An Organic nuclear reactor or Organic-cooled reactor (OCR) is a class of nuclear reactor in which the primary coolant or neutron moderator is an organic fluid.Starr, Chauncey, “Organic Coolants in Power Reactors.” Transactions of the American Nuclear Society, Vol. 15, 1971
Organic coolants have some advantages over light water, including low-pressure operation and decreased corrosion; however they suffer from flammability and degradation under neutron bombardment. Four demonstration organic-cooled reactors were constructed in the mid-20th century, but the technology was never commercialized.
Design principles
Organic reactors use an organic hydrocarbon fluid, such as terphenyl, to cool the reactor core or to moderate neutrons. Several designs use the organic coolant as the neutron moderator, similar to a pressurized water reactor (PWR). This is the simplest solution from a construction and operational point of view, and saw significant development in the US, where the PWR design was already common. Alternatively, a separate moderator such as graphite can be used, or heavy water as in the WR-1. Other possible moderators include beryllium, beryllium oxide, and zirconium hydride.
Similar to a boiling water reactor (BWR), a boiling organic-cooled reactor is also possible, aided by the fact that suitable organic fluids superheat on their own when they expand into gas. Such a reactor could operate on a direct cycle at significantly lower pressure than a BWR.
Advantages Organic coolants have high boiling points and low vapor pressures, allowing reactor operation at temperatures comparable to light-water reactors without requiring high pressure.Webb, J. G. “Development of Organic-Cooled Reactor Systems in Canada.” Nuclear Engineering Review, Vol. 7, No. 3, 1968.
Nuclear fuel must be inserted into a cladding tube to contain the fission products produced in the fuel. Because water dissolves most structural metals, fuel must be inserted into a zirconium alloy cladding tube while the rest of the reactor has to be built out of materials such as stainless steel or Inconel that are both corrosion resistant and resistant to the effects of neutron embrittlement. In contrast, many common organic fluids are less corrosive to metals, allowing the fuel assemblies to be much simpler and the coolant pipes to be built of less-expensive carbon steels. In most examples the fuel was metallic uranium with a simple cladding of stainless steel or aluminum.
Some organics also have the advantage that they do not flash into gas in the same fashion as water, thus there is no real possibility of a steam explosion. Other potential explosion sources in water-cooled designs also include the buildup of hydrogen gas caused when the zirconium cladding heats; lacking such a cladding, or any similar material anywhere in the reactor, the only source of hydrogen gas in an oil-cooled design is from the chemical breakdown of the coolant. This occurs at a relatively predictable rate, and the possibility of a hydrogen buildup is extremely remote. This greatly reduces the required containment systems.
Disadvantages Organic-based coolants have a few disadvantages which include their relative heat transfer capability, roughly half that of water, which requires increased flow rates to remove the same amount of energy. Another issue is that they tend to decompose at high temperatures and none could be expected to operate for extended periods above 530°C. Several organic coolants are also flammable which presents operational and safety considerations.
Another issue is that for oil moderators, the moderating capability of the fluid decreases as its temperature increases. This in turn causes the overall reaction rate of the reactor to slow and further cool the reactor. In many reactors, such a negative temperature coefficient is an important safety feature. However, in the case of an oil moderator, the temperature coefficient is so strong that it can rapidly cool the reactor, making load following difficult.
The largest problem for hydrocarbon coolants is that they decompose when exposed to radiation, also known as radiolysis. In contrast to heat-based decomposition, which tends to make lighter hydrocarbons, the outcome of these reactions is highly variable and results in many different reaction products. Water also undergoes decomposition due to radiation, but the output products are hydrogen and oxygen, which are easily recombined into water again. The resultant products of the decomposition of oils are not readily recombined, and have to be removed.
One particularly worrying type of reaction occurred when the resulting products polymerized into long-chain molecules. The concern was that these would form large masses within the reactor, and especially its cooling loops, and might "exert significant deleterious effects on the operation of a reactor". It was polymerization of the coolant sticking to the fuel cladding that led to the shutdown of the Piqua reactor after only three years of operation.
History Early experiments Early theoretical work on the organic cooled concept was carried out by the United States Atomic Energy Commission (AEC) at Argonne National Laboratory between 1953 and 1956. As part of this work, Mine Safety Appliances studied a variety of potential biphenyl coolants. In 1956–75, Aerojet conducted studies on the rate of "burnout" of polyphenyl coolants, and in the following two years, Hanford Atomic Products carried out several studies of polyphenyl irradiation.
Monsanto began operating a single coolant loop in the Brookhaven Graphite Research Reactor beginning in 1955 to study heat transfer, and in 1958 began to consider coolant reclamation and studies on boiling diphenyl coolant loops. Atomic Energy of Canada Limited (AECL) began similar studies around the same time, with an eye to the design of a future test reactor.
