Annex 4 - Spent Fuel Storage Technologies in Canada
4.1 Wet storage technology
Spent fuel discharged from a nuclear reactor is stored initially in wet bays or water pools. The wet bays, together with the cooling and purification systems, provide containment of the spent fuel and associated radioactivity and provide good heat transfer to control fuel temperatures. The water also provides shielding and allows access to the fuel, via remotely operated and automated systems, for handling and examination. The bay structure and structural elements (such as fuel containers and stacking frames) provide mechanical protection.
The walls and floor of CANDU reactor water pools are constructed of carbon steel reinforced concrete that is approximately two metres thick. Inner walls and floors are lined with a watertight liner, consisting of stainless steel, a fiberglass reinforced epoxy compound or a combination of the two. The bay structure is seismically qualified, so that the structures and bay components maintain their structural form and support function both during and following a design basis event. Other structural design considerations include load factors and load combinations (including thermal loads) for which upper and lower temperature limits have been established.
4.1.1 Bay liners
The bays are designed to prevent bay water leaking through any possible defects in the concrete into the environment. The bay's inner liner is the primary barrier against outward leakage. The bays also have a leakage collection system to ensure that any leakage that does occur is captured and conducted to a controlled drainage system. The design has provisions for leak detection and tracing.
4.1.2 Storage in wet bays
A number of designs are used to hold spent fuel for storage in wet bays. OPG has a standardized site-specific, storage-transportation module that stores the fuel compactly. To reduce handling, the storage-transportation module is also suitable to hold the fuel during transportation. Baskets, trays and modules are stacked vertically in the bays using seismically qualified stacking frames.
4.1.3 Water pool chemical control
In all storage bays, water is circulated through cooling and purification circuits. A combination of ion exchange columns, filters and surface skimmers is used to control water purity within design limits. A typical purification system also includes resin traps, sample points and instrumentation to indicate when filters and ion exchange columns are exhausted, as well as when resin traps must be cleaned out. Water-pool chemical control has the following objectives:
- minimize corrosion of metal surfaces,
- minimize the level of radioisotopes in the water, and reduce radiation fields and radioiodine levels in the bay area, and
- maintain clarity of the bay water for ease of bay operation.
To ensure purity, de-mineralized water is used.
4.2 Experiences with wet storage
Early operating experiences at both the AECL research reactor spent fuel bays (which have been in operation since 1947) and at the NPD and Douglas Point reactors have provided a basis for the successful operation of the spent fuel bays in this current generation of power reactors. Those experiences, along with the development of high-density storage containers, inter-bay fuel transfers and remote handling mechanisms, have contributed to the establishment of current safe storage techniques.
Good chemical control has been achieved in Canadian spent fuel bays. Radioactivity in the water has been kept to very low or non-detectable levels, resulting in low radiation levels in the bay area. Overall fuel bundle defect rates are low. During early operations, defective fuel was canned (e.g., stored in a sealed cylinder). With more operating experience, canning has been found to be generally unnecessary, due to minimal release of fission products from most defective bundles. In some cases, known defective fuel is held temporarily in the fuel handling system before being passed to the bay. Known defective fuel is generally stored in a designated part of the fuel bay.
As noted above, an epoxy polymer liner is in place at a number of the stations. With extended operating lifetimes and continual exposure to radiation, there has been some radiation-induced deterioration of the liner at the Pickering Nuclear Generating Station-A (Pickering NGS-A) Primary Bay (where the first epoxy liner was used).
Locating and repairing the potential leaks were included when Pickering NGS-A was returned to service after an extended shutdown. Techniques have been developed for underwater repairing by using an underwater-curing epoxy. Extensive repairs were completed in 2002/2003 at various locations in the Pickering NGS-A Primary Bay.
4.3 Dry storage technology
There are currently three basic designs used for the dry storage of spent fuel in Canada:
- AECL Concrete Canister,
- AECL Modular Air-Cooled Storage System (MACSTOR ), and
- OPG Dry Storage Container.
