Power Exhaust Concepts and Divertor Designs for Japanese and European DEMO Fusion Reactors

Concepts of the power exhaust and divertor design have been developed, with a high priority in the pre-conceptual design phase of the Japan-Europe Broader Approach DEMO Design Activity. A common critical issue is the large power exhaust and its fraction in the main plasma and divertor by the radiative cooling. Different exhaust concepts in the main plasma and divertor have been developed for JA and EU DEMOs. JA proposed a conventional closed divertor geometry to challenge large Psep/Rp handling of 30-35 MWm-1 in order to maintain the radiation fraction in the main plasma at the ITER-level (fradmain = Pradmain/Pheat ~0.4) and higher plasma performance. EU challenged both increasing fradmain to ~0.65 and handling the ITER-level Psep/Rp in the open divertor geometry. Power exhaust simulations have been performed by SONIC (JA) and SOLPS5.1 (EU) with corresponding Psep = 250-300 MW and 150-200 MW, respectively. Both results showed that large divertor radiation fraction (Praddiv/Psep 0.8) was required to reduce both peak qtarget ( 10MWm-2) and Te,idiv. In addition, the JA divertor performance with EU-reference Psep of 150MW showed benefit of the closed geometry to reduce the peak qtarget and Te,idiv near the separatrix, and to produce the partial detachment. Integrated designs of the water cooled divertor target, cassette and coolant pipe routing have been developed in both EU and JA, based on the tungsten (W) monoblock concept with Cu-alloy pipe. For year-long operation, DEMO-specific risks such as radiation embrittlement of Cu-interlayers and Cu-alloy cooling pipe were recognized, and both foresee higher water temperature (130-200 °C) compared to that for ITER. At the same time, several improved technologies of high heat flux components have been developed in EU, and different heat sink design, i.e. Cu-alloy cooling pipes for targets and RAFM steel ones for the baffle, dome and cassette, was proposed in JA. The two approaches provide important case-studies of the DEMO divertor, and will significantly contribute to both DEMO designs

Conceptual Design for Higher Capability of the Tritium Production by the Honeycomb Structure Blanket of JA DEMO

The conceptual design of the breeding blanket with a honeycomb structure has been created with pressure tightness against in-box Loss-of-coolant accident based on a water-cooled solid breeder. In the previous design, the breeding area of the module was divided into 0.1-m-squared cells with rib structure. As a honeycomb structure is higher in pressure tightness than a square prism structure, the area for filling the mixed pebbles breeder of Li2TiO3 pebbles and Be12Ti ones can be enlarged. Then, the overall TBR is improved to increase the packing ratio of the tritium breeding material.In the created blanket, the capabilities of the pressure tightness, tritium breeding and heat removal are studied using interaction analyses of the neutronics, stresses and fluid dynamics analysis. As a result, a rib with the thickness of 0.015 m is needed to withstand the design pressure of 17.2 MPa by a stress analysis. The packing factor of the mixed pebbles breeder increase to 77 % from 68 % by changing the rib structure from a square prism structure to a honeycomb structure. From the 3D neutronics analysis results, the target of the overall TBR (>1.05) is achievable. The cooling system for the created blanket is designed by fluid dynamics analysis based on the PWR water conditions which are the coolant temperature of 290 - 325 ºC and the operation pressure of 15.5 MPa, respectively. In addition, the tritium extraction system in the created blanket is proposed together with the purge gas system which does not clog the holes. The saturated time of the tritium extraction is also estimated to grasp the tritium inventory in the breeding area.14th International Symposium on Fusion Nuclear Technology (ISFNT-14

Effect of dragged magnetic field lines into RAFM steel blanket modules on first wall heat load

The blanket modules in DEMO are made of reduced-activation ferritic martensitic (RAFM) steel F82H. This material is ferromagnetic and it drags the magnetic field lines into the first wall (FW). Because of this, the heat load by the plasma heat flux, which goes along the magnetic field line will become higher. In this research, the first analysis of such effect has been done. The extra magnetic field Bm made by RAFM wall becomes higher at inner midplane, and the heat load at the module front surface becomes 1.3 MW/m2 to 5 MW/m2. Additionally, near the toroidal gaps, BM becomes high. Thus, at the top of the FW, magnetic field lines are dragged into the toroidal gaps directly because, the magnetic flux surface is not closed. This makes high (about 10MW/m2) heat load concentration at the moduel edge. The effect of the NBI port is also analyzed. Also near the port, Bm becomes high and the orbit of the magnetic field lines are changed. The effect of this doesn\u27t occur near the port, but far region such as inner midplane or top of the FW. The heat load becomes 6 MW/m2 at inner midplane. These results indicate that the effect of RAFM steel on the FW heat load is not negligible, and more detailed analysis is necessary

Deuterium Permeation Behavior in Fe Ion Damaged Tungsten Studied by Gas-Driven Permeation Method

The deuterium (D) permeation behavior for 1 displacement per atom Fe2+ damaged tungsten (W) was studied by the gas-driven permeation method and compared with undamaged W. The results of thermal desorption spectroscopy showed that dislocation loops and voids were formed in damaged W. It was found that the D permeation behavior in W was affected by irradiation defects. The effective diffusivity and permeability in the damaged W were lower than that in undamaged W. However, the difference in effective diffusivity and permeability between the undamaged sample and the damaged sample was reduced with increasing the heating temperature. Under 965 K, which was enough for D detrapping from voids, the permeability for damaged W was consistent with that for undamaged W