Applicability of tungsten/EUROFER blanket module for the DEMO first wall

doi:10.1016/j.jnucmat.2013.01.089

Journal of Nuclear Materials

Volume 438, Supplement, July 2013, Pages S440-S444

https://doi.org/10.1016/j.jnucmat.2013.01.089 Get rights and content

Abstract

In this paper we analyse a sandwich-type blanket configuration of W/EUROFER for DEMO first wall under steady-state normal operation and off-normal conditions, such as vertical displacements and runaway electrons. The heat deposition and consequent erosion of the tungsten armour is modelled under condition of helium cooling of the first wall blanket module and by taking into account the conversion of the magnetic energy stored in the runaway electron current into heat through the ohmic dissipation of the return current induced in the metallic armour structure. It is shown that under steady-state DEMO operation the first wall sandwich type module will tolerate heat loads up to ∼14 MW/m². It will also sustain the off-normal events, apart from the hot vertical displacement events, which will melt the tungsten armour surface.

Introduction

A sandwich type first wall (FW) blanket module made of W-clad EUROFER steel (see Fig. 1) is examined against the normal and off-normal operation heat loads expected in DEMO reactor. The module consists of a helium coolant tube of rectangular cross-section within the EUROFER matrix that is used as heat diffuser. The plasma material interface in DEMO is more challenging than in ITER, due to the requirements for approximately four times higher heat flux from the plasma and approximately five times higher average duty factor [1]. We consider here the DEMO design of ∼1 GW of electric power, the major radius R = 7.7 m, the aspect ratio A = 3, the toroidal magnetic field B = 6 T and the safety factor q_a = 4.5 [2]. The heat load to the FW under normal steady-state operation is expected to be in the range of 0.5–15 MW/m² and a considerable amount of energy (>90%) is radiate by light impurities injected into plasma boundary [3], [4]. Heat loads above 0.5 MW/m² could mainly be expected due to interaction with hot charge-exchange neutral atoms and due to convective radial plasma losses, associated with unstable convective cells in the SOL region. To achieve sufficient cooling efficiency under such an excessive heating helium gas as a coolant must be employed, which has no limitation on heat flux value like water, considered previously [4].

Below we consider two types of off-normal events: a loss-of control “hot” and following a disruption “cold” vertical displacement events (VDEs) and runaway (RE) generation that can occur during the current quench following a disruption. Both, VDE and RE energy deposition would affect mostly the first wall [5]. The consequent erosion due to excessive power and particle loads on plasma facing components (PFCs) is expected in DEMO, particularly, because of a huge amount of poloidal magnetic energy (∼1.2 GJ) which will eventually dissipate in the material structure. We evaluate here the conversion of magnetic energy into heat due to mainly ohmic dissipation of return current, induced during the penetration of RE beam into the tungsten armour.

Although W/EUROFER bound is of “low-activation” type, it has relatively low creep temperature (823 K) which could be the main drawback of EUROFER as a structural material. To assess proper design parameters of the FW module, calculations were performed with the Monte Carlo Energy Deposition code ENDEP together with the upgraded version of MEMOS code [6], which takes into account helium as a coolant and the RE magnetic field energy convertion into heat. The details of the RE modelling by means of ENDEP code are described in [3], [4].

Section snippets

Energy loads on the FW DEMO during off-normal events

The characteristics of off-normal events in DEMO can be assessed based on scaling arguments by extrapolating data envisaged for ITER [1], [3], [4]. In the case of VDE which may occur due to accidental loss of control in DEMO, we assume that ⩽2 GJ (∼0.7 GJ of plasma thermal energy and ∼1.2 GJ of magnetic energy) will eventually deposit on the FW structure. The resulting energy density can be estimated in the range of ∼50–100 MJ/m², which includes toroidal and poloidal peaking factors similar to ITER

Conversion of the RE magnetic energy into heat

The particular interest occurs when RE impinging on the FW and depositing their kinetic and magnetic energy into tungsten armor. The correct evaluation of deposited energy is important for assessment of surface erosion and plasma contamination. Usually, the evaluation of stopping power takes into account only the kinetic energy of impinging electrons. Here we consider the mechanism of inductive losses of the RE beam in tungsten armour. When an RE beam intersects a tungsten surface, the beam

Helium active cooling of the FW module

Here we analyse the helium coolant heat removal capability for the FW blanket module under DEMO conditions based on a model of turbulent flow in rectangular channel. In our model the helium coolant flows through a rectangular channel with square cross-section ∼15 mm × 15 mm. The channel passes through the EUROFER and positioned with wall thickness of 3.5 mm on the plasma facing side and 11.5 mm on the side facing the breeding units (Fig. 1). To stay within the allowed temperature window for the

Numerical results and analysis

Calculation where performed for armour thickness Δ_w = 3 mm and for EUROFER thickness Δ_EUROFER = 4 mm. and at high helium cooling efficiency (u = 150 m/s and 190 MPa). Fig. 4 shows the tungsten armour surface temperature and the maximum EUROFER temperature (interlayer temperature) for different heat loads in steady-state regimes of operation. Both temperatures increase with the increase the heat load. For heat loads above 14 MW/m² the EUROFER temperature exceeds the creep point T_c = 823 K and EUROFER loses

Conclusions

(1)
Under steady-state normal operation and helium cooling the FW W/EUROFER blanket module can tolerate expected in DEMO heat loads without W armour melting and EUROFER thermal destruction. For Δ_w ∼ 3 mm, Δ_EUROFER ∼ 4 mm the maximum tolerable heat flux is about 14 MW/m².
(2)
To achieve efficient heat transfer required for helium cooling of the FW blanket module in DEMO, a high flow velocity (⩾100 m/s) should be achieved by increasing the pressure drop (∼up to 200 MPa inlet pressure). This, unfortunately, could

Acknowledgments

This work, supported by the European Communities under the contract of Association between EURATOM and Karlsruhe Institute of Technology, EURATOM and CCFE, was carried out within the framework of the European Fusion Development Agreement. The views and opinions expressed herein do not necessarily reflect those of the European Commission.

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¹: Presenting author.

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