Forsmark 1 was shut down after a sample taken from one of three rubber pannels in the reactor’s outer housing was found to have lost its required elasticity, spokesman for Forsmark’s operator FKA, Claes-Inge Andersson, told AFP.
“The rubber had become stiff … it needs to be elastic,” Andersson said.
Further tests were to be carried out on the reactor on Sweden’s east coast.
Later on Saturday initial tests on Forsmark 1’s sister reactor, Forsmark 2, also led to its shutdown for similar analysis.
“We will do the same work on Forsmark 2 as we are doing on Forsmark 1,” Andersson said.
It was not known when either of the reactors would resume power production, he added.
The twin shutdowns follow the emergence earlier in the week of a damning internal report into safety standards at Forsmark, made public six months after a serious incident at the plant.
The report blamed lax security on a series of “potentially fatal accidents”, including a nitrogen gas leak, employees handling live electrical wires, falls in the workplace and employees sent home for failing sobriety tests.
An electricity failure at the facility on July 25, 2006, led to the immediate shutdown of Forsmark 1 after two of four backup generators, which supply power to the reactor’s cooling system, malfunctioned for about 20 minutes.
Some experts have suggested that a catastrophic reactor meltdown was narrowly avoided.
The incident prompted authorities to temporarily shut down five of Sweden’s 10 reactors for security checks and maintenance. Some of the reactors remained shut down for several months.
The Swedish Nuclear Power Inspectorate has asked prosecutors to investigate whether FKA broke the law in its response to the malfunction at Forsmark.
On Friday a prosecutor announced the launch of a preliminary investigation into FKA’s handling of the incident.
Nuclear power accounts for nearly half of Sweden’s electricity production.
The country has shut two of its 12 nuclear reactors since 1999 as part of a plan to phase out nuclear power over the next 30 or so years, or when the reactors’ lifespan expires.