National Report of the Slovak Republic - Radiation and Nuclear Safety , September 1998

2.3.3 Mochovce Units Safety Improvements


The aim of the safety improvements through the safety measures (SM) is to achieve a safety standard for NPP Mochovce to meet the requirements of "in-depth safety" concept according to IAEA - INSAG 3. Cooperation of SE-EMO with other countries having interest in improving the safety standard may be expressed by the following activities:

NPP safety improvement is an ongoing concern of the SE-EMO operator. A logical continuation of the activities of EdF in the area of safety reviews as summarized in "Safety Improvement Report" (SIR) which was opened to public commenting, is the "NPP Mochovce Safety Improvement Program" developed in 1995. This program has been conceived as a long-term one. It however is aimed at reaching a safety standard upon the NPP commissioning at the level corresponding to the internationally recognized requirements and standards included in the IAEA safety guides accepted by ÚJD, and at setting up good conditions for continuous safety improvements in the future. This approach is in compliance with the actual world trend of safety improvements, extension of life cycle, upgrading, and raising the outputs of already constructed NPP; it is based on a continuous monitoring of world progress in the field of nuclear safety.

The NPP Mochovce safety improvement program is based on the document entitled "Safety Issues and their Ranking for NPP WWER 440/213"; other basic documents include the outcomes of the safety review conducted by RISKAUDIT in 1994, as well as conclusions at the IAEA Safety Improvement of Mochovce NPP Project Review Mission - SIRM taking place at Mochovce in June, 1994.

The classification of the safety issues is based on the IAEA document "Safety Issues and their Ranking for NPP WWER 440/213", the only exception being the safety measures added based on the specific situation of NPP Mochovce which are not classified according to IAEA. The following Table shows the description of the various categories. As will be shown below, there was no safety issue concerning the power plant Mochovce classified among the most serious category IV.

The NPP Mochovce safety improvement program is based on the In-Depth Protection concept, and its aim is to verify and demonstrate the fulfillment of "general principles of nuclear safety" as defined in the IAEA document INSAG-3. This approach is deterministic in nature, it however also combines with the probabilistic approach in accordance with the technical safety objective as defined by INSAG-3:

Table 2.3.1

Definitions of safety issue categories according to IAEA

Category I

Deviation from recognized international practices. Remedial measures are advisable.

Category II

Safety significant problems. In-depth protection weakened. Remedial measures are needed.

Category III

Problems of high safety significance. In-depth protection insufficient. Immediate remedial measures are to be taken. Temporary measures may be necessary.

Category IV

Problems of highest safety-related importance. In-depth protection unacceptable. The issue requires immediate intervention and substitution measures have to be immediately taken until complete resolution of the problem.

Probabilistic safety review (Level 1 PSA study and Shutdown PSA) is a part of the NPP Mochovce´s safety improvement program. Based on the results of the analyses already conducted within the PSA studies for WWER-440/V213 type NPP, mainly PSA for Bohunice V-2 units, as well as based on the interim results of the PSA study for NPP Mochovce, it could be stated that:

Based on the above facts, the safety-related objective of the NPP Mochovce safety improvement program from the aspect of active core damage is to achieve a value lower than 10-5 per reactor. year operation.

The operator of the plant in cooperation with VÚJE, a.s. developed a set of technical specifications for 87 safety measures (TSSM) to be implemented under the "NPP Mochovce Nuclear Safety Improvement Program", with taking into account specific measures as identified by the RISKAUDIT and SIRNM Reports and experience with Bohunice V-2 and NPP Dukovany units (see Table 2.3.2). This has introduced certain differences between the "NPP Mochovce Safety Improvement Program" and the IAEA document "Safety Issues and their Ranking for NPP WWE 440/213" (certain measures have been added characterized as no-category measures). The differences are as follows:

Table 2.3.2

AREA

Category III

Category II

Category I

No Category

General

1

2

Active zone

1

Component integrity

1

4

1

Technological systems

2

12

3

Control and management systems

8

2

1

Electric systems

2

3

2

Containment

1

3

1

1

Internal risks

2

4

3

External risks

1

1

1

Accident analyses

5

8

Operation

11

Total 87

8

42

22

15

 

TSSM became the basis for contract negotiations with the contractors responsible for individual safety-related issues. The following organizations have been selected to implement the various parts of the TSSM: EUCOM (a consortium of FRAMATOME, France and SIEMENS, Germany), ŠKODA Praha a.s., ENERGOPROJEKT Praha a.s., Russian organizations representing the original designer and constructor under the umbrella of ATOMENERGOEXPORT (Atomenergoprojekt, Kurchatov Institute, Moscow, Zarubezhatomenergostroj, VNIIAES Moscow, SNIIP Systematom, Tenzor Dubna), and VÚJE a.s.

