S. Sandri

Collective dose at ITER feat

The safety study performed until December 2001 to assess the collective dose to the workers based on the International Thermonuclear Experimental Reactor (ITER) Fusion Energy Advanced Tokamak (FEAT) project, is presented in this work together with the relevant results. All systems located in the tokamak building of ITER FEAT facility important from the radiological point of view are considered. Radiological source terms with an important collective dose impact are airborne tritium and activated corrosion products (ACP) in the coolant of the water cooling system (WCS). A suitable computer code is used to assess the ACP inventory in the different WCS components. The dose rate is then assessed by considering walls attenuation with the 3D transport code MCNP, a Monte Carlo code that performs gamma ray transport calculation. Working activities needed to operate, maintain and replace each component of systems under study are partly provided by system designers and partly derived from the experience developed in operating nuclear fission reactors. In conclusion the collective dose result is shown and partly compared with that of previous ITER design stages considering the different systems and solutions and pointing out ALARA improvements.

Radiation environment in the ITER neutral beam injector prototype

We study the radiation environment in MITICA, the prototype of the ITER neutral beam injector, during deuterium operation. ITER is a scientific challenge: one of the several critical issues to solve is the compatibility to ionizing radiation of diagnostics components placed near the plasma vessel. In fact, non negligible fluxes of neutrons and photons are expected from deuterium-deuterium reactions, also from a major peripheral component such as the Neutral Beam Injector. After evaluating the results of Monte Carlo simulations on the expected environment, we determine the risk of malfunctions due to ionizing radiation in control and diagnostic electronics placed close to the injector vessel. The risk on different families of electronic devices is assessed, focusing separately on displacement damage, total ionizing dose, and single event effects.

Neutron Flux and Activation Calculations for A High Current Deuteron Accelerator

Neutron analysis of the first Neutral Beam (NB) for the International Thermonuclear Experimental Reactor (ITER) was performed to provide the basis for the study of personnel safety during normal operation and maintenance. The first ITER NB is a 1 MeV negative deuterium ions accelerator. The daily average beam current is 13.3 A. To assess neutron transport in the ITER NB structure a mathematical model of the components geometry was implemented into MCNP (Monte Carlo N-Particle transport code system) computer code. The neutron source definition was outlined considering both D-D and T-D neutron production. FISPACT code was used to assess neutron activation in the material of the system components. Radioactive inventory and contact dose rate were assessed considering the potential operative scenarios.

Radiological and Nuclear Events: Challenges, Countermeasures and Future Perspectives

Over the last few years a broad array of organizations have practiced terrorism with the aim to achieve political, criminal, religious, and ideological goals. These acts have revitalized awareness of the threat of attacks involving chemical, biological, radiological or nuclear weapons. In particular radiological and nuclear methods are likely to be pursued by well organised terrorist groups, particularly those which have access to financial resources. The objective of this paper is to provide the reader with basic knowledge of possible radiological and nuclear events and the potential risks they pose. The document focuses on the characteristics of radiologic and nuclear agents as well as on the basics of response. Ultimately, this article explores how emerging technology has been infusing additional complexity into the global radiological and nuclear threat scenario.

Personnel Dose Assessment at the PRIMA Neutral Beam Test Facility

Abstract PRIMA consists of two experiments which will test at the same time the main components of the final system and the whole system. The facilities are named respectively SPIDER (Source for Production of Ion of Deuterium Extracted from RF Plasma - ion source only) and MITICA (Megavolt ITER Injector Concept Advanced - the main system). Both injectors accelerate negative deuterium ions with a maximum energy of 1 MeV for MITICA and 100 keV for SPIDER, and a maximum beam current of 40 A for both experiments. Following D-D and D-T reactions on the calorimeter panels, important neutron and photon fields are generated around the injectors; such secondary radiation fields represent a relevant issue from the radiological safety point of view. Major radiation protection issues are the activation of materials and components around the injectors, among which the vessel itself and the corrosion products formed in the cooling loops (by the chemical reaction between metal and water). Both these radiation sources may contribute to personnel dose during maintenance operation. In addition, radioactive tritium is produced inside the vessel (mainly in the MITICA facility) which is likely to be poured in the environment during operation phases thus representing a possible contamination pathway for workers and for the population living in the area surrounding the facility. Finally, important penetrations for the ventilation, the power supply and the auxiliary systems were arranged inside the facilities. Tunnels in the underground region, with relatively large dimensions, were needed for both MITICA and SPIDER bunkers in order to allow personnel access for inspection and maintenance of cables and ducts. Each of these shielding weaknesses may provide important dose contribution to radiation workers thus requiring specific safety analyses. In the present paper all major safety issues and relevant radiological concerns are analyzed with a detailed assessment of dose contribution to personnel working inside the facility. Specific dose evaluations were performed through Monte Carlo simulations. Radiation shielding and radiation protection criteria were realized in order to meet the Italian regulatory limit for non radiation workers, ie. below 1 mSv/yr. Our analysis and project evaluations confirm that this constraint is never exceeded during operating phases of the injectors.

