Accrued and expected radiation doses to personnel during manual access to the JET vessel

Abstract The neutron-induced radioactivity of the torus vacuum vessel presents a significant exposure source to in-vessel workers. Assessment of the torus radiation field is of prime importance in accurately predicting the likely doses. Correlation of the ambient dose-rate, the time spent in-vessel, and the actual doses to individuals show an apparent inconsistency, with the effective dose being approximately half of the expected dose. This paper aims to show that simple comparison of the ambient dose-rate, and the effective dose-rate is not appropriate in this situation because of the unique isotropic irradiation geometry that applies to tokamak vessel structures. The relationship between these two dosimetric quantities is compared, based on data from published sources, and discussed in relation to observations of doses to JET in-vessel workers. The measured effective dose to ambient dose ratio, which is found to be in the range 0.5–0.6, agrees with calculated values. Future shutdowns at JET in 2001 and 2003 will continue to involve manned entries to install new components and perform upgrades. For continued demonstration of JET policy on ALARP, and for optimising manpower and equipment resources, it is important to have a firm basis for predicting the doses to vessel workers.