THE DISASTER at the Fukushima nuclear power plant in Japan in March, which was caused by an earthquake and subsequent tsunami, raised the question of the safety of nuclear power plants worldwide. In response, the operators of nuclear plants within the European Union were required to undertake comprehensive risk and safety assessments under the supervision of their national regulatory authorities. In Slovakia, these are being done by Slovenské Elektrárne (SE), the operator of nuclear power plants in Jaslovské Bohunice and Mochovce, under the supervision of Slovakia’s Nuclear Regulatory Authority (ÚJD). In mid September, the authority delivered to the European Commission its interim national report on stress tests of the country’s nuclear power plants. The deadline for submitting the closing national report is December 31.
The power plants subject to the tests are the two units of the Jaslovské Bohunice V2 nuclear power plant and the two operating units at the Mochovce nuclear power plant, plus the two further units in Mochovce which are currently under construction. All the nuclear power plants in Slovakia have so far passed all the tests, a result which came as no surprise to the ÚJD.
“The stress tests have not identified any deficiencies requiring immediate remedial measures or a shutdown of operating nuclear power plants,” the ÚJD wrote in its press statement. “The response of the tested power plants to the stress tests corresponds with the required safety level.”
Peter Uhrík, chief executive of the ÚJD, said on September 14, when the interim report was sent to Brussels, that his organisation is convinced that the nuclear power stations in Slovakia are robust enough, but that the tests had shown space for improvement and that authority did not therefore consider them to have been useless or worthless.
“These tests have shown that it is necessary to test some non-standard routes and have highlighted what a nuclear power station needs to have in place in order to be supplied with electricity, because one of the worst possible events for a nuclear power station is when it loses its power supply, a situation known as a station blackout.”
What are stress tests for nuclear power stations?
After the Fukushima disaster, in which a major earthquake and subsequent tsunami disabled the power supply and cooling to three reactors leading to a nuclear accident, the European Council on March 24 and 25 declared that “the safety of all EU nuclear plants should be reviewed, on the basis of a comprehensive and transparent risk assessment", which it dubbed ‘stress tests’.
The European Nuclear Safety Regulatory Group (ENSREG) and the European Commission defined stress tests as a targeted re-assessment of the safety margins of nuclear power plants in the light of the events which occurred at Fukushima: extreme natural events challenging plant safety functions and leading to a severe accident.
Uhrík specified that stress tests do not focus on the safety of nuclear power plants per se but on the plants’ safety margins, or how far a nuclear power station is able to cope with an event beyond its original design parameters.
“The objective of the stress tests is to determine which level of severity of an external hazard the nuclear power plant can withstand without severe damage to nuclear fuel (in the reactor core or in the spent fuel) or without significant releases of radioactive materials into the environment,” the interim report wrote. “Previous studies did not have this as an objective, since normally the plant structures, systems and components were designed to cope with the loads caused by external hazards within the plant design basis covering all events with non-negligible frequency of occurrence.”
This also means that in the stress tests the nuclear power plants are assessed regarding the margins they have to cope with in connection with extremely unlikely hazards not originally considered in their design.
Under the stress tests extraordinary external events such as earthquakes, floods and other events that could lead to potential loss of multiple plant safety functions are tested and analysed. A combination of such events is also considered, including a power supply interruption, long-term interruption of (cooling) water supply as well as a loss of power supply caused by extreme climate conditions, according to the nuclear authority.
Uhrík explained that the stress tests also verified the functionality of systems that are not routinely tested, and said the equipment was tested in a configuration that was not considered in the plants’ original design. The tests, for example, looked at how water could be supplied to the spent fuel pond by natural flow, i.e. by relying on gravity instead of circulation pumps; supplying steam generators with water from fire-fighting trucks located tens of metres from the main block was also tested.
While some tests were carried out physically, others could be done only ‘on paper’ as it is impossible to test for a real earthquake or a flood beyond the original design. These are tested analytically by calculations and estimations.
One of the principal objectives of the assessment is to indicate opportunities to increase the robustness of the plant to withstand conditions arising from extreme natural events.
“We expect, however, that all our nuclear power plants will continue to implement further future improvements,” said Marta Žiaková, chair of the ÚJD, adding that each accident at a nuclear facility is closely monitored by national nuclear authorities. The results are scrutinised in depth to see what happened and what lessons can be learned to prevent a similar occurrence.
The stress tests started on June 1 and the ÚJD’s interim report described the approach used and current status of the facilities in meeting the stress tests. The deadline for SE to complete the stress tests and deliver its final report is October 31. Afterwards, the ÚJD will draw up the closing national report and deliver it to the European Commission by December 31. In order to enhance the credibility and accountability of the process, the national report should be subject to a peer review process.
Uhrík stressed that the reports should not be used to compile a safety ranking of nuclear power stations, but only to show the robustness of the nuclear power stations’ designs and their safety margins.
“[The process] has shown that safety margins are more than sufficient in the case of both nuclear power stations [in Slovakia],” Uhrík said.
According to Uhrík, the tests have shown that the type of nuclear power station used in Slovakia comes out of the tests relatively well because of its design. Slovakia uses pressurised water reactors – unlike Fukushima, where boiling water reactors were installed – with a small output, which means that the active zone and the amount of fuel are small. By contrast, the amount of coolant in the primary as well as the secondary circuits is huge.
“This means that the safety margins and inherent safety of the nuclear power stations are big from the very beginning,” said Uhrík. ”What was a weak point of the power stations – or what was considered to be a weak point – at the beginning was their control systems, which have already been replaced completely in both nuclear power stations.”
