The generic safety issues for PHWRs compiled in Section 4 reflect the broad experience of Member States in resolving safety concerns and in maintaining improvements to current practice. As reflected in the “Measures taken” for these issues, most issues have been successfully addressed. The Section 4 compilation also includes those safety issues which are currently considered “pending”, i.e. the root causes and their resolutions are in the process of being developed. For these pending issues, interim judgments have been made by Member States with respect to continued safe plant operation. The hallmark of the Nuclear Industry is to review all aspects of the performance on a continuous basis and keep effecting improvements. These characteristics seem to be well reflected by the nature and variety of the Generic Safety Issues included in this document.
This comprehensive compilation has been found to be an appropriate basis to look for broader insights, lessons learned and trends, which is the subject of this section. Many issues are not specific to PHWRs but are applicable to the whole Nuclear Industry, with a few of them being applicable to other industries as well. These are included to make this document a “stand alone” and comprehensive one. Some issues arise due to the unique nature of PHWRs and their design differences such as on-power refuelling and the use of fuel channel technology, and operational differences such as the management of heavy water and tritium. These are reflected in issues pertaining to tritium, complexity, health management of coolant channels, additional systems and equipment, on-power fuelling, etc.
The source or sources for each issue are provided in Section 3: that is, whether operational experience, a deviation from current standards and practices, or analysis gave rise to the concern.
Nearly half the issues had operational experience as one of their sources. This is to be expected after over 30 years of worldwide nuclear power generation. The source areas of deviation from current standards and practices and analysis were noted to be in over half of the issues. The use of analysis, particularly PSA, to identify potential issues of concern which have not been observed during plant operation is considered a sign of health with respect to industry and regulatory body safety oversight.
The following discussion is organized by grouping generic observations on the issues by the primary source of the trend or common trends observed.
3.1 OBSERVATIONS BASED ON OPERATIONAL EXPERIENCE
Almost all the issues in the management, surveillance and maintenance, training, emergency preparedness and radiation protection areas are the result of operational experience. Generally, these issues are generic to the nuclear industry and are elements or attributes of a nuclear management programme. Shortcomings in these elements can give rise to issues pertaining to the effectiveness of a nuclear management programme which, in turn, can have either direct or indirect nuclear safety implications. The elements of a nuclear management programme can be broadly classified into the following focus areas:
Operate the Plant: includes issues on elements such as conduct of operations, control of plant status and operations procedures (i.e. MA3: Adequacy of shift staffing).
Maintain the Plant: includes issues on elements such as conduct of maintenance, outage planning and management, work management, materials management and calibration (i.e. MA4: Control of outage activities to minimize risk).
Manage Risk: includes issues on elements such as radiation protection, PSA, fire protection, emergency preparedness and worker protection (i.e. TR2: Training for severe (beyond design-basis) accident management procedures).
Manage Engineering Design: includes issues on elements such as protecting the asset (e.g. fuel channels, steam generators, chemistry, NDE, in-service inspection, procurement) and engineering change control (i.e. MA1: Replacement part design, procurement and assurance of quality).
Manage Performance: includes issues on elements such as worker practices, corrective actions, independent assessments and managed systems (i.e. MA7: Human and organizational factors in root cause analysis).
Manage People: includes issues on elements such as promoting and supporting a strong safety culture, training, human performance management and workforce planning (i.e. MA2: Fitness for duty).
One area where operational experience indicates a common trend is changing technology as it is reflected in I&C systems. This has resulted in the unavailability of identical replacement parts because of changes in industry product lines but has also, more importantly, posed new types of safety questions for plant owners and regulatory bodies. This is particularly true in the area of the introduction of digital technology. There are substantial safety advantages to the use of digital hardware/software in diagnostics, control and safety function actuation. The verification and validation of digital system and software are challenging tasks. It is unlikely that all problems can be considered in advance, as evidenced by the history of the original plant control and safety systems.
A concerted effort to agree on safety standards for digital technology should be combined with close monitoring of plant experience and reflection of lessons learned into system design and regulatory practice. There are issues connected with digital technology in two other areas as well, i.e. deviations from current standards and analysis insight.
3.2 OBSERVATIONS BASED ON DEVIATIONS FROM CURRENT STANDARDS AND PRACTICES
For PHWRs the pressure boundary includes the coolant channels. A lot has been learned on various aspects of coolant channel behaviour and several steps have been taken by member states to strengthen coolant channel health management.
Section 4 brings out only a few issues which are linked to ageing of the plants. Many NPPs have been operating for many years and have started showing signs of ageing. As such, ageing management of these operating plants is an important item in their periodic safety review (IAEA Safety Guide 50-SG-O12, Periodic Safety Review of Operational Nuclear Power Plants (1994), which, in the meantime, has been revised and is now available as the Safety Standards Series No. NS-G-2.10, Periodic Safety Review of Nuclear Power Plants (2003); and INSAG-8, A Common Approach to Judge NPPs Built to Earlier Standards) and in the extension of operation past the end of the original design lifetime.
Ageing studies sponsored by the IAEA and Member States as well as periodic plant safety reviews may bring out generic safety issues of PHWRs needing specific attention to assure safety. For example, the monitoring of some parameters, such as plant water chemistry, becomes important when considering plant history with respect to the feasibility of extended safe plant operation. PHWRs however are comparatively “younger” and life cycle management needs more effort as more plants age. In some cases, components unique to PHWR require special studies which cannot be obtained from work on other types of reactors.
Several safety issues result from deviations from current safety standards and practices. Some of these apply to reactors of older design which have not regularly taken into account the operating experience of other plants and the resultant adjustment to international standards and practices. Attention is warranted to assure that conscious decisions are made by Member States with regard to the applicability and significance of new safety issues either as they arise or through periodic reassessments. These are generally in areas where substantial advances are made worldwide.
Examples are fire protection, seismic qualification, internal and external hazards etc.
In this regard, the substantial participation in this effort and the improvement measures already taken by countries with older reactors is noted as a positive development to achieve and maintain high standards of reactor safety.
3.3 OBSERVATIONS BASED ON ANALYSIS INSIGHTS
The completion of PSA analysis for a large number of plants has allowed a more comprehensive and systematic assessment of the safety of the plants. Systems and situations, to which not much attention was paid before, are now considered relevant to safety (i.e. analysis of accidents in modes other than full power and during outages, station blackout, specific configuration situations, maintenance and outage management, external hazards).
Although plant specific PSAs will continue to identify vulnerabilities for particular plants, a large number of additional analysis issues with widespread applicability is not expected. Attention will need to continue, however, with respect to the adequacy of computer modeling of accident phenomena and the applicability of reliability data used in the PSA analyses. Efforts would be required in areas of severe accident management; symptom based emergency operating procedures, taking fuller advantage of PSA especially during operational phase etc.
There is also an opportunity, given current analytical capabilities and monitoring techniques, of significantly improving component reliability by the use of plant specific experience data to adjust maintenance and replacement programmes for active plant equipment.
4. GENERIC SAFETY ISSUES FOR PRESSURIZED HEAVY WATER REACTOR NUCLEAR