Potential of Copper Alloys using a Divertor Heat Sink in the Helical Reactor FFHR-d1 and their Brazing Properties with Tungsten Armor by using the Typical Candidate Filler Materials

Abstract

A tungsten block is supposed to be used as a divertor armor material on the helical reactor FFHR-d1. On the other hand, material selection of the heat sink and bonding technique between armor and heat sink are currently under investigation. On the material selection, copper alloy has a large advantage for the thermal conductivity, but its material properties such as toughness and thermal conductivity, are dramatically decreased due to the neutron irradiation. However, from the assessment of the neutronics environment on the divertor region of the FFHR-d1, copper alloys could be used for a heat sink especially at the outer divertor. In the ITER case, copper alloy (CuCrZr) pipes are joined by a brazing technique with Nicuman37 filler material. This combination has not been optimized for the FFHR-d1, because the toughness of the CuCrZr at high temperature over 450 • C is dramatically decreased with increasing the temperature. As such, another candidate is an oxide dispersion-strengthened copper alloy (ODS-Cu) such as GlidCop R . For the bonding technique, a reliable brazing combination between "two kinds of copper alloys" and "three kinds of filler materials (MBF-20, BNi-6, Nicuman37)" were investigated from a viewpoint of mechanical strength. The most superior fracture strength among the three filler materials was BNi-6 with GlidCop R .

Direct Tungsten/Copper Bonding for Divertor Application

“…Due to such surface corrosion, particularly in ambient air, bonding at too high pressure for long duration may rather weaken the mechanical bonding strength. We have thus obtained W/Cu bonding in ambient air at a temperature of 500 • C, significantly lower than those of earlier studies [7,8,15], about 1000 • C. The mechanical bonding strength of our samples, about 1 MPa, was however far lower than the state-of-theart reported values, over 100 MPa. This weakness issue is to be resolved.…”

“…Whilst, these characteristics are seemingly not very sensitive to the bonding temperature for the Cu side. Overall, although W/Cu bonds formed even at a temperature as low as 500 • C and a bonding pressure as low as 0.1 MPa, the resulted interfacial mechanical strength is quite low relative to those obtained by the state-of-the-art techniques such as high-temperature brazing [7,8]. While bonding in ambient air is practically convenient, the technical trade-off with the oxidation issue has to be accounted for, depending on the requirement of each situation.…”

“…For the bond between the armor and heatsink segments, various techniques have been investigated, such as brazing [7,8], electron beam welding [9,10], explosive welding [11,12], and friction stir welding [13,14]. At present, W/Cu brazing at temperatures around 1000 • C in vacuum is thought as the first candidate [7,8]. Meanwhile, W/Cu direct bonding has been studied also at around 1000 • C in hydrogen atmosphere [15].…”

Design Concept of Supercritical CO₂Gas Cooled Divertors in FFHR Series Fusion Reactors

“…Since the recrystallization temperature of tungsten armor tiles is 1500 to 1800 K [11][12][13], it is desirable that the armor tar surface temperature during operation be maintained below this temperature range.…”

“…It has been pointed out that in the actual divertor, material deterioration occurs when it is exposed to heavy neutron irradiation of 14 MeV with a high heat load. In particular, it has been pointed out that the ODS material causes significant deterioration in characteristics that interferes with long-term operation when exposed to irradiation of about 20 dpa within the expected operation period [3,[9][10][11][12][13].…”