Internal steam generator leak at Temelín NPP

On 26 June at 2:27 am, during the preparation of power reactor start-up of Unit 2 after the fuel loading, an untightness between the primary and secondary circuit (steam generator No. 4) was identified. At that moment, the reactor had been shut down and the unit reheating after the planned outage for refueling was underway (Regime 3); for stabilization, an auxiliary condenser was connected.

The leakage rate was estimated at around 2-3 m3/h.  The leak was compensated with sufficient reserve by systems of normal refilling/make up.

Following the identification of the steam generator leakage, the unit was cooled down to Regime 6 (cold shut-down) as per operating procedures through the bypass valves to atmosphere at “healthy” steam lines while the leaky steam generator was isolated.

The leakage was caused by a small defect of a small tube at the steam generator venting system (the small tube passes from the primary collector through the secondary side out of the steam generator).

Measured dose rates outside the controlled area on 26 June were as follows:

Hot condensate collecting tanks: Tank No.1= 0,25μSv/h; Tank No.2=0,3μSv/h.

Secondary water purification system (filters): Filter No.1=2,1μSv/h; Filter No.2=5,1μSv/h.

Steamline No.4=0,6μSv/h.

During the event, an activity leaked from the primary circuit into the secondary circuit and other systems.

At present, active media from individual technological systems are step by step treated (cleaned) by a standard procedure and released by a standard way into the river. Discharges remain deeply below the authorized limits. As for tritium activity, the same amount of activity would be released anyway.

The activity leaked into the Unit 2 cooling water system, and from there, a small amount of activity penetrated into the rain sewage. The activity was detected in a rain water safety basin and afterwards in a holding basin (the pond “Mlýnský rybník“), to where water passes from the rain water safety basin (from the holding basin, water flows into the reservoir “Hněvkovice”).

In the rain water safety basin, the tritium activity successively decreases from the value of 272 Bq/l on 27 June to 105 Bq/l on 2 July. On the same day, the Cs-137 activity was 0.2 Bq/l and the Cs-134 activity was 0.3 Bq/l.

In the holding basin, the tritium activity gradually increased from 20 Bq/l on 27 June to 56 Bq/l on 1 July, and then started to decrease – 53 Bq/l on 2 July. All these values are below the guidance level for drinking water (100 Bq/l). Artificial gamma-radionuclides’ activity (including Cs-134 and Cs-137) is below the detection level.

In the Hněvkovice reservoir, the tritium activity is on the standard environmental level – about 2 Bq/l. Artificial gamma-radionuclides have not been detected.

Cleaning of Unit 2 secondary circuit continues with the use of secondary water purification system BUK. During the detailed check of radiation situation after the event extremely low contamination  (0,8 µSv per hour) of one part of the reactor building roof was found nearby the bypass valve to atmosphere PSA-4. These radiation levels are well below the authorized limits.

Preliminary rating of event is INES = 1

Anybody interested in radiation situation at Temelín site and its vicinity can check it anytime on-line at MonRaS - Monitoring of Radiation Situation.