The WR-1 (Whiteshell Reactor-1) was a Canadian experimental organic-cooled, heavy-water-moderated research and demonstration reactor operated by the Atomic Energy of Canada Limited (AECL) at the Whiteshell Laboratories (WNRL) in Manitoba, Canada. Originally known as Organic-Cooled Deuterium-Reactor Experiment (OCDRE) [1] , it was built to evaluate an alternative to conventional water or sodium coolants with a CANDU-type reactor that replaced the heavy-water coolant with a high-boiling organic heat-transfer fluid. Commissioned in 1965 and operated until 1985, WR-1 was the most successful organic-cooled power reactor ever built and provided critical data for AECL’s advanced reactor development programs [2] [3] The WR-1 reactor successfully demonstrated the key advantages of the organic-cooled reactor concept:
An effort to commercialize the design began in 1971 but ended in 1973 when the heavy-water cooled CANDU became the standard. From then on WR-1 operated at reduced power limits for irradiation experiments and heating the WNRE site. WR-1 was shut down for the last time on 17 May 1985, was defuelled, and as of 2013 [update] is undergoing decommissioning scheduled to be completed in 2023. The decision to shut down WR-1 was due to shifting national programmatic research priorities to consolidate resources behind the heavy-water cooled CANDU reactor program and not to any technical rationale for the organic cooled reactor concept.
The 60 MWth reactor was designed and built by Canadian General Electric for a cost of $14.5 million CAD. The construction started 1 November 1962. [4] It achieved criticality on 1 November 1965 [4] and full power in December 1965. WR-1 was designed to evaluate whether an organic coolant could provide high operating temperatures at low pressure, enabling improved thermodynamic efficiency with simplified containment requirements. Its design included:
The reactor served as a testbed for alternate coolants, fuel bundle geometries, corrosion and materials research, and coolant chemistry control programs [2] [5] .
Natural uranium consists of a mix of isotopes, mostly 238U and a much smaller amount of 235U. Both of these isotopes can undergo fission when struck by a neutron of sufficient energy, and as part of this process, they will give off medium-energy neutrons. However, only 235U can undergo fission when struck by neutrons from other uranium atoms, allowing it to maintain a chain reaction. 238U is insensitive to these neutrons and it thus not fissile like 235U. While 235U is sensitive to these neutrons, the reaction rate is greatly improved if the neutrons are slowed from their original relativistic speeds to much lower energies, the so-called thermal neutron velocities. [6]
In a mass of pure natural uranium, the number and energy of the neutrons being released through natural decay are too low to cause appreciable fission events in the few 235U atoms present. In order to increase the rate of neutron capture to the point where a chain reaction can occur, known as criticality, the system has to be modified. In most cases, the fuel mass is separated into a large number of smaller fuel pellets and then surrounded by some form of neutron moderator that will slow the neutrons, thereby increasing the chance that the neutrons will cause fission in 235U in other pellets. Often the simplest moderator to use is normal water; when a neutron collides with a water molecule it transfers some of its energy to it, increasing the temperature of the water and slowing the neutron. [6]
The main problem with using normal water as a moderator is that it also absorbs some of the neutrons. The neutron balance in the natural isotopic mix is so close that even a small number being absorbed in this fashion means there are too few to maintain criticality. In most reactor designs this is addressed by slightly increasing the amount of 235U relative to 238U, a process known as enrichment. The resulting fuel typically contains between 3 and 5% 235U, up from the natural value of just under 1%. The leftover material, now containing almost no 235U and consisting of almost pure 238U, is known as depleted uranium. [7]
The CANDU design addresses moderation by replacing the normal water with heavy-water. Heavy-water already has an extra neutron, so the chance that a fission neutron will be absorbed during moderation is largely eliminated. Additionally, it is subject to other reactions that further increase the number of neutrons released during operation. The neutron economy is improved to the point where even unenriched natural uranium will maintain criticality, which greatly reduces the complexity and cost of fueling the reactor, and also allows it to use a number of alternative fuel cycles that mix in even less reactive elements. The downside to this approach is that the 235U atoms in the fuel are spread out through a larger fuel mass, which makes the reactor core larger for any given power level. This can lead to higher capital costs for building the reactor core. [4]
To address the cost issue, CANDU reactors use a unique reactor core layout, allowing online refueling and operation on natural or low-enriched uranium while Pressurized Water Reactors (PWRs) use light water as both coolant and moderator in a high-pressure closed primary loop with steam generated in separate steam generators and Boiling Water Reactors (BWRs) also use light water but boil the coolant directly in the reactor vessel to generate steam for the turbine, eliminating steam generators but introducing steam into the reactor core and containment systems. At the time CANDU was being designed, Canada lacked the facilities to fabricate such large pressure vessels, especially ones large enough to run on natural uranium. The solution was to enclose the pressurized heavy water within smaller tubes and then insert these into a much larger low-pressure vessel known as the calandria. One major advantage of this layout is that the fuel can be removed from the individual tubes which allow the design to be refuelled while operating, while conventional designs require the entire reactor core to be shut down. A small disadvantage is that tubes absorb some neutrons as well, but not nearly enough to offset the improved neutron economy of the heavy water design. [4]
