Two maintenance workers at Switzerland’s Beznau 2 were installing lamps in the space beneath the reactor pressure vessel when they received doses of 37.8mSv and 25.4mSv.

Utility Nordostschweizerische Kraftwerke posted the event on the IAEA’s INES database and rated it at level 2, ‘incident’.

It explained what happened:

“On 31 July 2009, Unit 2 of the Beznau NPP was shut down for maintenance. As part of the scheduled maintenance, it was planned to test the pressure in the primary circuit – this is done every 10 years – and as preparation for the later use of a camera two employees were installing lamps in the enclosed space beneath the reactor pressure vessel. Running through this space are double-walled tubes through which probes can be fed to measure the neutron flux in the core. Whilst the two employees were working under the reactor pressure vessel, the inner tubing was withdrawn from the reactor pressure vessel by employees located in a different room so that other work could be carried out. The tubing and probes in the reactor normally emit high levels of radiation and so the resultant local dose rate beneath the reactor suddenly jumped to a dose rate of probably more than 1,000 mSv per hour. The two employees left the scene as quickly as possible. One employee received an individual dose of 37.8 mSv and the other a dose of 25.4 mSv.”

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