The IAEA Spent Fuel Storage Glossary [1] defines extended storage as
“storage of fuel units beyond the period of time originally planned”. In numerous countries, “beyond the period of time originally planned” evolved to be the only interim option. It is important that the effects of extended operation be anticipated and monitored to ensure that the excellent safety
record of fuel storage technology is maintained. At several fuel storage facilities, expensive recovery operations have been the consequence of failure to effectively manage degradation of fuel and/or facility components.
The generally accepted parlance in nuclear technology for the effects of extended operation is ‘ageing’, defined as a “general process where character-istics of a system, structure or component (SSC) gradually change with time or use” [2]. The effects of ageing may be beneficial, as in the curing of concrete, or they may be deleterious, as in the corrosion of metal or degradation of insulating material due to the effects of radiation. The same concepts and terminologies that apply to nuclear reactor ageing are relevant to ageing in fuel storage facilities.
2.1. AGEING TERMINOLOGIES
The sources of ageing terminologies are provided in Refs [2, 3]. Selected terms are defined below.
Ageing. General process in which the characteristics of a system, structure or component (SSC) gradually change with time or use.
Natural ageing. Ageing of an SSC that occurs under pre-service and service conditions, including error induced conditions.
Error induced conditions. Adverse pre-service or service conditions produced by design, fabrication, installation, operation or maintenance errors.
Ageing degradation. Ageing effects that could impair the ability of an SSC to function within acceptance criteria. Examples are reduction in diameter from the wear of a rotating shaft, loss in material strength from fatigue or thermal ageing, swelling of potting compounds and loss of dielectric strength or cracking of insulation.
Ageing effects. Net changes in the characteristics of an SSC that occur with time or use and are due to ageing mechanisms. For examples of negative effects, see ageing degradation; examples of positive effects are increase in concrete strength from curing and reduction of vibration from wear-in of rotating machinery.
Ageing mechanism. A specific process that gradually changes the character-istics of an SSC with time or use. Examples are curing, wear, fatigue, creep, erosion, microbial fouling, corrosion, embrittlement and chemical or biological reactions.
Artificial ageing. Simulation of natural ageing effects on SSCs by application of stressors representing plant pre-service and service conditions, but perhaps different in intensity, duration and manner of application.
Figure 1 shows the ageing sequence, the key elements that lead to age related degradation or failure. The sequence illustrated is for the case of through-wall cracks that developed in a fuel storage pool (FSP) stainless steel pipe during service [4]:
(a) Pre-service conditions: sensitization of welds due to lack of a proper weld procedure and possible overstressing when the ends of matching pipe ends are forced together.
(b) Operating conditions: borated water at about 30°C; possible contami-nants.
(c) Ageing stressors: stress in weld area and abnormal metallurgy (sensiti-zation) in heat affected zones.
(d) Ageing mechanism: intergranular stress corrosion cracking (IGSCC).
(e) Ageing effect: through-wall crack development (nine locations).
(f) Ageing degradation: degradation of pipe wall in heat affected zones.
(g) Degraded condition or failure: degraded pipe that had to be replaced.
Figure 1 provides spent nuclear fuel storage facility (SNFSF) staff with a concise insight into the elements that contribute to degradation and/or failure of components in their facilities. The sequence emphasizes that ageing processes may be influenced by histories that precede the onset of service of an SCC.
Appendix II provides a concise compilation of terms on the following topics [2, 3]:
FIG. 1. Ageing sequence [2].
(ii) Life cycle:
— Life;
— Failure.
(iii) Ageing management:
— Maintenance;
— Condition assessment.
The groupings of related terms lead to an understanding of the various elements of ageing. Definitions of each term are found in Refs [2, 3]. Figure 2 provides a concise view of the elements of maintenance, refurbishment and repair, which are key factors in effective ageing management.
While the relatively benign conditions in most SNFSFs place maintenance on a less stringent basis than that of safety related SSCs in high temperature service, the elements of maintenance deserve attention at the level demanded by the potential vulnerabilities in SNFSF SSCs. A facility that can show evidence of a systematic approach to ageing management will have a sound basis on which to address extension of its operating period, if that becomes necessary.
As previously indicated, a major key to the effective control of ageing degradation in any facility is to have a staff that is attuned to the important concepts that comprise the understanding and managing of ageing. Operators of SNFSFs can use Appendix II and Figs 1 and 2 to facilitate examination of their programmes to understand and effectively manage ageing degradation in their facilities.
MAINTENANCE Degraded SSC
UNFAILED
FAILED
PREVENTIVE MAINTENANCE
REFURBISHMENT
CORRECTIVE MAINTENANCE
REPAIR
● Periodic maintenance
● Planned maintenance
● Predictive maintenance
FIG. 2. Terms related to maintenance [2].
When it is necessary for SNFSF staff to design an AMP specific to their facility, guidance to develop the programme is provided in the GALL (Generic Aging Lessons Learned) report [5], which is based on ten programme attributes that have been identified by staff of the United States Nuclear Regulatory Commission (NRC). The ten AMP attributes should be applied with judgements regarding relevance to how the AMP is to be implemented in the specific SNFSF; for example, if there is no relevant operating experience for the SSC in the SNFSF, operating experience from similar facilities may be useful to assess potential vulnerabilities.