A significant problem with using any sort of water as a coolant is that the water tends to dissolve the fuel and other components and ends up becoming highly radioactive as these materials are deposited in the water. This is mitigated by using particular alloys for the tubes and processing the fuel into a ceramic form. While effective at reducing the rate of dissolution, this adds to the cost of processing the fuel while also requiring materials that are both non-corrosive while also being less susceptible to neutron embrittlement. More of an issue is the fact that water has a low boiling point, limiting the operating temperatures. [4]
This was the basic premise of the organic nuclear reactor design. In the CANDU layout, the moderator and coolant both used heavy water, but there was no reason for this other than expediency. Since the bulk of the moderation occurred in the calandria mass, replacing the small amount of heavy water in the fuel tubes with some other coolant was straightforward, unlike conventional light water designs where some other moderator would have to be added. [a] Using oil meant the issues with corrosion were greatly reduced, allowing more conventional metals to be used while also reducing the amount of dissolved fuel, and in turn, radiation in the cooling system. The organic liquid that was selected is a mixture of terphenyls treated catalytically with hydrogen to produce 40 percent saturated hydrocarbons. The terphenyls are petrochemical derivatives that were readily available and were already in use as heat transfer media. [4]
Additionally, by using a material with a higher boiling point, the reactor could be operated at higher temperatures. This not only reduced the amount of coolant needed to remove a given amount of energy, and thereby reduced the physical size of the core, but also increases the efficiency of the turbines used to extract this energy for electrical generation. WR-1 ran with outlet temperatures up to 425 °C, [4] compared to about 310 °C in the conventional CANDU. This also meant that there is no need to pressurize the cooling fluid beyond what is needed to force it through the cooling tubes at the required rate, whereas water must be held under high pressure to allow it to reach higher temperatures. This allowed the fuel tubes to be made thinner, reducing the number of neutrons lost in interactions with the tubing, and further increasing the neutron economy. [4]
The reactor had vertical fuel channels, in contrast with the normal CANDU arrangement where the tubes are horizontal. The reactor did not use conventional control rods, but relied on control of the level of the heavy water moderator to adjust the power output. The reactor could be shut down quickly (SCRAMed) by the rapid dumping of the moderator. [4]
WR-1 employed an organic heat-transfer fluid as it's primary coolant which avoided the mechanical stresses, corrosion, and embrittlement concerns seen in high-pressure PWRs and BWRs. This coolant allowed WR-1 to achieve near-steam-cycle temperatures without PWR/BWR pressure boundaries and demonstrated that an organic coolant could be integrated into a pressure-tube, heavy-water moderated reactor design with stable heat removal characteristics.
During sustained operations, WR-1 experienced some radiolysis and thermal degradation of the organic coolant caused by neutron and gamma irradiation and prolonged high-temperature exposure. Neutron activation of the WR-1 organic coolant was minimal and operationally insignificant while corrosion rates in the primary system remained very low with proper materials selection. As a result, operational issues were dominated by coolant chemistry management and maintenance rather than structural degradation of pressure boundaries. These processes produced lighter hydrocarbons, dissolved gases, acidic species, and heavier polymeric by-products, leading to coolant darkening and particulate buildup in pumps, valves, and heat-exchange surfaces. AECL mitigated these effects through online purification systems, filtration, and periodic coolant reconditioning, maintaining system operability within operational limits.
Operational radiation surveys at WR-1 confirmed that dose rates in the organic coolant system were driven mainly by trace fission product contamination and surface plate-out, rather than by neutron activation of the coolant itself. This significantly reduced radiation exposure during maintenance and inspection activities compared with water-cooled reactor primary systems, where activated coolant is a dominant contributor to occupational dose. Unlike light-water reactor coolants, the organic fluid did not contain oxygen or nitrogen as intrinsic constituents, eliminating dominant activation pathways such as the production of ¹⁶N that drives high gamma dose rates in PWR and BWR primary systems. Furthermore, the measured tritium levels and other activation products associated with the organic coolant were orders of magnitude lower than those observed in light-water reactor primary systems.
