The Swedish Radiation Safety Authority (SSM) has on 18 December approved the independent core cooling systems implemented by the operators of the country's nuclear power plants. The operators were required to introduce such systems following the accident at Fukushima Daiichi. The systems were to be in place by the end of this year in order for the reactors to continue operating.

All European nuclear power plants were required to undergo stress tests following the March 2011 accident at Japan's Fukushima Daiichi plant. Recommended safety improvements were drawn up through peer reviews of these stress tests.

In October 2014, SSM announced a two-stage set of upgrades it wanted to see at Sweden's operating nuclear power reactors. By 2017, all reactors were required to have independent systems to ensure power and water are available for emergency cooling for a period of 72 hours. SSM said this requirement could be met by means such as mobile diesel generators and external water storage. By the end of 2020, the regulator required the plants to have a "robust permanent installation that includes power supply and systems for pumping of water and an external water source independent of those used in existing emergency cooling systems."

The system must also be able to handle extreme external influences, such as significantly stronger weather conditions, seawater levels and earthquakes than the current safety system. The installation of such systems required engineering deep within the reactor building and potentially its primary coolant circuit.

The regulator has now completed a review of work carried out by Forsmark Kraftgrupp AB, OKG AB and Ringhals AB to install independent core cooling systems at Forsmark units 1-3, Oskarshamn unit 3 and Ringhals units 3 and 4.

SSM said the plant operators have predominantly met the set conditions and requirements. However, it identified a number of shortcomings in the licensees' reports, including the current methodology for analysis of earthquake resistance, reporting of resistance to extreme temperatures and reporting probabilistic safety analyses. SSM has instructed the power companies to remedy the identified deficiencies but noted these "do not directly affect the physical installation and are not considered to have a negative impact on existing safety functions."

"The introduction of the independent core cooling means a large increase in safety and the nuclear power plants' resistance to extreme external influences has been significantly strengthened," said Karin Liljequist, SSM's project manager for the review. "Thus, the licensees are judged to meet the conditions for operation of the reactors after the year 2020 that the Swedish Radiation Safety Authority decided on in 2014."

"The introduction of systems for independent core cooling is the largest single measure in Sweden's national action plan after EU stress tests," added Michael Knochenhauer, head of SSM's department for nuclear safety. "Most of the measures in the plan have now been implemented and we expect to be able to report to the EU in 2021."

Vattenfall said it has invested about SEK3 billion (USD363 million) in extra safety systems for the Ringhals 3-4 and Forsmark 1-3 reactors to meet the requirements. It noted the solutions used at the Forsmark and Ringhals plants are different as the Forsmark units are boiling water reactors while the Ringhals units are pressurised water reactors.

"The approval means that Ringhals and Forsmark have added an independent safety system to already safe facilities," said Torbjörn Wahlborg, production manager at Vattenfall. "Now we can continue to plan for continued operation of our nuclear power reactors for further decades."

Researched and written by World Nuclear News