Please cite this article in press as: R. Fetzer, et al., Efficiency of water coolant for DEMO divertor, Fusion Eng. Des. (2014), http://dx.doi.org/10.1016/j.fusengdes.2014.11.012 ARTICLE IN PRESS G Model FUSION-7646; No. of Pages 4 Fusion Engineering and Design xxx (2014) xxx–xxx Contents lists available at ScienceDirect Fusion Engineering and Design jo ur nal home p age: www.elsevier.com/locate/fusengdes Efficiency of water coolant for DEMO divertor Renate Fetzer ∗ , Yuri Igitkhanov, Boris Bazylev Karlsruhe Institute of Technology, Karlsruhe, Germany a r t i c l e i n f o Article history: Received 2 September 2014 Received in revised form 15 October 2014 Accepted 24 November 2014 Available online xxx Keywords: DEMO Tungsten monoblock Water cooled divertor ELMs a b s t r a c t Up to now, water-cooled divertor concepts have been developed for limited incident fluxes without taking into account transient power loadings. In this paper we analyzed the efficiency of water as a coolant for the particular PFC tungsten monoblock shield with a cooling tube made from Cu alloy (Cu OFHC) as a laminate adjacent to W and a low activation martensitic steel (Eurofer) as inner tube contacting the coolant. Thermal analysis is carried out by using the code MEMOS, which simulates W armour damage under the repetitive ELM heat loads. We consider cooling conditions which allow one to keep relatively high material temperatures (in the range 300–600 ◦ C) thus minimizing Eurofer embrittlement under neutron irradiation. Expected DEMO I and DEMO II heat loads including type I ELMs are found to cause melting of the W surface during unmitigated ELMs. By mitigation of the ELMs melting of W is avoided. DEMO I operation under these conditions is save for cooling at water pressure 15.5 MPa and temperature 325 ◦ C, while for DEMO II with mitigated ELMs the critical heat flux is exceeded and safe operation is not provided. © 2014 Elsevier B.V. All rights reserved. 1. Introduction The proper choice of coolant and cooling conditions is important for a Fusion Power Plant DEMO. Selection of a suitable coolant for the blanket and divertor in DEMO is made difficult by the number of factors that must be considered, such as an efficient removal of the high amount of energy at operating temperatures com- patible with structural materials, thermal and radiation stability, required pumping power, coolant availability, economics, etc. The most credible option for DEMO nowadays seems to be helium [1]. However, helium as a coolant has some drawbacks. The high tem- perature He-cooling technologies are still not developed for fusion reactor conditions and it is far from certain that He-related R&D will be successfully completed in time for DEMO. Therefore it is appropriate to consider the credibility of a water cooled DEMO as a back-up option. Up to now, water-cooled divertor technologies have been devel- oped for incident heat fluxes limited to 15–20 MW/m 2 [2,3]. DEMO concepts that use steel (e.g., Eurofer) as a heat sink material and tungsten as armour must be tested for incident fluxes at least up to 25 MW/m 2 in case of transient heat loads. One of the main concerns in this case is the Eurofer embrittlement at around 300 ◦ C under neutron irradiation [4] and there would be ∗ Corresponding author. Tel.: +49 72160824263. E-mail address: renate.fetzer@kit.edu (R. Fetzer). significant benefits in operating DEMO at relatively high Eurofer temperatures (in the range 300–500 ◦ C). It should be noted that heat sink material temperatures in all tokamaks presently oper- ating or under construction (including ITER) are below 300 ◦ C. No foreseen device will therefore provide demonstration of in-vessel operation in DEMO relevant ranges of plasma facing component temperatures. This is a significant gap in the fusion programme worldwide, which needs to be covered by at least numerical modelling. In this paper we analyzed the efficiency of water as a coolant for the particular PFC tungsten monoblock shield with a cooling tube made from Cu alloy OFHC (oxygen free high conductivity Cu) as a laminate adjacent to W and a low activation martensitic steel (Eurofer) as inner tube contacting the coolant. Eurofer has been selected because it is the reference candidate structural material due to its expected capability of withstanding neutron damages higher than 70 dpa, corresponding to 3–4 years of DEMO divertor continuous operations. The design of the DEMO divertor module is taken as suggested by Li-Puma et al. [4], see Fig. 1. The diameter of the coolant tube is 6 mm, the thicknesses of the Eurofer and Cu OFHC layers are, respectively, 0.42 mm and 1 mm. The W armour is 3.5 mm thick at its thinnest position. The described design has been found to be the optimum in terms of thermo-mechanical properties for stationary heat loads of 10 MW/m 2 and water cooling with inlet water tem- perature 325 ◦ C, pressure 15.5 MPa, and velocity 20 m/s [4]. Water temperature and pressure correspond to the cooling conditions in http://dx.doi.org/10.1016/j.fusengdes.2014.11.012 0920-3796/© 2014 Elsevier B.V. All rights reserved.