APPROACH TO MANAGING BDBA CONDITIONS

The approach used to establish the normal design basis reflects the large body of codes, standards, established practices, and operating experience which provides the underlying framework of NPP design and operation. The standard processes related to plant modifications have been developed to stipulate a high degree of rigour to ensure consistency with the design basis of the station. Consequently there is limited latitude for relaxation when addressing BDBA conditions.

For low frequency, high consequence event sequences, where significant uncertainty around the plant conditions which could challenge safe operation exists, strict adherence to

“normal” practices may prove to be problematic. For example, in NPP operation, the

“symptom based approach” is adopted to ensure that operational strategies provide adequate flexibility. Performance requirements must be met. Yet cost and schedule flexibility are also a consideration.

Figure 1 presents an overview of the OPG approach, and shows the relationship between the design, operations, and regulatory considerations. The referenced procedures in Figure 1 (eg, N-PROC-MP-0090 – Modification Procedure, and N-GUID-01130-10000 – Modifications for BDBA) and regulatory documents (CNSC RD-310 – Safety Analysis for NPP, Siting Guide (SG), and CNSC Consultative Document (C6)) are specific OPG and Canadian regulatory references which delineate process and analysis requirements and safety limits for Canadian NPP.

In developing the Canadian approach to BDBA modifications, regulatory guidance (Canadian Nuclear Safety Commission (CNSC) RD-337 [1] and RD-310[2] were considered – as they provide general guidance regarding the treatment of BDBA from the perspective of plant design requirements and the associated Nuclear Safety Analysis (NSA). CNSC Guidelines (e.g., G-306 [3]) also provide supporting guidance. US NEI 12-06 [4] provides guidance on the “FLEX” approach to BDBA. In addition, IAEA guidelines (e.g., SS-R-2/1 [5] and SS-R-2/2 [6]) also provide guidance on BDBA considerations.

Consistent with the current approach in IAEA guidelines, the strategy developed aligns with BDBA Design Extension Conditions (DEC) and Safety Features for DEC.

3.1 Graded Approach

A graded approach to modifications for BDBA conditions has been adopted within OPG. The graded approach is implemented because it is anticipated that some relaxation of conservatisms is warranted, or even necessary, to address the BDBA conditions in a reasonable and cost-effective manner. These changes from the normal full change control processes are always reviewed and approved, and in all cases, the processes and the resultant modifications are consistent with the Power Reactor Operating Licences of the stations, appropriate engineering codes and standards, and the overall practice of engineering.

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FIG.1. Beyond design basis overview.

The guide that has been developed prescribes four categories of modification:

1. Existing engineering SSCs which can be called upon to manage BDBAs. This category includes all permanently installed station SSCs, where operation outside and/or beyond the design basis of the SSC may be required under BDBA conditions.

This category covers documentation changes, and updated analyses to address BDBA conditions, but does not require a physical change to the SSC.

91 2. Equipment upgrades installed on existing engineered SSCs to manage BDBAs. This

category includes any modification (either in terms of additional equipment or upgraded equipment) in any permanently installed station SSC, which is required to maintain essential functionality under BDBA conditions.

3. New engineered SSCs added for the sole purpose of managing BDBAs. This category includes all new SSCs which are permanently installed specifically to manage and/or mitigate BDBA conditions.

4. Portable SSCs which can be attached to an existing SSC to manage BDBAs. This category includes all portable (or any temporarily connected) equipment required to manage and/or mitigate BDBA events.

The objective of the categorization is to ensure that the highest degree of rigour (least flexibility) is applied to the lower number categories, and increasing flexibility is applied to the higher numbered categories.

3.2. Examples of Modifications within Categories

To clarify the categorization of modifications for BDBA, the following examples are provided as representative of the categories:

Category 1: The assessment of the survivability under greater mechanical or thermal stresses which the SSC might be subjected to under a set of specific BDBA conditions.

Category 2: Seismic reinforcement of the deaerator to ensure that the deaerator inventory is available to the Steam Generators following an Extended Loss of all AC Power (ELAP) event.

Category 3: The addition of a new permanently installed Containment Filtered Venting System to enhance containment venting capability under BDBA conditions.

Category 4: The addition of Emergency Mitigating Equipment (EME) (including portable pumps and generators) to provide fuel cooling and essential monitoring following an ELAP.

3.3 Accountabilities and Approvals

The application of the guidance for modifications for BDBA conditions requires that the modifications packages are prepared by appropriately qualified staff and reviewed by the accountable manager. Deviations and variances from the full engineering change control procedures are approved by the accountable Design or Program Authority and are documented via signature. Design Authority approval for all BDBA modifications is required.

4.0 APPLICATION OF GUIDANCE

For Categories 1, 2, and 3, the standard rigorous engineering change control processes apply, with only limited deviations to simplify and streamline the process. For Category 4 – the portable equipment that is tied in to support BDBA response, there exists the largest degree of flexibility. This can be seen in its application to particular aspects of plant modification change control, commissioning, and operation.

4.1. Design

In the design process for BDBA – Category 1, 2, and 3 modifications follow the detailed design engineering change control process, with all of the associated screens and

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considerations and rigors based on the design basis of the system. Category 4 has the largest degree of design flexibility. Distinctions are as follows:

- Category 1, 2, 3, 4: Analysis supporting BDBA capability is conducted based on more realistic initiating conditions rather than considering limit of envelope conservative assumptions typically included in design basis analysis. This is consistent with the treatment of safety analysis for BDBA.

- Category 1, 2, 3, 4: Interfacing components must be designed to system requirements of the parent system (for design basis considerations).