SE-EMO has kept the responsibility for 7 safety-related measures which are dealt with by own staff or in cooperation with other organizations. As an example, a joint team of SE-EMO experts and those of the Brussels branch of WESTINGHOUSE developed new guides for the management of emergency situations. The safety measures focus on the organization of operations (area "Operation" according to "Safety Issues and their Ranking for NPP WWER 440/213").

An implementation model has been developed for the various safety measures; the model respects the original guarantees for the NPP project and its realization. From the nuclear safety viewpoint, this means keeping and upgrading of the original safety concept. The CM implementation was split into the following stages:

    1. Study and Analytical Phase
    2. The individual safety-related issues were analyzed by the investigators and concepts of technical solutions (so-called "basic design") have been suggested if there was a need to modify the NPP design at this stage. The results of the analyses and the suggested basic designs were reviewed by all organizations involved, including VÚJE, and were discussed with ÚJD on a continuous basis.

    3. Project Development Phase
    4. As soon as the basic solution had been approved, the change in NPP design was accepted based on the corresponding QA programs and the Decree No. 105/81. The "amendment procedure" was instituted, and the general designer developed addendums to the initial project. The task of the general designer (EGP) was to coordinate the technical solution with the original concept of NPP Mochovce, including links to other technological systems (CMS, electrical systems) and implications for the construction part. Addendums to the initial design were submitted to ÚJD for approval. Construction plans were then prepared based on the approved addendums, to the initial project serving the implementation of adjustments to the existing technological systems and buildings.

    5. Implementation of Safety Measures
    6. Safety measures resulting in adjustments of technological systems or buildings (i.e. with "hardware" implications) were implemented by the general contractor of the construction and/or technological part and final suppliers. Results of safety measures without "hardware" implications have been documented in the safety report and/or additional supporting documentation.

      Of the total amount of safety measures, 70% were implemented prior to the first unit being put into operation, category III and II safety measures (which all were implemented) being taken as top priority. The remaining safety measures will expectedly be implemented before the second unit is put into operation in 1999.

      Seismic Upgrading

      The original project of NPP Mochovce was developed based on the knowledge of seismic risks of the site acquired during the preparation and design of the construction of NPP Mochovce in the 80s, accounting for MSK scale grade VI at which a safe shut-down of the reactor has to be secured upon an earthquake with a horizontal acceleration value of PGA = 0.06 g. The legislative development represented by the IAEA Safety Guide 50-SG-D15 has recommended for nuclear power plants a minimum acceleration value in horizontal direction of 0.1 g.

      Owing to the above, "Selected Buildings and Technological Systems" have bee reassessed from seismic aspects, and improvements of buildings are gradually implemented by setting supports to the beams of existing walls, reinforcing light walls with steel profiles and wire mesh; new pillars to were erected reinforce floors in some buildings. The upgrading of the seismic behavior of the technological equipment mainly consists in reinforcing their anchoring and in reinforcing the tanks.

       

    7. Pre-Operation Safety Report
    8. The operation of NPP Mochovce 1 was licensed based on the pre-operation safety report prepared in accordance with internationally recognized standards. The overall concept has been based on US NRC RG 1.70; for accident analysis the IAEA document "Guidelines for Accident Analysis for WWER Nuclear Power Plants" has been used in this report, taking into account the applicable Slovak legislation. ÚJD approved, based on positive position on POSR and the checks performed by ÚJD´s inspection teams, and in accordance with the C SKAE Decree No. 6/1980 on nuclear safety upon start-up and operation of nuclear energy facilities, the starting of the physical and power start-up. At the same time, POSR represents the basic background document for the issuance of ÚJD´s approval of the commissioning protocol (commercial operation licence).


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