Measurements of Activation and Decay Heat Produced in Materials Irradiated with D-T Neutron and Comparison with EASY-2007 Code Predictions

Abstract Neutron activation of materials produces an energy release during the subsequent radioactive decay. In a fusion power plant this energy release is of the order of MWs. Accurate prediction of this decay heat is fundamental for the design of a fusion power plant, especially for the safety analysis. A very efficient detector system able to measure both electron and photon heats simultaneously and separately has been developed at ENEA Frascati and has been already used to validate the predictions of computer codes developed to calculate neutron activation energy release. In this paper we report measurements on some elements (tin, tantalum and lead) that have been irradiated with the D-T fusion neutrons produced by the Frascati Neutron Generator FNG. These elements could be present in ITER materials and give a significant contribution to the total radioactive inventory, especially if they produce long-live radionuclides. The scope of this study is to validate the general purpose code European Activation code System EASY-2007 comparing the results of the measurements with code predictions. The results are presented in terms of C/E (Calculation vs. Experiment) together with the associated uncertainties.

Occupational radiation exposure during inspection and maintenance of the ITER water cooling system. A comparison between two working strategies

The international thermonuclear experimental reactor water cooling system (ITER WCS) is divided into 20 loops with the aim to reduce the consequences of a loss of coolant accident (LOCA). This makes the ITER WCS a very complicated system with many components requiring periodical inspection and maintenance. The main radiological concern for the staff involved in such activities is due to the activated corrosion products (ACPs) generated by corrosion of the inner wall of the piping under the neutron flux. In the present study the collective dose due to the ACP for one loop of the ITER WCS is assessed. Two different maintenance and inspection (M/I) approaches (or working strategies) for the ITER WCS components are to be considered. The first one is usually applied in American and European designs of pressurised water reactors (PWRs) that have cooling loops similar to those envisaged in ITER. The other one is the strategy proposed by the ITER WCS designers that takes into account the actual plant layout and technology. Both strategies have been implemented in the occupational radiation exposure (ORE) assessment process and two results have been obtained. The similarity is then performed pointing out the differences between the two approaches, trying to indicate what is the best one from the ORE point of view.

Dose rate around a primary cooling loop of the ITER plant with two different operational scenarios

To assess the activated corrosion products (ACP) inventories in the primary cooling loop of the ITER plant two operational scenarios are considered that have been studied in different approaches: the M5a and the BPP acp. Then external dose rates around the hot leg are calculated using the MCNP code and the resulting surface concentrations as source terms. A direct comparison of the dose rates calculated with both scenarios is made considering a specific component of the first wall loop. All the assessment procedure is described, from the ACP inventory evaluation to the dose rate calculation. Results are then discussed considering the radiological impact on the workers as well as the potential risk for the ACP inventory in the coolant (leakages and accidents).

ORE Assessment Due to ACP in the PHTS of the Point Design Phase of the ITER Project

The work presented here dealt with the revision and the updating of the ORE (Occupational Radiation Exposure) assessment for the ITER PHTS (Primary Heat Transfer System). The data used come from the Point Design Documents and refers to the ITER design of the first half of 1996. The MCNP computer code was adopted to perform the shielding calculation. In addition, an accurate approach to evaluate the photon flux during maintenance and inspection activities was followed and recently published photon-flux-to-dose-rate conversion factors were applied to obtain the corresponding dose rate. The ACP inventory was taken from the relevant calculation performed with the PACTOLE code for the Point Design. A special ACP calculation was performed for each PHTS circuit and the related results are used in the respective dose rate calculations. The collective dose for the main activities performed to maintain the PHTS components is reported. The dose result for each activity type is shown and the comparison with a reference fission plant is discussed.

Radiological safety during maintenance of the primary heat transfer system of the ITER plant

The primary heat transfer system (PHTS) of the ITER plant is devoted to the heat removal from different plasma facing components: the first wall, the blanket, the divertor and the vacuum vessel. The system requires a scheduled or regular maintenance that involve component checking and changing, and a special maintenance that has a non periodic nature. During the maintenance operations some shielding barrier has to be partly or completely removed and workers are likely to be exposed to an unusual radiological dose. In the present work the occupational radiation exposure (ORE) for the maintenance activities performed at the PHTS has been assessed. The problem of the activated corrosion product (ACP) transport assessment inside the coolant has been faced and solved with the computer code PACTOLE. The working procedure protocols have been taken from the fission PWR plant experience. The shielding effectiveness of the PHTS components has been evaluated with a well tested computer code. The final result presented in the work is the collective dose for regular and special maintenance at the ITER PHTS, compared with typical data coming from the PWR operational experience.