Office for Nuclear Regulation

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Graphite moderator core of the Hinkley Point B

Date released
29 April 2016
Request number
201603195
Release of information under
Freedom of Information Act 2000

Information requested

Referring to the graphite moderator core of the Hinkley Point B Units 1 and 2 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.

Referring to the category fault incidents so defined by the Safety Assessment Principles (SAPs):

4. for the credible or reference incident (for example, the guillotine failure of a boiler tube) the i) residual strength of the assembled core and the ii) maximum lateral deflection of any one fuel channel and iii) control rod channel; and

5. for the beyond-design-basis incident (such as a boiler superheater tailpipe failure) the i) maximum lateral deflection of any one fuel channel and ii) control rod channel;

Referring to post-incident shutdown for scenarios 4) and 5):

6. if it [is] assumed that the secondary and final criticality shutdown system (i.e. boron beads and nitrogen purge, if fitted) will operate successfully in the immediate aftermath of the incident.

Relating to the incidents of 4) and 5) under the fuel core condition achieved by 6) [we have renumbered subsequent questions as there were two question 6s]:

7. the nature and content of any radiological release involving fission products from failed full pin cladding;

8. the assessment of the numbers of morbidity and mortality for members of the public  assumed for a) effective implementation of the off-site countermeasures under the Radiation (Emergency Preparation and Public Information) Regulations 2001

Finally, please indicate 

9. 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,

10. 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.

You further wrote to provide the following clarifications

Item 4) is seeking to establish if, in account of the graphite brick fracturing, the core residual strength and lateral distortion in the event of a sudden pressure change across the core - the example given is a guillotine failure of a boiler tube accompanied by a corresponding rise in reactor pressure under the baffle dome which I assume is one of the credible or reference accident scenarios assumed in the nuclear safety case but, again for example, the credible accident could be a rapid depressurisation of the reactor. Similarly, item 5) seeks to establish the core residual strength and lateral distortion of a fuel/control rod channel for the beyond design basis accident, such as a superheater tailpipe failure being given as example. Put another way, I am trying to establish whether, for both 4) and 5) events, the articulated control rods would continue to operate is the lateral axial distortion of the control channel is greater than the maximum design value which I believed to have been 50mm and, similarly, if the fuel stringer would jam in a distorted fuel channel and how this would affect fuel cooling and, separately, lateral interbrick flow locally

The first item 6) is endeavouring to establish if either or both 4) and 5) scenarios the nitrogen purge and, separately, boron dust/beads would be impeded if the core flow regime was disrupted by excessive core movement and distortion.

For the second item 6) and item 7) [which we have renumbered as 7 & 8] I am endeavouring to establish if a radioactive release (of fission product from failed [fuel] cladding) is foreseen and if, in consequence of this, the radiological impact has been quantitatively assessed.

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. It may be also be helpful to add that the word "significant", used to describe the observations in the two right hand columns of the tables, means that the width of these cracks is of the order of one to two millimetres.
  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. For the credible or reference incident (for example, the guillotine failure of a boiler tube) the i) residual strength of the assembled core and the ii) maximum lateral deflection of any one fuel channel and iii) control rod channel. ONR does not hold information that would answer this question. ONR continues to inspect compliance against the requirements of the safety cases relating to water ingress, depressurisation and refuelling operations and have seen no reason to suggest that the plant would not behave according to its original design parameters. Plant improvements such as the introduction of Super-Articulated Control Rods and a new seismically qualified nitrogen plant have increased the resilience to fuel and control rod channel deflection.
  5. For the beyond-design-basis incident (such as a boiler superheater tailpipe failure) the i) maximum lateral deflection of any one fuel channel and ii) control rod channel Please see response to 4. ONR does not hold information that would answer this question.
  6. If it is assumed that the secondary and final criticality shutdown system (i.e. boron beads and nitrogen purge, if fitted) will operate successfully in the immediate aftermath of the incident. It is assumed that the nitrogen criticality hold-down system will operate. This system has recently been replaced with a new seismically qualified nitrogen plant. Hinkley Point B is not fitted with boron bead injection.
  7. The nature and content of any radiological release involving fission products from failed full pin cladding. ONR does not hold information that would answer this question. However, ONR requires the operator to adequately address scenarios which may give rise to the release of fission products from the core. 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 similar to those identified above could have significant consequences (dose band 5) if they were allowed to occur. Therefore, 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. In terms of boiler leaks, ONR requires that safety measures are put in place to limit the amount of water that can enter the core and that the likelihood of significant boiler tube failure is shown to be very small. The overall requirement is that ageing of the reactor core does not significantly change the likelihood of a dose band 5 release.
  8. The assessment of the numbers of morbidity and mortality for members of the public assumed for a) effective implementation of the off-site countermeasures under the Radiation (Emergency Preparation and Public Information) Regulations 2001. ONR does not hold information that would answer this question. Calculations of the effectiveness of countermeasures such as sheltering, potassium iodate and evacuation are made by Public Health England, who's advice is taken into account in developing effective off-site emergency arrangements.
  9. 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.
  10. 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.

Exemptions applied

None.

PIT (Public Interest Test) if applicable

N/A