atw - International Journal for Nuclear Power | 05.2021

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atw Vol. 66 (2021) | Issue 5 ı September Plant status N. events Root cause N. times OPERATION AND NEW BUILD 20 Power operation 54 Startup 7 Hot standby 2 Hot shutdown 2 Cold shutdown 12 Refuelling 20 Other or Unknown 16 Dectection of events N. events Periodic test / In service inspection 29 Fault report in control room 65 Work surveillance 6 Supplementary inspection 4 Other or Unknown 9 Affected system N. events Primary reactor systems 41 Reactor auxiliary systems 12 Essential service systems 1 Essential auxiliary systems 15 Electrical systems 24 Feed water, steam and power conversion systems 5 I&C systems 5 Service auxiliary systems 3 Structural systems 5 Other 2 Affected comp. N. events Passive mechanical components 43 Active mechanical components 38 I&C components 10 Electrical components 18 Structural components 4 Direct cause N. events Mechanical failure 83 Electrical failure 17 I&C failure 9 Structural failure 3 Other 1 Absent Ageing Managemet Programme 12 Deficiencies in Ageing Management Programme 25 Deficiencies in maintenance or surveillance 55 Wrong material selection 15 Equipment specification, manufacture, storage and installation 18 Deficiencies in design 30 Other or unknown 4 Ageing mechanism N. times Swelling 1 Relaxation 3 Fatigue 28 Thermal ageing 13 Irradiation damage 2 Corrosion 38 Wear 15 Erosion 4 Electrical ageing 15 Creep 1 Chemical ageing 1 Other 13 Unknown 3 Consequences N. events Degradation (damage) 28 Deformation 12 Embrittlement and cracking 36 Material loss 30 Other or Unknown 7 Corrective Actions N. times Equipment replacement or repair 112 Monitoring and/or inspection improvement 39 Changes in operation modes 8 Changes in maintenance programme 50 Changes in ageing management programme 18 Design modification 25 Other 2 | Tab. 2. Number of events/times per family. deficiencies in design and in ageing management programmes. To this respect, we infer that the establishment of an effective ageing management programme, as early as possible in the lifetime of the plant, will significantly contribute to preventing events and the resulting consequences. Ageing mechanisms The category “ageing mechanism” was split in 13 families, as indicated in Table 2, making it possible to allocate several (maximum three) ageing mechanisms to a single event. Figure 7 shows that corrosion (38 times) is the main cause of failure, followed by fatigue (28 times). Other important contributions are coming from thermal ageing, wear and electrical ageing. As it will be showed later in the section on lessons learned, many events only appear after long term operation of an aged component or material, and the main cause was a deficiency in design that was latent. Figure 8 put some light on this issue and illustrates that fatigue is the main degradation mechanism in relation to hidden deficiencies in design. Figure 9 correlates deficiencies (or absence) in ageing management programme with the ageing mechanism. In this case electrical ageing is the most relevant contributor to failure. This indicates the need for improvement of the ageing management programmes of electrical and I&C components. Consequences and corrective actions Figure 10 shows the distribution of events among different consequences. 36 events were related to embrittlement and cracking and 30 events to material loss (mainly due to corrosion). Figure 11 illustrates the corrective actions. A maximum of three corrective actions were allocated to a single event. As expected, the main corrective action was the replacement or repair of equipment. Changes in maintenance programme was the second Operation and New Build Operating Experience from Ageing Events Occurred at Nuclear Power Plants ı Antonio Ballesteros Avila and Miguel Peinador Veira
atw Vol. 66 (2021) | Issue 5 ı September | Fig. 2. Plant status (left) and detections (right) means versus number of events. OPERATION AND NEW BUILD 21 | Fig. 3. Number of events (%) per system affected. | Fig. 4. Number of events per component affected. | Fig. 5. Number of events per direct cause. | Fig. 6. Distribution of root causes. most usual corrective action followed, in this order, by monitoring or inspection improvement, design modification and changes in ageing management programme. Lessons learned The extraction of the lessons learned from the operating experience has been completed in two steps. First (step 1), low level lessons learned were retrieved from the IRS database, or developed in some cases, for a large number of the 113 analysed events. A total of 110 low level lessons learned were obtained. They are given, together with a short summary of the events, in Annex 2 of reference [ Ballesteros Avila A., 2019]. Several lessons are allocated to the same event in many cases. These low level lessons learned are very specific, so that they would have a too limited applicability. To address this issue, the low level lessons learned were grouped under similar topic or underlying key message to get a high level lesson learned (step 2). In the following paragraphs the high level lessons learned are presented: Lesson learned #1 – Appropriate measures should be taken and design features should be introduced in the design stage to facilitate effective ageing management throughout the life of the plant. Lesson learned #2 – Ageing Management Programmes as well as maintenance programmes should be reviewed and updated to take into account modifications in the current licensing bases. Lesson learned #3 – The monitoring of the environmental conditions, as information source for ageing management, is of high importance. In particular, a review of possible changes in environmental conditions (e.g. temperature, radiation, etc.) that could affect ageing should be performed in case of operational changes or structures, systems and components (SSC) modifications. Lesson learned #4 – The maintenance and inspection programmes should be evaluated and, if considered necessary, updated (frequency, Operation and New Build Operating Experience from Ageing Events Occurred at Nuclear Power Plants ı Antonio Ballesteros Avila and Miguel Peinador Veira
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