Preparing Hinkley Point B for a seismic event28 September 2001
The latest seismic upgrade work at Hinkley Point B has reached the final stage.
In the UK every nuclear installation is subjected to a periodic safety review (PSR) every 10 years as part of its site licence requirements. The PSR reviews the existing safety case and provides confirmation that the plant will operate for at least the period until the completion of the next PSR. The operating history of the plant is reviewed and factors that could potentially affect safe operation are identified and evaluated. A general assessment of the plant and operational experience is performed and recommendations for safety improvements are identified. The latest PSR for Hinkley Point B concluded that there were no major shortfalls in the design of the plant due to the large amount of safety enhancement that was performed during the previous 10 years. However, one recommendation of the PSR was that safety significant plant and structures should be qualified against a defined seismic event to ensure that following a seismic event the reactors could be shut down safely.
Detailed inspections of these safety significant plant items were carried out, followed by a process of analysis and assessment against prescribed seismic criteria. The seismic walkdowns were performed in accordance with the generic implementation procedure (GIP) for the seismic verification of nuclear power plant equipment, prepared by the Seismic Qualification Utilities Group (SQUG). The GIP provides screening evaluation work sheets (SEWS) for various categories of equipment, which are completed during a plant walkdown. This provides a convenient summary and checklist of the seismic evaluation criteria and identifies the shortfalls in the seismic safety case that require resolution. These shortfalls were compiled to form a schedule of potential additional work needed (SPAWN), which listed the items that required potential modification to meet seismic qualification status. The extent of the SPAWN list was significant and covered almost 2000 plant items ranging from electrical cubicles, battery racks and piping inside the station buildings, to tanks outside. The seismic upgrade work was divided into a number of packages, covering a variety of tasks:
• Review of the SEWS information.
• Initial site surveys to formulate potential design solutions.
• Production of detailed designs.
• Production of plant modification proposals (PMPs).
• Procurement and installation.
• Installation approval.
• Provision of a lifetime records package (LTR).
NNC has been at the forefront of earthquake engineering in the UK nuclear industry for more than 20 years and played a key role in the seismic design of the only UK nuclear power stations that have been designed to withstand earthquakes, Heysham 2, Torness and Sizewell B. The initial contract was NNC’s first seismic upgrade work for retrofitting, rather than design. NNC’s performance in this initial contract was such that it was awarded the remainder of the work, which has been performed over a period of more than three years.
Design base earthquake
The seismic safety case for Hinkley Point B states that the risk from earthquakes will be tolerable and as low as reasonably practicable. This was achieved by qualifying one line of protection for the essential functions of trip, shutdown, post-trip cooling and monitoring for an infrequent seismic event – a probability of 1 in 10,000 years (10-4 per annum). All plant items required to operate these functions were classified as bottom line plant. A second line of protection, qualified to a lower level, was provided to give defence for more frequent seismic events – a probability of 1 in 1000 years (10-3 per annum). The plant items required to operate this second line of protection were classified as second line plant. This second line protection is generally diverse from that provided by the bottom line plant.
The Design Base Earthquake (DBE) was defined as a hard site design response spectrum scaled to a peak horizontal ground acceleration of 0.14g for bottom line plant and 0.1g for second line plant. Location-specific spectra were supplied from building integrity analyses, which accounted for the dynamic response of the buildings.
Various factors of conservatism were included in the seismic loading used for design purposes, the first of which was a value of 1.25 as specified by the GIP to account for potential inaccuracies in the method used to derive the in-structure response spectra. Further factors of safety were included as specified by seismic design guidelines and the design code assessment of the supporting steelwork for the design modifications or the GIP for any anchorage into concrete.
Non-intrusive design solutions were used where possible as part of the design strategy adopted, as the use of structural adhesives remove the need for drilling into the sides of electrical cubicles. The NNC contract was for the design and implementation of the proposed modifications to seismically qualify essential plant items, as identified from the SQUG walkdown and reported in the SEWS. Overseeing the work was a senior engineer, who was appointed by the client as the seismically competent person (SCP), whose design team were responsible for the production of proposed design modifications. Due to the size of the contract the proposed work was grouped into plant areas with dedicated design teams assigned to each individual or group of areas. Each design team consisted of a number of engineers and design draughtsmen and was headed by a task engineer.
When required the design team employed expert personnel such as structural analysts, pipe designers and welding inspectors to assist in the design process. In addition there were dedicated PMP authors, who were solely responsible for the production of the safety documentation (which justified the proposed modification work in terms of nuclear safety). NNC also made use of its own manufacturing facility to produce much of the seismic restraint steelwork, along with its own dedicated site team, who were responsible for the installation.
The design and installation process consisted of the following stages:
This consisted of extracting the relevant SEWS (within the scope of supply of the contract) from a database. The SEWS and supporting documentation (calculations and photographs) were then grouped according to the plant area and assigned to the design teams for review. The purpose of this review was to clearly identify the potential seismic shortfalls and identified interactions, review any supporting calculations and familiarise the design team with the plant item. A preliminary design was then produced on the basis of this review, which also considered the requirements of the GIP and the seismic design guidelines.
The design team conducted a site survey to confirm the details of the SEWS and to gather further information about the plant item and layout, in the form of sketches and photographs. At this stage the SCP reviewed the SEWS and the preliminary design, formulated at the review stage, and considered other options, which could have been required due to local plant differences and restrictions. This process also involved the NNC site manager and the station liaison officer who were informed of the design intent and then provided information on local station matters that may have affected the design proposal. These included, for example, plant access/lay down requirements and potential access difficulties when implementing the proposed modifications.
