Waste Resin Holding Containers (WRHC)
| Contract: | Waste Resign Holding Containers (WRHC) |
| Customer: | Rosyth Dockyard |
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| Details: |
The basis of this contract was to provide the client with a cost effective means of providing new pressure containers for the on-going long term storage of active waste resins currently stored on site in carbon steel tanks which have been subjected to years of corrosion. The existing RCTs (Resin Catch Tanks) are essentially Ø1200 mm Carbon Steel pressure vessels set centrally within a larger Ø1900 mm steel shell and the void filled with concrete for shielding purposes. Resins were originally pumped into the vessel, the flange sealed and a concrete capping poured over the top to the same thickness as the sides. These vessels were originally intended for sea-dumping but legislation in later years prevented this, leading to the tanks being stored in a dedicated facility on site. Over the years it was speculated that the inner pressure vessel would have been subject to corrosion which may now be reaching a critical level. As it was not possible to verify the exact degree of corrosion, a fast and economical solution to on-going storage had to be found. NES provided a solution to this problem whereby a new slightly smaller stainless pressure vessel would be inserted inside the existing steel vessel and the units then re-used. This approach would keep costs of new materials and disposal of old, contaminated equipment to a minimum by re-using much of the original structure and retaining its proven shielding and protective properties. The new vessels were designed to pressure vessel code BS5500 and the whole new assembly was the subject of ‘dropped load’ calculations and ‘thermal withstand’ calculations in a 538ºC fire for 20 minutes. To physically install the new vessels into the existing RCT also required the use of several special machines to overcome the task at minimal cost. Firstly the vessels were emptied of resin and washed-out and the concrete shielding removed from the top of the vessel using pneumatic hammers. The exposed vessel lid was a flat circular 30 mm thick steel plate welded into the vessel shell, it was also unique in that six of the 40 mm steel tie bars were welded through the lid, down into the vessel and welded again onto the vessel bottom plate. To remove the lid a special cutting machine was sourced that powered two ‘parting’ tools around a diameter 10 mm less that the lid diameter to cut through the lip entirely leaving the lid supported on the size tie bars. A bridging support was then bolted across the rim and each of the Tie Bars machined around with a ‘broaching tool’ to release them from the lid. After physically removing the lid the remaining bars were cut off close to the vessel base plate with a special hydraulic ‘cropping tool’ and each bar fully removed. The new vessels were then set inside at a specific height and a pre-assessed quantity of cement grout poured in and allowed to set to fully support the new vessel just across its bottom domed end. The incorporation of thick steel plates on top of the new vessel topped by an additional shield plate meant that it was not necessary to replace the concrete capping. Finally a close fitting disc of special thermal insulation was added to protect the top against the fire withstand requirement, the Resin was then transferred back into the vessel and the unit moved back into storage. Radiological scans of the new vessel now renamed as a Waste Resin Holding Container later showed that the shielding properties were better than the original RCT and well in excess of requirements. |