4.3.1 AECL Concrete Canisters
The AECL Concrete Canister Fuel Storage Program was developed at the Whiteshell Laboratories in the early 1970s to demonstrate that dry storage for irradiated reactor fuel was a feasible alternative to water pool storage. Owing to the success of the demonstration program, concrete canisters were used to store Whiteshell Reactor-1 used fuel. Thanks to the success of the AECL Concrete Canister Fuel Storage Program, the AECL concrete canister design was used at the CRL, the Point Lepreau Generating Station and the partially decommissioned Douglas Point and Gentilly-1 Nuclear Generating Stations.
The main components of the canister system are:
- the fuel basket,
- the shielded workstation,
- the transfer flask, and
- the concrete canister itself.
The fuel basket is constructed of stainless steel, and comes in two sizes. One can hold 54 bundles (used for fuel from Douglas Point, Gentilly-1 and Nuclear Power Development) and one can hold 60 bundles (in use at Point Lepreau). The fuel basket is designed to provide storage for spent fuel that has been in wet storage for six years or more, and consists of two assemblies: the basket and the basket cover.
A shielded workstation is equipped to dry a loaded fuel basket and to weld the basket cover to the basket base plate and central post assembly. It is composed of a number of subassemblies used for lifting, washing, drying, seal welding and inspecting the spent fuel baskets. The shielding provided by the workstation is sufficient to reduce the radiation fields and ensure the safety of workers.
The fuel basket transfer flask is used to shield the basket when it is moved from the shielded workstation at the generating station to the dry storage canister at the waste management facility.
The concrete canister is a cylindrical reinforced concrete shell with an internal liner. To provide additional shielding, a two-piece loading plug is used until the canister is filled. Provision is made for IAEA safeguard seals to be placed on top of the canister plug, so that it cannot be removed without breaking the seals.
Two small diameter pipes allow the air between the liner and the fuel baskets to be monitored in order to confirm the integrity of the confinement barriers. The concrete canisters are supported on reinforced concrete foundations above the water table. Each canister holds 6, 8, 9 or 10 baskets, depending on the specific needs of the station.
The transfer of spent fuel from the storage bays to dry storage canisters always begins with the oldest fuel first. Therefore, the nominal age of the spent fuel in dry storage is usually older than seven years, which adds a measure of conservatism to the assumptions and overall safety of the dry storage of irradiated fuel.
Three barriers (defence-in-depth) ensure the containment of the radioactive products:
- the fuel sheath,
- the fuel basket, and
- the internal liner.
4.3.2 AECL MACSTOR™ module
The AECL MACSTOR module is a variant of the canister storage technique. Currently, it is only being used in Canada at the Hydro-Québec Gentilly-2 Used Fuel Dry Storage Facility. Seven modules have been constructed since 1995.
A typical MACSTOR module, such as the one used in Gentilly-2, is 8.1 metres wide, 21.6 metres long and 7.5 metres high. It stores 20 watertight galvanized carbon steel cylinders, arranged vertically in two rows of 10. Each cylinder holds 10 baskets of 60 spent fuel bundles, for a total of 12,000 bundles per module. Each cylinder is secured to the top slab of the module, and two sampling pipes, which extend to the outside of the MACSTOR module, are provided at its base. These pipes allow confirmation of the integrity of confinement.
The heat of the spent fuel is dissipated primarily by natural convection, through ventilation ports that extend through the concrete walls. The ventilation is provided by 10 large air inlets in each longitudinal wall near the base of the module (five on each side), and by 12 large air outlets located slightly below the top of the module (six on each side). The air inlets and outlets are arranged in a series of baffles to avoid direct gamma radiation.
To enhance cooling, the storage cylinders of the MACSTOR module are in direct contact with the air circulating in the module. All the surfaces of the storage cylinders are hot galvanized to protect the storage cylinders from ambient air.
The loading operations for the MACSTOR module are identical to those of the concrete canister. Both use the fuel basket, shielded workstation and transfer flask concept. The only essential difference between the two is the storage structure itself.
4.3.3 Ontario Power Generation dry storage containers
OPG currently operates three spent fuel dry storage facilities - at the Pickering Waste Management Facility (PWMF), the Western Waste Management Facility (WWMF) and the Darlington Waste Management Facility.
