Radiation Protection Today Summer 2021 | Page 26

Lessons From History

In this section of the magazine , we aim to highlight events that provided lessons that we can learn from . Events may be from all over the world and all sectors . The first of these is shared by Editorial Team member Maureen McQueen CRadP MSRP .
Maureen worked as a radiation safety professional in Canada and the USA for 28 years before returning to the UK in 2021 .
In 2009 , over 550 contract workers undertaking refurbishment work on the pressurised water reactor at a nuclear power plant in North America were potentially exposed to airborne alpha activity . The event response led to a significant change in working methods and also to the sharing of lessons learned throughout North America , through The Institute of Nuclear Power Operation ( INPO ), The World Association of Nuclear Operators ( WANO ) and the Electric Research Power Institute ( EPRI ). Ultimately , this led to the re-writing of the industry guidance document Alpha Monitoring and Control Guidelines for Operating Nuclear Power Stations , which outlined enhanced standards for the identification and control of alpha contamination .
The exposure was identified when long-lived alpha contamination was confirmed during off-site laboratory analysis of routine air samples taken during the performance of welding activities on the laid-up reactor coolant system . The low ratios of beta to
alpha contamination indicated that exposures had been potentially undetected over a period of six weeks .
DACh estimates were made of personnel exposures using available air sample results and the beta gamma to alpha activity ratio . Bioassay was conducted using urine samples analysed by Thermal Ionisation Mass Spectrometry ( TIMS ), a very sensitive technique which could detect exposure in workers long after the event had occurred . The event was found to have resulted in a collective internal dose from alpha emitters of 510 mSv . The highest individual internal exposure was 6.9 mSv CEDE which , while well within regulatory limits , was preventable and not ALARP .
How did this happen ? While indications were available of the presence of transuranics in the reactor system , the impact of the alpha activity which was fixed inside the reactor coolant system pipework , and would be released as a result of welding activities , had been underestimated . Prior assessments had focussed on the low level of beta-gamma activity which was present . As a result , there was inadequate monitoring of alpha
26 Radiation Protection Today www . srp-rpt . uk