Office for Nuclear Regulation

Licencing conditions for Sizewell C

Date released
9 April 2021
Request number
202102068
Release of information under
Freedom of Information Act 2000

Information requested

  1. Demographics information, for example that used to determine the offsite emergency arrangements.
  2. Map of the SZC site with grid references including the location of buildings. Map of site and grid references should include the Green and Blue Planning lines for the Eastern (North Sea) boundary if these are available (Historic and part of Sizewell B Planning consent). Original consent indicating green line https://community.magnoxsocioeconomic.com/wp-content/uploads/2014/10/EDF-Energy-SSG-Actions-Nov-2014-Attachment-No.-2-2014.pdf.
  3. Information that explains how the site will be constructed, for example approximately what depth will be excavated, over what area, how will it be built back up before the nuclear structures are constructed and approximate foundation depth. This information may wish to include the existing flood defence features consented for Sizewell B which may be disturbed during construction of SZC.
  4. A cross section(s) illustrating the geology on site under the key buildings or at key points on site.
  5. Information on the cut-off wall.
  6. Information on how the edge of the site will be constructed given it appears to be built up above the natural land, for example reinforcement around the edge of the site. Proposed Western boundary and northern site access currently obstructs or corrupts the main water course Leiston river. How this will be engineered in association with the reinforcement and avoid increased flood risk to Leiston town/sewage works and main site access road.

Background to concerns 3 and 6 relate to comments made at Hinkley C where the Site Manager is reported to have said he was only responsible for flood protection of HPC not the surrounding area. If EDF do not build SZC themselves this situation could arise at Sizewell.

Information released

I can confirm that under Section 1 of the FOIA we do hold some of the information requested. Please find a response to each of your questions in turn below.

1. Demographics information, for example that used to determine the offsite emergency arrangements.

The information you requested is available on the March 2018 update to the Residential Layer on the National Population Database.

Details of how to access the database can be found on the Health and Safety Executive Science and Research Centre. This data is still suitable for use in the subsequent Sizewell C (SZC) assessment.

2. Map of the SZC site with grid references including the location of buildings. Map of site and grid references should include the Green and Blue Planning lines for the Eastern (North Sea) boundary if these are available (Historic and part of Sizewell B Planning consent).

We do not currently hold a map of the SZC site with grid references and planning lines. However, we do hold a map of the proposed site with building locations that was provided as part of the Nuclear Site Licence application. Please see attachment 1 which is a drawing from section 6.3 of the licence application dossier.

3. Information that explains how the site will be constructed, for example approximately what depth will be excavated, over what area, how will it be built back up before the nuclear structures are constructed and approximate foundation depth. This information may wish to include the existing flood defence features consented for Sizewell B which may be disturbed during construction of SZC.

Information that explains how the site will be constructed

Firstly, it is important to note that the detailed design of SZC, including its civil engineering design, will continue past the point of any potential nuclear site licence grant. Therefore, some of the information that we currently hold is preliminary and will change. Post any potential nuclear site licence grant, there will be a number of regulatory “hold points” that prevent the licensee from starting certain construction activities without our agreement. This allows us to assess aspects of the detailed design when it is mature whilst still allowing work to continue in other areas.

At the current time, the focus of our ongoing civil engineering assessment for informing our nuclear site licensing decision is to ascertain whether the licensee sufficiently understands the geotechnical conditions of the site and presents a viable solution for the long term support of structures, systems and components, including a viable outline construction method to achieve this. This also includes consideration of the site size.

Site excavation depth and area

Our current understanding is that a cut-off wall will be constructed near the perimeter of the proposed nuclear licenced site boundary in reinforced concrete, with the function of forming a watertight box around the main construction area as well as performing an earth retaining function. The area within this cut off wall will be excavated to varying levels depending on the structures. Figure 1 shows the current theoretical bottom of the  excavation area.

Figure 51: Change of excavation geometry

For most of the site, the excavation depth is -10mAOD. The areas under the reactor buildings is -12mAOD (with some localised deeper excavation), pumping station -16.10m AOD, and areas of shallower depth excavation on the south side of the site. The aim of this excavation is to reach the more competent crag deposits beneath the site.

How will it be built back up and approximate foundation depth

The prospective licensee’s current intention is to build up engineered fill from these levels to improve bearing capacity under structures (a proportion of which will be reclaimed excavated material) to foundation levels. Backfill material from foundation level to platform level (7.3mAOD) is expected to mostly be made up of excavated material. Final foundation depths for the structures have not yet been finalised and vary structure to structure. Figure 2 shows a simplified schematic of the backfill strategy, the excavation levels and how the site will be constructed. R1, R2 and R3 relate to the different type of engineered fill specified for construction.

Figure 2. Schematic of the backfill strategy (May 2020)

Figure 2. Schematic  of the backfill strategy (May 2020)

Flood defence features for Sizewell B (SZB)

We do not hold any information regarding any potential impact on the Sizewell B (SZB) flood defences. As part of nuclear site licensing, NNB GenCo SZC’s claim is that operations (including construction) on the SZC site will not adversely affect the ability to maintain an adequate safety case for the adjoining nuclear licensed site SZB. We are considering this claim as part of the nuclear site licensing assessment. Detailed construction information is not yet available for the SZC site. Prior to start of construction, we expect NNB GenCo (SZC) to consider the potential impact of construction on SZB, including, if relevant, on the existing SZB flood defence features.

4. A cross section illustrating the geology on site under the key buildings or at key points on site.

Please find attached three geotechnical cross sections (as the site currently exists) that illustrate the geology on site.

5. Information on the cut-off wall.

The cut-off wall design continues to be developed; the information we hold is relevant as of February 2021 and is subject to change. The detailed design of the cut off wall is not part of the nuclear site licensing assessment.

Our current understanding is that the cut-off wall will be constructed 1.5m thick in reinforced concrete. The plans indicate a depth of -48mAOD for the piles (minimum 3m into the Thames group layer – see geotechnical cross sections). This will allow dewatering of the site to -32mAOD. Figure 3 presents a position previously shared with us on the location of the cut off wall; however, it has been indicated that this work is ongoing, and the positioning may change. Our current understanding is that the cut off wall will remain in place after the plant is constructed.

6. Information on how the edge of the site will be constructed given it appears to be built up above the natural land, for example reinforcement around the edge of the site. Proposed Western boundary and northern site access currently obstructs or corrupts the main water course Leiston river. How this will be engineered in association with the reinforcement and avoid increased flood risk to Leiston town/sewage works and main site access road?

The information we hold indicates that the sloped edges of the site will be strengthened using a sheet pile wall prior to the downhill slope to natural ground level. We do not hold any information on potential impacts on Leiston river/drain.

This information is not necessary for nuclear site licensing, but may be considered in future assessments if foundations of any nearby structures rely upon the strengthening measure to provide stability.

Figure 3. Location of the cut-off wall (May 2020)

Figure 3.  Location of the cut-off wall (May 2020)

Exemptions applied

None

PIT (Public Interest Test) if applicable

N/A