Regulatory oversight of nuclear safety in Finland. Annual ... - STUK

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4.1 Loviisa NPP protective elements in the outer circles of the reactors of Loviisa 1 and Loviisa 2. Consequently, the spring packs of all protective elements were inspected during the 2011 annual maintenance outages before fuel transfers. Two protective elements were replaced with eight-year-old protective elements that still had a sufficient spring force. Fortum has initiated investigations into the reasons for the jamming. Reactor coolant pump seal The purpose of the shaft seal of the reactor coolant pump is to prevent the primary circuit water from escaping into the room space. The shaft is sealed with two successive sealing systems, the so-called hydraulic and mechanical seals. The mechanical seal contains antimony as the packing material. Some antimony enters the sealing water by dissolving and abrasion, and it is activated when it ends up in the primary circuit when carried there by sealing water circulation. According to a survey carried out by Fortum, more than half of the radiation dose rate caused by the primary circuit comes from antimony. For this reason, Fortum has planned that it will replace the material of the mechanical seals of the reactor coolant pumps with another graphite-based material where resin is used as the packing material instead of antimony. The seal construction itself remains the same, but the new material will not be activated. During the annual maintenance outage of 2011, Fortum replaced the mechanical seal on the first reactor coolant pump at Loviisa 2. However, an installation error was detected in the hydraulic seal of the primary circuit during ramp-up. This resulted in the mechanical seal being damaged, and the old antimony-impregnated seal was re-installed. Fortum intends to test the use of a resin-impregnated mechanical seal in one of Loviisa’s reactor coolant pumps in the 2012 annual maintenance. Inspection of the material of valve bonnet stud bolts A broken valve bonnet stud bolt was discovered during the 2010 annual maintenance outage at Loviisa 2. The bolt had broken because the manufacturer had used the wrong material and incorrect heat treatment. Following the discovery, Fortum inspected in 2011 the stud bolts of 58 valves at Loviisa 1 and 40 valves at Loviisa 2 using a mate- STUK-B 147 rial analyser. The above deviations were detected in a total of five bolts, which were replaced. The inspections will continue during the 2012 annual maintenance. Periodic inspection programmes In 2011, Loviisa 1 and Loviisa 2 had short refuelling outages, which means that the scope of periodic inspections was also limited. No significant faults with nuclear safety implications were observed in the inspections. The dimensions of the indication earlier detected in one weld joint of the reactor pressure vessel cover head were measured using an improved ultrasonic inspection technique. The technique has been developed after the 2010 annual maintenance outage with the aim of being able to measure the dimensions of the fault more accurately. The size of the fault was determined to be larger than the 2010 results indicated, and it was found to start at the interface between the cladding and the base material. The power company submitted the updated strength calculations to STUK for approval. STUK has approved the indication. According to Fortum, the indication will be next inspected using the improved technique in 2014. The deterministic periodic inspection programme of Loviisa 2 pipelines has been replaced by a risk-informed in-service inspection programme for the next 10-year inspection period of 2011–2020. At Loviisa 1, the change was already implemented for the 10-year inspection period of 2008–2017. Following the introduction of the risk-informed periodic inspection programme, new inspection subjects were added to the programme and subjected to basic inspections. Delayed periodic inspections of registered pressure equipment STUK observed in connection with the pressure equipment inspections made during the annual maintenance outage at Loviisa 1 that inspections of registered pressure equipment required to be carried out during power operation had been omitted and were several months overdue. STUK required Fortum to immediately investigate the root cause of the event and to assess the procedures related to the administration of pressure equipment inspections. In order to prevent similar occurrences, the Loviisa power plant must develop the 33
STUK-B 147 4.1 Loviisa NPP system for monitoring the inspections of registered pressure equipment and the actual inspections so that they are carried out by the set deadlines. Ageing management Measurements of loads inducing fatigue to pipe material have been carried out at the Loviisa power plant in the system designed for blowing out secondary circuit impurities accumulating in the steam generator. STUK required the power company to verify further these observations by non-destructive tests on the subject pipeline sections. STUK also paid attention to the possibility of strains caused by fluctuating flow conditions when assessing the replacement of pressurizer spraying lines at the Loviisa power plant, and required that a report be submitted on the strain measurements carried out in connection with the replacement. In addition, the inspection revealed that the Loviisa power plant has not recorded the interrupted heatups of the primary circuit from the cold shutdown state to operating state as primary circuit loading situations. STUK required the power company to assess if the interrupted heat-ups might have caused significant additional strains on the primary circuit. The Loviisa plant has had several development projects improving ageing management: the locations of faults in steam generator tubes and the spots where deposits from the secondary circuit will accumulate can now be visualised three-dimensionally, and their causal links can be shown. The material studies on parts decommissioned from the Greifswald plant were utilised for analysing the ageing mechanisms and realistic accident loads of inner parts of the reactor pressure vessel. Furthermore, the power company has studied ageing of cables, reactor support cage screws, diesel generators and concrete structures, and investigated the possibilities for increasing the pH of the secondary circuit. In deviation from the other main components of the primary circuit, no inner parts of the reactor pressure vessels are included in the most critical group of the ageing management programme of the Loviisa power plant, but the power company has allocated resources for studies concerning them. The main focus of the ageing management programme at the Loviisa power plant is on the 34 main components that are difficult to replace and determine the service life of the plant. When maintaining other components important to safety, the safety implications are taken into account through criticality classification based on the Operating Limits and Conditions. STUK required a further analysis to be carried out to establish whether the said method is sufficient for managing the ageing of these components. The power company must also analyse if the ageing management-related allocations of responsibility and resources at the Loviisa power plant are sufficient for all components important to safety. In relation to ageing management, the periodic inspections of registered pressure equipment were carried out according to plans during the annual maintenance outages at both plant units. A total of 10 inspections were carried out at Loviisa 1; of these, two inspections were in STUK’s inspection domain. A total of 17 inspections were carried out at Loviisa 2; of these, six inspections were in STUK’s inspection domain. In addition, periodic inspections were carried out for 16 pieces of registered pressure equipment at both plant units. These pieces of pressure equipment are in the licensee’s inspection domain. No observations restricting the use of pressure equipment were made during the inspections. Condition monitoring The condition monitoring inspections of pipelines at the Loviisa NPP were performed according to plans at both plant units, and no observations requiring repairs or restricting their use were made. No significant deviations were observed when monitoring the condition of the spent fuel storage and the fuel pools. The power company monitored the condition and leak tightness of the containment without observing any significant deviations. The monitoring of leak tightness requirements of NPP containments is discussed in greater detail in Section A.III.3 of Appendix 1. On the basis of the analyses made following the Fukushima accident, STUK required Fortum to update the safety assessments of spent fuel storages and fuel pools with a view to earthquake situations. Fortum has announced that it will update its assessment after the supplementary analyses are completed on 30 April 2012.
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