- Category 1, 2, 3, 4: Review Level Conditions (RLC) – seismic (10^-4 probability), wind (F4 tornado), flood (probable maximum precipitation with wave overtopping

considerations) must be applied to credited systems. RLCs are established to be appropriate estimates of worst case conditions and are typically events at lower probability than design basis conditions. Systems must be designed to their nominal design basis requirements. Robustness to withstand these RLC must be demonstrated.

In some cases this qualification/robustness is further extended – for example – in a situation with a containment filtered venting design, seismic qualification to an even higher level than nominal may be prescribed to provide a larger safety margin.

- Category 4: Design process must be established by the accountable manager and approved by the Design or Program Authority, in accordance with the BDBA Modifications Guide. It should be documented, and auditable, and include

consideration of the elements of full design change control. (e.g., the Modification Outline Form does not have to be used prescriptively, but the document should be consulted to ensure appropriate areas of consideration are captured). Pressure boundary code requirements do not apply specifically. Codes and standards appropriate for portable equipment should be used and documented.

- Category 4: Equipment should be designed for two tie-in points, at least one of which is an engineered tie-in point. The second tie-in point may require some additional system modification at the time of installation (e.g., removing a spool piece), provided sufficient instruction and fittings are included as part of the modification process.

4.2. Procurement

Procurement processes are also variable depending on the categorization.

- Category 1, 2: Full procurement rigor – application of catalogue identification, procurement engineering, technical specifications, etc. apply.

- Category 3: As above, but typical commercial / industrial process can apply beyond system isolation tie-in points. Deviations approved by the appropriate Design Authority.

- Category 4: Commercial/industrial processes apply and manufacturer’s standards apply. Equipment will typically be tested by the manufacturer and inspected by OPG upon receipt. May be treated as transportation and work equipment if appropriate records are maintained and the process is auditable.

In the cases above, the use of commercial/industrial equipment allows the use of equipment that is readily available and with a proven commercial performance record. This can reduce costs and speed deployment. Functional performance capability is demonstrated through manufacturer’s specifications and testing.

Spare parts considerations must be included in the procurement process. For portable equipment – under Category 4 – there are requirements for some spare parts to be maintained on site – for running spares and for N+1 capability (where N is the required number of

93 components to support all units at a site) is a requirement. This is consistent with US requirements for FLEX equipment [4].

4.3. Installation, Testing, Commissioning and Availability for Service

Installation, testing, commissioning, and availability for service of the modifications for BDBA mitigation is according to the following guidance.

- Category 1, 2: Normal installation, testing and commissioning process per station procedures.

- Category 3, 4: Commercial/industrial processes apply for installation, testing and commissioning – with sufficient rigour to demonstrate functional performance requirements are met.

- Category 1, 2, 3: Available for service / Operations Acceptance processes consistent with all permanent station modifications.

- Category 4: Availability for service should follow station Operations Acceptance process. Where deviations are required – demonstration of equivalent level of review and acceptance is required.

- Category 4: Equipment should be stored in an area that will not be impacted by the accidents that the equipment is being procured to provide mitigation for.

In all cases, associated training, parts lists, operating documentation, maintenance documentation, spare parts, etc. are confirmed as part of the availability for service process.

4.4. Operations and Maintenance

Operations, Maintenance, Testing, Routines and Call-ups are required to support the modifications installed – to provide confidence in their ability to continue to perform to meet their functional requirements.

- Category 1, 2, 3: Operating instructions are typically incorporated into Abnormal Incidents Manuals, Emergency Operating Procedures, and Severe Accident Guidelines.

- Category 4: Operating Instructions are included in Emergency Mitigating Equipment Guidelines and Severe Accident Guidelines.

- Category 1, 2, 3, 4: Maintenance and testing is performed based on manufacturer’s requirements, maintenance standards, and regulatory requirements. For Category 4, testing is conducted to demonstrate functional performance for BDBA is achieved.

To ensure that Category 4 equipment is maintained available, where equipment redundancy exists, equipment can be taken out of service for maintenance for up to 90 days. Where no equipment redundancy exists, equipment can be taken out of service for up to 14 days for maintenance. A longer restoration period requires approval of the Operations and Maintenance Director.

5. SUSTAINABILITY

It is important to ensure that processes are in place to ensure that station modifications to support BDBA response is maintained available after it is installed. Processes must be established based on similar practices for design basis equipment, and additional considerations must be put in place.

- The technical basis for modifications for BDBA response capability should be formally documented and periodically reviewed to ensure that it remains current.

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- Regular maintenance and testing is controlled through a predefined process consistent with regular station equipment – or using alternate tracking schemes for portable equipment. In all cases this must be documented and audited.

- BDBA response capability maintenance is audited by Nuclear Safety Division – program owners of the Nuclear Safety Program.

- BDBA response capability must be revisited when new safety analysis is done – to ensure that the assumptions and analysis that went into the development of BDBA capability remains valid with the new assumptions.

- Station modification / change control processes must ensure that beyond design basis capabilities are not inadvertently altered.

- Documentation of BDBA response requirements for SSC (e.g., for ELAP event credits) should be documented in a Beyond Design Basis Functional Safety Requirements document (based on similar Operational Safety Requirements documents for design basis credits). This will allow subsequent design changes to easily confirm that changes will not impact BDBA response capability.

- Station transient material (laydown areas, etc.) processes must include controls to ensure that access to tie-in points and staging locations for Emergency Mitigating Equipment are not blocked.

- Maintenance and Outage Management processes must account for capability to implement BDBA response. Such schemes may include recall times, controlling outages of redundant tie-in points, pre-staging equipment where required, etc.