Voici comment le réacteur SLOWPOKE-2 a tracé la voie de la recherche nucléaire au Canada
As governments worldwide consider how small modular reactors (SMRs) and nuclear energy could potentially power our future, reactors of the past may provide some insight into ways to move forward, both safely and sustainably.
The SLOWPOKE-2 (Safe LOW-POwer Kritical Experiment) nuclear research reactor is a good example. It is a low-energy, tank-in-pool reactor that is both inherently safe and extremely small. Surrounded by a protective barrier, the core itself is the size of a soccer ball.
A nuclear reactor operates by nuclear fission—the process by which a heavy atom (such as uranium-235) splits into smaller fragments, releasing neutrons and energy in the process. The neutrons released by one uranium-235 atom interact with other uranium-235 atoms, causing them to split, which in turn releases more neutrons and energy, resulting in a self-sustaining chain reaction. The rate of the reaction and amount of energy produced can be controlled by several factors:
- The amount of uranium-235 present
- The configuration of the reactor core
- The presence of materials that increase the reactivity of the system (e.g., moderators and reflectors)
- The presence of materials that reduce the reactivity of the system (e.g., control rods)
The concept of the SLOWPOKE originated in a paper from the Los Alamos National Laboratory, which explored the smallest mass of uranium-235 that could be configured to sustain a chain reaction. Inspired by this paper, Dr. John Hilborn of Atomic Energy of Canada Limited (AECL) designed the SLOWPOKE reactor in the late 1960s.
The first commercial example began in 1971 at AECL’s Commercial Products Division in Ottawa, ON. Between 1976 and 1984, seven SLOWPOKE-2 reactors with highly enriched uranium (HEU) fuel were commissioned in six Canadian cities and in Kingston, Jamaica. In 1985, the first SLOWPOKE-2 reactor fuelled by low-enriched uranium (LEU) was commissioned at the Royal Military College of Canada in Kingston, ON.
SLOWPOKE is an inherently safe system because the heat it produces limits its reactivity, so it can never get out of control. Its low power output (maximum of 20 kW) means it cannot be used as a power generator, but it does produce a steady, stable stream of neutrons that can be used for a wide range of applications, including neutron activation analysis and isotope production.
A Unique Opportunity for the Saskatchewan Research Council
The Saskatchewan Research Council (SRC) has housed one of Canada’s SLOWPOKE-2 reactors since 1981.
In the late 1970s, three separate events were about to come together:
- The uranium industry in Saskatchewan was growing.
- SRC was looking to build a new facility for its analytical laboratory.
- AECL was promoting the SLOWPOKE to universities and research establishments.
At the time, Dr. Gene Smithson, director of the SRC Environmental Analytical Laboratories, recognized that a nuclear reactor in Saskatchewan could be a good fit with the growing uranium industry and the role SRC could play in bringing the idea to fruition. In 1980, a new building was constructed to house both the new analytical lab and a SLOWPOKE reactor.
The first commercial sample was processed at SRC’s SLOWPOKE reactor on March 25, 1981. The reactor proved to be a great resource for the uranium industry as it provided a method to rapidly analyze the thousands of exploration samples that the industry was producing. The analysis technique used by the SLOWPOKE had the advantage of not requiring any sample preparation beyond drying and grinding; solids did not have to be dissolved with various acids as they did with other analysis techniques.
Slowdown in Uranium Exploration Makes Way for Other Uses of the Reactor
The initial few years were the busiest for SRC’s SLOWPOKE—over 22,000 samples were analyzed in the first two years of operation. A slowdown in exploration in subsequent years meant fewer samples for uranium analysis. As a result, the lab began analyzing samples for other elements using a technique known as instrumental neutron activation analysis (NAA).
Many analytical techniques require that samples be dissolved or digested into a liquid medium for analysis. However, NAA is a non-destructive technique, so it was the method of choice for analysis of materials that were difficult to put into solution. As an added benefit, samples could be returned to the client following analysis. For example, an archaeologist may want an old shard of pottery analyzed, but would also like to retain the shard for their collection.
Many elements can be detected by NAA, from rare earth elements to precious metals. Over the years, eager prospectors, hoping to find gold or other valuable commodities, would bring their samples to SRC to be analyzed using fire assay, a technique that involves extensive sample preparation. If those results came back negative and the prospector was interested in a second opinion, they would then be directed to the SLOWPOKE, where the sample would only have to be ground prior to analysis. NAA was able to detect the presence of even a few nanograms of gold or other commodity, so prospectors would have a definitive answer regarding their sample.
In later years, one of the main uses of the SLOWPOKE-2 reactor was rapid and inexpensive analysis of oilfield waste and other environmentally sensitive materials for halogenated organic compounds (such as chlorine, bromine, or iodine) bound to hydrocarbons and other compounds. The concentration of these compounds affects how waste is handled and disposed of. The ability to determine their presence or absence for little cost was of great benefit to industry clients.
One unique use for the reactor was to produce tracer diamonds for use in the recovery of diamonds from kimberlite—a process that was optimized by SRC’s Geoanalytical Laboratories. When irradiated, industrial diamonds change colour from relatively colorless to green or steel gray. A known number of these irradiated diamonds could then be added to the kimberlite ore at the start of a recovery process; at the end of the recovery process, the number of tracer diamonds present would be proportional to the number of natural diamonds in the ore to begin with.
