Soviet submarine K-431
The Soviet submarine K-431 (originally the Soviet submarine K-31) was a Soviet
Soviet Union
The Soviet Union , officially the Union of Soviet Socialist Republics , was a constitutionally socialist state that existed in Eurasia between 1922 and 1991....

 nuclear-powered submarine
A submarine is a watercraft capable of independent operation below the surface of the water. It differs from a submersible, which has more limited underwater capability...

 that had a 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...

 accident on August 10, 1985. An explosion occurred during refueling of the submarine at Chazhma Bay, Vladivostok
The city is located in the southern extremity of Muravyov-Amursky Peninsula, which is about 30 km long and approximately 12 km wide.The highest point is Mount Kholodilnik, the height of which is 257 m...

. TIME magazine has identified the accident as one of the world's "worst nuclear disasters".

Nuclear reactor plant disaster

The K-431, completed around 1965 as unit K-31, was a Project 675 (Echo II
Echo class submarine
The Echo class submarines were nuclear cruise missile submarines of the Soviet Navy built during the 1960s. Their Soviet designation was Project 659 class for the first five vessels, and Project 675 for the following twenty-nine...

) class submarine with two pressurized water reactor
Pressurized water reactor
Pressurized water reactors constitute a large majority of all western nuclear power plants and are one of three types of light water reactor , the other types being boiling water reactors and supercritical water reactors...

s, each 70 MWt capacity and using 20% 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...

 as fuel. On 10 August 1985, the submarine was being refueled at the Chazhma Bay naval facility near Vladivostok
The city is located in the southern extremity of Muravyov-Amursky Peninsula, which is about 30 km long and approximately 12 km wide.The highest point is Mount Kholodilnik, the height of which is 257 m...

. The submarine had been refueled and the reactor tank lid was being replaced. The lid was laid incorrectly and had to be lifted again with the control rods attached. A beam was supposed to prevent the lid from being lifted too far, but this beam was positioned incorrectly, and the lid with control rods were lifted up too far. At 10:55 AM the starboard reactor became prompt critical
Prompt critical
In nuclear engineering, an assembly is prompt critical if for each nuclear fission event, one or more of the immediate or prompt neutrons released causes an additional fission event. This causes a rapid, exponential increase in the number of fission events...

, resulting in a criticality excursion of about 5x1018 fissions
Nuclear fission
In nuclear physics and nuclear chemistry, nuclear fission is a nuclear reaction in which the nucleus of an atom splits into smaller parts , often producing free neutrons and photons , and releasing a tremendous amount of energy...

 and a thermal/steam explosion. The explosion expelled the new load of fuel, destroyed the machine enclosures, rupturing the submarine's pressure hull and aft bulkhead, and partially destroyed the fueling shack, with the shack's roof falling 70 meters away in the water. A fire followed which was extinguished after 4 hours, after which assessment of the 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...

 began. Most of the radioactive debris fell within 50-100 meters of the submarine, but a cloud of radioactive gas and particulates blew to the northwest across a 6 km stretch of the Dunay Peninsula, missing the town of Shkotovo-22, 1.5 km from the dock. The contaminated forest area was later surveyed as 2 square km in a swath 3.5 km long and 200 to 650 meters wide. Initial estimates of the radioactive release were about 2 MCi of noble gas
Noble gas
The noble gases are a group of chemical elements with very similar properties: under standard conditions, they are all odorless, colorless, monatomic gases, with very low chemical reactivity...

es and 5 MCi of other fission product
Fission product
Nuclear fission products are the atomic fragments left after a large atomic nucleus fissions. Typically, a large nucleus like that of uranium fissions by splitting into two smaller nuclei, along with a few neutrons and a large release of energy in the form of heat , gamma rays and neutrinos. The...

s, but most of this was short-lived isotope
Isotopes are variants of atoms of a particular chemical element, which have differing numbers of neutrons. Atoms of a particular element by definition must contain the same number of protons but may have a distinct number of neutrons which differs from atom to atom, without changing the designation...

s; the estimated release inventory one hour after the accident was about 1000 Ci
CI or Ci may refer to:-Criminology and military:* Counter-intelligence* Confidential Informant* Confidential Information* Crazy Ivan, a U.S...

 of non-noble fission products. In part because the reactor did not contain spent fuel, the fraction of biologically active isotopes was far smaller than in the case of the Chernobyl reactor accident.

M. Takano et. al. suggest that only 29 GBq of I-131 was released, but larger amounts (620 GBq of I-133 and 1840 GBq of I-135). The same source suggests that the total release was about 259 PBq but due to radioactive decay this decreased to 43 TBq after 24 hours. The same source suggests that the fission yield was 5e18 fissions which would deliver 156 MJ of heat into the reactor.

Ten naval personnel were killed (8 officers and 2 enlisted men), probably by the explosion itself and not from radiation
In physics, radiation is a process in which energetic particles or energetic waves travel through a medium or space. There are two distinct types of radiation; ionizing and non-ionizing...

 injuries. Radiation injuries were observed in 49 people, with 10 developing radiation sickness
Radiation Sickness
Radiation Sickness is a VHS by the thrash metal band Nuclear Assault. The video is a recording of a concert at the Hammersmith Odeon, London in 1988. It was released in 1991...

; the latter figure included mostly firefighters, some of whom sustained doses up to 220 rad
Rad (unit)
The rad is a unit of absorbed radiation dose. The rad was first proposed in 1918 as "that quantity of X rays which when absorbed will cause the destruction of the malignant mammalian cells in question..." It was defined in CGS units in 1953 as the dose causing 100 ergs of energy to be absorbed by...

 (2.2 Gy
Gray (unit)
The gray is the SI unit of absorbed radiation dose of ionizing radiation , and is defined as the absorption of one joule of ionizing radiation by one kilogram of matter ....

) external and 400 rem
Röntgen equivalent man
Named after Wilhelm Röntgen , the roentgen equivalent in man or rem is a unit of radiation dose equivalent...

 (4 Sv
The sievert is the International System of Units SI derived unit of dose equivalent radiation. It attempts to quantitatively evaluate the biological effects of ionizing radiation as opposed to just the absorbed dose of radiation energy, which is measured in gray...

) to the thyroid gland. Of the 2,000 involved in cleanup operations, 290 were exposed to high levels of radiation compared to normal standards.

High-level waste gathered during clean-up operations was placed in temporary disposal sites. Due to the rapid decay of most of the fission products and the cleanup operations, some dockyard facilities were able to resume operations four days later. About two months post-accident the radioactivity in water in the cove was comparable to background levels
Background radiation
Background radiation is the ionizing radiation constantly present in the natural environment of the Earth, which is emitted by natural and artificial sources.-Overview:Both Natural and human-made background radiation varies by location....

, and 5–7 months post-accident the radiation levels were considered normal throughout the dock area. The damaged submarine was towed to Pavlovsk Bay and berthed
Berth (moorings)
A berth is a location in a port or harbour used specifically for mooring vessels while not at sea.-Locations in a port:Berth is the term used in ports and harbors to define a specific location where a vessel may be berthed, usually for the purposes of loading and unloading.Most berths will be...

The source of this article is wikipedia, the free encyclopedia.  The text of this article is licensed under the GFDL.