Facts concerning the issue of Temelin NPP containment leakage
Recently, a new point of interest occurred concerning insufficiencies in the automatic containment insulation at the Temelin Nuclear Power Plant which may result in leakage and in the case of an accident to a higher release of radioactivity to the environment. Number of formal deficiencies of the plant operator occurred during assessment of the discordance between the original Russian and the upgraded American design of the containment system, which are subject to an administrative proceeding of the State Office for Nuclear Safety. Nevertheless, the situation is not as serious as it may be understood from certain statements and articles in media. The last was the article in Euro weekly magazine dated July 17, 2006. We tried to clarify number of „misunderstandings“included in the article at the SUJB web site. This notice only concerns the containment insulation issue.
The containment, including other safety systems, fulfils number of very important functions at the nuclear power plant:
it encloses all components of the primary circuit including other systems and protects them from external effects,
it protects the nuclear power plant surroundings from radiation both during normal operation and in the case of accidents,
in the case of an accident the containment retains most of the released radioactive substances and contributes to significant reduction of radioactivity. This results from natural processes (radioactive decay, settling) as well as from activities of special safety systems (pressure reduction and heat removal system etc.).
The containment is the last stage of the physical barriers separating radioactive products from the environment. Among such barriers belong the fuel itself, fuel rods cladding as well as the reactor vessel and the primary circuit walls.
It is obvious that the safety function of the containment is very important and therefore the design and construction of the containment is subject to strict requirements. Parameters are selected in such manner that the containment must maintain its tightness at any circumstances considered by the design and fulfill its function.
The Temelin nuclear power plant containment is based on the Russian VVER 1000/320 design. It is a classic single full-pressure structure made of pre-stressed concrete with 1.2 m wall thickness and 8mm thick internal steel liner ensuring tightness.
There are many pipes penetrating the containment, the diameter of which varies from tens of cm [Fig. 1] to several mm [Fig.2]. They are required for safe and reliable operation of the nuclear power plant. Many of them are necessary also for coping with eventual accidents and mitigation of consequences. Most pipes penetrating the containment wall are immediately automatically sealed by so called quick-acting valves [Fig, 3] in the case of accident and so the internal part is separated from the environment. In addition there are pipes related to the safety systems, which are located outside the containment. These are not equipped with any closing mechanism since safety systems intended for accident management must not be separated under any circumstances at any power plant.
During an accident it is required to have available as detail information as possible concerning the situation inside the containment, including fuel state, primary circuit and climate in the containment. Pipes of the post-accident monitoring system (so called routes) serve for collection of such information. They enable collection of liquid and gas samples (they also penetrate the containment wall). There are several of them (5routes, each of them with 3 valves) and diameter of monitoring pipes is about 15 mm. Inside of them, there is a nozzle decreasing pressure inside the pipes and reducing the internal diameter to 1, 3 mm and 1 mm. In the case of accident they are not closed automatically but they can be sealed by an operator from the control room by pressing a button of the valve electric drive remote control. The objective of the post accident sampling and monitoring is to receive timely and as accurate as possible information concerning the plant status and radiation situation inside the equipment after accident and so support the protection measures in the surroundings. The original Russian design did not consider monitoring of situation inside the containment and therefore required automatic closing of such routes.
What would happen after an accident associated with a need of the containment isolation? In the worst case, the above mentioned 15 valves (or at least one at each route - i.e. at least 5 valves) would not be closed by the operator for whatever reason. In such case radiation would release through the reduction membrane with diameter appr. 1 mm to the sampling equipment and the consequential chemical analysis equipment. Such equipment is not analyzed for emergency conditions, i.e. their manufacturer does not guarantee non-proliferation of radioactivity to other premises. And what premises does it concern? It so called controlled zone which is a space in the containment enclosure building where certainly a very strict radiation monitoring is performed. Therefore it can be very realistically assumed that the operator would close the valve at latest upon signaling of radioactivity level increase. Nevertheless even in the case that the operator would make a faulty analysis of the situation and did not close the valves, radioactivity would not leak to the environment. This is because air in the controlled zone is discharged through filtration ventilation unit which would stop any eventual radioactive aerosols.
This would be the worst case but even that can't occur anymore. Upon discussion between the State Office for Nuclear Safety and CEZ power company 9 valves of the post-emergency containment sampling system are permanently closed since July 2006. The remaining 6 pipes on the routes of reactor media uninterrupted sampling can not be closed permanently. In the case of emergency conditions in the containment they will be immediately closed by an operator from the unit control room using the above described method and in accordance with the existing procedures.