Stress tests of Russian nuclear power plants confirmed that their designs ensure protection against external natural and man-induced impacts and, for some plants, provide strength margins allowing to withstand beyond design basis seismic impacts. This is said in a Rostechnadzor’s report on a degree of protection of Russian NPPs.
“The stress tests conducted have confirmed that the Russian NPPs are protected against external impacts subject to consideration in the design bases of NPPs. At the same time, for some NPPs it is required to carry out additional refining calculations of stability of building structures with regard to tornado and extreme snow loads,” the document says.
Stress tests for individual power units of NPPs, in particular, for units with VVER-1000 reactors, demonstrated that there are “strength margins with regard to seismic impacts which exceed the impacts subject to consideration in the NPP design bases.” In case of containments of NPPs with VVER-1000 reactors such margin is about 1 point in excess of the safe shutdown earthquake as per MSK-64 intensity scale.
Rafael Arutyunyan, First Deputy Director of the Nuclear Safety Institute of RAS, said in an interview with an ITAR-TASS correspondent that upgrading of nuclear reactors “is a nearly endless process” because it is carried out not for an economic benefit but for safety enhancement. “The aspects which were not considered at the time of the design development, when there were no such requirements, have to be compensated for. This is achieved by various additional measures, accident management systems,” the expert explained.
However, some countries do not do these, according to Arutyunyan. He gave the example of Fukushima-Daiichi NPP which had suffered an accident in 2011. “It looked like an upgrading was done there, but in reality it came out that it had been on paper only. Since in case of tsunami, it is not enough to expect just a rise of water level.
One has to be ready for a dynamic load associated with a wave approach. And the height should not be 3 meters but 10-meter high waves should be considered. It is one of Fukushima lessons that says upgrading should be continuous,” Arutyunyan emphasized.
The Deputy Director of the institute noted that in Russia NPP upgrading is a continuous process and the stress tests were passed successfully owing to this fact, while Rostechnadzor’s recommendations will entail new actions to enhance safety.
Arutyunyan drew attention to the fact that it is necessary to upgrade not only old but also modern NPPs, for example, of the Russian project AES-2006. “It is the matter of proactive upgrading of the Russian designs,” he said. So the passive protection system, which ensures reactor cooling without an off-site power source, in the past were required to operate for 24 hours, now it is 72 hours, according to him. “In this country it is not just following regulations and the letter of the law, it is the safety strategy,” the expert is confident.
Arutyunyan believes that “a frequency of accidents does not depend on who owns the plant, a private entity or the state.” “The economic consideration, when they economize on modernization, may prevail but many things depend on the system. For example, in the USA nuclear power plants are operated by private companies but there is a very strong regulator there. While in Japan, as Fukushima NPP demonstrated, they did not take seriously severe accidents and possibility of their occurrence; both the operator and regulator,” Arutyunyan concluded.
He emphasized that “after the stress tests Rosenergoatom has planned for implementation of a set of measures to strengthen the safety systems.” In particular, this concerns the hydrogen explosion-protection system, monitoring of a gas concentration. “It is not something new; it is just the thing that has worked for a long time is retested. Because they started to install recombiners /hydrogen protection system/ 8 years ago,” the expert clarified.
“In terms of nuclear power development, the priority is given to safety not because plants a priori unsafe. The matter is that in spite of the fact that plants are safe in terms of their impact on humans and the environment, situations where an off-site release affects a significant part of the public, when economic consequences already arise, are absolutely unacceptable. The society does not like radiation, resettlement, fear for relatives etc. All these have to be considered, if a country wants to develop nuclear power and business. So the Generation III plants and that of III+ are a message that one has to meet not only purely technical requirements but also that of public perception.