Serious incident at Hungarian Paks-2 reactor

Nuclear Monitor Issue: 

(April 25, 2003) A serious incident has happened in cleaning fuel elements at the Hungarian Paks-2 reactor. A majority of the 30 fuel elements in a 'washing' machine got severely damaged and radioactivity was released into the environment. The incident was initially classified at level 2 ("incident") of the International Nuclear Event Scale (INES), but later reclassified to level 3 ("serious incident").

(586.5507) WISE Amsterdam - The incident was discovered by the plant at 22.30h on 10 April, when a sudden increase in released radioactive gases through the exhaust stack was observed. Radiation monitors inside the reactor hall also reached alarm levels. The reactor hall was evacuated immediately after the alarm signals were activated. Radiation levels decreased when technical measures were taken at a fuel cleaning system with 30 spent fuel elements inside (1).

In the cleaning system, dust and corrosion particles are removed from fuel elements. The system was hired from Framatome ANP (a joint company of French Areva and German Siemens). The cleaning system is placed on the bottom of the spent fuel pool, next to the reactor, and consists of a container in which fuel elements are placed (2).

The cleaning system was first used at Paks-2 in 2001. At that time it was even the first ever trial in the world of this new CORD UV/AMDA technology. Paks-2 had experienced problems with Russian fuel elements due to the presence of corrosion deposits. These deposits resulted in coolant flow problems which had resulted in an unscheduled refueling outage. In 2001, the new CORD UV/AMDA chemical cleaning system was used for the first time and 148 fuel elements and 22 control rods were cleaned (3).

The 467 MW Paks-2 was taken offline on 28 March for its annual refueling and maintenance period. The fuel elements from the reactor were stored in the spent fuel pool next to the reactor. Part of the fuel elements were to be cleaned to operate more effectively during the further operation of the reactor (4).

When the incident at 10 April started, radioactive gas discharges were detected as coming from the cleaning system. The operators think that the radioactivity escaped due to insufficient cooling of the fuel elements inside the system. The release of radioactive gases through the NPP's chimney continued during some days but were lower than during the first hours of the incident (5).

On 11 April Paks personnel tried to open the lid of the cleaning container but the cable of the crane broke and the lid was half open. That allowed radioactive gases to escape from the container, bubble up from the water, and enter the reactor hall and eventually escape into the environment. According to the Hungarian Atomic Energy Authority (HAEA), radioactivity outside the plant would be only 10% higher than normal (6).

Fuel damaged
When the head of the cleaning container was completely removed on 16 April, video inspections revealed that most of the elements (probably even all of them) had been damaged (7). The extent of the fuel damage was reason to newly classify the incident at level 3 of the INES scale (8). The HAEA however had doubted to uprate the incident at level 3 because it feared that such a rating could cause unwarranted public "excitement". But they had to admit that a level 3 was justified "because the situation in the tank was worse than expected" (9).

The damage to the 30 fuel elements is very serious as it appeared that elements had been broken and its uranium pellets had fallen to the bottom of the cleaning system. The damage of the elements might have occurred when the operators discovered that the cooling inside worked improperly (after having observed an increase in radioactivity). A sudden flow of cold water into the system could have broken the fuel claddings due to thermal stress.

The amount of fuel pellets at the bottom of the system raises another concern: that a criticality accident could occur. In that case, a chain reaction will start when sufficient fissionable material is put together. Neutron absorbing borated water has been injected into the fuel pool to prevent this (10).

According to an official of the Hungarian Nuclear Inspectorate, the restart of Paks-2 could be delayed by one year (11). Hungarian press agency MTI has reported that the incident could have been that serious that Paks-2 possibly never get permission to restart (12).

The plant has established eight teams of experts to make a plan for the removal of the fuel. The teams include experts from the plant, the Hungarian university and specialists from Germany and Russia (13).

The Energy Club has urged the responsible authorities to reveal the details about the accident and the releases of radioactivity into the environment. Until now they have not been satisfied with communication from the plant itself. They criticize the Hungarian government which is venting statements like "no problem" and "calm down" and thus neglecting the seriousness of what happened at Paks (14).

The Austrian environmental NGO Global 2000 sent its Radiation Monitoring System (RAMOS) team to Hungary when it learned from the incident in the neighbouring country. On 17 April they measured the radiation levels next to the plant and found 90 to 130 nanoSievert per hour, which is a level of normal natural background radiation. But at that time, a week after the incident, discharges from the chimney were back to normal. Radiation levels immediately after the incident could have been much higher. Therefore the RAMOS team wanted to collect soil samples near the plant (to check for radioactive fallout) but were interrupted by the police. RAMOS has demanded full openness by the Hungarian authorities and the release of all data on radiation levels after the incident (15).

Four Russian designed VVER 440-213 reactors are located at the Paks NPP. The three other Paks reactors continued to operate after the incident at Paks-2 (16).


  1. INES Event Rating Form,, 11 and 17 April 2003
  2. Information from Ada Amon, Energy Club Hungary, 23 April 2003
  3. Advanced Nuclear Power, Framatome ANP, August 2001
  4. INES Press Release Detail,, 14 April 2003
  5. INES Press Release Detail,, 17 April 2003
  6. Nucleonics Week, 24 April 2003
  7. INES Event Rating Form,, 17 April 2003
  8. INES Press Release Detail,, 17 April 2003
  9. Nucleonics Week, 24 April 2003
  10. Information from Ada Amon, Energy Club Hungary, 23 April 2003
  11. Magzar Hirlap, 22 April 2003
  12. MTI (Hungarian News Agency), 23 April 2003
  13. MTI (Hungarian News Agency), 19 April 2003
  14. Information from Ada Amon, Energy Club Hungary, 23 April 2003
  15. Global 200 press release, 18 April 2003
  16. Nucleonics Week, 17 April 2003

Contact: Ada Amon at Energy Club Hungary, 1056 Budapest, Szerb utca 17-19, Hungary
Tel: +361 411 3534
Fax: +361 411 3529