Office for Nuclear Regulation

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Graphite cracks at Hinkley Point B

Date released
7 March 2019
Request number
201902059
Release of information under
Freedom of Information Act 2000

Information requested

I am applying under the FOI Act for the results of the fuel and control channel inspections undertaken during the most recent outage at Hinckley B specifically
 
i) the number and location of cracked graphite blocks,
 
ii) identifying these to be a) radial, b) axial and c) keyway slot fractures or cracks,
 
iii) the reduction of the axial and radial residual strengths of the moderator ore as a result of the existence of cracked - a), b) and) foregoing,
 
iv) the results of i), ii) and iii) foregoing as found by inspection compared to the model predictions for the moderator core;
 
v) and if, as a result of i), ii) and iii) foregoing there has or will arise a need to revise the seismic hazard pga.

I would also like to be told if it is the intention to permit Unit B1 Hinkley Point to recommence criticality and power operation at the completion of the EDF outage and before the results of the inspection have been analysed and submitted to ONR.

Information released

Question i) and ii):

i) the number and location of cracked graphite blocks, 

ii) identifying these to be a) radial, b) axial and c) keyway slot fractures or cracks.

ONR’s response:

The last inspection of a graphite core at Hinkley Point B (HPB) was performed in April 2018 in Reactor 4 when 31 fuel channels and 1 control rod channel were inspected. 

The results of the fuel channel inspections were:

No cracks were observed in the control rod channel

The total number of observed cracked fuel bricks in HPB Reactor 4 are:

Question iii):

iii) the reduction of the axial and radial residual strengths of the moderator ore as a result of the existence of cracked - a), b) and) foregoing,

ONR’s response:

The residual strength of the graphite in a core with cracks is the same as that in a non-cracked core of similar age.  The problem that cracked bricks present to the core is that they may allow more freedom of movement of the core structure, particularly during faults such as an earthquake. This potential behaviour is what EDF model to support their safety case. Currently the number of cracks revealed by inspection of HPB Reactor 4 has been shown to have no effect on how the core moves.

Question iv):

The results of i), ii) and iii) foregoing as found by inspection compared to the model predictions for the moderator core;

ONR’s response:

EDF’s model predictions made prior to the outage predicted that 6 new circumferential cracks as the most likely outcome, with a range from 2 to 11 being totally in line with expectations and between 1 and 13 being unsurprising.

In terms of bore initiated axial cracks, EDF’s model predictions prior to the outage were that the most likely outcome is 2 new singly-cracked and no new doubly-cracked bricks. Up to 7 additional singly-cracked bricks or 3 doubly-cracked bricks would be totally in line with expectations. Up to 9 additional singly-cracked bricks or 4 additional doubly-cracked bricks would not be totally surprising.

In terms of keyway root cracks, EDF’s model predictions prior to the outage were that the most likely outcome was 0 to 2 keyway root cracks would be in line with expectations, and 3 to 6 would not be totally unexpected.

Question v):

And if, as a result of i), ii) and iii) foregoing there has or will arise a need to revise the seismic hazard pga.

I would also like to be told if it is the intention to permit Unit B1 Hinkley Point to recommence criticality and power operation at the completion of the EDF outage and before the results of the inspection have been analysed and submitted to ONR.

ONR’s response:

The seismic withstand requirements for the core have not been revised and must be demonstrably met by the safety case.
HPB Reactor 4 was returned to service following completion of the EDF outage in 2018. This was in line with arrangements put in place by ONR at HPB that all graphite core inspection results should be submitted to ONR prior to restart and that EDF must consult ONR before restarting the reactor.  Reactor restart is dependent upon EDF having a valid safety case that justifies the safety of the reactor for the next period of operation.

Please note the next statutory outage for HPB Reactor 3 is planned for March 2019. Full details are included on the EDF website.

Reference material:

Further details are included in EDF’s Hinkley Point B Reactor 4 Inspection results 2018.

https://www.edfenergy.com/energy/power-station/daily-statuses

Exemptions applied

N/A

PIT (Public Interest Test) if applicable

N/A