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After its operation in the reactor core, spent nuclear fuel is unloaded to the reactor cooling pools to be stored for 4 to 5 years to decrease residual energy release.
Residual energy release is a process induced by radioactive decay of fission products.
After cooling in the reactor pools, spent fuel is loaded into special containers that ensure its safety in transportation and is sent to a spent fuel storage facility.
The current state of science and technology does not permit the final conclusions on further spent fuel management. Hence, there are several approaches to spent fuel management in the world:
1. Processing. There are two types of processing – local or in other countries:
local processing provides for spent fuel processing to obtain components and substances whose use is economically sound (Great Britain, India, Russia, France, Japan);
processing in other countries provides for processing of spent fuel and return of high-level waste to the owner country (Bulgaria, the Netherlands, Switzerland);
2. Disposal is intended for spent fuel cooling and burial in deep geological formations (USA, Finland, Sweden).
3. Deferred decision is intended for long-term spent fuel storage that permits a decision on subsequent spent fuel management taking into account future technologies and economic factors. The deferred decision is used by Argentina, Denmark, Spain, Canada, Lithuania, Germany, Norway, South Korea, Poland, Slovakia, Hungary, Czech Republic, and Croatia.
According to design decisions for WWER-1000 NPPs (there are 13 operating units of WWER-1000 type in Ukraine), spent fuel was to be transported to a stationary storage facility to Russia.
However, it became evident, even in the former USSR, that the storage facility had limited capacities, could not be expanded, a spent fuel processing facility could not be constructed in the immediate future and that would significantly affect NPP performance.
Estimates showed that the Zaporizhzhya NPP would be under the most pressing conditions requiring shutdown of the units.
In this connection, the USSR Ministry of Energy issued Order No. 361 of 6 October 1988 to approve the design for the second stage of the Zaporizhzhya NPP, including a spent fuel storage facility.
After the USSR broke up, spent fuel ceased to be transported to Russia in 1993-1995. The Zaporizhzhya NPP analyzed the dynamics of filling the cooling ponds and started a search for alternatives for spent fuel storage in 1993.
In view of the economic part, possibility to purchase components from Ukrainian producers, minimize modernization efforts at power units and use available handling equipment, the project proposed by the US Duke Engineering & Services Inc. (DES), which had obtained a license of the US NRC at that time, was chosen.
In 1996, the Zaporizhzhya NPP started implementing the spent fuel dry storage project.
The project is based on dry ventilated storage casks (VSC) for vertical storage of spent fuel assemblies (SFA). Dry storage is efficient since SFAs are stored in the cooling pool for no less than 5 years where their residual energy release and radioactivity greatly decrease. This fuel can safely be stored on-site in dry VSCs that effectively remove heat from SFAs and ensure adequate shielding from radiation for personnel, the public and the environment.
Prior to commissioning of the dry spent fuel storage facility (DSFSF), the working design, DSFSF Safety Analysis Report and DSFSF Environmental Impact Assessment were developed and reviewed, ecological reviews were carried out and pre-commissioning tests were performed at Zaporizhzhya-1–6.
Following an analysis of pre-commissioning tests and documentation submitted by the Zaporizhzhya NPP, the NAEK Energoatom was granted a license for trial commercial operation of the nuclear installation on 16 July 2001.
Zaporizhzhya DSFSF site. First containers
On 24 August 2001, the first WWER VSC was placed on the DSFSF site. Since then the trial commercial operation of the DSFSF has stated. On 10 September 2004, the Zaporizhzhya NPP obtained a license for operation of the Zaporizhzhya NPP nuclear installation including the DSFSF.
It should individually be noted that the Zaporizhzhya NPP developed more than 270 documents within the licensing procedure. More than 50 reviews and assessments were conducted.
The spent fuel dry storage system used at the Zaporizhzhya NPP is conditionally divided into three areas:
The loading area is intended for the safe loading of SFAs into a basket, handling operations in sealing, drainage, vacuum drying and filling of a multi-place sealed basket with helium and loading of the basket into a ventilated concrete cask. The loading area is located directly in reactor compartments.
Available handling equipment is used for DSFSF components.
The transportation area is a network of paths for WWER VCS transportation to the storage area with a storage cask transporter.
The storage area is intended for safe storage of WWER VSCs for no less than 50 years. The storage area includes the storage site formed with a reinforced concrete plate for WWER VSCs. The storage area has its own physical protection fence.
The dry spent fuel storage system is designed for 380 ventilated storage casks that can hold more than 9000 SFAs.
The storage site can house spent fuel for the entire operational period of the Zaporizhzhya NPP. The license permits storage of spent fuel only from the Zaporizhzhya NPP.
The main components of the dry spent fuel storage system are:
multi-assembly sealed basket (MSB);
transfer cask (TC);
ventilated concrete cask (VCC);
ventilated storage cask (WWER-VSC), including SFA in MSB placed into VCC.
To ensure safe operation of the dry spent fuel storage system, the ventilated storage casks, equipment used and buildings and structures of the storage system are continuously monitored.
Regular radiation monitoring is conducted in compliance with regulatory and production documentation at all handling and transportation stages, beginning from spent fuel transfer for storage at the DSFSF and to storage on the DSFSF site.
The WWER VSCs are placed for storage on the DSFSF site under individual SNRCU permits. Each cask is placed so as to minimize the dose rate at the site boundaries and to minimize radiation on construction personnel during installation and construction. After the shielding structure outside the site perimeter to ensure radiation protection of personnel, the public and the environment had been completed in December 2005, it was no longer needed to calculate the location of WWER VSCs on the DSFSF site.
The gamma dose rate at reference points at a distance of 50 meters from the site external fence is 0.11-0.12 µSv/h (11-12 µR/h), which corresponds to background radiation.
Radiation monitoring of samples of well water, atmospheric precipitations and atmospheric air for the entire operational period shows that the content of radionuclides in the DSFSF area corresponds to the background and global contamination level.
The radiation state around the casks is generally stable. Since spent fuel loaded into the casks has different characteristics, the total dose rate and neutron radiation from the center of inlet ventilation ducts varies from 13.4 to 155.9 µSv/g in casks. The neutron dose rate from the VSC lateral surface is detected at no more than one meter. The absence of radioactive contamination, inert gases and aerosols confirms that the casks are hermetically sealed.
Radiation monitoring on the DSFSF site established that maximally permissible values were not exceeded.
Dose analysis, which was carried out after WWER VSCs were loaded and stored on site, showed that administrative process levels of external (15 mSv/y) and internal (3700 Bq/y) exposure were not exceeded.
Visual examination and inspection of the WWER VSC outer surface showed that there were no inadmissible defects in concrete for the entire DSFSF operation.
Plugging of inlet and outlet WWER VSC ventilation channels was not observed during the entire DSFSF operational period.
Indirect parameters, such as the difference between the air temperature at the WWER VSC ventilation outlets and ambient temperature, are used to monitor the state of fuel stored at the DSFSF.
Temperature monitoring over the entire DSFSF operational period has shown that the maximum difference between the air temperature at the ventilation outlets and ambient temperature was 59 °C for WWER VSC No. 73, which is lower than normal operation limit 61°C justified in SAR.
Spent fuel of the Rivne, Khmelnitsky and South Ukraine NPPs is shipped to Russia. WWER-1000 spent fuel is sent for storage and WWER-440 spent fuel (Rivne-1, 2) for processing.
1. The State Nuclear Regulatory Committee of Ukraine. Nuclear and Radiation Safety in Ukraine. Annual Report 2009/