Security & Safety
All nuclear reactor units in Sweden should have a system of independent core cooling installed by 31 December 2020, the Swedish Radiation Safety Authority (SSM) has said.
In a statement on its website SSM said independent core cooling reduces the risk of meltdown and major radioactive release in the event of an accident. “We are raising the requirements for all reactors,” said Michael Knochenhauer, SSM’s director of the department of nuclear safety.
Given the long lead time for the installation of an independent core cooling system, SSM has also decided that nuclear operators must register a “transitional solution” for independent core cooling by 31 December 2017. The transitional solution would use independent water and power supplies, SSM documents say, without giving details.
By 30 June 2015, operators must submit proposals for transitional measures to be implemented by 2017. An implementation plan for the final independent core cooling system must be submitted by 31 December 2015.
Independent core cooling will mean the installation of an independent power source that directs water into the reactor core if other cooling systems fail to function, which can occur for reasons such as an extended loss of the external power supply.
SSM originally announced the proposal for independent core cooling in April 2014, but has only now confirmed the timetable.
A proposal for independent core cooling was put forward in Sweden in the early 2000s. That proposal said that in order to reduce the risk of core melt and reactor pressure vessel melt-through, it should be possible to add water to the reactor pressure vessel by connecting an independent water reservoir located outside of the reactor containment.
The need for independent core cooling received further attention after a Forsmark-1 reactor scram in July 2006 caused by a short circuit in a switchyard, as well as after the Fukushima-Daiichi accident in 2011. At Fukushima-Daiichi, the cores of three nuclear reactors melted after a tsunami disabled power supplies and cooling systems.
The idea at the heart of the proposals is to maintain a water reservoir outside the reactor containment that can be connected to the reactor pressure vessel. The pumping of the contents can be started independently of the reactor protection system and is backed by a separate power feed.
The Swedish national action plan in response to stress tests carried out in Europe following Fukushima-Daiichi assumes two new worst-case, simultaneous design requirements for emergency core cooling:
– Protracted loss of AC power and present steam-generated motor power for at least 72 hours;
– Protracted loss of ultimate heat sink for at least 72 hours.
Independent core cooling is also one of the Western European Nuclear Regulators’ Association’s reference requirements for nuclear plants. It is one of the post-Fukushima upgrades being undertaken in many countries in the EU and beyond.
Sweden has 10 nuclear reactor units in commercial operation at three nuclear sites: Forsmark, Oskarshamn and Ringhals.