The art of cooling a nuclear power plant

The Forsmark nuclear power plant is idyllically situated on Sweden’s Uppland coast. And the Baltic Sea waters play an important role in the plant’s function. The cooling process for the safety switches is one example.

When the reactor is to be shut down, Baltic Sea water is flooded in to each of the Forsmark reactors’ cooling systems. The water cools down another, closed water system, which transports the heat when the reactor has to be turned off.

In the two oldest reactors, Forsmark 1 and 2, this system has been modernised to enhance safety.

“The facilities are already very safe, but now we are raising the bar further and making sure these two reactors are updated in accordance with the new regulations that have been issued by the regulatory body (SSM) during the 2000s,” explains Ulf Sandberg, reactor safety engineer at the Forsmark plant.

For the cooling systems, this means that Forsmark 1 and 2 are now moving from two to four process subdivisions. This entails that the pumps and heat exchangers that take in sea water are situated in four parallel and independent subdivisions, instead of two.

“This is called redundancy,” says Sandberg. “Two of the four subdivisions are needed to shut down the reactor. We can thus have a malfunction in two subdivisions and still be able to safely shut down the reactor. It is part of what we call ‘deep defence’.”

The work on expanding the cooling system to four channels is just one of hundreds of measures that are now being taken at Vattenfall’s Swedish nuclear power plants in Forsmark and Ringhals. In all, Vattenfall is investing - together with the other owners - approximately SEK 50 billion in a programme 2003-2030 to raise the plants’ safety and extend their useful life. It involves everything from power feed-in and modernisation of control equipment to diversification measures in the cooling systems.

“The cooling system at Forsmark 1 was refurbished in 2005, while at Forsmark 2 we began work in 2006 and are almost finished. Not replacing all the subdivisions at once is also a safety aspect, since if something were to go wrong, we can correct it in due time,” Sandberg explains.

Although a modern heat exchanger in a nuclear power plant is no larger than 3 x 3 x 3 metres, replacing the components is no easy process.

“We can’t shut down everything - part of the plant must always be in operation. And there are hundreds of metres of pipe that have to be replaced and welded.”

Replacing and upgrading the plant is actually an everyday matter for Forsmark’s safety engineers. Updates are continuously being made to the reactors, and new components have to be installed.

“We are constantly raising our level of ambition with respect to safety,” says Sandberg. “Our plants will be even better at withstanding unforeseen events - everything must still be operational.”