No details as to where in the system the seawater infiltrated.
From Channel News Asia, Quoting AFP (12/21/2012; emphasis is mine):
Swedish nuclear reactor shut after sea water infiltration
STOCKHOLM - A reactor in Sweden's biggest nuclear plant was stopped on Thursday after an infiltration of sea water, the operator Vattenfall and the national nuclear industry watchdog said.
"There is no safety problem" at Reactor 4 at the Ringhals plant near Gothenburg in the country's southwest, nuclear authority inspector Jan Gällsjo told the national TT news agency.
He added that the presence of salt water in the pressurised water system was nonetheless an anomaly which needed to be fixed.
Earlier this month, a reactor at another Swedish nuclear plant was shut down after the nuclear watchdog said the operator had failed to comply with its safety requirements, safety officials said.
The Oskarshamn plant's number two reactor was taken out of service after the operator was unable to show the Swedish Radiation Safety Authority that standard maintenance work had been performed on two diesel generators used for emergency power supply.
Some 35 percent of electricity in Sweden is generated from nuclear power.
In July this year, Reactor 2 at the same nuclear power plant shut down 9 hours after it was restarted, for causes unknown.
Hamaoka Nuclear Power Plant in Shizuoka Prefecture had a problem of having seawater in the pressurized water reactor system last year. It turned out to be a very big problem.