Solid Radioactive Waste Strategy Report.pdf - UK EPR

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EPR UK N° NESH-G/2008/en/0123 REV. A PAGE 216 / 257 FIGURE 83: SKETCH VIEW OF THE TN DUO FLASK IN STORAGE CONFIGURATION AND IN TRANSPORT CONFIGURATION The canister (the first independent containment barrier) contributes also to the mechanical strength of the flask and to the shielding. In its storage configuration, a monitoring system including three pressure sensors located on the side of the flask overcomes the requirement to remove the anti-aircraft crash cover in case of repair. An anti-aircraft crash cover is fitted on the top of the flask and protects the package in case of aircraft crash. For transport, the TN DUO is equipped with shock absorbers which are suitable for rail and road utilisation. Containment The TN DUO solution offers two independent containment barriers. The primary (inner) containment boundary consists of: · A rolled and welded steel shell; · A thick bottom steel plate which is welded to the cylindrical body; · A forged steel top flange which is welded to the cylindrical body; · A top lid is bolted to the flange with bolts. This lid is equipped with two gaskets (metallic gaskets and/or elastomer gaskets). The secondary containment boundary is designed on the same concept as the primary, with the following differences: · Two rolled and welded steel shells (instead of one) are shrink-fitted together; · The top lid is bolted to the flange with bolts. This lid is equipped with two gaskets (metallic gaskets and/or elastomer gaskets). During storage, the secondary lid forms a line of defence in case of failure of the primary lid. The inter-lid space is pressurised with helium and continuous monitoring of the pressure allows detection of any decrease of leak-tightness performance long before any radioactive release is possible. The pressurisation of the monitored space ensures that no gas can flow
EPR UK N° NESH-G/2008/en/0123 REV. A PAGE 217 / 257 from the inner cavity to the atmosphere. Both primary and secondary lids are equipped with a single orifice (used for draining water from the cavity, venting, drying, vacuum drying and back-filling with helium). Multiple penetrations are thus avoided. The flask cavity is filled with helium under atmospheric pressure. All the confinement sealing surfaces are covered with stainless steel overlay for enhanced corrosion protection. The two containment boundaries are independent of each other. All challenges that could result from accident conditions would be prevented by the two-layer-thick secondary barrier. The primary containment would thus be subjected to reduced stress therefore ensuring leaktightness. Criticality control The flask is fitted with a basket designed to fulfil three functions: · Mechanical support for the spent fuel assemblies; · Conduction of decay heat to the cavity surface; · To ensure sub-criticality of the spent fuel. The basket consists of slotted aluminium and steel plates. These two types of plates are stacked in a criss-crossed structure to form an array of cells. Basket-length boron aluminium plates are inserted flush with the side surface of these lodgements to ensure sub-criticality. Assembly of the basket is thus simplified. FIGURE 84: CONSTRUCTION OF THE TN DUO BASKET
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Solid Radioactive Waste Strategy Report.pdf - UK EPR

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