Technical Assessment Guide – Containment for reactor plant

T/AST/020

Issue date:: 04/02/2008
Review date:: 04/02/2012
Issue No:: 002
Approved by:: Andrew Coatsworth

Purpose and scope
SAPs addressed
Relationship to licence and other relevant legislation
Advice to assessors

1. Purpose and scope

1.1 The purpose of this technical assessment guide (TAG) is to provide assessors with guidance on the interpretation and application of HSE Safety Assessment Principles for Nuclear Plant 1992 (SAPs) that relate to containment of nuclear reactor plant. For the purposes of this document the term "containment" refers to structures, associated sub systems and equipment which are provided around nuclear reactor plant to prevent the escape of nuclear matter to the environment under normal and fault conditions. The components of a "containment" include: structures; isolating systems; penetrations and pipework that constitute the containment boundary. A related guide (AG42B / T/AST/021) provides similar guidance on containments for nuclear chemical plant.

1.2 This guide lists relevant SAPs; provides a commentary on these principles; identifies areas assessors should consider when reviewing a safety case; and lists relevant supporting references. Although this guide specifically addresses the sub section of the SAPS concerned with containment, assessors are also referred to the assessment guide that provide guidance on:

Key Engineering Principles;
Safety Categorisation;
Special Case Procedures;
Plant Ageing;
Reliability; and,
Structural Integrity.

The guidance in this document is intended for use in the assessment of existing and new reactor plant.

1.3 This TAG contains guidance to advise and inform NSD inspectors in the exercise of their professional regulatory judgement. Comments on this guide, and suggestions for future revisions, should be recorded on the appropriate registry file.

2. SAPs addressed

2.1 The containment SAPs P222 to P238 are contained within the Engineering Principles under a sub section entitled, "Plant Specific Principles". The SAPs cover reactor plant and chemical plant containments but not all principles apply to both containment types. SAPs P230 to P231 and P237 to P238 apply to chemical plant only. AG 42B / T/AST021 provides guidance on containments for chemical plant. The remaining principles which are therefore considered relevant to power reactor containments are P222 - P229 and P232 - P236.

2.2 The rest of this section outlines the intent of the SAPs that cover containments for reactor plants. Where appropriate the background to and the topics which should be considered are described. In general the intent of the SAPs is the same for a new and existing plant. An important objective of the assessment of existing plant is to identify any shortfalls in respect of the SAPs and judge their significance.

2.3 When interpreting the SAPs it is important for the assessor to recognise that the general lack of adequate reliability data for structural components has led to assessment being based primarily on established engineering practice. As a result, although the radiological consequences of failure of structures may be significant it is often not possible to calculate the risk for inclusion in the Probabilistic Safety Analysis.

2.4 The following commentary considers the intent of individual SAPs.

2.5 P222 - This principle indicates that a containment and associated systems should be provided to limit radioactive releases under normal and fault conditions and to protect against hazards. To achieve this objective it is necessary to determine the normal and fault conditions and establish the safety function and categorisation of structures, plant and equipment necessary to contain any nuclear matter arising. The performance of a new containment should be demonstrated by a combination of analysis, design and pressure test. In addition the design should be sufficient to protect the plant within the containment from external hazards.

2.6 P223 - This principle applies to the situation following an incident in which nuclear matter from the reactor plant is released into the reactor containment, e.g. post accident release into PWR containment.

2.7 P224 - The principle states that containment boundaries should be defined and that it should be capable of withstanding external and internal hazards in accordance with P119 to P143 (T/AST/013 & 014). Thus the containment boundaries of the component parts are to be identified: e.g. structures, isolating systems, and penetrations for access and pipework. The safety functions of the containment boundary under normal and fault conditions should be defined together with appropriate criteria to measure their ability to fulfil these functions. The assessment should consider whether the design has demonstrated that the containment's ability to meet these criteria is not impaired by the effects of internal and external hazards.

2.8 P225 - This principle can be split into two parts: firstly there should be provision for reactor shutdown following an incident; and secondly equipment should be available for decontamination and post incident entry. An assessment of the integrity of the containment boundary is to include those components that, following an accident and completion of the shut down process, define the containment boundary of mechanical services that penetrate the containment building, e.g. penetrations and isolating valves. The design of containment access is to permit safe entry by personnel and equipment following an accident.

2.9 P226 - This principle relates to penetrations passing through the containment which could, if breached, lead to an escape of radioactivity. Penetrations in containment boundary for access, services, pipework etc. are a potential source of leakage. Thus the analysis, design, installation, maintenance and testing of penetrations should ensure that appropriate standards of leaktightness commensurate with the containment function are maintained. In addition pipes, ducts and drains that penetrate the wall or floor of a containment should be equipped with appropriate monitoring, alarm and isolating systems to detect and isolate any potential breach.

2.10 P227 - The principle relates to service penetrations that rely upon the activation of an isolating valve under accident conditions to contain a potential release of nuclear matter. By avoiding the use of such ducts the risk of potential leakage will be reduced. The reasoning behind this that the reliability of the isolation systems installed within the ducts to maintain the integrity of the containment barrier under accident conditions should have a reliability consistent with the containment performance requirements.