OPG dry storage facilities employ standard dual-purpose dry storage containers. These are massive, transportable containers, with an inner cavity for fuel containment. Each one is designed to hold 384 fuel bundles, and weighs approximately 60 tonnes when empty and 70 tonnes when loaded.
The containers are rectangular in design, with walls of reinforced concrete sandwiched between interior and exterior shells made of carbon steel. The inner liner constitutes the containment boundary, while the outer liner is intended to enhance structural integrity and facilitate decontamination of the surface of the dry storage container. Helium is used as a cover gas in the dry storage container cavity, to protect the fuel bundles from potential oxidation. OPG dry storage facilities are indoor, while the AECL storage concepts are outdoor. For both, there are no anticipated radiological releases under normal operating conditions.
4.4 Experiences with dry storage
Research programs have assessed the behaviour of spent fuel when stored in dry and moist air conditions, and in a helium environment. The programs concluded that CANDU fuel bundles, whether intact or with defects, could be stored in dry storage conditions for up to 100 years or more without losing integrity. Additional research is ongoing.
The experience achieved at licensed dry storage facilities, which have been in operation for several years, provides a high level of confidence that CANDU dry storage facilities can be operated safely and without undue risk to workers, the general public and the environment. Dry storage containers have been used successfully and safely at the PWMF since 1996. The safety performance of the facility has been excellent over the entire period. Dose rates have remained below regulatory limits. Collective occupational radiation exposures have been less than predicted, by 30 percent or more. Emissions from the processing area have remained below regulatory limits. The PWMF operates contamination-free, and there have been no effluent releases resulting from dry storage.
Thermal and shielding analyses, carried out for design and safety assessment purposes, have been found to be conservative. Analysis and measurements carried out at the PWMF indicate that the maximum fuel cladding temperature does not exceed 175°Celsius in dry storage. In addition, results of neutron dose rate calculations have demonstrated that, as expected, the dose rates produced by neutrons are negligible compared to those generated by gamma radiation. This result is due to the heavy concrete used as shielding in the dry storage containers.
To verify the results of the thermal analysis, an experimental thermal performance verification program was carried out in the summer of 1998. A dry storage container, instrumented with 24 thermocouples at various locations on the inner and outer liners, was loaded with six-year cooled fuel and placed within an array of dry storage containers containing ten-year cooled fuel. Temperatures were also measured at the interspaces between the dry storage containers, in addition to indoor and outdoor ambient temperature measurements. The results demonstrated the conservatism of the temperatures predicted analytically.
4.5 Spent fuel storage facilities
After a cooling period of six to ten years in the storage bay (the exact cooling period is site-specific), spent fuel is then transferred to an interim dry storage facility. All transfers of spent fuel to dry storage are conducted under IAEA surveillance. All loaded dry storage containers in interim storage are also under the surveillance of the IAEA through the application of a dual sealing system.
4.5.1 Pickering Nuclear Generating Station
Pickering hosts two NGSs (Pickering NGS-A and NGS-B). Both stations consist of four CANDU Pressurized Heavy Water reactors. Pickering NGS-A commenced operation in 1971 and continued to operate safely until 1997, when it was placed in voluntary lay-up as part of what was then Ontario Hydro's nuclear improvement program. In September 2003, Unit four was returned to commercial operation. Unit one was returned to commercial operation in November 2005, while units two and three remain in a safe shutdown state.
Pickering NGS-B commenced operation in 1982 and continues to operate today. OPG has begun a Pickering B Refurbishment Study to determine the feasibility of refurbishing the units at Pickering B in order to extend their operating lives until 2050-2060.
The spent fuel waste generated at both Pickering NGS-A and Pickering NGS-B is stored in the irradiated fuel bays for a minimum of 10 years before the spent fuel is transferred to the PWMF.
4.5.2 Pickering Waste Management Facility - Used Fuel Dry Storage
OPG's PWMF is located within the protected area of the Pickering NGS. In operation since 1996, the primary purpose of the PWMF is to store spent fuel from the reactors at the Pickering A and B NGS. It is expected that the PWMF will be in operation until at least 10 years after the shutdown of the last Pickering reactor unit.
