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PILOT DISMANTLING OF THE WAGR. DISMANTLING OF TOP BIOSHIELD REFUELLING STANDPIPES, VESSEL TOP DOME, HEAT EXCHANGER, REMOTE DISMANTLING OF HOT BOX, REMOTE WASTE PACKAGING

Objective

The Windscale Advanced Gas-cooled Reactor (WAGR) had a capacity of 33 MWe and was operated from 1962 to 1981. Dismantling of the plant has started and is planned to be completed in 1996.

Considering that the experience to be gained from the dismantling of the first large-scale nuclear installations in the Community should be made available to all Member States, the Commission selected WAGR as a pilot dismantling project for the 1989-93 R&D programme on the decommissioning of nuclear installations. The Commission, through shared-cost participation in specific parts of the project, is promoting the use of advanced techniques and the performance of collateral investigations, in order to enhance the production of useful knowledge and experience to serve in subsequent decommissioning tasks. In particular, the generation of specific data on costs, working hours and job doses as well as on the amount of created secondary waste is an important objective of this project.

As a gas-cooled reactor, WAGR provides opportunities for testing decommissioning techniques against the specific requirements of such reactors, which represent the majority of the first-generation nuclear power reactors to be decommissioned in the Community in the near future. The first phase of the contract involves in particular the dismantling of the top biological shield, of refuelling standpipes and of the reactor pressure vessel top dome as well as inactive trials of the remote dismantling machine. The second phase covers dismantling of the hot box, one steam generator and the remote packaging of waste.

The estimated radioactive inventory is in the order of 10 superscript 5 Ci; estimated dose rates are in the range of 0.1 to 1.5 mSv/h.
As a gas cooled reactor the Windscale advanced gas cooled reactor (WAGR) provides opportunities for testing decommissioning techniques against the specific requirements of such reactors, which represent the majority of the first generation nuclear power reactors to be decommissioned in the Community in the near future. The first phase of the contract involves in particular the dismantling of the top biological shield, of refuelling standpipes and of the reactor pressure vessel dome as well as inactive trials of the remote dismantling machine. The second phase covers dismantling of the hot box, one steam generator and the remote packaging of waste.

Refuelling standpipes have been cut in 5 locations. Cut 4 permitted the task of top dome removal to commence. Following removal of the top dome, the fifth and final cut (prior to the start of remote dismantling) reduced the standpipes to within 300 mm of the hot box. The top dome has been separated from the reactor pressure vessel, removed from the reactor vault, size reduced and sent for disposal as low level waste. Associated with inactive trials of remote dismantling machine, construction of the test facility has been completed and installation of the remote dismantling machine is in progress. An option study for the dismantling of a WAGR heat exchanger has been carried out from which three possible methods are being considered. An initial preparatory phase in dismantling the hot box has been completed in cutting 130 stay tubes which served to provide additional stiffness to the top and bottom plates of the hot box.
WORK PROGRAMME

1. Dismantling of the top biological shield (TBS), a 60 t disc-shaped steel and concrete structure, by thermic lancing after moving it into a ventilated containment placed on the refuelling floor.

2. Cutting and handling of the refuelling standpipes, ie, 253 pipes of 6.3 m length penetrating the upper part of the reactor block, by 4 cuts, with an internally rotating plasma arc torch.

3. Cutting and dismantling of the pressure vessel top dome, a complex steel structure of 6.5 m diameter and 98 mm maximum thickness, by in-situ segmentation in 2 parts using a semi-remote operated oxy-gas cutter placed on a tractor followed by post-segmenting in a temporary containment placed on the refuelling floor.

4. Inactive trials of the remote dismantling system, comprising a rotating floor shield, an extendable mast carrying a telemanipulator arm, and a remotely operated conveying system, in a test facility representing a 30C sector of the reactor pressure vessel.

5. Generation of specific data on costs, radioactive job doses, working time and secondary waste arisings, derived from the execution of items 1, 2, 3, 6, 7 and 8.

6. Dismantling of a WAGR heat exchanger, beginning with the top outer shell to gain access to the tube banks. Protected by temporary shielding, the tube bank and the stainless steel thermal insulation packs of the economizer, evaporator and superheater sections will be decontaminated and removed successively.

7. Remote dismantling of the hot box, using the remote dismantling system (see 4). This operation requires severing the refuelling tubes and the 130 small diameter stay tubes remotely.

8. Remote packaging of intermediate level waste, consisting of the transfer, monitoring, size reduction and cement encapsulation of activated pressure loop components, hot box sections and operational waste.

Funding Scheme

CSC - Cost-sharing contracts

Coordinator

United Kingdom Atomic Energy Authority (UKAEA)
Address
Sellafield Laboratory
CA20 1PF Seascale
United Kingdom