Development of accident management procedures

4.2.1. Development and writing

Development and writing of accident management guidance involves various closely related activities, each influencing the quality of the final prod-uct. There are quality assurance (QA) related requirements such as several independent reviews of each EOP by development team members as well as CR staff. There may be a need to assess modifications to strategies or devel-opment of new strategies which may influence already written procedures.

Clarity of language, consistency of terminology and other style related require-ments should also be given proper consideration. Therefore, careful planning of the project with enough control points and effective interface between the individual phases is important.

A critical part of development is feasibility assessment. This may be done while strategies are being developed and the guidelines are being written and includes, but may not be limited to, evaluation of:

(a) The availability of information and instrumentation, (b) Equipment performance under severe accident conditions, (c) Accessibility of equipment,

(d) The necessity/possibility of overriding safety related instrumentation and control (I&C).

For each severe accident management strategy, the existing instrumenta-tion should be analysed to check whether informainstrumenta-tion necessary to monitor safety functions, entry criteria to the relevant guideline, execution of the guideline and success criteria for the guideline are available. Another important aspect is the reliability and accuracy of the information in view of environmental conditions of instrumentation and sensors. Procedures should include diversity and redundancy of measurement of some parameters and provide alternative information sources for instruments that may have failed.

A prerequisite to execution of SAMGs is that equipment and instru-mentation used in any of these guidelines and strategies will perform as intended under the expected environmental conditions. It should be decided whether the proposed strategy should be implemented if this performance has not been confirmed. If it is decided to implement the strategy even if suc-cess cannot be guaranteed, a minimum requirement would be that the infor-mation used to initiate and monitor execution and success of the strategy is sufficiently reliable and the fact that equipment performance has not been verified is known to the operator.

One of the major differences between procedures and SAMGs is the increased need for on-site actions in different plant compartments, e.g. restora-tion of power to active components, repair of malfuncrestora-tioning driving devices or components, and operation of valves which are not power driven. When such on-site actions are included in the guidelines, a careful examination should be conducted to determine whether the equipment concerned can be accessed without exposing personnel to excessive radiation, temperature and other potential dangers. Such an examination could also result in backfitting measures to reduce hazards and improve physical accessibility.

Since SAMGs are often based on the non-conventional use of existing operating or safety systems, it is possible that the proposed staff intervention will be prevented by the safety related I&C which has priority, or individual component protection I&C. The operator must be able to deactivate these functions without major delays when it is sufficiently clear that they would prevent him or her from executing the required accident management guideline.

An important aspect of accident management planning is to ensure that adequate administrative controls are in place to prevent premature or inappro-priate execution.

Depending upon the approach adopted, the following guidelines and documents need to be provided in parallel with the development and writing of the SAMGs:

(i) Guidance for the main CR operators during a severe accident.

(ii) Guidance for the TSC (or equivalent support staff), including the chosen methodology for recording implemented strategies and listing and addressing long term concerns.

(iii) Computational aids to allow support staff to understand plant conditions based on the available instrumentation (not necessarily computerized) and to guide them in the selection and execution of suitable strategies; several examples of CAs and their application are given in Appendix III.

(iv) Calculation of the plant specific set points required by the SAMGs.

If the generic procedures and guidelines are written in another language, suitable provisions must be made to prevent confusion arising from language differences.

Based on the severe accident management strategies, procedures are to be as user friendly as possible. Extensive use of flow charts, figures, tables and diagrams should therefore be considered.

4.2.2. Preparation of background material and documentation

Background material is prepared in parallel with the development and writing of the individual procedures as it has to document all the changes, improvements and modifications to generic strategies and procedural steps which were agreed to at the time of writing of the accident management guidance. All grounds and justifications should be recorded to allow for future reviews and for the updating of procedures and guidelines in the light of new knowledge of plant behaviour or modification of plant systems.

Because of the greater complexity of severe accidents compared with DBAs, as well as the continuously increasing knowledge base on plant behaviour under severe accident conditions, consideration should be given to creating background documents in a way that allows easy upgrades and extensions and allows their use as reference and training materials.

Background material should include:

(a) The technical basis for strategies,

(b) Detailed description of instrumentation needs, (c) Results of supporting analyses,

(d) The basis and detailed descriptions of procedure and guideline steps, (e) Set point calculations and their basis.

The preparation of good background documentation is very important.

It has three primary functions:

(1) It is a self-contained source of reference,

(2) It demonstrates compliance with the relevant QA requirements, (3) It provides support material to be used in training courses for technical

support staff and operators.

The reference and training course support functions of a background document may require it to be supplemented by additional analyses for illus-tration, even though generic preventive and mitigatory strategies have been applied and the assessment did not require a plant specific analysis.

Additionally, a ‘deviation document’ may be prepared which lists the differences between the plant specific procedures and guidelines developed and the generic ones, and explains the reasons for the differences.

4.3. SUPPORTING ACCIDENT ANALYSIS FOR DEVELOPMENT OF