Organic nuclear reactor

An organic nuclear reactor, or organic cooled reactor (OCR), is a type of nuclear reactor that uses some form of organic fluid, typically a hydrocarbon substance like polychlorinated biphenyl (PCB), for cooling and sometimes as a neutron moderator as well.

Using an organic fluid had a major advantage over conventional designs using water as the coolant. Water tends to corrode and dissolve metals, both the nuclear fuel and the reactor as a whole. To avoid corrosion of the fuel, it is formed into cylindrical pellets and then inserted in zirconium tubes or other "cladding" materials. The rest of the reactor has to be built out of materials 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 normal carbon steels instead of more expensive corrosion-resistant metals. Some organics also have the advantage that they do not flash into gas in the same fashion as water, which may reduce or eliminate the need for a containment building.

These benefits are offset to a degree by the fact that organics also generally have a lower specific heat than water, and thus require higher flow rates to provide the same amount of cooling. A more significant problem was found in experimental devices; the high-energy neutrons given off as part of the nuclear reactions have much greater energy than the chemical bonds in the coolant, and they break the hydrocarbons apart. This results in the release of hydrogen and various shorter-chain hydrocarbons. The polymerization of the resulting products can turn into a thick tar-like state. Further, many suitable coolants are naturally flammable and sometimes toxic, which adds new safety concerns. Many uses of PCBs were banned beginning in the 1970s as their environmental toxicity was better understood.^[1]

The OCR concept was a major area of research in the 1950s and 60s, including the Organic Moderated Reactor Experiment at the Idaho National Engineering Laboratory, the Piqua Nuclear Generating Station in Ohio, and the Canadian WR-1 at Whiteshell Laboratories. The US experiments explored the use of organics for both cooling and moderation, while the Canadian design used a heavy water moderator, as did the unbuilt EURATOM ORGEL and Danish DOR designs. Ultimately none of these would be used for commercial generators, and only the small experimental reactors at Piqua in the US and Arbus at the Research Institute of Atomic Reactors in the USSR ever generated power, and then only experimentally.