Decommissioning the Steam Generating Heavy Water Reactor

The reactor, which operated from 1967 until 1990, is similar in design to CANDU reactors. Light water was pumped over the fuel elements, boiled and the steam passed directly to the turbine. The absence of heat exchangers meant the turbines and cooling system as well as the reactor became contaminated. Heavy water controlled the reactivity in the Calandria.

Following initial fuel element removal and decommissioning of the reactor by Cavendish Nuclear's subsidiary, the Nuclear Decommissioning Authority turned to Cavendish Nuclear in 2011 for a strategy to complete its decommissioning. The rate at which SGHWR could be dismantled determined the date for remediation of the whole site; thereby it was on critical path.

Cavendish Nuclear's strength in depth and broad experience allowed us to put together a team of specialists capable of giving the customer the optimal solution, using a number of criteria – simplicity, minimal new construction, minimal dependencies, use proven technology, has high confidence levels and is consistent with regulatory requirements and site end-state criteria.

Our programme of work to deliver this included development of technical and waste management methodologies, underpinning schedule and cost data, and associated stakeholder and implementation management plans.

The Steam Generating Heavy Water Reactor was defueled in 1991 and its D2O coolant removed. In 1997, it was placed in care and maintenance.

In 2005, Cavendish Nuclear developed a two-phase strategy to perform decommissioning:

• Remove secondary containment and decontaminate to low levels (completed in 2007).
• Remotely remove core and package as ILW, remove primary containment, decontaminate all areas, demolish all structures, and restore site.

This work was accelerated to achieve an end date for the site in 2021 and includes:

• Remove Primary Containment
• Design, build, commission, operate and decommission new plant (Core Segmentation, ILW Processing)
• Remove all other equipment and materials
• Decontamination to meet release criteria
• Demolition and Site Restoration
• Maintenance of assets pending disposal

The existing reactor building structure is being modified to create a number of new compartments;

• Segmentation Cell
• Core Jacking Area
• ILW Processing Plant / Grouting Plant
• Decontamination Cell
• Service Cell
• Central Control Room

The main compartment is the Segmentation Cell, where the core was cut up using two remotely-operated demolition machines and four hydraulic lifting arms which segment the core from outside-in. Intermediate level waste was encapsulated with grout and stored in a 6m3 ILW box, with a total weight once conditioned of 50 tonnes. Approximately 75 waste boxes were produced.

An area of the primary containment was cleared and adapted to accommodate a reactor core jacking mechanism which raised the core incrementally into the Segmentation Cell.

The objective was to clean structures to an underground depth of 1m and backfill with recycled concrete and masonry from the decommissioning. Total waste generated- 410 tons of ILW, 4,200 tons of LLW and 9,200 tons of clean, excluded or free release waste.