The used fuel dry storage area of the PWMF is comprised of a dry storage container processing building and two storage buildings. The Pickering spent fuel dry storage system is designed to transfer spent fuel from wet storage in the Pickering A and B irradiated fuel bays into a dual-purpose (storage and transport) concrete dry storage container designed by OPG. Prior to transfer to the PWMF, each loaded dry storage container is drained, its cavity is vacuum dried, and the container surface is monitored for loose contamination. If necessary, decontamination is carried out.
At the processing building in the PWMF, once the dry storage container loaded with spent fuel is received, the transfer clamp and the seal are removed, and the lid is seal-welded to the dry storage container body. The lid weld is subsequently inspected for defects using x-ray radiography. The vent port is also welded, and a weld dye penetrate inspection is performed. The dry storage container undergoes final vacuum drying and helium backfilling. Subsequently, the drain port is welded, inspected and helium leak testing is performed. The dry storage container is monitored to ensure that no loose contamination is present; if contamination is found, the container is decontaminated.
Finally, touch-up paint is applied to scuffs or scrapes on the container's exterior. Prior to being introduced into the storage buildings, IAEA seals are applied to each container. The PWMF can process approximately two dry storage containers (or 768 spent fuel bundles) per week.
The PWMF can store up to 650 dry storage containers or 249,600 fuel bundles in the two existing storage buildings. An application to expand the facility - to include two additional storage buildings for storing a further 1,000 dry storage containers - has been approved for construction. While the two storage buildings will be constructed within the Pickering nuclear site - but some distance from the PWMF - the buildings will be part of the PWMF licence. Construction of storage building 3 has started. The two buildings will be operated within an established protected area.
In 2007, the PWMF (spent fuel dry storage area and re-tube components storage area combined) reported releases of less than 0.001 GBq to air and 0.12 GBq to water. It is important to note, however, that activity released from the PWMF is included in the total releases reported for the Pickering NGS.
4.5.3 Bruce Nuclear Generating Stations A and B
The Municipality of Kincardine, Ontario hosts the Bruce nuclear site, which contains two NGSs (Bruce NGS-A and NGS-B). Bruce NGS-A consists of four CANDU Pressurized Heavy Water reactors. Currently, only Units 3 and 4 are in operation; Units 1 and 2 are being refurbished.
Bruce NGS-B consists of four CANDU Heavy Water reactors. This station commenced operation in 1984, and continues to operate today. Bruce Power Inc. leases and operates both Bruce NGS-A and NGS-B.
4.5.4 Western Waste Management Facility - Used Fuel Dry Storage
OPG's Western Used Fuel Dry Storage Facility, which is part of the WWMF, began operations in February 2003. The WWMF Used Fuel Dry Storage Facility was designed to provide safe storage for the Bruce NGS-A or NGS-B spent fuel until all of it is transported to an alternative long-term spent fuel storage or disposal facility. It can provide dry storage for about 750,000 fuel bundles produced at Bruce NGS-A and Bruce NGS-B. The spent fuel is stored in dual-purpose concrete dry storage containers, identical to those currently in use at the PWMF. The processing of dry storage containers is carried out in a manner similar to that at the PWMF.
The WWMF can process three to four dry storage containers per week. OPG is authorized to store up to 750,000 spent fuel bundles, or approximately 2,000 dry storage containers, at the facility.
In 2007, the WWMF (used fuel dry storage area and L&ILW storage area combined) released 13,400 GBq to air and 80.8 GBq to water. The activity released from the WWMF is typically less than one percent of the total activity released from the BNPD site.
4.5.5 Darlington Nuclear Generating Station
The Darlington NGS, operated by OPG, consists of four CANDU Pressurized Heavy Water reactors. The station commenced operation in 1989 and continues to operate today. All of the spent fuel produced at the Darlington NGS is currently stored in the water-filled storage bays.
