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

4.5.4 ÚJD Requirements with Respect to NPP Bohunice WWER 440/V-230 Units Safety Upgrade

The safety upgrading program for V-230 type reactors was first started in 1985 based on recommendations from the former Soviet Union. Main attention was paid to embrittlement of reactor pressure vessel and to seismic upgrading.

Based on a review of the nuclear safety of reactors V-230, Czechoslovak Atomic Energy Commission (CSKAE), the predecessor of ÚJD, by CSKAE Resolution No. 5/1991, defined measures to upgrade the safety (known as "Small Reconstruction"), in response to the safety shortcomings identified. The "Small Reconstruction" was completed in 1993. It focused on the most urgent safety aspects, in particular on the tightness of the confinement, seismic upgrading, reactor vessel integrity, emergency sources of electric feeding, reliability of steam generators feed water systems, the application of "leak before beak" principle, on the reduction of fire risk and the capacity of the fire extinguishing equipment, etc. The implementation of the mentioned safety-related measures was a condition for the licensing of further operation through the year 1995.

Being established in 1993, ÚJD took additional measures to upgrade V-230 reactor safety. Based on the inception safety report for so-called "Gradual Reconstruction of NPP V-1", ÚJD issued Resolutions Nos. 1/94 and 110/94 in 1994, which regulated the conditions for further operation of NPP V-1 after 1995 until the end of the designed life cycle. ÚJD Resolution No. 1/94 contained additional 59 safety measures to be gradually implemented within 1996-1999.

ÚJD Resolution 1/1994 had five topics-based parts. In each of them, ÚJD prescribed measures to upgrade safety level; any further operation of the power plant would be conditional on the implementation of those measures.

Part one took measures to maintain primary circuit integrity including analyses of the integrity of reactor pressure vessel. Part two contained measures focusing on a safety improvement of the reactor core cooling systems during the operation and accidents. Part three, Reactor Core Cooling during LOCA, set forth conditions for an improved core cooling to manage a new level of design basic accident, i.e. when a primary circuit pipe of the diameter of 200 mm bursts (the original design basic accident only considered the bursting of a primary circuit pipe of a diameter equivalent to 32 mm). Part four contained measures to improve the safety level of confinement , in particular by improving its tightness and demonstrating integrity upon LOCA. Part five focused on an improvement of the safety level of auxiliary systems of the nuclear power plant such as process water, electric feeding, and control and monitoring system.

Resolution No. 1/1994 was also based on recommendations for safety level upgrading contained in IAEA documents developed as a part of the TECDOC 640 document: Ranking of Safety Issues for WWER 440/230 Reactors. Because of the extent of the project, the deadlines for the measures to be implemented have been spread over the period of four years. ÚJD Resolution No.110/1994 complemented CSKAE Resolution No. 5/1991 and ÚJD Resolution No. 1/1994, by defining conditions to be met by operator to obtain license for further operation of the respective unit. The license validity has been restricted to one year.

In assessing the safety level of nuclear power plants, results of probabilistic safety analyses (PSA) have been used since 1995; they are also used to assess the benefits of suggested safety improvements.

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