Slovenské elektrárne (SE) is a major supplier of electricity in Slovakia, generating 85% of domestically produced electricity. Besides thermal and water power plants, SE currently operates two nuclear power plants – Bohunice (4xVVER-440) and Mochovce (2xVVER-440) – which account for over a half of its generation capacity. The first of the Mochovce units commenced operation in 1998, and the second unit in 1999. Nuclear fuel for all the units in Slovakia is supplied by the Russian Federation.

The safe handling of radioactive wastes (from plant operations and the Bohunice A1 decommissioning project), including the long-term storage of spent nuclear fuel (SNF), is a major concern for SE. In 1996 a subsidiary of SE, the Nuclear Installations Decommissioning, Radwaste and Spent Fuel Management (SE-VYZ), was established to handle the decommissioning of nuclear installations, treatment of radioactive wastes and storage of spent nuclear fuel in Slovakia.

Transport interrupted

It was originally assumed that SNF from VVER-440 units would be transported back to the USSR after three years storage in spent fuel ponds. Later, the Russians changed the cooling period to 10 years. As a result, the interim storage facility at Bohunice was designed and constructed for SE.

In February 1984, the USSR proposed the transportation of Czechoslovakian SNF back to the USSR for reprocessing. Due to these new circumstances the Czechoslovakian government cancelled the construction of similar interim storage facilities at Dukovany (now in the Czech Republic) and Mochovce. The decision was such that all SNF from all Czechoslovakian reactors would be stored in the Bohunice interim storage facility and, after a five-year cooling period it would be transported to the USSR for reprocessing.

This decision meant that SNF was to be transported from Dukovany to the Bohunice interim storage facility. A total of 1176 fuel assemblies were transported during 1989-1992. The transport of SNF to the USSR for reprocessing was not accomplished because of political and economical changes in Central and Eastern Europe at the beginning of 1990s.

The return of SNF back to Dukovany started in 1995 and was finished in 1997.

Bohunice store

The interim storage facility (ISF) at Bohunice is a wet store, where SNF is stored in three storage pools and a fourth storage pool is kept empty for emergency use.

The designed storage capacity of Bohunice ISF was 600t of uranium – 5040 fuel assemblies. SNF is stored in the special storage baskets of T-12 type with 30 fuel assemblies in each. Failed fuel assemblies, which are isolated in special hermetic canisters, are stored in T-13 storage baskets. Fifty six storage baskets, or 1680 fuel assemblies, can be stored in one pool.

The original design enables capacity enlargement by an additional building of 2-3 storage pools.

The ISF is equipped with high capacity pool water-cooling and clean-up system, as well as an air conditioning system with continuous monitoring of aerosols in off-gases. The capacity of the air conditioning system is 127,000m3/h.

Loading of SNF into the ISF began in 1987. Spent nuclear fuel is transported from each unit by using three C-30 transport containers and special railway carriages.

Transport of Czech fuel assemblies back to the newly constructed dry store at Dukovany began in 1995.

There had been 11 transports of spent fuel from Dukovany to the Bohunice ISF using TK-6 and TK C-30 transport containers, and 14 transports from Bohunice ISF back to Dukovany using transport containers TK C-30 and Castor 84/440.

At present there are only on-site transports from the Bohunice reactor units to the ISF. Each transport is evaluated in a separate report that is sent to the regulatory authorities. Specified tests of selected equipment as well as tests of railways carriages are carried out regularly.

ISF reconstructed

At the end of 1993 and during 1994 the international tender for the supply of a long-term store (or possibly two stores) for SNF from Bohunice and Mochovce was carried out.

The management of SE decided to cancel this tender in 1994 and look into the possibility of storage capacity enlargement of the existing Bohunice ISF. The decision was taken to upgrade the existing ISF, when it was shown to be technically possible and cheaper. Three main goals were set:

• Seismic upgrading of the ISF – upgrade of building and technological systems to level 8 on the Richter scale.

• Increase the storage capacity to cope with all fuel from all the Bohunice units.

• Prolong the lifetime of the ISF for 40 years from the end of the upgrade work.

Seismic upgrading

Because of worldwide changes in the approach towards seismic safety of nuclear facilities, SE decided to re-evaluate the seismic risk of its older nuclear facilities.