A similar program began in the UK at Harwell in the 1950s. This soon focused on radiation damage to organic compounds, specifically polyphenyls. Around 1960, Euratom began studies of such designs as part of their ORGEL project. A similar but separate project began in Italy under the direction of the Comitato nazionale per l'energia nucleare, but their PRO design was never built. Likewise, a major study carried out in Denmark considered the heavy water-moderated reactor.
Major experiments The first complete organically cooled and moderated reactor design was the Organic Moderated Reactor Experiment (OMRE), which began construction at the Idaho National Laboratory in 1955 and went critical in 1957. This used biphenyl and Santowax (commercial name of an isomeric mixture of terphenyl) for coolant and moderation and operation was generally acceptable. The reactor was a very low-energy design, producing 15 MW thermal, and operated for only a short period between 1957 and 1963. During this time the core was rebuilt three times to test different fuels, coolants and operating conditions from 260 to 370 C. It was planned that a larger 40 MW design, the terphenyl-cooled Experimental Organic Cooled Reactor (EOCR), would take over from the OMRE. It began construction at Idaho in 1962, but was never loaded with fuel when the AEC shifted their focus mostly to light water reactors.
The next major reactor was a commercial prototype, the Piqua Nuclear Power Facility, which began construction in 1963 at Piqua, Ohio. It was funded as a private/public venture as part of the AEC's Power Reactor Demonstration Program. This used the same Santowax coolant as the original OMRE, but was as large as the EOCR, producing 45 MW thermal and 15 MW electrical. It ran on 1.5% enriched fuel formed into annular tubes that were clad in finned aluminum casings. It ran only for a short time until 1966, when it was shut down due to films building up on the fuel cladding, formed from radiation degraded coolant. The reactor was prepared to receive a second core for future research, but this plan was cancelled in 1968 after the cancellation of the HWOCR program and the reactor was decommissioned instead.
Although various European nations did development work on organic reactor designs, only the Soviet Union built one. The ARBUS (АРБУС) reactor was an experimental organic-cooled research reactor operated by the Research Institute of Atomic Reactors. Work on the 5 MW thermal ARBUS (АРБУС) NPS began in Melekess, Russia in 1963 and it ran until 1979. It produced a maximum of 750 kW of electricity. The research reactor was used to investigate coolant stability, radiolysis, and materials compatibility under irradiation, as well as the use of standard carbon steels in reactor construction. In 1979 it was rebuilt as the AST-1, this time to deliver 12 MW of process heat instead of electrical power. It ran in this form until 1988. Like its Western counterparts, the program identified coolant degradation and chemistry control as limiting factors.IAEA Organic Cooled Reactor Technology. Technical Report Series No. 61, Vienna, 1966.IAEA Review of Operating Experience with Organic-Cooled Reactors. IAEA-TECDOC-432, Vienna, 1987.
The most powerful ONR was the Canadian 60 MW thermal WR-1. AECL had investigated the use of organic coolants for the next generation of CANDU reactors, and commissioned an organic-cooled heavy-water reactor to explore the concept. It began construction at the newly formed Whiteshell Laboratories in Manitoba in 1965 and went critical late that year. Midway through construction, the concept was abandoned for use in CANDU reactors, however AECL continued operation of the WR-1 to advance research into the technology elsewhere, particularly for the US-Canadian HWOCR program. WR-1 used heavy water as the moderator and terphenyls as the coolant, and did not suffer from the problems with coolant breakdown seen in the US designs. WR-1 achieved reliable sustained operations for nearly 20 years as a materials test reactor with an availability factor of 85%. It operated until 1985, by which time AECL had standardized on using heavy water for both the moderator and the coolant, and the organic cooled design was no longer being considered for development. It is generally regarded as the most successful organic-cooled reactor.
Heavy Water Organic Cooled Reactor (HWOCR) Program
The Heavy Water Organic Cooled Reactor (HWOCR) program was a joint U.S.–Canadian reactor development effort in the late 1950s and 1960s led by the US AEC and Canada’s AECL.
At the time, the AEC was concerned by the limited supply of uranium ore compared to the predicted increase in nuclear power demand by the end of the century. This concern drove research into the breeder cycle and other ways of extending the nuclear fuel supply such as using thorium as fertile material. Heavy-water reactors (HWR) were already under consideration by the AEC due to their excellent neutron economy, and the HWOCR program was devised to improve the operating characteristics of HWRs using lessons from the Piqua facility. While the LWR consumed enormous amounts of uranium ore for its fuel, fast breeder reactors (FBR) or thorium-HEU converter reactors like the HWR or the high-temperature gas-cooled reactor tested at Peach Bottom would use significantly less uranium until FBRs producing plutonium or molten-salt breeders using a thorium fuel cycle could be developed. A heavy-water organic-cooled reactor was flexible enough to use either thorium-HEU or natural uranium.