* Vattenfall may put it in 5 reactors, seek exemptions for 2
* OKG still analysing, to present assessment on May 5
By Nerijus Adomaitis
OSLO, April 2 (Reuters) - Sweden's nuclear reactors must be equipped with independent core cooling systems by around 2020, taking into account lessons from the 2011 Fukushima disaster, the regulator said on Wednesday.
Installing the new systems will require additional investment by power plant operators and may mean longer maintenance times, which could drive up Nordic power prices.
"Swedish nuclear power plants will have to install an independent core cooling system if they want to operate longer. There is no final decision yet on its design or the time frame, but we are thinking about 2020 now," Tomas Jelinek, an official at the Swedish Radiation Safety Authority (SSM), told Reuters.
"If power plant operators consider this to be too costly, they might (be able to) run reactors for some time but will have to shut down eventually," he added.
Independent core cooling is activated to direct water into the reactor core if other cooling systems fail to function, which can occur for reasons such as a loss of the external power supply.
"This area has once again been brought to the fore following the severe nuclear power accident in Japan," Jan Hanberg, head of a section at SSM, said in a statement.
The cores of three nuclear reactors at the Fukushima plant melted after a tsunami disabled the power supply and their cooling systems.
Swedish state-owned utility Vattenfall said it was still assessing the impact of the regulator's proposal on power production and the need for investment.
"The preliminary assessment it that most of the system can be installed during a reactor's normal operation, and it will not require any additional time beyond the maintenance that has been already planned," Magnus Kryssare, a Vattenfall spokesman, said.
Vattenfall operates seven nuclear reactors at the Forsmark and Ringhals power plants. Two of its oldest reactors - Ringhals-1 and Ringhals-2 - are already expected to be shut in the next decade.
"We are focusing on installing the system at five reactors. We might ask for an exemption for Ringhals-1 and Ringhals-2," Kryssare added.
OKG, operator of the Oskarshamn nuclear power plant with three reactors, said it was still analysing the regulator's proposal and planned to present its assessment by May 5.
The plant, which includes Sweden's two oldest reactors, is owned by Germany's E.ON and Finland's Fortum . (editing by Jane Baird)