The original Russian design of the ISF – according to which this facility had been constructed – took into account seismic resistance in the store up to 6 on the Richter scale. The geographic position of the locality of Bohunice is in the zone with possible earthquake with intensity of 7-8 on the Richter scale.

Because the former design did not fulfil these requirements it was decided to seismically upgrade building and technology of the interim storage facility to the appropriate level.

Capacity extension

The need for the higher capacity of the ISF resulted from the amount of existing stored fuel assemblies, as well as from calculated amounts of spent fuel assemblies produced in the future operation of four Bohunice units.

Storage capacity enlargement has not been solved by the construction of additional storage pools but by so-called “compactization” – replacement of the old T-12 storage baskets by new, compact KZ-48 storage baskets – together with necessary adjustment of the technological and safety systems.

The old type of the storage basket allows storage of 30 fuel assemblies. One storage pool can store 56 of these baskets. The newly designed KZ-48 storage basket allows for the storage of 48 VVER-440 fuel assemblies where enrichment is below 4.4%. The average burnup of the fuel assemblies in a storage basket must be less than 42MWd/kgU and maximum burnup of a fuel assembly can be 52.5MWd/kgU. The shape of this

basket enables closer storage of these baskets in the storage pool in such a way that there can be as many as 94 KZ-48 baskets in one storage pool.

In this way the previous ISF capacity of 5040 fuel assemblies could be increased to 14,112. This increase in capacity also leads to an increase in the heat generation from a maximum level of 516kW to 1990kW. The increase of the heat generation is proportional to the number of the stored fuel assemblies. This requires higher performance of the cooling system, which must be fitted with new plate heat exchangers and pumps. The autonomous cooling water distribution system with its own cooling towers has been constructed in order to ensure independence in the supply of water from the Bohunice distribution system.

Apart from the modifications of the original building design and technology of the ISF, which resulted in requirements for seismic upgrading and storage capacity enlargement there have been also other changes and adjustments, which have significantly increased the technical standard of the ISF.

• Installation of MAAP-400 manipulator for fuel handling from T-12 baskets to KZ-48 baskets.

• Constructional adjustments of radiation zone entry at elevation 0.00m.

• Construction of the new radiation zone entry for visitors at elevation of 3.60m.

• Structural adjustments to the

entry hall.

• Change of position of the pressurised air and nitrogen reduction stations together with piping adjustments.

• Air conditioning system adjustment.

• Renewal of the filtration system of pool water.

• Modernisation of the decontamination tank for small particles.

• Supplementation of the control system for tightness control of fuel assemblies and corrosion monitoring of storage pools.

• Modernisation of radiation control systems.

• Aerosol detection system has been supplemented with detection in more rooms.

• Electricity supply to the ISF is from Bohunice V1 by two independent 6kV power distributors. Diesel generator has been installed for the event of total black-out of system voltage.

• Control system SIMATIC S5 is hierarchic, with a two-level control system with adequate communication software for the operator. The Siemens ETHERNET SINEC L1 allows communication between both levels.

Lifetime extension

The condition of the fuel assemblies, construction and technological parts all influence the overall lifetime of the ISF. To assess the present conditions the operator must know the initial condition, present condition and be able to predict the future development of all components and systems of this nuclear facility.

For monitoring the condition of building and equipment there are systems that will be used for:

• Evaluation of stability and lifetime of the building and technologies.

• Evaluation of the condition of the fuel assemblies.

• Evaluation of safe and reliable operation of the ISF.

The upgrading process is still continuing. Seismic upgrading of the building, steel construction parts and technological systems was finished in 1999. Adjustments of the storage pools and transport systems required for higher storage capacity were also finished in 1999. The exchange of the storage baskets will be sequential. Supply of 210 KZ-48 storage baskets is contracted through 2007. The maximum number of 294 KZ-48s can be reached later. Currently fuel assemblies from old T-12 storage baskets are being transferred to new KZ-48 baskets. Disposal of used T-12 storage baskets began in 2000.

The upgraded pool water cooling system is now fully operational.

After completion of all ISF upgrading work, SE will have enough capacity for storage of 14,112 fuel assemblies (approximately 1700tU) for approximately 50 years.