With additional information from the initial site inspection a series of generic design solutions where produced, covering different types of modifications such as base and overhead anchorage systems, modifications to existing steelwork, additional supports for plant items (including pipework, HVAC and cabling). These generic designs were then collated into a preliminary design statement (PDS) and issued to the client for approval.
Design production and verification
Following approval of the PDS the formal design modification was produced in accordance with the GIP, the seismic design guidelines and BS 5950 and issued to the client for approval, in the form of design drawings, design reports and supporting calculations. The document package was internally verified in accordance with NNC’s approved ISO 9001 QA procedures, with a further site inspection performed to ensure the proposal could be installed.
Production of the plant modification proposals
As part of the main PMP, NNC were contracted to produce the relevant stage submissions to cover the proposed modifications within their scope. The stage submission addresses the nuclear safety issues and asks the ‘what if’ questions regarding the design and installation, thus justifying the proposed modification in terms of nuclear safety. The PMP is deemed to be a high level document, without which proposed modifications cannot be implemented. The PMP was then approved by the client in accordance with the Nuclear Safety procedures.
NNC’s procurement department was responsible for the purchasing of the majority of the proprietary items that were required as part of the design and installation process. This included structural steelwork (including grade 55 steel), pipework (including stainless steel), structural adhesive and pipe clamps, supports, snubbers and bellows expansion units, as well as routine tooling equipment.
There was a substantial amount of steelwork fabrication required throughout the contract and, in particular, for the two statutory reactor outages that occurred during the timescale of the seismic upgrade work. To assist the dedicated site team, much of the steelwork was pre-fabricated at NNC’s manufacturing facility.
NNC’s dedicated site team implemented the modifications, employing civil, scaffolding/lagging and electrical contractors as required to meet the programme and budgetary requirements. The site team was responsible for raising all of the site documentation required to perform the modifications, – work order cards (WOC), quality plans (QPs), risk assessments, procurement, and so on. A further responsibility was the raising of technical queries (TQs), which were required for any design variations. The site team liased closely with the design team, discussing technical issues and providing further site surveys to supplement any contract variations. During intense periods of installation – the statutory reactor outages – the site team was supplemented with members from the design team, who assisted with the resolution of technical difficulties on an individual basis. This ensured that the tight programme during these periods was met.
Inspection and lifetime records
At the completion of the installation the modifications were inspected by SCPs from NNC, using relevant drawings and TQs, to confirm that no seismic concerns remained. These inspections were independent of the inspections performed by suitable site personnel to confirm the completion of the site installation works. The QP was signed by all parties and the relevant WOC and safety documents were cancelled. The SCP then produced a closeout report to confirm that all the modifications had been installed satisfactorily, resolving the seismic concerns highlighted in the SEWS.
NNC also produced a complete LTR package containing SCP reports, closed out TQs, welding schedules and procedures, and adhesive schedules.
Throughout the work NNC were contracted to seismically qualify hundreds of plant items, which had potential concerns as indicated in the SEWS, all of which required modifications to restrain or support them. The following are examples of the types of plant items that required modification:
• Electrical switchgear and instrumentation cubicles.
• Pumps and motors.
• Storage vessels (containing CO2, water, air, oil, etc).
• Electrical batteries.
• Piping (containing water, steam and gas).
• Electrical cabling and conduit.
• HVAC and lighting.
• Suspended ceilings and raised floors.
• Rainwater pipework.
• Mobile units and storage racks.
The design team based their designs on the SEWS and further site survey information. The design method adopted was to establish the geometry, mass and centre of gravity of the plant items and apply pseudo-static seismic loading. Appropriate in-structure response spectra were applied together with any further design considerations specified by the seismic design guidelines and the design codes used for assessment purposes. All three orthogonal seismic forces – the two horizontal forces and the vertical force were applied to assess the extent of rocking/overturning and sliding that individual plant items would experience. The loading at proposed restraint positions was established and the anchorage system was designed in accordance with BS 5950 and the GIP.
The major contributing factor in the type of restraint system adopted was that the majority of the proposed modifications had to be implemented outside a statutory outage, thus the plant items were live. Also, the plant items were often essential plant containing sensitive equipment such as relays, which are susceptible to damage from drilling or other similar intrusive attachment methods.
To overcome these problems the design team used a proven non-intrusive design approach. Restraints were normally bonded to the plant items using structural adhesive and connected to adjacent concrete or steelwork. In addition existing lifting features were also used when attaching restraints to plant items.
The design solutions generally fell into one of the following categories:
• Base restraint system – steelwork bonded to the base of the plant item and anchored to the floor.
• Top restraint system – steelwork attached to the top of a plant item and anchored to the ceiling above or floor below.
• Frame restraint system – the use of structural members to enclose the plant and prevent movement during the defined seismic event.
Approval of installation
The SCP approval, following inspections, was reported at the completion of the project. NNC’s approval documentation is to be incorporated into the overall seismic safety case submitted to the National Nuclear Inspectorate (NII). The NII’s approval of this safety case, against the requirements of the defined seismic event is the final stage of the latest seismic upgrade work for Hinkley Point B.
|Abbreviations used in this article|
|PSR periodic safety review GIP generic implementation procedure SQUG Seismic Qualification Utilities Group SEWS screening evaluation work sheets SPAWN schedule of potential additional work needed PMP plant modification proposal LTR lifetime records package DBE design base earthquake SCP seismically competent person PDS preliminary design statement WOC work order card QP quality plan TQ technical query RSA rolled steel angle|