While most of the analyses performed by SRC’s SLOWPOKE were fairly routine, occasionally a strange request would come in. On one occasion, the lab was asked to analyze glass chips that a client had, allegedly, nearly ingested and compare them to the glass mugs used at a well-known fast food establishment. On another occasion, an army widow requested that her deceased husband’s ashes be analyzed for depleted uranium to determine if he had been exposed to that substance during his tour(s) of duty.
The SLOWPOKE Slows to a Halt
As time went on, new methods became available to dissolve materials that were previously considered insoluble. This, along with advances in technology, led to analytical methods that could be performed more quickly and to lower detection limits. These newer technologies were faster and more economical than NAA, so it fell out of favour and was gradually replaced by the new methods.
By the early 2000s, use of the SLOWPOKE was limited to a few different tests and fewer samples for each test. The total number of irradiations performed per year dropped from a peak of over 11,000 to about 4,000. Occasional use by faculty and students from the University of Saskatchewan for teaching purposes and student projects, as well as use for the production of isotopes for research purposes, helped extend the life of the reactor.
By the early 2010s, SRC was nearing a decision point in terms of the future of the SLOWPOKE. It had been operating for over 30 years on its original fuel, and the fuel was nearing the end of its useful life. It would soon be necessary to either refuel the reactor or decommission it. At the same time, under the Global Threat Reduction Initiative, the United States was endeavouring to repatriate the HEU that it had sold to other countries, and this was the source of fuel for SRC’s SLOWPOKE.
By 2015, plans were also being made to move SRC’s Environmental Analytical Laboratories into a new facility. After thorough consideration of all factors, the decision was made to decommission the reactor. It operated for the last time on April 30, 2019. Decommissioning began in July 2019 and is expected to be complete in 2020.
Four more of Canada’s SLOWPOKE-2 reactors have also been decommissioned:
- MDS Nordion (formerly AECL) in Kanata, ON, in 1992
- University of Toronto in 2001
- Dalhousie University in 2011
- University of Alberta in 2018
A fifth SLOWPOKE-2 reactor, at Polytechnique Montréal, was converted to LEU in 1997.
Tests that used to be done at SRC using the SLOWPOKE are now done by either existing methods or new capabilities acquired by the lab. Uranium, thorium, and other metal analyses are done by inductively coupled plasma mass spectrometry (ICP-MS), and organic halide analysis is now done by combustion and microcoulombic titration.
A Legacy of Safety and Trouble-Free Operation
Safety has always been an overriding priority for SRC, and its work with the SLOWPOKE was no different. Throughout the reactor’s 38-year history, SRC’s SLOWPOKE operated without trouble.
SLOWPOKEs were a popular choice in the 1970s due to their high level of safety. Their design is failsafe and does not require an active mechanical safety system. Heat produced by the reactor limits its reactivity and operation. In addition, there are several auxiliary safety shutdown systems to keep employees and the facility safe.
As low as reasonably achievable (ALARA) is the guiding a principle of radiation protection. The SLOWPOKE operated within the guidelines defined by SRC’s occupational health and safety policy and radiation safety policy, as well as regulations set forth by the Canadian Nuclear Safety Commission (CNSC) and its predecessor, the Atomic Energy Control Board. Operating under these policies and regulations ensured radiation, contamination, and exposure levels were kept to non-detectable or nearly non-detectable levels throughout the life of the reactor. At no time were regulatory limits or action levels approached. In fact, it was deemed unnecessary to designate any of the employees as nuclear energy workers, as exposures were well below those allowable for the general public.
Over its lifetime, the SLOWPOKE had eight certified operators and 47 authorized users who were specially trained to work with the reactor.
Certified operators were legally permitted to
- operate the reactor,
- prepare and irradiate samples,
- perform routine maintenance,
- perform basic repairs to auxiliary systems outside the reactor container, and
- perform other functions defined in the SLOWPOKE operating licence.
Certified operators were trained in-house using a standardized training program approved by CNSC. The program had 14 modules that each consisted of classroom and on-the-job training. At the conclusion of each module, trainees demonstrated their understanding of the course material by completing a practical exam, performing reactor operations, and answering oral questions. At the end of the training, which typically took about a year to complete, a three-hour written exam was administered and a score of 90% or better was required to pass. Results of the training program and exams were submitted to CNSC, who, upon approval of the training results, would certify candidates.
Authorized users are permitted to
- prepare and irradiate samples and
- assist a certified operator with some of their duties.
Authorized users were also trained in-house, following a training program that included radiation safety and operation of irradiation systems. Users were authorized by the chair of SRC’s internal SLOWPOKE committee.
All together, these employees operated SRC’s SLOWPOKE for over 20,000 hours and irradiated almost 250,000 samples, primarily for Saskatchewan’s uranium industry.
While SRC’s SLOWPOKE-2 nuclear research reactor will soon be fully decommissioned, its legacy in Canada’s uranium industry and the lessons learned by the nuclear reactor industry (through the design, build, operation, maintenance, and decommissioning of SRC’s and the other SLOWPOKEs) will live on. Hopefully, some of these lessons will guide us as we move into the future of nuclear energy in Saskatchewan, Canada, and the world.