Paks Nuclear Power Plant

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The Paks Nuclear Power Plant, located 5 km from Paks, central Hungary, is the only operating nuclear power station in Hungary. Between them, its four reactors produce more than 40 percent of the electrical power generated in the country.

Station	Type	Net capacity	Construction date	Grid date
PAKS-1	VVER-440/V213	437 MWe	01-Aug-74	28-Dec-82
PAKS-2	VVER-440/V213	441 MWe	01-Aug-74	06-Sept-84
PAKS-3	VVER-440/V213	433 MWe	01-Oct-79	28-Sept-86
PAKS-4	VVER-440/V213	444 MWe	01-Oct-79	16-Aug-87

VVER is the Soviet designation for a pressurized water reactor. The number following VVER, in this case 440, represents the power output of the original design. The VVER-440 Model V213, was a product of the first uniform safety requirements drawn up by the Soviet designers. This model includes added emergency core cooling and auxiliary feedwater systems as well as upgraded accident localization systems.

Each reactor contains 42 tons of slightly enriched uranium dioxide fuel. Once the fuel has been used, which takes four years per reactor, the fuel rods are stored for five years in a cooling pond adjacent to the reactor before being removed from the site for permanent disposal.^[1]

[edit] The 2003 incident

An INES level 3 event ("serious incident") occurred on 10 April 2003 at the Unit 2 reactor. The incident occurred in the fuel rod cleaning system located under 10 metres of water in a cleaning tank next to the spent fuel cooling pond, located adjacent to the reactor in the reactor hall. The reactor had been shut down for its annual refuelling and maintenance period on 28 March and its fuel elements removed.^[2]

The cleaning system had been installed to remove dirt and corrosion from fuel elements and control rods during shutdown, as there had previously been problems with magnetite corrosion products from the steam generators being deposited on the fuel elements which affected the flow of coolant. The sixth set of thirty partially spent elements were in the tank having been cleaned, the cleaning having been finished at 16:00. At 21:50, radiation alarms mounted on the cleaning system detected a sudden increase in the amount of krypton-85. The suspicion was that one of the fuel rod assemblies was leaking. At 22:30, the reactor hall was evacuated because of elevated radiation levels in it and its ventilation stack.^[3]

At 02:15 the following morning, the hydraullic lock of the cleaning vessel lid was released, and immediately the dose rate increased significantly (6-12 millisieverts/hour) around the spent fuel pond and the pool containing the cleaning machine, and the water level dropped for a short time by about 7 cm. Water samples from the pond showed contamination due to damaged fuel rods. The lid on the cleaning machine was winched up at 04:20, but one of the three lifting cables attached to it broke; and it was not finally removed until 16 April.

The incident was initially given an INES rating of 2 ("incident"). However a video examination of the damaged fuel elements following the succesful removal of the lid caused the rating to be raised to 3 ("serious incident"). This revealed that cladding on the majority of the 30 fuel elements had been broken so that uranium fuel pellets, containing fission products, had fallen out of them and dropped to the bottom of the cleaning tank. Apart from the release of radioactive material, a concern was that the accumulation of a compact mass of fuel pellets could lead to a criticality accident, as the pellets were in a tank of neutron moderating water. Water containing neutron absorbing boric acid was added into the tank to raise its concentration to 16 g/kg to prevent this. Ammonia and hydrazine were also added to the water to help with the removal of radioactive iodine-131.

An investigation by the Hungarian Atomic Energy Agency concluded that the cause of the incident was inadequate cooling of the fuel elements which were heated through the radioactive decay of short-lived fission products. These were kept cool by water circulated by a submerged water pump. However the cooling was inadequate, leading to the damage to some elements through a build-up of steam around them which deprived them of most of their cooling. The investigation proposed that the severe damage probably occurred when the lid was released, causing thermal shock to cladding because of the sudden entry of cool water into the system, and explosive steam production. ^[3]

The discharge of radioactive gases through the stack continued for several days after the incident, although the Hungarian Atomic Energy Agency determined that the radiation levels adjacent to the plant were only about 10% above normal. However, the reactor remained out of service for over a year, finally resuming commercial electricity production in September 2004.^[4]