Office for Nuclear Regulation

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Data relating to the AGR power stations Dungeness B, Hartlepool, Heysham 1, Heysham 2, Hunterston B, and Torness - Graphite

Date released
21 June 2016
Request number
201605274
Release of information under
Freedom of Information Act 2000

Information requested

I am applying under the Freedom of information Act for the following data relating to the AGR power stations Dungeness B, Hartlepool, Heysham 1, Heysham 2, Hunterston B, and Torness.
For each station please provide;

1)   the number of  cracked and/or fractured graphite bricks both a) partial and b) complete in the i) circumferential and ii) axial modes;

2)   the number of graphite keys sheared or otherwise failed; and

3)   the location of the graphite components of 1) and 2) in the a) depth and b) horizontal planes of the  moderator core.

4)   all recent (last 10 years) Licence Amendments (or similar) and/or changes in the Operating Rules imposed on the operating regimes of the Hinkley Point B Units 1 and 2 arising as direct result of progression of the fracturing, cracking, etc of the graphite core components of 1) and 2); and again related to the progression of the graphite component fracturing,

5)   Please outline whether the quantitative risk of core failure and/or misalignment fuel and control rod channel would result in a lateral displacement of 50mm or greater has changed over the past 10 years.

6) Please can you define your  'worst case reference accident/fault' in relation to Hinkley B . And please explain how the current cracked moderator cores in terms of the overall residual strength of the core assembly would structurally respond.

 7) Please provide copies of any studies that have been undertaken to model the lateral displacement of a core in a severe default condition.  And provide the projected  number of fuel pin failures arising because of displacement and or melt.

I would also like to see the result of any further inspection of Hinkley Point B if they have been carried out subsequent to my request 201603195.

Information released

  1. Number of cracked and/or fractured graphite bricks both a) partial and b) complete in the i) circumferential and ii) axial modes

    See information in tables attached indicating number of crack types, first observation and position in core.  To put this into context, there are around 6000 graphite bricks in each reactor core. In order to assist in interpretation of the tables see the following definitions
    • IIIA = Partial Crack
    • IIIC = Full (>80%) height/circumference of brick
    • Circ. = Circumferential crack
    • Axial = Axial crack
    • e.g. IIIA Circ. = Partial Circumferential crack
    • ‘Significant cracks’ in the tables for Heysham 1 and Hartlepool mean axial cracks the full (>80%) height of the brick
    • The word ‘significant’ in the tables for Hunterston B means full (>80%) height/circumference of brick
    We do not hold any information on the total numbers of cracks at Dungeness B in the form of tables.  However, we know that inspections have found 0 full height cracks, 6 partial axial cracks and 1 circumferential crack between the two reactors.  The level of bore cracking is much less at Dungeness B since the bricks are smaller in cross section and hence the differential stresses are less.  Furthermore, despite construction of Dungeness B starting at the same time as Hinkley Point B and Hunterston B, delays of several years in its construction and lengthy periods of non-operation during its early life mean that the core at Dungeness B has the lowest cumulative irradiation of the entire AGR fleet.  Therefore, it is not currently expected that the core will operate long enough for the mechanism of keyway root cracking to occur.

  2. The number of graphite keys sheared or otherwise failed.

    The end face keys between fuel bricks are visible by inspection of the fuel channels.  To date inspections have shown that these keys remain intact.  It is currently not possible to directly inspect the keys at the outside of the fuel bricks.  Instead, the integrity of these keys is supported in the safety case by structural integrity assessments that argue that the internal and external stresses that these components are subject to are insufficient to cause them to fail.  Furthermore, the measurement of core distortion through monitoring and inspection supports the argument that the keying system is intact and functioning correctly.  Finally, because there are several thousand keying components in the core, the safety case demonstrates that there is significant redundancy associated with the functionality of the keying system and it is therefore tolerant to damage.

  3. The location of the graphite components of 1) and 2) in the a) depth and b) horizontal planes of the moderator core. 

    See information in tables attached indicating number of crack types, first observation and position in core.  In the tables, the number in brackets after the channel location code is the graphite brick layer (vertical location), where layer 1 is at the bottom of the core.

  4. All recent (last 10 years) Licence Amendments (or similar) and/or changes in the Operating Rules imposed on the operating regimes of the Hinkley Point B Units 1 and 2 arising as direct result of progression of the fracturing, cracking, etc of the graphite core components of 1) and 2); and again related to the progression of the graphite component fracturing. 

    No amendments to the Operating Rules have been made as a result of graphite matters.  However, there have been changes to the plant operating periods and maintenance schedules to allow more frequent inspections of the graphite core to be completed in between the three-yearly statutory outages.  N.B.  Although not asked for in the FOI request, please note that this statement equally applies to Hartlepool and Heysham 1.  In due course, we would expect increased inspections to become a requirement of the safety case for Heysham 2 and Torness.

  5. If the quantitative risk of core failure and/or misalignment fuel and control rod channel would result in a lateral displacement of 50mm or greater has changed over the past 10 years.

    ONR is not aware of any change in the past 10 years.  Much of the inspection, analysis and assessment work being done by EDF and by ONR is intended to demonstrate that the risks from continued operation are as low as reasonably practicable and are effectively unchanged by further irradiation of the graphite core.

  6. Please can you define your worst case reference accident/fault in relation to Hinkley B and please explain how the current cracked moderator cores in terms of the overall residual strength of the core assembly would structurally respond. 

    ONR requires the operator to adequately address scenarios which may give rise to the release of fission products from the core; for example, severe accidents such as earthquakes. The magnitude of any such release is referred to by dose bands 1 to 5 (5 being the worst case and equivalent to a dose of >1000 mSv - Target 8 of ONR’s Safety Assessment Principles).  Scenarios such as earthquakes could have significant consequences (dose band 5) if not properly mitigated against and demonstrably so.  ONR requires that the total frequency of such a high consequence release is shown to be very low (i.e. lower than the Basic Safety Level of 10-4 per year but with a Basic Safety Objective of 10-6 per year and risks as low as reasonably practicable (ALARP)) by the provision of adequate safety measures.  In the case of the graphite core, ONR requires that the operator demonstrates that ageing (cracking of the graphite bricks) within its operating envelope will not have a detrimental effect on the high reliability of the core shutdown system.  The overall requirement is that ageing of the reactor core does not significantly change the likelihood of a dose band 5 release.  In the case of an earthquake ONR requires that the licensee demonstrates that the structure is tolerant to an extent of cracking in excess of the operating limits and that, compared to an intact core, there is no increase in risk of fission product release.  The licensee has demonstrated this tolerance by extensive studies using computer models and scaled rig experiments.

  7. Please can you provide copies of any studies that have been undertaken to model the lateral displacement of a core in a severe default condition.

    The number of studies undertaken over the last 30 years or more make this request manifestly unreasonable.  In terms of the projected number of fuel pin failures arising because of displacement and or melt this aspect of the consequences of core response is considered by ONR in the way outlined in 6.  That is, such a scenario must be shown to occur at a suitably low frequency as defined by its potential dose band release.  

    With regards to further inspection results from Hinkley Point B if they have been carried out subsequent to my request 201603195, no further inspections at Hinkley Point B have taken place since your request 201603195.

Exemptions applied

N/A

PIT (Public Interest Test) if applicable

N/A