a new roadmap for this project. Orano has thus abandoned the centrifugation scenario and undertaken to conduct new studies in parallel aiming firstly to look into the sludge treatment and conditioning solutions in more detail, and secondly to put in place an intermediate storage facility (new silos) under suitably safe conditions, enabling the retrieval and safe storage of these sludges to be separated from their final packaging. Orano sent ASN the DOS associated with this project to create new sludge storage silos (project called “NABUCO”) in December 2023. Silo 130 Silo 130 is a reinforced concrete underground storage facility, with carbon steel liner, used for dry storage of solid waste from the reprocessing of Gas-Cooled Reactor (GCR) fuels, and the storage of technological waste and contaminated soils and rubble. The silo received waste of this type as from 1973, until the 1981 fire which forced the licensee to flood the waste. The leak-tightness of the water-filled silo is only ensured at present by a single containment barrier consisting of a steel “skin”. Furthermore, the civil engineering structure of silo 130 is weakened by ageing and by the fire that occurred in 1981. The water is therefore in direct contact with the waste and can contribute to corrosion of the carbon steel liner. One of the major risks for this facility concerns the dispersion of radioactive substances into the environment (infiltration of contaminated water into the water table). The leak-tightness of silo 130 is monitored by a network of piezometers situated nearby. Another factor that can compromise the safety of silo 130 is linked to the nature of the substances present in the waste, such as magnesium, which is pyrophoric. Hydrogen, a highly inflammable gas, can also be produced by phenomena of radiolysis or corrosion (presence of water). These elements contribute to the risks of fire and explosion. The WRC scenario comprises four steps: • retrieval and conditioning of the solid GCR waste; • retrieval of the liquid effluents; • retrieval and conditioning of the residual GCR waste and the sludges from the bottom of the silo; • retrieval and conditioning of the soils and rubble. Orano has built a retrieval unit above the pit containing the waste and a new building dedicated to the storing and conditioning operations. The licensee validated industrial commissioning of the waste recovery process in 2022, further to the tests carried out in 2020 and 2021. Quantitatively speaking, 2023 saw the recovery of some fifty additional drums of waste, bringing the quantity of waste recovered since the start of the operations in 2020 to about 17% of the total. The licensee has nevertheless had problems with recovery equipment reliability (failure lasting from August 2022 till March 2023) and the recovery rate also remains below the initial target. To increase the rate of waste recovery, the licensee took various measures in 2023, such as setting up a maintenance team dedicated to silo 130, and having the recovery teams work three 8-hour shifts per day instead of two since November 2023. ASN considers these measures to be positive, but it will keep a close track on their effectiveness and impact on the rate of waste recovery. Lastly, in December 2023, ASN issued the authorisation for the second stage of waste recovery from silo 130, corresponding to the liquid effluents. ASN Report on the state of nuclear safety and radiation protection in France in 2023 77 Regional overview of nuclear safety and radiation protection • NORMANDIE •
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