Lucens reactor
Encyclopedia
The Lucens reactor at Lucens
Lucens
Lucens is a municipality in the Broye-Vully district in the canton of Vaud in Switzerland.-History:Lucens is first mentioned in 964 as in villa Losingus. It was formerly known by the German name Losingen....

, Vaud
Vaud
Vaud is one of the 26 cantons of Switzerland and is located in Romandy, the French-speaking southwestern part of the country. The capital is Lausanne. The name of the Canton in Switzerland's other languages are Vaud in Italian , Waadt in German , and Vad in Romansh.-History:Along the lakes,...

, Switzerland, was a small pilot
Pilot plant
A pilot plant is a small chemical processing system which is operated to generate information about the behavior of the system for use in design of larger facilities....

 nuclear reactor
Nuclear reactor
A nuclear reactor is a device to initiate and control a sustained nuclear chain reaction. Most commonly they are used for generating electricity and for the propulsion of ships. Usually heat from nuclear fission is passed to a working fluid , which runs through turbines that power either ship's...

 destroyed by an accident in 1969.

In 1962 the construction of a Swiss-designed pilot
Pilot plant
A pilot plant is a small chemical processing system which is operated to generate information about the behavior of the system for use in design of larger facilities....

 nuclear power plant
Nuclear power plant
A nuclear power plant is a thermal power station in which the heat source is one or more nuclear reactors. As in a conventional thermal power station the heat is used to generate steam which drives a steam turbine connected to a generator which produces electricity.Nuclear power plants are usually...

 began. The heavy-water moderated, carbon dioxide
Carbon dioxide
Carbon dioxide is a naturally occurring chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom...

 gas-cooled, reactor was built in an underground cavern and produced 30 megawatts of heat (which was used to generate 8.3 megawatts of electricity). It became critical in 1966 and the plant was decommissioned in 1968. It was fueled by 0.96% enriched uranium
Enriched uranium
Enriched uranium is a kind of uranium in which the percent composition of uranium-235 has been increased through the process of isotope separation. Natural uranium is 99.284% 238U isotope, with 235U only constituting about 0.711% of its weight...

 alloyed with chromium
Chromium
Chromium is a chemical element which has the symbol Cr and atomic number 24. It is the first element in Group 6. It is a steely-gray, lustrous, hard metal that takes a high polish and has a high melting point. It is also odorless, tasteless, and malleable...

 cased in magnesium
Magnesium
Magnesium is a chemical element with the symbol Mg, atomic number 12, and common oxidation number +2. It is an alkaline earth metal and the eighth most abundant element in the Earth's crust and ninth in the known universe as a whole...

 alloy (magnesium with 0.6% zirconium
Zirconium
Zirconium is a chemical element with the symbol Zr and atomic number 40. The name of zirconium is taken from the mineral zircon. Its atomic mass is 91.224. It is a lustrous, grey-white, strong transition metal that resembles titanium...

) inserted into a graphite
Nuclear Graphite
Nuclear graphite is any grade of graphite, usually electro-graphite, specifically manufactured for use as a moderator or reflector within nuclear reactors...

 matrix. Carbon dioxide gas was pumped into the top of the channels at 6.28 MPa and 223 °C and exited the channels at a pressure of 5.79 MPa and at a temperature of 378 °C.

It was intended to operate until the end of 1969, but during a startup on January 21, 1969, it suffered a loss-of-coolant accident, leading to a partial core meltdown and massive radioactive contamination
Radioactive contamination
Radioactive contamination, also called radiological contamination, is radioactive substances on surfaces, or within solids, liquids or gases , where their presence is unintended or undesirable, or the process giving rise to their presence in such places...

 of the cavern, which was then sealed.

The accident was caused by water condensation forming on some of the magnesium alloy fuel element components during shutdown and corroding them. The corrosion
Corrosion
Corrosion is the disintegration of an engineered material into its constituent atoms due to chemical reactions with its surroundings. In the most common use of the word, this means electrochemical oxidation of metals in reaction with an oxidant such as oxygen...

 products from this accumulated in some of the fuel channels. One of the 73 vertical fuel channels was sufficiently blocked by it to impede the flow of carbon dioxide coolant so that the magnesium alloy cladding melted and further blocked the channel. The increase in temperature and exposure of the uranium metal fuel to the coolant
Coolant
A coolant is a fluid which flows through a device to prevent its overheating, transferring the heat produced by the device to other devices that use or dissipate it. An ideal coolant has high thermal capacity, low viscosity, is low-cost, non-toxic, and chemically inert, neither causing nor...

 eventually caused the fuel to catch fire in the carbon dioxide coolant atmosphere. The pressure tube surrounding the fuel channel split because of overheating and bowing of the burning fuel assembly, and the carbon dioxide coolant leaked out of the reactor.

No irradiation
Irradiation
Irradiation is the process by which an object is exposed to radiation. The exposure can originate from various sources, including natural sources. Most frequently the term refers to ionizing radiation, and to a level of radiation that will serve a specific purpose, rather than radiation exposure to...

of workers or the population occurred, though the cavern containing the reactor was seriously contaminated. The cavern was decontaminated and the reactor dismantled over the next few years.

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