Calandrino Riccardo, Manenti Simone, Groppi Flavia, Broggi Francesco, Bergamaschi Carlo, Ferrari Andrea, Manenti Simona, Nizzi Massimiliano, Loria Alessandro, Del Vecchio Antonella
Medical Physics Department, Ospedale San Raffaele, Via Olgettina 60, I-20132 Milano, Italy.
Department of Physics, University of Milan, Via Celoria 16, I-20133 Milano, Italy.
J Radiol Prot. 2021 Dec 6;41(4). doi: 10.1088/1361-6498/ac28f0.
The present article describes the decommissioning of a self-shielded 18 MeV medical cyclotron IBA Cyclone 18/9 after 14 years of operation. A Monte Carlo simulation of the possible nuclear reactions was performed in order to plan the decommissioning activities. During the cyclotron dismantling, the activities of the cyclotron components, concrete wall and floor samples were measured. Residual activities were analysed by means of an HPGe detector and liquid scintillation counting, and compared with simulation data. Dosimetry of the staff involved in the decommissioning procedure was monitored by individual TL dosimeters and/or digital dosimeter. The cyclotron component analysis confirmed the presence of gamma and pure beta emitters,Na,Mn,Co,Zn,Bi,Fe,Ni at different values of specific activity, depending on the positioning of the sample point and on the alloy of the sampled part. In these components the presence of gamma and pure beta emitters was measured 5 years after the shutdown at levels far above clearance limits as defined by the 'Recommended radiological protection criteria for the recycling of metals from the dismantling of nuclear installations' (RP89) guidelines. The simulation, carried out by FLUKA Code (version 2020.0.5) on the cyclotron components, provided good agreement with measurements, with a maximum discrepancy of the same order as the uncertainties. Four engineers of the cyclotron maintenance staff were involved in the dismounting of the hottest components and rigging of the cyclotron in the deposit 6 months after shutdown and two engineers were involved during the drilling phase 3.5 years after shutdown. The measured dose from external exposure of the involved staff was lower than 100Sv personduring the first phase and lower than 20Sv personduring the final drilling phase. Measured doses from intake were negligible. In conclusion, the decommissioning of the 18 MeV cyclotron does not represent a risk for the involved staff, but, due to the presence of long-lived radioisotopes, the cyclotron components are to be treated as low level radioactive waste, and stored in an authorised storage area for at least 25 years after shutdown.
本文描述了一台自屏蔽式18兆电子伏医用回旋加速器IBA Cyclone 18/9在运行14年后的退役过程。为了规划退役活动，对可能的核反应进行了蒙特卡罗模拟。在回旋加速器拆除过程中，对回旋加速器部件、混凝土墙和地板样本的放射性活度进行了测量。通过高纯锗探测器和液体闪烁计数法对剩余放射性活度进行了分析，并与模拟数据进行了比较。参与退役程序的工作人员的剂量测定由个人热释光剂量计和/或数字剂量计进行监测。回旋加速器部件分析证实存在γ和纯β发射体，钠、锰、钴、锌、铋、铁、镍，其比活度值不同，这取决于采样点的位置和采样部件的合金。在这些部件中，停机5年后测量到的γ和纯β发射体的存在水平远远高于《核设施拆除金属回收推荐放射防护标准》（RP89）指南规定的清除限值。由FLUKA代码（2020.0.5版本）对回旋加速器部件进行的模拟与测量结果吻合良好，最大偏差与不确定性处于同一量级。回旋加速器维护人员中的四名工程师在停机6个月后参与了最热点部件的拆卸以及回旋加速器在存放处的吊装工作，两名工程师在停机3.5年后的钻孔阶段参与其中。在第一阶段，所涉工作人员的外部照射测量剂量低于100Sv/人，在最后钻孔阶段低于20Sv/人。摄入剂量的测量值可忽略不计。总之,18兆电子伏回旋加速器的退役对所涉工作人员不构成风险，但是，由于存在长寿命放射性同位素，回旋加速器部件应作为低水平放射性废物处理，并在停机后在经授权的储存区域储存至少25年。