4.5.6 Darlington Waste Management Facility
The Darlington Waste Management Facility (DWMF) is located at the Darlington NGS site. It provides safe storage for the Darlington NGS used fuel until this fuel is transported to an alternative long-term spent fuel storage or disposal facility.
The current DWMF is made up of a processing building and storage building that can house up to 500 dry storage containers. The facility, however, is designed to provide a storage capacity for up to 575,000 fuel bundles produced at the Darlington NGS after two additional storage buildings are constructed in future. The spent fuel is stored in dual-purpose concrete dry storage containers, identical to those currently in use at the PWMF and WWMF. The processing of dry storage containers is also identical to the operations at the PWMF and the WWMF.
4.5.7 Gentilly-2 Nuclear Generating Station
The Gentilly-2 Nuclear Generating Station, which is operated by Hydro-Québec, houses a CANDU pressurized heavy water reactor. The station went into service in 1982, and began commercial operation in 1983.
The spent fuel generated here is first stored in a pool in irradiated fuel bays. After a period of cooling in the storage bays, the spent fuel is transferred to the dry storage facility. The transfer of the spent fuel into baskets is done directly in the pool. The loaded baskets are then transferred to a shielded workstation, in which the contents are dried and the basket lids are welded on. Once the work on the baskets has been completed, the baskets are transported to Hydro-Québec's spent fuel dry storage facility.
4.5.8 Hydro-Québec Used Fuel Dry Storage Facility
In operation since 1995, the Gentilly-2 Used Fuel Dry Storage Facility provides additional storage capacity in CANSTOR modules, which is an AECL-designed technology MACSTOR . This facility has been authorized to build a total of 20 CANSTOR modules, with a total storage capacity of 240,000 spent fuel bundles. By the end of 2007, seven CANSTOR modules had been built and were in service. The eventual number of these modules will depend on a decision made concerning the refurbishment of the reactor.
Currently, the storage baskets are transferred on an as-needed basis, with transfers normally held between April and December each year. Approximately 4,500 spent fuel bundles are transferred to storage each year. At all times, the licensee makes sure that dose rates at the fence line of these facilities stays within the authorized limit of 2.5 µSv/h.
4.5.9 Point Lepreau Nuclear Generating Station
The Point Lepreau NGS, operated by New Brunswick Power Nuclear Corporation, consists of one CANDU Pressurized Heavy Water reactor. The station commenced operation in 1982, and continues to operate today. The spent fuel generated at the Point Lepreau NGS is initially stored in the irradiated fuel bay, and is then transferred to Point Lepreau spent fuel dry storage facility, where it is stored in concrete canisters.
4.5.10 Point Lepreau Used Fuel Dry Storage Facility
In operation since 1990, the Point Lepreau spent fuel dry storage facility provides additional storage capacity for the Point Lepreau NGS in aboveground concrete canisters. The facility is authorized to construct 300 canisters for a total of 180,000 spent fuel bundles. By the end of 2007, the facility had constructed 180 canisters. Approximately 5,000 spent fuel bundles are transferred to dry storage each year, depending on the power output of the Point Lepreau nuclear reactor.
Samples of surface run-off from the spent fuel dry storage facility - as collected and analyzed in 2007 - have shown that tritium concentrations varied between 19 and 405 Bq/L. The average dose for the year at the spent fuel storage facility perimeter fence was 927.5 µSv, which is equivalent to an average dose rate of 0.11 µSv/h.
The Point Lepreau Generating Station is currently preparing for a major refurbishment outage, starting in April 2008. This work will enable the station to operate for another 25 to 30 years. To handle the spent fuel resulting from the extended operational life of the station, land was prepared to permit the construction of up to 300 additional canisters, depending on upcoming needs.
4.5.11 Douglas Point Used Fuel Dry Storage Facility
The AECL Douglas Point Used Fuel Dry Storage Facility is located at the Bruce NGS. The prototype CANDU power reactor at Douglas Point was shut down permanently after 17 years of operation. Decommissioning began in 1986, and approximately 22,000 spent fuel bundles were transported to concrete canisters in late 1987. The concrete canisters are currently in storage-with-surveillance mode. The Dry Fuel Storage Canister Air Sampling program showed gross beta activity levels were less than 1.0 Bq/canister in 2007.
