(a)   Fuel damage or baffle jetting

(b)   Fire issues

(c)   Leaks and leakage

(d)  Ageing management

(e)   Human performance

(f)   Seismic issues

(g)   Ineffective use of operating experience

(h)   Degraded cooling

Safety Report 2012-2014

The participants reviewed the IRS events reported during the 2012-2014 period and identified and selected those events with lessons and information of interest to the broader nuclear community. The selected events were grouped into common categories, for example Teaks and leakage’, or common plant activities, such as ‘maintenance’.

This is a brief overview of some of the event categories.

1. It is the group of events related to human errors. Most of these events were the result of several contributing factors. It is important to thoroughly investigate the circumstances in which human error occurred during an event because other contributing factors or deeper safety culture issues will most likely be found.

For many of the reported events, human error either had significant consequences for the plant or exacerbated the course of the events. Some reported events did not have direct consequences but could have had serious consequences if a loss of core coolant or a seismic event had occurred.

Most of the events were triggered by errors made during non-routine maintenance activities performed either by licensee staff or contractors, rather than being triggered by the errors of regular operating personnel. The descriptions of these events indicate that maintenance or contract workers are not consistently informed of the potential consequences of their work on plant operation. Careful planning and close monitoring of the work done by maintenance staff and contractors could reduce the number of events significantly.

2. There has been a notable increase in events related to ageing in recent years as plants get older and many are granted life extensions. In many cases, subsidiary equipment and components that have been present since construction have not been considered for ageing issues and have therefore not been part of an ageing management programme. In one instance the primary service water system that delivered water to the heat sink for several systems was compromised, and in another instance the concrete structure that acted as a barrier to the radioactive material and protected personnel from ionizing radiation during normal and accident conditions had been degraded.

3. Issues associated with leaks and leakage in nuclear power plants have started to increase recently and pose challenges to nuclear and operational safety. Leaks that are not corrected in a timely manner can deteriorate over time, compromise the reliability of safety systems and cause damage to associated equipment. In the reporting period, many leaks occurred in subsidiary systems not considered significant; however, these leaks affected other more important systems.

It is essential that leaks, particularly those that occur in important safety systems, are identified quickly and that the necessary actions are taken to remedy the situation in a timely manner to avoid adverse effects on operational safety and plant reliability.

4. The inspections and design reviews of seismically qualified structures continued to reveal deficiencies in the reporting period. Frequently, these deficiencies could be traced back to the original design. In other cases, design modifications or changes in operating practices invalidated the assumptions made by the original design. Constant efforts to reassess the seismic hazard and other external events should be maintained.

5. Weaknesses in design and plant procedures contributed to several of the fire events reported during this period. A design deficiency in a main turbine lubricating oil heating system led to the single point failure of a heater and a fire in one system. Another significant fire occurred during containment air testing because of a procedural violation that led to arcing in an electrical motor connection. Two other reported events were related to seismically induced fires.

6. Maintaining reactor core and spent fuel pool cooling capability is one of the main safety objectives in the operation of nuclear power plants. Two degraded cooling related events were reported during the 2012-2014 period. Both events took place during outages, and in both cases, operator response to the loss of core and spent fuel cooling would have been significantly facilitated if preapproved plans had existed to deal with outage situations in which the only equipment readily available for core cooling fails in service. Experience shows that this situation can and does happen in shutdown and outage conditions.

7. A number of reported events might have been avoided if previous similar operating experience had been adequately considered. For several events, an assessment of external or internal operating experience had identified recommendations; however, the recommendations had either not been implemented at all or not been implemented in a timely manner. Also, a major contributor to some events was a failure to complete a thorough operating experience assessment. It is important for any organization dedicated to safety to take advantage of all available information to prevent the occurrence of safety significant events.

8. Three of the events highlighted in this publication report the recurrence of baffle jetting related fuel failure. Baffle jetting fuel failures were first identified in the early 1970s, and heightened industry awareness of this issue over the following decades resulted in plant operators making modifications designed to eliminate this failure mechanism. However, several units that were identified with baffle jetting related damage during outages in the reporting period were thought to be resistant to baffle jetting because they incorporated an improved design with additional bolting of the baffle plates. Moreover, two units that made baffle jetting related reports were at two of the plants previously believed to be at low risk for baffle jetting. The plant operator of one of those units stated that changes in the material properties of the baffle plates and bolting due to ageing mechanisms may have resulted in the gaps widening at the baffle joints over time and set up a condition in which baffle jetting could occur.