Stop Hinkley Press Release:

26 October 2007

Renewed calls for closure following new reactor core problems

A top nuclear consultant has called for Hinkley's immediate closure following a regulator's report stating the reactor core support system is potentially unsafe.

Key structural engineering components supporting Hinkley's cracked reactor core have been discovered to be more prone to failure than previously assumed which could affect the safety of the reactor both in normal operation and in an emergency, prompting renewed calls for Hinkley's closure by leading expert John Large.

Embrittlement has been found on the flexible restraining garters which protect the reactor core in case of pressure surges and other events. As the graphite reactor core is already weakened by the corrosion caused by long-term radiation, heat and pressure, the consequences of a garter failure could be severe.

The Nuclear Installations Inspectorate last week issued a notice stating "the effects of irradiation embrittlement on core restraint components are more severe than previously assumed...they may be degraded to the extent there may be a significant number of latent failures, which may affect the function of the support system in normal operation and faults...potentially undermining the safety case for the graphite core."

The implications of this latest discovery at the Hinkley (and Hunterston) AGR reactors raises profound nuclear safety issues. As already reported by the nuclear regulator in June 2006, a number of the graphite bricks in the cores are known to be cracked and many more are likely to have cracks under development. (2)

This cracking has resulted in bricks cleaving in two, a situation for which it is difficult to assess the remaining residual strength of the core as a whole structure.

Moreover, this development comes on top of severe boiler tube corrosion discovered at Hinkley last September, which shut the twin reactors down till late May this year. The regulators demanded a thirty percent cut in operating output to limit the likely rupture of the repaired but old boiler tubes. However the boiler system has had trouble accommodating this precise limitation and now operates with a greater forty percent output reduction.

The nuclear safety regulator has issued the following statement: (1)

"British Energy has discovered that the effects of irradiation embrittlement on core restraint components [i.e. the turnbuckles] are more severe than previously assumed. The properties of the core restraint components may be degraded to the extent there may be a significant number of latent failures, which may affect the function of the support system in normal operation and faults. This new information potentially undermines the existing safety case for the graphite core. The matter has been reported to HMNII and a safety justification made by the Licensee for continued operation in the short-time. A more detailed safety case is being developed to secure the long term position. HMNII will continue to monitor this issue closely."

Although the NII have sanctioned a short-term case for continued operation a nuclear consultant expressed his concerns. John Large of Large Associates said:

"The Hinkley Point and Hunterston reactors were permitted to continue operating, albeit at a derated output, with an acknowledged weakened core and doubtful boiler tube integrity only because the restraint tank garter system was then believed to be intact and capable of functioning under abnormal core pressure surges. Now we are informed that this last line of core integrity defence has weakened over the years and there is uncertainty over how effectively it would function under adverse conditions. This is a most unsatisfactory approach to Defence in Depth and all four Hinkley and Hunterston reactors should be immediately closed until the longer term nuclear safety case has been demonstrated to be adequate".

Jim Duffy, spokesman for Stop Hinkley said:

"Common sense says one problem at the heart of an ancient reactor is bad enough, two is very worrying, three is begging for trouble. Hinkley are playing down this discovery perhaps to keep their shares from collapsing any further after this week's tumble when half of their reactors were off-line. But safety should come first. Let's see this reactor shut for good, right now."

Jim Duffy. Stop Hinkley, 07968 974805

Technical analysis by John Large

In the Hinkley Point reactor the reactor core is, essentially, a loose assembly of graphite bricks, of about 12 meter diameter and 10m height, with about 330 channels of 225mm diameter running top to bottom of the core and which receive the nuclear fuel stringers and up through which passes the carbon dioxide coolant gas. The core sits inside an open topped restraint tank which stabilises and restrains the core assembly via steel turnbuckle links that allow for differential temperature and pressure induced movements of the core and tank. The core 'garter' formed by the turnbuckles also fulfils a vital nuclear safety role of maintaining alignment of the core fuel and control rod channels in the event of an abnormal pressure surge occurring in the reactor circuit. Such a pressure surge can and has not that infrequently occurred following the failure of a single tube of the hundreds of tubes making up each of the four boilers that are also located inside the reactor pressure circuit.

 

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Useful links
  1. HSE website: Reports for Local Stakeholder Groups - www.hse.gov.uk

  2. For further details of the graphite core cracking and for the general design and function of the core and its restraint system see: www.largeassociates.com

  3. British Energy response in today's news section of its website www.british-energy.com/

  4. Guardian article on Inspectors' warnings on Hinkley cracks: 'Unexplained cracks in reactor cores increase likelihood of accident, say government inspectors', The Guardian, Wednesday July 5, 2006.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Page Updated 17-Dec-2007