4.5.12 Gentilly-1 Used Fuel Dry Storage Facility
The AECL Gentilly-1 Nuclear Power Station became operational in May 1972. It attained full-power for two short periods in 1972 and was then operated intermittently for a total of 183 effective full-power days until 1978. In 1984, AECL began a two-year decommissioning program, during which a total of 3,213 spent fuel bundles were transferred to concrete canisters. The concrete canisters are currently in storage-with-surveillance mode. The Dry Fuel Storage Canister Air Sampling program showed gross beta activity levels were 0.37 Bq for each canister in 2007.
4.5.13 Chalk River Laboratories - Area G - Used Fuel Dry Storage Area
The Waste Management Area G at AECL CRL is a used fuel dry storage area and contains concrete canisters as described in section 4.3.1. NPD was a demonstration reactor operated by Ontario Hydro (now OPG) from 1962 until 1987, at which time it was decommissioned. As part of the decommissioning program, the spent fuel was transferred to concrete canisters located at the AECL CRL used fuel dry storage area. At this site, AECL has stored 68 full and partial spent fuel bundles from Bruce, Pickering and Douglas Point, as well as 4,853 fuel bundles from the NPD reactor, in 12 dry storage concrete canisters. The concrete canisters are currently in storage-with-surveillance mode.
Two concrete canisters were constructed on the existing concrete support pad to store calcined waste from the processing of radioisotopes separated in the New Processing Facility at CRL. The final purpose of the canisters, however, may be changed after the recent cancellation of the Dedicated Isotope Facility.
4.5.14 Whiteshell Laboratories (WL) Used Fuel Storage Facility
The WL was established at Pinawa, Manitoba in the early 1960s to carry out nuclear research and development activities for higher-temperature versions of the CANDU reactor. The initial focus of research was the Whiteshell Reactor-1 Organic Cooled Reactor, which began operation in 1965. Whiteshell Reactor-1 continued to operate until 1985.
The Concrete Canister Storage Facility, or Whiteshell spent fuel storage facility, was developed at WL to demonstrate that dry storage was a feasible alternative to water pool storage for irradiated reactor fuel.
Because of the success of the demonstration program, concrete canisters have been used to store all remaining WR-1 spent fuel. In addition, a number of spent fuel bundles from CANDU stations are stored in the WL facility after undergoing Post-Irradiation Examinations in the WL shielded facilities. The facility provides storage for 2,268 irradiated fuel bundles from both the WR-1 operation and CANDU reactor origin. Some spent fuel waste from operations prior to the 1975 canister development program is buried in standpipes in the WMA. (Further details on the Whiteshell decommissioning program can be found in Annex 7.1.)
4.5.15 NRU Research Reactor
The NRU Research Reactor is a thermal neutron, heterogeneous, heavy water moderated and cooled reactor. It was designed for operation with natural uranium metal fuel rods and converted to operation with enriched driver fuel rods in 1964. Gradual conversion to LEU fuel began in 1991.
Initial storage of the spent fuel rods is in water filled bays located within the NRU. After an appropriate time to allow for radioactive decay and cooling, the spent fuel is generally transferred to tile holes at Waste Management Area ‘B' at CRL. The tile holes are also used to store the spent fuel from the NRX Reactor, which was shutdown in 1992.
4.5.16 McMaster Nuclear Reactor
The McMaster Nuclear Reactor (MNR) is a pool-type reactor, with a core of enriched uranium fuel moderated and cooled by light water. The reactor was upgraded to operate at powers up to five MW. MNR has recently been converted to LEU, some of which comes from France. All MNR used fuel (HEU and LEU), irrespective of its origin, is sent to Savannah River in the United States.
The MNR is the only Canadian medium-flux reactor in a university environment. The MNR's neutrons are used in nuclear physics, biology, chemistry, earth sciences, medicine and nuclear medicine. All fuel generated at the McMaster Nuclear Reactor is stored in a water environment.
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