UWFDM-1331 Analysis of Radiation Streaming Through ARIES-CS He-Access Pipes using Two- and Three-Dimensional Analyses

Neutron streaming through penetrations compromises the shielding performance of fusion devices. To characterize the problem, the effects of neutron streaming through the divertor Heaccess pipe on the peaking of radiation-induced damage across the ARIES-CS power plant components were compared to the nominal values for the different components. The results show that due to streaming the damage exceeded the limits near the pipe for the manifolds, vacuum vessel, and magnet. Certain precautions should be taken that include changing the pipe design and orientation, avoiding rewelding the manifolds and vacuum vessel near the pipe, and/or relocating the magnet away from the pipe. This problem also provided a good test for both the deterministic and the Monte Carlo approaches. The analysis demonstrated the applicability of the deterministic transport codes, DANTSYS and Attila, to perform 2and 3-D nuclear analyses of complex fusion systems and in particular the problem of radiation streaming. The analyses likewise showed that Attila cannot be used as a black box. Thought must be given to the generation of a workable CAD model using experiences gained from discrete ordinates deterministic codes before importing it into Attila. Negative fluxes in Attila results and discontinuities were noticed near the interface of the pipe and surrounding components. Some results obtained by Attila were compared to the University of Wisconsin newly developed CADbased Monte Carlo code, DAG-MCNPX. The results compared well with the results of the Attila code being slightly lower than those from the DAG-MCNPX code, particularly near the end of the 2.7 m pipe. Introduction The ARIES Compact Stellarator (ARIES-CS) fusion power plant depicted in Fig. 1 is a geometrically complex fusion device that requires a detailed three-dimensional (3-D) model to assess the neutronics performance of the critical structures in the device. Generally, stellarators promise disruption-free operation with reduced recirculating power due to the absence of current-drive requirements. The compactness of ARIES-CS mandates that all components provide a shielding function [1]. Besides breeding tritium, the blanket protects the shield for the plant life (40 full power years). Along with the blanket, the shield is designed to protect the welds of the manifold and the vacuum vessel (VV). All four components help protect the superconducting magnet that operates at 4 K. The fusion power of the ARIES-CS is ~2400 MW, the average neutron wall loading (NWL) is 2.6 MW/m, and the net electric power is 1000 MWe. All materials were carefully chosen to enhance the shielding performance and minimize the long-term environmental impact [1]. Penetrations are necessary for vacuum pumping, coolant supply lines, plasma control, and maintenance ports. Such penetrations jeopardize the effectiveness of the shield as neutron streaming through these penetrations enhances the damage near the interfaces of the shield, manifolds, VV, and magnet components. The most serious streaming issues are related to the large divertor He-access pipe. Of specific interest are the displacements per atom (dpa) and He production at the ferritic steel structure of the shield, manifolds, VV and pipe wall, the fast neutron fluence at the magnet, and the flux level behind the magnet. If the damage level in the structure is too high, these components will have to be replaced in order to maintain the integrity of the machine. The He production rate level should not exceed 1 appm at the manifolds and VV to be able to reweld these components during operation where cutting/rewelding is necessary to maintain the replaceable components (divertor and blanket). In addition, the displacement per atom should not exceed 200 dpa at the shield at any time during the expected 40 full power years (FPY) plant lifetime. Furthermore, the fast neutron fluence to the Nb3Sn superconductor of the magnet should remain below 10 n/cm @ 40 FPY, the neutron flux outside the magnet should not be excessive to protect the externals, and the atomic displacement level should not exceed 10 dpa at the outer screws (that adjust the divertor plates during operation) to minimize the neutroninduced swelling. As Table I indicates, in absence of pipes, the design satisfies the radiation limits [1] for the 2.7 MW/m peak NWL in the divertor region. The aim of this study is to examine the effectiveness of the shielding scheme proposed for the He-access pipes, estimate the radiation damage at the shield, manifolds, VV, and magnet, and assess the radiation environment behind the magnet due to neutron streaming through the He-access pipe. We began the multi-dimensional analysis by modeling a simple straight pipe with 30 cm inner diameter (ID) using the DANTSYS code [2] in 2-D R-Z geometry. Based on the preliminary 2-D results, the pipe configuration was modified with shielding plug and inserts, hoping to eliminate the streaming problems. A 3-D analysis was judged necessary to handle the more complex, new pipe configuration. The main transport code used for the 3-D analysis is the Attila 3-D deterministic code [3]. The results of Attila for the damage along the pipe wall were compared to the 3-D DAG-MCNPX Monte Carlo code, currently under development at the University of Wisconsin-Madison [4]. This comparison was performed to demonstrate the applicability of