Passive autocatalytic recombiner (PAR) is a device that removes

hydrogen Hydrogen is the chemical element with the symbol H and atomic number 1. Hydrogen is the lightest element. At standard conditions hydrogen is a gas of diatomic molecules having the formula . It is colorless, odorless, tasteless, non-toxic ...

from the

containment Containment was a geopolitical strategic foreign policy pursued by the United States during the Cold War to prevent the spread of communism after the end of World War II. The name was loosely related to the term '' cordon sanitaire'', which w ...

of a nuclear power plant during an

accident An accident is an unintended, normally unwanted event that was not directly caused by humans. The term ''accident'' implies that nobody should be blamed, but the event may have been caused by unrecognized or unaddressed risks. Most researche ...

. Its purpose is to prevent hydrogen explosions. Recombiners come into action spontaneously as soon as the hydrogen concentration increases. They are passive devices because their operation does not require external energy. Hydrogen may be generated in a nuclear accident if the reactor

fuel A fuel is any material that can be made to react with other substances so that it releases energy as thermal energy or to be used for work. The concept was originally applied solely to those materials capable of releasing chemical energy bu ...

overheats and

zirconium Zirconium is a chemical element with the symbol Zr and atomic number 40. The name ''zirconium'' is taken from the name of the mineral zircon, the most important source of zirconium. The word is related to Persian '' zargun'' (zircon; ''zar-gun'' ...

cladding of the fuel rods reacts chemically with steam. If the hydrogen is released from the reactor to the containment, it may get mixed with air and form a flammable or even explosive mixture. A hydrogen explosion could break the containment and cause radioactive materials to be released to the environment. Recombiners aim at removing hydrogen and thereby preventing explosions. Inside a recombiner there are plates or pellets that are coated with

platinum Platinum is a chemical element with the symbol Pt and atomic number 78. It is a dense, malleable, ductile, highly unreactive, precious, silverish-white transition metal. Its name originates from Spanish , a diminutive of "silver". Pla ...

palladium Palladium is a chemical element with the symbol Pd and atomic number 46. It is a rare and lustrous silvery-white metal discovered in 1803 by the English chemist William Hyde Wollaston. He named it after the asteroid Pallas, which was itself nam ...

catalyst Catalysis () is the process of increasing the rate of a chemical reaction by adding a substance known as a catalyst (). Catalysts are not consumed in the reaction and remain unchanged after it. If the reaction is rapid and the catalyst recyc ...

. On the surface of the catalyst, hydrogen and oxygen molecules react chemically at low temperature and low hydrogen concentration. The reaction generates steam. The reaction starts spontaneously when the hydrogen concentration reaches 1–2 percent. Burning of hydrogen in air requires at least 4 percent hydrogen concentration, and even higher for an explosion. Therefore a recombiner is able to remove hydrogen from the containment before a flammable concentration is reached. A recombiner is a box that is open from the bottom and from the top. The catalyst is located at the lower part of the box. The reaction of hydrogen and oxygen on the catalyst surface generates heat, and temperature in the recombiner reaches hundreds of degrees Celsius. Hot steam is lighter than the air in the containment, so buoyancy is caused inside the recombiner, much like in a chimney. This causes a strong airflow through the recombiner, feeding hydrogen and oxygen from the containment to the device. Hundreds of kilograms of hydrogen may be generated in a few hours during a severe reactor accident. The most efficient recombiner made by

Framatome Framatome () is a French nuclear reactor business. It is owned by Électricité de France (EDF) (75.5%), Mitsubishi Heavy Industries (19.5%), and Assystem (5%). The company first formed in 1958 to license Westinghouse's pressurized water reacto ...

(formerly Areva) removes slightly over five kilograms of hydrogen per hour when the hydrogen concentration is four percent. Therefore many recombiners are needed. For example the containment of

Olkiluoto 3 The Olkiluoto Nuclear Power Plant ( fi, Olkiluodon ydinvoimalaitos, sv, Olkiluoto kärnkraftverk) is one of Finland's two nuclear power plants, the other being the two-unit VVER Loviisa Nuclear Power Plant. The plant is owned and operated b ...

EPR in Finland has 50 recombiners. Manufacturers of passive autocatalytic recombiners include Framatome,

SNC-Lavalin SNC-Lavalin Group Inc. is a Canadian company based in Montreal that provides engineering, procurement, and construction (EPC) services to various industries, including mining and metallurgy, oil and gas, environment and water, infrastructure, ...

(formerly Atomic Energy of Canada Ltd, AECL), and German Siempelkamp-NIS.

References

{{reflist Catalysis Nuclear power plant components Nuclear safety and security Hydrogen

See also

References