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

2.1.3 Bohunice V-1 Units Safety Improvement Programs


Numerous improvements and modifications of the equipment have been implemented since the very beginning of the operation of the units, aimed at improving the safe operation (more than 1,200 technical adjustments). Safety improvements have become a special focus since 1995 when the first comprehensive program was developed predominantly oriented towards reducing the risk of brittle fracture of the reactor pressure vessel (installation of shielding assemblies around the core, low-neutron-leakage fuel management, measures to reduce thermal stress of the reactor pressure vessel during operational transients, etc.), and increasing of fire safety.

"Special V-1 NPP Regime" was introduced as of April 1, 1989, based on the recommendations of nuclear authorities of countries where WWER-440/230 reactors are operated. This regime reduced to a minimum performance changes of the reactor and required securing of the unit operation at a constant nominal output. Criteria for the exclusion of suspect assemblies were tightened. The originally four-year control cycle of selected equipment was shortened to two years. Moreover, documentation quality criteria, staff training, modifications and methods of repairs of equipment, testing programs, etc. were added.

A safety improvement program, so-called "Small Reconstruction" was performed on both units within 1991-1993, contained in 81 points of the Resolution No. 5/91 issued by the former Czechoslovak Atomic Energy Commission; it included:

Based on the safety report developed on the "Small Reconstruction", JD set, in its Resolutions No. 1/1994 and No. 110/1994 the areas and the procedures for taking and implementing measures enabling further operation of V-1 units - so-called "Gradual Reconstruction" program implemented within 1994 - 1999. The V-1 units safety improvement program has been based on both probabilistic and deterministic objectives:

probabilistic:

deterministic:

 

The V-1 units safety upgrading process shall be implemented gradually during extended outages for refueling and general overhauls within 1996-1999 (this is why it is called "Gradual Reconstruction"), and shall include, among others, the following safety improvements:

Fig. 2.1.2 shows results of PSA studies for the individual safety improvement stages of the initial project - IP of V-1 units. After gradual reconstruction including the implementation of symptom-oriented emergency instructions (SOEP) , the CDF value is 5.39 E-05.

Fig. 2.1.2 Results of the probabilistic assessment of Bohunice V-1 units, and reconstruction objectives

 

Fig. 2.1.3 Results of the Bohunice V-1 units burst can tightness improvement

Certain projects have a general effect on the entire Bohunice site, as evident from the following implemented projects:

Fig. 2.1.4 Diagnostic systems in place at the NPP Bohunice

 

Fig.2.1.5 Teledosimetric system in place at the NPP Bohunice

The objective of the implementation of all the measures under the Gradual Reconstruction Program by the REKON syndicate (a syndicate of the company Siemens and VJE Trnava, a.s.) is to achieve an internationally acceptable safety level with respect to Bohunice V-1 units and to create technical conditions for their operation throughout their planned life cycle.


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