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NPP Mochovce consists of four units of WWER 440 reactors, V-213 type, with improved safety. The first unit was commissioned in 1998, on unit 2 individual phases of commissioning took place, units 3 and 4 are in construction phase, which was interrupted in 1994. For NPP Mochovce mainly the end of 1999 was very demanding with regard to inspection and assessment activity due to a parallel outage for refueling linked with overhaul and start up of unit 1, with commissioning of unit 2. During the first refueling for the first time a new profiled fuel with higher average enrichment was loaded into the reactor of Unit 1. During the planned outage of unit 1 also part of the safety improvement program was realized, which was imposed by the UJD. After the outage, on 7 December 1999, the reactor of unit 1 was put into operation. At the end of 1999 Unit 1 operated at a decreased output of 55% due to repairs on the turbine generator. Completion of this repair is planned for January 2000.

In 1999 the works were intensified to complete the construction works on Unit 2, part of which was also implementation of safety measures. The scope of realized safety measures on Unit 2 was the same at the beginning of commissioning as on Unit 1 at the time after the end of outage for refueling. Significant milestones during commissioning of Unit 2 in 1999 was completion of phase 1 and 2 of the expanded hydro-tests. Approval for starting phase 1 of physical start-up, i. e. loading the fuel into the active zone, was issued by UJD 's Decision-Making Act of October 4, 1999. The first minimum controlled output of reactor was achieved on December 1, 1999, and on December 14, 1999 UJD issued a Decision-making Act to launch the phase of power increasing of the start-up of the unit. Connecting the first turbine-generator of Unit 2 to the grid was realized on December 21, 1999. By the end of 1999 tests of power increasing of the start-ups at the output levels of 20 % and 35 % were realized on Unit 2 of NPP Mochovce and part of the tests at the output level of 50 % . The design and the operational safety of NPP Mochovce units were assessed in a complex and detailed way and were evaluated within a PHARE project by the expert and consulting association of the European Commission, Riskaudit, composed of experts from the IPSN (France) and GRS (Germany) , organizations cooperating with experts from ANPA (Italy) , CIEMAT (Spain) , KFKI (Hungary) and VUJE (Slovakia). Expert organizations came to a conclusion that the proposed and realized safety measures, complemented with recommendations of Riskaudit, are adequate for meeting western requirements for nuclear safety. Also at the International conference EUROSAFE, held in November 1999 in Paris, it was stated about NPP Mochovce, that it is the first NPP of a Soviet design built in CEE countries, which has achieved safety standard comparable with the western standards.

Assessment activity

In 1999 UJD assessed changes of POSAR related to use of new type of nuclear fuel at Unit 1. Part of the approval from UJD for its use was also the program of refueling of Unit 1 of NPP Mochovce.
Special attention was paid in 1999 to limits and conditions of safe operation. Based on the experience from commissioning of Unit 1, the operator submitted a proposal for changes in limits and conditions. Before starting the 2nd fuel campaign UJD reviewed and approved the use of new, symptom oriented operational procedures, which allow the operating personnel to apply an effective dealing with a possible failure.

A separate category of assessment activity was represented by review of documentation of safety measures contained in the Program of safety improvements for NPP Mochovce, which were realized during the outage of Unit 1.
During the outage of Unit 1, the relevant part of in-service inspections program for selected equipment was realized, in the scope corresponding to the first year of the first cycle of operational inspections. During in-service inspections only non-significant faults having a nature of mechanical damage were found on connecting material and sealing surface of flange joints. Based on results of in-service inspections, the technical conditions of equipment on Unit 1 can be evaluated as good.
From the beginning of the year, assessment activity of UJD on Unit 2 of NPP Mochovce focused on preparation of the unit for commissioning. UJD evaluated the safety documentation for Unit 2 submitted in compliance with the "Atomic Act "and also assessed readiness of the unit equipment for individual phases of commissioning. Part of this was also review of POSAR in the part related to storage and fresh and spent fuel management.
From the side of UJD adequate emphasis was placed on realization of pre-operational control and test program on important selected equipment of Unit 2 of NPP Mochovce. During inspection of the reactor vessel from outside, a modern technology was applied for ultrasound test, which due to its higher sensibility resulted in finding indications, which upon intervention of UJD had to be specially analyzed, assessed and incorporated into the program of in-service inspections on the reactor vessel with the purpose of their monitoring during future operation. During active start up, the assessment process by UJD continued focusing on results of non-active tests and for meeting the success rate criteria. Those tests, where the criteria were not met, or where the results were not within the tolerance required, had to be repeated. The following phase could be started only upon meeting the success criteria from the previous phase and the approval of UJD to continue with the next phase of commissioning.
When assessing operational events it is necessary to state that in 1999 there were 89 operational events recorded on Unit 1 of NPP Mochovce, 12 of which were categorized as INES 0, and 77 were belove of INES scale.

The automatic reactor protection system on Unit 1 reactor was actuated 6 times during 1999. Total number and the nature of events on Unit 1 NPP Mochovce does not exceed the framework of common technical failures and their common reasons for new equipment. When assessing these events, UJD recorded a higher rate of events caused by human factor.
Analysis of reasons for a higher number of automatic shut down of Unit 1 in 1999 indicates that after removing design faults, the unit has all the preconditions to achieve higher reliability during the future operation as it was in 1999.
On Unit 2 (during commissioning) there were 27 operational events in total recorded in 1999, of which 2 events were categorized as INES 0 and 25 events were below the INES scale. During the year there was no violation of limits and conditions during the operation. Systems and equipment important with respect to nuclear safety worked reliably throughout the year.

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© Nuclear Regulatory Authority of the Slovak Republic, 2000