The internal report, written by Forsmark technicians and released late on Monday, cites “a degradation of the company’s security culture over a long period of time”.
An electricity failure at the plant on July 25, 2006, led to the immediate shutdown of the Forsmark 1 reactor 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 potentially catastrophic reactor meltdown was narrowly avoided at the plant, located on Sweden’s east coast.
But Swedish authorities have classed it a level-two incident on a scale from zero to seven.
The internal report said lax security has led to “potentially fatal accidents”. It cited among other things a nitrogen gas leak, employees handling live electrical wires, falls in the workplace and employees sent home for failing sobriety tests.
The Swedish Nuclear Power Inspectorate said it has asked prosecutors to investigate whether the Forsmark operator, FKA, broke the law in its response to the malfunction.
When the power supply failed, the reactor was kept warm until the next day. Under regulations the reactor is supposed to be cooled down as soon as possible.
Green Party spokeswoman Maria Wetterstrand called on Tuesday for an international independent inquiry into Swedish nuclear safety and said Forsmark’s chief executive ought to be replaced.
The incident at Forsmark 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, although all are now up and running again.
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.