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:

• annealing of reactor pressure vessels of both units,
• validity verification of the LBB (Leak Before Break) method - seismic upgrading of the equipment,
• improved sealing of pressurized spaces,
• installation of quick-closing armatures at the steam generator pipelines,
• installation of diagnostic systems at the primary and secondary circuit equipment,
• reconstruction of the containment spray system delivery routes,
• interconnection of unit emergency feeding of the primary circuit,
• reconstruction of the pressurizer relief valves,
• installation of emergency control room panels,
• reconstruction of the unit own consumption electric feeder system including the installation of an additional diesel generator and an accumulator battery,
• replacement of the fire signaling system,
• extension of the stable fire extinguisher system,
• fire resistance upgrade - replacement of cables for fire-proof ones, fire-proof coats,
• redrafting of operating regulations for emergency conditions,
• development of level 1 probabilistic safety assessment study,

Based on the safety report developed on the "Small Reconstruction", ÚJD 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:

• probability of safety systems failure must be below 10^-3/year,
• probability of reactor core damage must be below 10^-4/year,
• probability of reactor protection systems failure must be below 10^-5/year.

• design basis accident (LOCA 2 x F 200 mm) must be coped with using conventional approach, and non-design basis accident (LOCA 2 x F 500 mm) using best-estimate method,
• the confinement must be sealed tightly so as to secure that dose equivalents be not exceeded even upon design basis accident or non-design basis accident (whole-body: 50 mSv, thyroid gland: 500 mSv for LOCA 2 x F 200 mm, or whole body: 250 mSv and thyroid gland: 1500 mSv for BDBA LOCA 2 x F 500 mm),
• the facilities are to be seismically upgraded to 8^o MSK-64.

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:

• primary circuit integrity upgrade,
• confinement integrity improvement and installation of separation valves at the confinement interface (the improvement brought about so far is evident from Fig. 2.1.3),
• strength improvement of the burst can structures to withstand maximum overpressure during LOCA 2 x F 500 mm,
• measures taken on emergency high-pressure feeding systems and shielding spray systems,
• measures taken to improve the reliability of super-emergency coolant injection systems,
• installation of a new process water system,
• control and monitoring system upgrade,
• installation of a hydrogen detection system and a hydrogen combustion system,
• implementation of safety improvements of electric systems - installation of new motor generators, diesel generators,
• significant seismic upgrade of key safety equipment,
• implementation of measures in the fire safety area.

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.