The Swedish Radiation Safety Authority has written a proposal regarding design bases for a new independent core cooling function that is to be built at nuclear power plants in Sweden. The proposal will serve as the basis of the Authority's requirement that nuclear power plants implement a core cooling system by 2020; it will help them determine impact assessments for implementing such a system.
For several years, since before Fukushima, the Authority (SSM) has been assessing the need for independent core cooling. This kind of function feeds the reactor core with water and is activated if other cooling systems fail to function. It also represents an additional level of protection for preventing core melt.
"This area has once again been brought to the fore following the severe nuclear power accident in Japan," says Jan Hanberg, head of section at the Authority. "Implementing independent core cooling is one of the measures highlighted in the action plans produced after the work on the stress tests."
The idea at the core of the proposals is to maintain a water reservoir outside of reactor containment that can be connected to the reactor pressure vessel, and the pumping of whose contents can be started independently of the reactor protection system, and from a separate power feed.
The Swedish national action plan in response to the stress tests assumes two new worst-case, simulataneous design requirements for emergency core cooling:
- Protracted loss of AC power and present steam generated motor power for at least 72 hours (ELAPSP - Extended Loss of AC Power and Steam Power).
- Protracted loss of ultimate heat sink for at least 72 hours (LUHS - Loss of normal access to Ultimate Heat Sink)
It also said that all external events with a frequency of occurrence greater than 10-6/year (once-in-a-million years) should be taken into account, in order to allow for beyond-design-basis events, the Agency said in a six-page dossier.
Photo: Sweden's Forsmark nuclear power plant