On two consecutive days – 21st and 22nd December 2023, State Enterprise Radioactive Waste successfully carried out a complex pilot operation of removal from Unit 3 SG compartment and transfer of the first of a total of 24 Steam Generators* from the 440-megawatt units of the nuclear power plant that are currently under decommissioning.
The apparatus weighs 156 tonnes and is 11.5 m long and about 4.5 m in diameter. It was raised to a height of 12 m and transferred through the Reactor Hall and thento the Turbine Hall (TH). The operation was led by the Chief Technologist of Specialized Division Decommissioning of Units 1 to 4, Borislav Iliev. He spoke about the challenges in its implementation: “The opening, through which the Steam Generator had to pass, is 9×5 metres in size. This means that given the apparatus overall dimensions, it had to be removed at an angle of about 45 degrees. It was therefore important, from an engineering point of view, to calculate the length of the slings extremely precisely. This was done by the Engineering Support Division’s team within the Specialized Division. There are a total of three openings in the Steam Generator compartment through which we are about to remove the remaining five Steam Generators. This means that some of them have to be moved within the compartment itself, which is also a challenge, but we have the right equipment delivered and I believe we will be successful.”
Following a rigorous dosimetric check, the Steam Generator is now laid down in a buffer storage area within the Turbine Hall. A permit to operate the storage area and store 12 Steam Generators has been issued by the Nuclear Regulatory Agency. By the end of the next year, Units 3&4 SG compartments must be completely emptied. Fragmentation of the SGs will begin in 2026. In the course of these two years, a Dry-cutting Workshop for dry fragmentation is to be built through financing by the Bulgarian National Funds. The equipment under the project will be financed by the Kozloduy International Decommissioning Support Fund (KIDSF) through the European Bank for Reconstruction and Development (EBRD).
The Steam Generator relocation activities started from Unit 3 for two reasons. Firstly, this is where a full-system chemical decontamination of the Primary Circuit via ‘Spider’ installation was completed earliest in time. Secondly, according to the Decommissioning Programme, it is planned that by 2030, the Steam Generator compartment of this unit be reconstructed into a storage facility for interim storage of large-tonnage containers (80 tonnes). They will be used to store fragmented components from the reactor internals and from the reactors themselves, with a single container being capable of holding up to 20 tonnes of materials segregated during dismantling. They are expected to remain there for storage for the next 50 yearsuntil a deep geological repository is built or until activities have fallen enough to meet the criteria for disposal in the National Disposal Facility for Radioactive Waste.
* The Steam Generator is a major component of the NPP electricity generating scheme. It is a heat exchanger for generating water steam of superatmospheric pressure at the expense of the heat of the primary coolant coming from the nuclear reactor. In the 440-megawatt units of the nuclear power plant, there are six Steam Generators for each reactor.