It started as a routine task: a German-French joint company had to clean the fuel elements at the Hungarian Paks-2 reactor. However, it almost ended in a Chernobyl-like nuclear disaster thanks to a series of grave human errors because of which radiation got into the environment.
Too much trust in the German-French company
In 1986, the biggest nuclear disaster in Chernobyl could happen because of the design problems of the RBMK-reactors and a lot of human errors. In Hungary, something similar happened during the cleaning of the fuel elements of the Paks-2 reactor on April 10, 2003.
According to atomcsapda.blog.hu, it all started with the fact that magnetite corrosion products started to coat the fuel rods of Paks-2 reactor, which affected the flow of coolant and thus, it brought down the efficacy of the reactor. Therefore, cleaning they had to be cleaned for which
a German-French joint company, the Framatome, was hired.
They had experience in such works; their cleaning vessel was already successfully used at Paks-2 in 2001.
The 467 MW Paks-2 was taken offline on 28 March for its annual refuelling and maintenance period during which some of the fuel elements were cleaned. In fact, the process happened under 10 metres of water but with a new cleaning vessel of Framatome never tried before.
On April 10, at 21:50, however, the radiation alarms turned on, and the operators thought that it has happened because one of the fuel rod assemblies was leaking. At 22:30, the reactor hall was evacuated because of increased radiation levels both there and in the cleaning system’s ventilation stack. At 02:15 the following morning, the hydraulic lock of the cleaning vessel lid was released to remove it but immediately
the dose rate increased significantly (6-12 millisieverts/hour).
As a result, radioactivity could get into the environment.
We could evade the nuclear disaster
Since one of the three lifting cables attached to the lid broke, it could have finally been removed only on 16 April. Scientists examining the cleaning tank observed that the fuel elements in the tank were damaged meaning that radioactive spent uranium fuel pellets from the fuel elements got into 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.
To prevent this, water containing neutron-absorbing boric acid was added into the tank together with ammonia and hydrazine to help with the removal of radioactive iodine-131. They were successful, and by the time the Austrian environmental NGO Global 2000’s Radiation Monitoring System arrived, the radiation level had already decreased below the stable background radiation.
According to the investigation of the Hungarian Atomic Energy Agency HAEA), the incident was caused by inadequate cooling of Framatome’s new cooling device which resulted in overheated fuel rods that broke the moment they were doused with 37 Celsius degree water when the lid was open. This could happen because the HAEA placed too much trust in the technology and knowledge of the Framatome Company; they wanted to save time, so they did not investigate documentation provided by the company deeply enough. Therefore, they allowed the procedure even though
it was clear that the cleaning system of the company had a fatal design flaw.
In fact, the incident was classified to level 3 (“serious incident”) by the International Nuclear Event Scale (INES). The Paks-2 reactor remained out of service until 2004 while the radioactive and broken fuel rods were transported only in 2014 to Russia.
Source: atomcsapda.blog.hu, wiseinternational.org