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Remote Handling & Decommissioning

The reactor core of the Windscale Advanced Gas-Cooled Reactor (WAGR) has been dismantled as part of an ongoing decommissioning project. The WAGR operated until 1981 as a development reactor for the British Commercial Advanced Gas cooled Reactor (CAGR) power programme.

Decommissioning began in 1982 with the removal of fuel from the reactor core which was completed in 1983. Subsequently, a significant amount of engineering work was carried out, including removal of equipment external to the reactor and initial manual dismantling operations at the top of the reactor, in preparation for the removal of the reactor core itself. Modification of the facility structure and construction of the waste packaging plant served to provide a waste route for the reactor components.

The reactor core was dismantled on a ‘top-down’ basis in a series of ‘campaigns’ related to discrete reactor components. This report describes the facility, the modifications undertaken to facilitate its decommissioning and the strategies employed to recognise the successful decommissioning of the reactor.

BACKGROUND

The WAGR was the prototype industrial scale development model for the CAGR nuclear power stations, the UK’s second generation of reactors. Constructed between 1957 and 1961 and operated successfully by the United Kingdom Atomic Energy Authority (UKAEA) until its shutdown in 1981, the reactor containment building (Figure 1) was an all welded hemispherical dome, 41m in diameter, providing a controlled area and pressure containment, to totally envelop the reactor and its associated biological shielding.

In 1993 a project review was undertaken involving the major stakeholders. Studies indicated that there would be less cost to the government to put the facility into care and maintenance than continue with the decommissioning. To ensure continuation of the project, the nuclear power generators agreed to contribute towards the cost. The review concluded that there should be a greater emphasis on cost effective decommissioning rather than development of techniques and data gathering. The project completion was re-defined as the completion of core and pressure vessel decommissioning, with demolition of the bio-shield and containment building deferred.

The aims of the decommissioning project were;

• To demonstrate the feasibility of dismantling a nuclear power generating reactor safely and at acceptable cost in terms of money and radiation exposure;

• To identify the engineering problems and to develop and adapt industrial techniques and equipment for their solution;

• To establish routes and appropriate authorisation procedures for disposal of the wastes arising;

• To acquire and record the information, data and expertise that would be of use in the design and subsequent decommissioning of nuclear power plants.

OVERVIEW OF WAGR DECOMMISSIONING

Campaign:

1 Preliminary Operations

2 Operational Waste

3 Hotbox

4 Loop Tubes

5 Neutron Shield

6 Graphite Core & Restraint

7 Lower Structures

8 Thermal Shield

9 Pressure Vessel & Insulation

10 Thermal Columns & Outer Ventilation Membrane

The tooling deployed from the 3te hoist consisted either purely mechanical devices or those being electrically controlled from a ‘pushbutton’ panel positioned at the operator control stations, there was no degree of autonomy in the 3te hoist or any of the tools deployed from it. Whilst the ATC was able to provide some tele-robotic capabilities these were very limited and required a good degree of driver input which due to the operating philosophy at WAGR was not utilised.

The WAGR box proved a successful waste package, adaptable through the use of waste box furniture specific to the waste-forms generated throughout the various decommissioning campaigns. The use of low force compaction for insulation and soft wastes provided a simple, robust and cost effective solution as did the direct encapsulation of LLW steel components in the later stages of reactor decommissioning.

Progress through early campaigns was good, often bettering the baseline schedule, especially when undertaking the repetitive tasks seen during Neutron Shield and Graphite Core decommissioning, once the operators had become experienced with the equipment, though delays became more pronounced, mainly as a result of increased failures due to the age and maintainability of the RDM and associated equipment.

Extensive delays came about as a result of the unsupported insulation falling away from the pressure vessel during removal and the inability of the ventilation system to manage the sub-micron particulate generated during IPOPI cutting operations, though the in house development of revised and new methodologies ultimately led to the successful completion of PV&I removal.

In a programme spanning over 12 years, the decommissioning of the reactor pressure vessel and core led to the production 110 ILW and 75 LLW WAGR boxes, with 20 LLW ISO freight containers of primary reactor wastes, resulting in an overall packaged volume of approximately 2500 cubic metres containing the estimated 460 cubic metres of the reactor structure.

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