2.11 P228 - 229 - These principles address the need for a pressure relief system if a safety advantage can be shown. The process of establishing the need for a pressure relief system will be dependent upon the containment type and the potential temperatures and pressures which could arise. If it can be shown sufficient margins are available in the containment design to cover all severe accidents then no advantage would be gained from a pressure relief system. There may be containments which do not have the available volume to resist temperatures and pressures. For these, the relief system should be capable of operating under the appropriate fault conditions.

2.12 P232 - This principle is concerned with the transport of nuclear matter from a containment. For a power reactor this would, for instance, be the movement of used fuel to the cooling ponds. The means for carrying out this activity and the measures to be taken during the operation are covered by items a and b of this principle. Therefore the safety analysis for the plant should show that these operations do not degrade containment performance.

2.13 P233 - The principle is concerned with sampling and monitoring systems which can be used to detect, locate and identify leakage from the containment. Provision of environmental monitoring around the plant is also asked for. It is therefore important to establish how the ventilation and drainage systems are used to detect leakage. The sampling and monitoring arrangements should be consistent with the safety analyses to demonstrate any radioactive release would be minimised by a robust detection system.

2.14 P234 - The principle relates to the need for access into a containment during normal operation and following an accident. To ensure access to the containment the plant should have the necessary reliability and safety systems to reduce the need for access to a minimum. Under accident conditions the plant should be "fail safe" and therefore the need for access to the containment would be limited.

2.15 P235 - This principle addresses the situation where routine access to the containment is necessary and indicates that emergency escape and rescue facilities should be made available. This is considered to be self explanatory.

2.16 P236 - The principle relates to the removal and reinstatement of shielding and containment for maintenance. For a power reactor such as a PWR this would relate to the shutdown condition where access for maintenance via the various access routes is required. The safety analyses should demonstrate that during this condition the potential for radioactive release is within acceptable limits.

3. Relationship to licence and other relevant legislation

3.1 Licence conditions applicable to the SAPs covered in the guide are as follows:

LC14 Safety documentation
LC15 Periodic review
LC19 Construction of new plant
LC23 Operating rules
LC28 Examination, inspection, maintenance and testing

4. Advice to assessors

4.1 In this section the key elements which an assessor should consider in a safety case submission from a licensee are identified for each of the relevant SAPs. The topics identified for consideration in the following paragraphs should not be considered as a check list but important areas which should be addressed when assessing a safety case. It is accepted there may be good reasons for a licensee not meeting a SAP. In these cases the assessor should ascertain the validity of the arguments presented.

4.2 Since the principles are not prescriptive the assessor will need to judge the extent to which the safety submissions presented satisfy the principles. For many areas this will rely on the skill and expertise of the assessor. In the case of existing plant there may well be some areas where the SAPs would not be satisfied. The assessor would be expected to judge the significance of the shortfall against the overall safety case for the plant and ALARP considerations.

4.3 In assessing a licensees submission the following should points should be considered when interpreting the SAPs.

4.4 P222 Containment provision

Is the safety function for a containment and its associated systems clearly identified?
Has P 69 which refers to safety categorisation been satisfied?
Have all normal and fault loading conditions been considered?
Will the containment protect the plant from external hazards?

4.5 P224 Containment boundaries

Have all elements of the containment boundary been defined?
Have external and internal hazards described in SAPs P 119 to 143 been determined and quantified for the containment design?
Has the containment design been carried out using the parameters for the hazards?
Has the design demonstrated that the plant can be maintained in a safe state as a result of hazard loading?

4.6 P225 Nuclear matter released within containment

Has the design of the containment made adequate provision for securing and maintaining the containment boundary following an accident involving the release of nuclear matter from the plant.
Has the design of the access hatch addressed safe entry by personnel and decontamination equipment following the accident.

4.7 P226 Containment leakage

Do arrangements exist to ensure nuclear matter does not escape via routes not installed for containment purposes such as pipework?
Have the number of penetrations been minimised?
Are all drains and ducting passing through the containment subject to containment and isolation provision?
Have monitoring and alarm systems been provided?

4.8 P227 Isolation systems

Do ducts passing through the containment have isolation valves which activate under accident conditions?
Does the performance of the isolation valves meet the containment performance requirements?
Are all the ducts passing through containment required?

4.9 P228 Provision of pressure relief systems

Has an assessment of the need for a pressure relief system been made?
Are there margins in containment design above the pressure relief system pressures?
Does this contain sufficient margin for uncertainty in pressure relief system operation?
Is the safety relief system sufficiently large enough to avoid overpressurisation of the containment?
Has the design of the safety relief system achieved the reliability assumed in the safety analysis of the plant?

4.10 P229 Filtered venting

Has an appropriate treatment system been provided in the pressure relief devices?
Is the system capable of operating under fault loading conditions?

4.11 P232 Removal of nuclear matter from containment

Is nuclear matter removed from containment e.g. fuel?
Are there arrangements for safe removal of the material?
Are the standards for removal consistent with the containment performance requirements?
Are the arrangements for removal carried out remotely?
Is additional shielding and ventilation needed to carry out the operation?

4.12 P234 Sampling and monitoring systems