Damage to the 30 fuel assemblies occurred inside a cleaning tank designed, manufactured and operated by Framatome ANP.
The incident was provisionally registered as level 2 on the International Nuclear Event Scale (INES). The incident has been reevaluated, and given a final rating of INES 3.
A statement issued by Paks said it appears that insufficient cooling of the fuel elements within the cleaning tank submerged in the transfer pond was responsible for their irreparable damage due to overheating. Five batches of fuel had been successfully cleaned before the incident occurred. It will take some time to remove the damaged fuel and make provision for it, thus extending the plant's outage at a cost of some $220,000 per day — plus around $450,000 to replace the written-off fuel.
The Hungarian Atomic Energy Authority (HAEA) has begun an 'independent investigation' into the incident. The HAEA said its decision to launch the investigation was 'contrary to routine practice' and that the 'data collection phase' had already been completed. The HAEA is due to publish its findings at the end of this month. A report by Paks into the incident was submitted to the HAEA earlier this month.
Order of events
Oxalic acid cleaning of the fuel assemblies was completed by at around 4:00pm on 10 April. Since work was being carried out on the reactor internals, the crane and the fuel handling machine were not available for removal of the cleaning tank lid and unloading of the fuel assemblies.
The cleaning tank was continuously maintained at 37 degrees C by Framatome ANP specialists by circulating the coolant using a submergible pump. The pressuriser tank was continuously filled through the vent line of the cleaning tank. At 7:20pm, measurements indicated that the pressuriser level had increased by around 70mm in about 20 minutes. This level change was also detected in the level measurement of the decay pool.
At 9:50pm, the cleaning system krypton-measuring display showed an increase in activity. At 9:53pm, the 'emergency' level was reached by the noble gas activity concentration monitors within the reactor hall, and then the operational dosimetry systems installed in the ventilation stack indicated an abrupt increase of noble gas activity (0.2x10E13Bq/10min). Soon after, the shift supervisor ordered an extraordinary maintenance work committee meeting for 1:00am.
At 11:30pm, dosimetry service personnel measured 20mSv/h dose rate in the cleaning unit by-pass tank. Due to the dose rates, the shift supervisor instructed workers to evacuate the area except the two Framatome ANP employees who had to monitor the cleaning tank cooling system. They were supplied with breathing apparatus.
The extraordinary maintenance work committee decided to open the cleaning tank in order to carry out its visual inspection and, if possible, to separate the leaking fuel assembly; they also decided to analyse the water in the decay pool. The first results of water chemistry analysis were obtained at 4:00am on 11 April. The fission products Cs-134 and -137, I-131 and -132, Xe-133, and Kr-85 were identified in the samples at activity level of 10E4-10E7Bq/kg.
At 2:15am, the hydraulic lid locking system of cleaning tank was unlocked. An immediate large increase in activity (3,1x10E7MBq/10min noble gas release) was observed in the dosimetry control and monitoring system. At the same time the level of the decay pool decreased from 13,410mm to 13,340mm and a pressuriser measuring location also indicated level decrease by around 70mm.
During the cleaning tank lid removal operation, one of the three cables of the lifting tackle broke, thus the lid removal failed. The covering lid of the cleaning vessel was lifted on 16 April, and a video camera was inserted. According to the images, the condition of the assemblies was significantly worse than expected. Most of the assemblies were deformed and claddings opened.
The Paks report states: "The direct reason of the emergency was the insufficient cooling. Prior to the investigation of the potential reasons it is absolutely necessary to evaluate the cooling requirement." According to a joint statement issued by the Hungarian Atomic Energy Authority (HAEA) and Governmental Coordination Committee: "There is no increase in the atmospheric emissions, and a coordinated programme for environmental measurements has started. There is no detectable increase in population doses. Currently, there is no need for the introduction of any countermeasures."