During a routine filter change, in the refuelling water storage and cooling system, three workers received whole body radiation doses of 50, 90 and 105 milli-Sieverts. The statutory annual whole body dose limit is 50 milli-Sieverts. A pre-job radiation survey performed on the filter had indicated a dose rate of approximately 2 milli-Sieverts per hour, therefore two maintenance workers started to remove the filter from its location on the plant (watched by another worker employed as a radiation monitor). After about 20 minutes one of the workers looked at his portable radiation indicator and saw that it was reading off-scale high. The work site was immediately cleared and the workers reported to the radiological area entry point for monitoring. A subsequent radiation dose survey indicated a contact dose rate of approximately 2400 milli-Sieverts per hour on the filter. Initial readings taken from the worker's legal radiation monitoring dosimetry indicated whole body doses of 50, 90 and 105 milli-Sieverts. Estimated doses to the hands of the maintenance workers are being calculated. Justification of rating: INES Manual II-2.2: level 2 an event resulting in a dose to a worker exceeding a statutory annual dose limit.