Mochovce on target for mid 1998 start-up

1 January 1998

Judging by presentations at the NUSIM 97 meeting held in Levice, Slovakia, 19-21 November 1997, notably that by Jaroslav Holubec, deputy director of the plant, start-up of Mochovce unit 1 in June 1998 looks achievable.

The results of testing carried out at Mochovce unit 1 so far have been encouraging. Commissioning of the unit started on 21 August 1997 and the first phase of hydraulic testing, the cold test, was successfully completed on 31 October. On 5 November pressure-tightness testing of the “hermetic zone” was carried out.

The units at Mochovce are the later V-213 type of Soviet-designed VVER-440. They therefore incorporate a number of improvements over the earlier, V-230, type and, for example, most safety systems are 3x100% redundant. There were originally intended to be four reactors at the site, but construction was stopped in 1990, after the “velvet revolution”, when units 1-4 were respectively 90%, 75%, 40% and 30% complete. Since the early 1990s the utility, Slovenske Elektrarne (SE), has been working to resuscitate the project, focusing initially on the safety upgrade (with Western assistance) and completion of units 1 and 2. A major step forward was taken in April 1996 with the signing of the major contracts paving the way for completion. The following month the Slovak government gave guarantees for loans and SE signed loan agreements to finance completion.

The original general designer (Energo-projekt Praha), as well as the original prime contractors for civils (Hydrostav Bratislava) and equipment (Skoda Praha), were retained in the new project structure to maintain the original warranties. The following contractors were added: EUCOM (Framatome/Siemens consortium) – bulk of the safety measures; Russian organisations, under Atomenergoexport – some safety measures and work on Russian-supplied special systems (eg reactor protection and in-core instrumentation); EDF – project management and preparations for future operation; VUJE – technical assistance on implementing safety measures and, as from May 1997, lead role in commissioning.

The utility has been motivated to press on with the projects by its belief that the VVER-440 type V-213 has some positive inherent safety features (even relative to state of the art Western PWR designs). For example: there are large thermal, hydraulic and mechanical margins in the design of fuel, core and cooling systems; the fuel operates at low temperature, low stored energy and low linear power; the secondary and primary system water inventory is large relative to unit size (meaning that operators have a long grace period for taking action in the event of total loss of heat removal); the core is small, without danger of xenon power oscillations; and there are a large number of primary loops. The technology therefore constitutes a good basis on which to achieve future safe operation.

SE already has two V-213s in operation, at Bohunice V2. With upgrading the utility believes that by the time it starts up Mochovce will have achieved a level of safety in line with international standards, providing a good platform for continuous safety improvement, uprating and modernisation in the future.

At NUSIM, Jaroslav Holubec argued that the performance of V-213s has been in some ways better than Western PWRs, for example in terms of scrams per year, INES reportable events and occupational dose.

Even before the current programme of safety improvements (which was drawn up in 1995), and in many cases from the outset of the original project, modifications were incorporated in the Mochovce 1 and 2 design which set these units apart from most of the previous V-213s. These modifications included seismic measures, adoption of a Siemens control system (as used at the modern German Isar 2 plant) and changes in primary and secondary circuit systems derived from experience at Bohunice and at Dukovany (Czech republic), in particular aimed at upgrading the steam generator feedwater system and improving residual heat removal (RHR) capabilities. Measures included: siting in a low seismic area; heating of ECCS tanks and hydroaccumulators to avoid vessel thermal shock; provision of isolating valve on pressuriser relief valve and emergency vent for the primary circuit; installation of ECCS heat exchanger for RHR; replacement of aluminium insulation with stainless steel to avoid hydrogen production; removal of water seals during long term RHR; provision of steam dump to improve RHR; removal of emergency feedwater pumps and tanks from turbine hall to reduce risk of common cause failure (turbine hall fire); installation of fire fighting water system; improvement in the reactor protection system; and provision of a full scope simulator.

The 1995 safety improvement programme aims to build on these positive features. It is essentially the culmination and synthesis of a number of international audits and assessments, involving no less than 2000 experts, which have been done since the early 1990s, in particular by EDF, IAEA and Riskaudit (joint venture of GRS and IPSN). Overall, these studies show that, for a type V-213 reactor, including Mochovce, a core damage frequency of 10-4 per reactor year can be reached without any major hardware modification, while it is possible to reach a core damage frequency of 10-5 per reactor year through hardware modifications.

The 1995 safety improvement programme was drawn up by the utility in collaboration with VUJE and sets out 87 safety measures. Using an IAEA scheme these have been classified into four categories: I - departure from recognised international practices; II - safety concern, defence in depth degraded; III - high safety concern, defence in depth insufficient; and IV - highest safety concern, defence in depth unacceptable. None of the measures fell into category IV.

Class III issues, and the relevant contractors were as follows:

• Equipment qualification (EGP, VUJE).

• Non-destructive testing (Skoda).

• ECCS sump screen blockage (Skoda, VUEZ).

• Feedwater supply (EGP).

• Bubbler–condenser strength and behaviour (EUCOM).

• Fire prevention (EUCOM).

• Internal hazards due to high energy (EUCOM).

• Seismic design (Skoda).

Examples of Class II issues were: classification of components; reliability analysis of safety class 1 and 2 systems; reactor pressure vessel integrity; steam generator collector integrity; steam generator tube integrity; pressuriser safety and relief valve qualification for water flow; steam generator safety and relief valve qualification for water flow; primary circuit venting under accident conditions; replacement of Hindukus and VK3 I&C systems; on-site power supply; emergency battery discharge time; bubbler condenser thermodynamic behaviour; maximum pressure differences on walls between compartments of hermetic boxes; systematic fire hazard analysis; fire detection and extinguishing.

Seven of the safety measures (for example relating to procedures, safety culture, experience feedback, QA and document management) are being tackled directly by the utility, collaborating with other organisations as necessary. For example the issue of emergency operating procedures is being addressed by a joint team drawn from the utility and Westinghouse Brussels.

The regulatory position regarding Mochovce safety enhancement, as desc-ribed in a paper to NUSIM by Miroslav Lipar, who has recently taken over from Josef Misak as head of UJD, the Slovak regulatory body, is that the most urgent measures must be implemented prior to unit 1 start-up, or their safety significance must be reduced to category I or II.

Overall, licensing is on the basis of a pre-operational safety analysis report (POSAR), in line with internationally recognised standards. The draft POSAR was submitted to UJD at the end of September 1997. The regulatory hold-points during commissioning are: hot functional tests; fuel loading; readiness for criticality; start-up tests; power increase; and operation.

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