Internal steam generator leak at Temelín NPP (Update)

On 26 June at 2:27 am, during the preparation of power reactor start-up of Unit 2 after the fuel loading, leakage 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 refuelling was underway (Regime 3); for stabilization, an auxiliary condenser was connected. The event was found out by the operator after the stabilisation of parameters based on performing balances of the primary circuit make up.
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 safely 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 with an outer diameter of 16 mm and wall thickness 2,5 mm which passes from the primary collector through the secondary side out of the steam generator).
During the event, as a consequence of the erosion-corrosion damage of the tube in the part which leads by the secondary side of the steam generator, activity penetrated from the primary to the secondary circuit and other systems.  
A significant part of the activity was captured in the steam generator blowdown system and in the system of turbine condensate treatment. A part of activity (mainly tritium) leaked into the Unit 2 circulation cooling water system, and from there, a small amount of activity penetrated into the rain sewage (channels of the cooling water system and rain sewage pipelines are made of concrete and thus not fully impermeable). The negligible activity was further detected in a rain water safety basin and afterwards also 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 by brook “Strouha” to “Hněvkovice” reservoir).
In the Unit 2 cooling water system, the highest activity of gamma emitting radionuclides was measured on 30 June 2015 - 15 Bq/l. Afterwards, it gradually decreased. Since 5 July 2015, the measured values have remained below 1 Bq/l. The highest activity of tritium in cooling water was measured on 26 June 2015 - 1035 Bq/l and afterwards, in consequence of the blowdown, it gradually decreased. The last value - 23 Bq/l - was measured on 26 July 2015. The treated blowdown tritium water is discharged (under the control) to the environment within all limits set by the SÚJB and the water management office. For this purpose, the SÚJB performs its own independent measurements of released water activity.
In the rain water safety basin, the tritium activity successively decreased from the original value of 272 Bq/l on 27 June 2015 to 13 Bq/l on 27 July 2015.

In the holding basin, the tritium activity gradually decreased from its maximum value of 66,5 Bq/l on 3 July 2015 to 38 Bq/l on 27 July 2015.
In Hněvkovice reservoir, on 28 June 2015, the tritium activity was 2,9  Bq/l  - i.e. the standard environmental level (which is also the minimum level measurable by the used method). During further measures, the values remained below the detection level.
During the detailed check of radiation situation after the event, low contamination (8 Bq/cm2 in maximum) of one part of the reactor building roof nearby the bypass valve to atmosphere PSA-4, limited part of the roof of the turbine hall and of a space under the exhaust of the expansion tank of the condensate collecting tank was found. No contamination was detected outside of the NPP. During the event, systems monitoring dose rates in the environment were measuring usual values corresponding to the natural background of Temelín site.
Although there was no significant release of activity during the event, real amount of the coolant leaked from the primary to the secondary circuit is being determined within the scope of its investigation, including the corresponding amount of activity released through one of the safety barriers.

As a consequence of activity detected on the Unit 2 roof, the investigation is ongoing with the goal to find out whether the leak could exist before the detection of  the primary circuit make up tank level decrease, i.e. already during the stabilisation in hot status or testing of PSA at the steam generator 4. This finding is important mainly for a more precise determination of total activity released out of the technology.
According to the existing conservative estimates of the activity released to the atmosphere and the Vltava river, the effective dose for an individual from the critical group of residents has been preliminarily estimated at 0,05 µSv for discharges to atmosphere and 0,05 µSv for discharges to the river. These values are insignificant from the viewpoint of the authorized limit (40 µSv for discharges to atmosphere and 3 µSv for discharges to the river).
The activity released during the event will be, after its specification, included in the yearly balance of NPP discharges, and in the assessment of their impact on population.
After the Unit 2 shutdown to Regime 6, the damaged part of the small tube of the steam generator 4 venting was replaced within the general repair. The repair was performed in accordance with the recommendation of the steam generator manufacturer – by the replacement of the entire small tube, in line with the approved procedures under the supervision of the SÚJB.
In order to eliminate occurrence of similar leak at the remaining steam generators, their inspection with focus on the small tubes of the venting system was carried out; other defects have not been identified. For preventive reasons, all small tubes of the steam generator venting system in the part passing through the secondary space of all steam generators at Unit 2 were replaced.
Following the replacement of the defective small tube, pressure test was performed at all steam generators, and showed no leakage. Both the defective small tube and parts cut out of all other small tubes have been sent for analysis to the ÚJV Řež with the aim to confirm the stated causes of their damage.
Following this event the decision was taken that, in the framework of the current refuelling outage and the general repair of Unit 1, the small tubes concerned will be checked also at all steam generators of Unit 1. After the evaluation of results of their checks, similar type of surface damage of small tubes at the steam generator venting system as at Unit 2 was found, but without the leakage. Therefore it was decided to replace all small venting tubes also at all Unit 1 steam generators.
Based on all the above mentioned activities as well as on the expert opinions of the steam generator manufacturer and ÚJV Řež which proved designed functionality of all these small tubes of the steam generator venting system, the SÚJB issued a license for Unit 2 to reach criticality and to perform start-up tests and connection of Unit 2 to the grid.