The SLOWPOKE (acronym for Safe Low-Power Kritical Experiment) is a low-energy, pool-type nuclear research reactor designed by Atomic Energy of Canada Limited (AECL) in the late 1960s. John W. Hilborn (now retired from AECL) is the scientist most closely associated with its design. It is beryllium-reflected with a very low critical mass but provides neutron fluxes higher than available from a small particle accelerator or other radioactive sources.
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The SLOWPOKE-2 uses 93% (originally) enriched uranium in the form of 28% uranium-aluminum alloy with aluminum cladding. The core is an assembly of about 300 fuel pins, only 22 cm diameter and 23 cm high, surrounded by a fixed beryllium annulus and a bottom beryllium slab. Criticality is maintained by adding beryllium plates in a tray on top of the core. The reactor core sits in a pool of regular light-water, 2.5 m diameter by 6 m deep, which provides cooling via natural convection. In addition to passive cooling, the reactor has a high degree of inherent safety; that is, it can regulate itself through passive, natural means, such as the chain reaction slowing down if the water heats up or forms bubbles. These characteristics are so dominant, in fact, that the SLOWPOKE-2 reactor is licensed to operate unattended overnight (but monitored remotely). Most SLOWPOKES are rated at a nominal 20 kW, although operation at higher power for shorter durations is possible.
The SLOWPOKE research reactor was conceived in 1967 at the Whiteshell Laboratories of AECL. In 1970 a prototype unit was designed and built at Chalk River Laboratories. It was primarily intended for Canadian universities, providing a higher neutron flux than available from small commercial accelerators, while avoiding the complexity and high operating costs of existing nuclear reactors. The Chalk River prototype went critical in 1970, and was moved to the University of Toronto in 1971. It had one sample site in the beryllium reflector and operated at a power level of 5 kW. In 1973 the power was increased to 20 kW and the period of unattended operation was increased from 4 hours to 18 hours.
The first commercial example was started up in 1971 at AECL's Commercial Products Division in Ottawa; and in 1976 a commercial design, named SLOWPOKE-2, was installed at the University of Toronto, replacing the original SLOWPOKE-1 unit. The commercial model has five sample sites in the beryllium reflector and five sites stationed outside the reflector.
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 Low-Enriched Uranium (LEU) fuelled SLOWPOKE-2 reactor was commissioned at the Royal Military College of Canada (RMC) in Kingston, Ontario. Since then several units have been converted to LEU.
AECL also designed and built a scaled-up version (2-10 MWth) called SLOWPOKE-3 for district heating at its Whiteshell Nuclear Research Establishment in Manitoba. The economics of a district-heating system based on SLOWPOKE-3 technology were estimated to be competitive with that of conventional fossil fuels. However, the market for this technology did not materialize.
A Chinese version of the Slowpoke exists, designated the Miniature Neutron Source Reactor (MNSR). This version is nominally rated at 27 kW with similar characteristics and performance.
During the mid-1980s Canada briefly considered converting its Oberon class submarines to nuclear power using a SLOWPOKE nuclear reactor to continuously recharge the ship's batteries during submerged operations. A good deal of work had been done on potential marine applications of the reactor at Royal Military College of Canada.
SLOWPOKE reactors are used mainly for neutron activation analysis (NAA), in research and as a commercial service, but also for teaching, training, irradiation studies, neutron radiography (at the Royal Military College of Canada) and the production of radioactive tracers. The main advantages are the reliability and ease of use of this design of reactor and the reproducibility of the neutron flux. Since the fuel is not modified at all for at least 20 years, the neutron spectrum in the irradiation sites does not change and the neutron flux is reproducible to about 1%.
Six of the original reactors are still in operation and one has been refuelled. Although all of the technical goals of this reactor were achieved, the lack of foreign sales was disappointing, the market being taken by the Chinese version.