While WR-1 did experience three documented coolant leaks over its near 20-year operating life, these incidents behaved like traditional hydrocarbon releases rather than radiological emergencies, as the coolant did not become significantly radioactive. Corrective actions included coolant containment or recovery, sediment cleanup in affected areas, and mechanical and procedural upgrades to prevent recurrence. In all cases, the incidents were handled as chemical release issues rather than nuclear safety events, demonstrating a key distinction between organic coolant behavior and traditional water-cooled accident scenarios.
WR-1 proved that organic coolants can be operated safely, effectively, and with stable heat transfer when the reactor design is purpose-built for them. Piqua failed not because the coolant concept was inherently flawed, but because the reactor system architecture was mismatched to the coolant’s operational performance requirements. In modern reactor design terms, WR-1 provided the proof-of-concept, while Piqua demonstrated the misapplication of the concept.
WR-1 operated well because the reactor design, operational envelope, and coolant performance through chemistry controls were aligned from the start. By combining a low-pressure organic coolant with a CANDU-like pressure-tube configuration and heavy-water moderation, AECL paired the coolant’s strengths—low pressure, thermal stability, and minimal corrosion—with a system that did not depend on high mechanical loads. It was a purpose-built match of coolant operational performance to reactor system architecture.
Piqua [13] [14] , by contrast, used an inferior organic coolant and moderator with poor melting/pour point characteristics into a reactor designed much more like a light-water reactor (LWR) than like WR-1, resulting in chronic contamination issues, coolant handling risks, and maintenance burdens the plant was never engineered to support. These design choices is the key reason why Piqua performed poorly. Where WR-1 validated the feasibility of organic cooling as a highly reliable nuclear generation concept, Piqua illustrated that the coolant could not simply be treated as a substitute for water in an existing reactor architectural layout.
| Criteria | WR-1 (Canada) Success Case | Piqua (USA) Failure Case |
|---|---|---|
| Design Fit | CANDU-like design matched coolant with operational envelope | Inferior organic coolant choice mismatched to operational envelope |
| Operations | 1966-1986 (20 years) | 1963-1966 (3 years) |
| Power Output | 60 MWth | 45 MWth |
| Organic Coolant | HB-40 | Santowax OMP |
| Moderator | Heavy Water | Santowax OMP |
| Coolant Melting/Pour Point | -24oC | 85oC |
| Coolant Chemistry Control | Dedicated purification and filtration systems & coolant monitoring | Insufficient chemistry management and coolant cleanup capability |
| Operational Performance | Achieved high reliability and sustained operational performance with >85% availability factor | Reliability and contamination issues, high maintenance burden, and poor availability led to early shutdown |
| Coolant-Reactor System Match | Organic fluid with low pressure operations yielded highly reliable operational performance | Poor organic coolant/moderator choice in a LWR inspired design yielded persistent operational issues |
There were three leakage events that took place at WR-1 over its 20-year lifetime [15] [16] [17] :
Despite sediment detection, river water samples generally showed organic coolant levels below analytical detection limits, indicating limited aqueous dispersion of dissolved contaminants. Biological sampling during follow-up monitoring did not show internal contamination or elevated mortality attributable to WR-1 organic coolant residues. Furthermore, multiagency reviews — including Environment Canada and Health Canada participation — concluded that river water downstream of the WR-1 site met Canadian drinking water guidelines, with no evidence that the coolant releases posed measurable human health risks. In 2006, AECL analyzed river sediment core samples at areas downstream of the site where deposits from the outfall were found. AECL concluded that there was no contamination of the river sediments that would have an ecological impact or affect human health. [15] [16] [17]
The WR-1 organic coolant contained only carbon and hydrogen and was not the neutron moderator, hence, neutron activation products were minimal, short-lived, and operationally insignificant—making coolant behavior chemical rather than radiological in both routine operation and leak scenarios. Because of this, the WR-1 leakage events behaved as chemical releases rather than radiological incidents. Environmental assessments following the coolant leaks concluded that the primary hazards were chemical, not radiological, and emergency response actions typical of activated water releases in PWRs/BWRs were not required. This distinction was repeatedly cited by AECL as a favorable operational characteristic of organic-cooled reactor systems.
In 1971 AECL initiated design engineering of a 500 MWe CANDU-OCR, based on uranium carbide fuel. Carbide fuels would corrode in water but not the oil coolant. Carbide fuels were much easier to produce than the more complex ceramics being used in most reactor designs. This design effort was shut down in 1973, but WR-1 tested the concept anyway. Another possibility was to use metallic fuel, which would increase the density of the fuel and offer higher burnup. The metallic fuel conducts heat better so that a higher power core could be used in the same space. [18]
WR1 was shut down for the last time for programmatic reasons, on May 17, 1985. The reactor is in an interim decommissioning stage, defuelled and largely disassembled. The site will be returned to greenfield status at the end of decommissioning.
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