Chapelcross nuclear power station

Remains of Chapelcross after demolition of towers

The Health and Safety Executive (HSE) granted consent to carry out decommissioning projects at Chapelcross under the regulations to Magnox Electric Ltd on 26 September 2005. The first visible sign of decommissioning was controlled demolition at 09:00 BST on 20 May 2007 of the four natural-draught concrete cooling towers, which were of the same hyperboloid design as conventional inland power stations such as Didcot, Drax, Ferrybridge and Fiddlers Ferry. The explosions were designed to remove a section of the towers' shells. Approximately two-thirds of the circumference and two-thirds of the shell legs were removed by the blasts, causing a controlled collapse of each tower. The charges were fired sequentially, reducing the 300 ft (91 m) high towers to an estimated 25,000 tons of rubble in less than 10 seconds. Those at Calder Hall were demolished on 29 September 2007.

Some local people (including site employees) were opposed to the obliteration of a symbol of the region's industrial heritage. The towers were considered a local landmark that could be seen from a distance of up to 50 miles (80 km) in good weather conditions. British Nuclear Group and the NDA prioritised conventional demolition over deplanting and post-operational clean-out (POCO) of the nuclear facilities on the site. A large part of the shell of tower 1 managed to resist the explosives despite having a visible bulge that resulted from a construction anomaly.[4]

By December 2012, three of the four reactors had been defuelled.[5] Defuelling was completed in February 2013.[6]

Removal of most buildings is expected to take until 2023–2024, followed by a care and maintenance phase from 2024 to 2089. Demolition of reactor buildings and final site clearance is planned for 2089 to 2095.[7]

Fuel in a single channel in Reactor 2 that was loaded with fuel elements under evaluation for the commercial reactor programme experienced a partial blockage, attributed to the presence of graphite debris (see fuel element failure). The fuel overheated and the Magnox cladding failed, causing contamination to be deposited in one region of the core. The reactor was restarted in 1969 after successful clean-out operations and was the final reactor to cease operation in February 2004.

BNFL was fined £200 in 1978 for a fatal accident at Chapelcross.[8]

Cracks associated with brackets in Heat Exchanger 6 on Reactor 2 were discovered during routine ultrasonic testing. Metallurgical examination of samples of the defect showed that:

• It originated during fabrication in the workshop and prior to an over-pressure test of 2.35 times the design pressure (a loading significantly in excess of a modern pressure vessel code requirement).
• There was no evidence of in-service fatigue crack growth.
• The material in which the crack was located was different from that specified in the design. Similar material was also identified in other heat exchangers, and no additional cracks of structural significance were revealed during comprehensive inspections. The NII considered the material to be adequate and within the bounds of the heat exchanger safety case.[9][10]

During refuelling operations on Reactor 2, an irradiated fuel element failed to release from the grab which holds an element while it is withdrawn from a reactor. Routine methods were used to release the grab. However, the irradiated fuel element snagged during the operation and was lifted out of its shielding, resulting in the operators on the pile cap being exposed to the intense radiation being emitted from the irradiated fuel element. Personnel responded quickly, and the radiological dose they received was small.

The event revealed shortfalls in the safety of the refuelling operation and the licensee took the immediate step of halting all refuelling while it investigated the event and reviewed the safety of the equipment. The NII investigated the event and judged that it was due to inadequate design and operation of the equipment.

The incident was classified as Level 1 (anomaly) on the International Nuclear Event Scale (INES).[11]

A small amount of Magnox depleted uranium (MDU) leaked from some corroded mild steel drums due to rainwater ingress and leaching. MDU is a dense yellow powder that is less radiologically toxic than naturally occurring uranium but chemotoxic in a similar manner to lead. Owing to its high density and low solubility, it does not tend to disperse far and dry spills are easy to clean up. This material was stored at the larger sites, including Capenhurst in mild steel drums. BNFL upgraded the fabric of the building and the original drums were overpacked into stainless steel drums and dispatched to Capenhurst for long-term storage.[12]

During routine defuelling activities on Reactor 3, a basket containing 24 low-rated irradiated Magnox fuel elements fell a few feet within the discharge machine onto the door at the top of the fuel discharge well. Remote TV camera inspections revealed that twelve of the elements had fallen just over 80 ft (24 m) down the discharge well into a water-filled transport flask at the bottom. The NII initiated an investigation because dropping irradiated fuel elements is a serious issue even when, as in this event, BNFL had advised NII that there had been no release of radiological activity.[13]

Because of known shrinkage of the graphite moderator bricks in the core due to in-service irradiation effects, some of the steel charge pans on top of them had become dislocated from their design position in the interstitial channel and were suspended from the Burst Can Detection (BCD) pipework. This was most prevalent in Reactor 1 because of the different core design to Reactors 2, 3 and 4. BNFL were unable to make an adequate safety case or effect an economic repair and therefore, Reactor 1 did not return to power from its annual outage in August 2001. The core of Reactor 4 was repaired but this reactor did not return to power after the repair.[12]