July 2006 incident
On 25 July 2006, one reactor was shut down after an electrical fault.[4][5] According to the Swedish Nuclear Power Inspection authority SKI, the incident was rated 2 on the International Nuclear Event Scale. Initially it was rated 1 since two generators remained online. But once it was discovered that all four generators could have failed due to the same fault, the event was upgraded to 2.
The first proximal cause of the accident was maintenance work in the adjacent high-voltage yard by the Swedish grid operator Svenska kraftnät. An incorrect interlock procedure caused a disconnector to open which sustained an arc that caused a two-phase short circuit in equipment directly adjacent to the plant. This short caused the station generators to disconnect from the grid and, due to the failure of further safety systems this disconnection, in turn, led to a large overvoltage on various supplies within the station. The overvoltage caused failure of the control circuitry of two of the four redundant UPS systems which supplied the safety critical equipment at the plant, including cooling pumps and control circuitry. Though diesel generators started correctly even on these two systems, the lack of control circuitry led to their being unable to engage with their corresponding circuits. The other two UPS systems functioned correctly, surviving the overvoltage, probably due to an undetermined subtle difference in wiring or equipment between the two pairs of units. The reactor fully and effectively scrammed immediately on detecting these supply failures, however staff relied on neutron detector readings to determine reactor state due to lack of information on control rod state. At all times effective cooling was maintained by the pumps operating on the two functioning circuits. Though a number of options remained to operators, had further equipment failed, a single cause (one short-circuit) leading to such a cascade of failures was seen as a challenge to the principle of redundancy and safety in depth.[6]
At the request of the Swedish Government, IAEA launched an OSART mission to Forsmark.
Lars-Olov Höglund, a former construction chief at Vattenfall, claimed it was the most serious nuclear incident in the world since the Chernobyl disaster and it was pure luck that prevented a meltdown.[7] Both the SKI and the safety chief of Forsmark power plant disagree with that opinion and state that the incident was serious but the description provided by Höglund was incorrect and there was no real risk of a meltdown.[8] Höglund has personally been involved in a legal dispute with Forsmark Nuclear powerplant for several years in connection with his private business.[9] On March 14, 2011, Höglund commented that the Fukushima-disaster parallels the Forsmark incident, i.e. failing UPS system backup, and repeated his statement from 2007 that "only luck" prevented a meltdown at the Swedish plant.[10]
However, Kjell Olsson, a researcher at SKI, later stated in an informal discussion with schoolchildren that a meltdown technically could have developed from the incident.[11] The agency later stated that the failing safety system proved to be linked together in a delicate, extremely serious way.[12]
February 2007 shutdown
On February 3, two units at Forsmark were shut down to inspect a rubber seal in one of the safety systems. On Forsmark 1 this seal needed to be replaced, a job that would take approximately one month.[13] Unit 2 was cleared by the regulator SKI and was free to restart.
In January an internal report made by a few employees at Forsmark who were concerned over a "degrading safety culture" was leaked to media who ran an extensive story on it. In the storm following the report the Forsmark CEO chose to resign. Forsmark was already under way to implement a 60-point program designed to improve safety culture, designed shortly after the event in July 2006.
