Collective Dose Modification upon Inversion of Primary and Secondary Side of the ITER Cooling System Heat Exchangers

In the current design of the ITER cooling system heat exchangers (HXs), the primary water flows in the shell side of the component and the secondary water in the tube bundle and the channel head. This is the inverse of the more classical design previously proposed for this ITER component. The reason for this change is basically the need to reduce the collective dose to the operators working inside the HX channel head. In order to evaluate the effectiveness of this change, the radiological dose accumulated by all the personnel involved in the different working activities connected with the HX operation was assessed. The collective dose was calculated by using a procedure already applied to assess the occupational radiation exposure (ORE) since the end of the ITER conceptual design phase (CDA). Two main sources of radiological dose for the primary heat transfer system (PHTS) of ITER were considered in the assessment: the tritium in the room atmosphere and the activated corrosion products (ACPs) in the cooling loops. In this paper the HX structures are described and two models are selected for the comparison. The working activities needed to keep the HXs in operation are identified and classified. ACPs and tritium concentrations data, evaluated with suitable computer codes or by specific analyses also made by other authors, are used to calculate the dose rate during the various working activities. The final collective dose evaluation for the personnel working at HXs is mainly based on the practice developed at the pressurized water reactors (PWRs) and uses many information and data coming from there. In fact, the ITER heat transfer system (HTS) has many similarities with the PWRs cooling system and the majority of its components are the same as those already used by these plants. Furthermore the working procedures required to inspect and maintain the HXs according to the above approach are presented and discussed. The conclusion of this work includes the results of the comparison between the two HX design models in terms of dose rate and collective dose and points out the benefits of the current design for the ITER staff. Nevertheless, some concern relevant to the inspection and maintenance activities is still present.