[en] Gentilly-2 NGS is experiencing high radiation fields in the fuelling machine vaults. These high fields make maintenance outages more expensive and their management more complicated. As part of the station refurbishment project, a task group was created to identify the cause of the high fields and make recommendations to prevent their reoccurrence in the second (post-refurbishment) operating cycle. To identify the root cause of the problem, the task group decided to analyse the primary heat transport system (PHTS), the fuel handling system and their inter-relation. Gentilly-2 has had to manage a unique (to CANDU) problem arising from antimony released from the main heat transport pump seals. Antimony deposits on in-core surfaces, becomes activated, and subsequently can be released, especially under oxidizing coolant conditions. It then becomes incorporated into the magnetite deposits on PHTS piping, including the steam generators and inlet feeders. Gentilly-2 has focused a great deal of effort on managing antimony over the last 15 years. As a result of these initiatives, radioantimony fields have been quite effectively managed since 1997, resulting in a decrease in their relative contribution to the total fields. The decrease in radioantimony fields highlighted the significant contribution of ⁶⁰Co cobalt activity; the high levels of both radioantimony and ⁶⁰Co differentiate Gentilly-2 from other CANDU 6 plants. Two types of ⁵⁹Co sources are present in the CANDU PHTS. High surface area materials such as steam generator tubes and feeder pipes contain trace concentrations of ⁵⁹Co as an impurity, which can be released by corrosion. Low surface area materials such as Stellites contain high concentrations of ⁵⁹Co that can be released as either corrosion or wear products. After assessing potential cobalt sources, the task group concluded that PHTS materials were not likely the origin of the high ⁶⁰Co fields. The major PHTS components identified as cobalt sources have the same specifications at Gentilly-2 as at other CANDU 6 plants. Consequently, the task group revisited operating and maintenance practices, going back through station operating history to identify operational practices that could have lead to increased ⁵⁹Co release and/or ⁶⁰Co production. The focus rapidly moved to the fuel handling system, as it had long been suspected, but never proven, that the Stellite ram balls were a major source of ⁵⁹Co; the fuelling machines provide a direct route for the introduction of corrosion or wear products into the reactor core. Data on the gamma fields measured in the fuelling machine D₂O supply room from 1995 to the present were reviewed. Water samples were taken and chemically analysed. The results confirmed the presence of ⁵⁹Co, probably generated by the wear and corrosion of the Stellite ram balls, and also confirmed the presence of antimony in the system. As part of the task group program, γ-spectrometry was performed in the course of a refuelling cycle. The results showed that variations in the general radiation fields observed in the fuelling machine D₂O supply room resulted from variations of the ¹²²Sb and ¹²⁴Sb concentrations in the process water. The presence of ¹²²Sb (t_1/2 3.2 d) suggested that the antimony was recently released from the reactor core. ⁶⁰Co was also detected but the level was constant; ⁵⁹Co from the ram balls is invisible to γ-spectrometry before it is activated to ⁶⁰Co. Remedial actions were implemented in mid-July, 2004. Shortly after, a significant reduction in the radiation fields in the Gentilly-2 fuelling machine ^D2O supply room was confirmed. During an unplanned outage in December 2004, a decrease in fields, in particular those due to ⁶⁰Co and radioantimony, was found at the reactor face. Activity monitoring scheduled for the 2005 annual outage should confirm if the source of the abnormally high fields has finally been identified. This presentation will describe the task force program, the remedial actions taken and the benefits achieved. This work, originally initiated to improve post-refurbishment operations, may bring benefits earlier than expected. (author)