Fig. 2-12 Screw tube for thermal fatigue test and leak test conditions We have proposed a screw tube as a high performance cooling tube having three features: superior heat removal capability, high mechanical strength, and reduced manufacturing cost. The inner cooling surface of the screw tube is threaded like a nut, which enhances heat transfer by promoting continuous mixing of cooling water near the inner tube surface. During service, the screw tube is exposed to cyclic heat loads; and the screw threads potentially are crack initiators under these loads. To evaluate its practical application, the thermal fatigue test of the screw tube formed from a Cu-alloy, CuCrZr, was performed under single-sided heat exposure conditions with heat fluxes of 20 and 30 MW/m2. These fluxes are more severe than the heat load conditions of a fusion device. The use of a screw tube mock-up with its monolithic lower structure accelerated these experiments because axial thermal expansion of the cooling channel was constrained and thermal strain at the channel wall was concentrated by the monolithic lower structure.

Fig. 2-13 Comparison of the prediction of the screw tube lifetime with the results of thermal fatigue experiments The lifetimes of the screw tube were predicted by a combination of Manson-Coffin's law and numerical analyses with a finite element method (FEM) under the applied experimental conditions. In the analyses, the mechanical strains that occurred in the screw tube wall were estimated. As a result, the predicted lifetime of the screw tube has good correlation with the experiments (, ).

Fig. 2-14 Fractographic observation of the test sample after 20 MW/m2 heat flux thermal cycles Microscopic observations of fatigue fractures at a water-leak point with a Scanning Electron Microscope (SEM) reveal that the fatigue crack starts from the heated outer surface and propagates toward the cooled inner surface. This indicates that the screw geometry does not act as a crack initiator under single-sided exposure to high heat flux.

In a fusion machine, a divertor is the only in-vessel component in contact with plasma. In this service, a divertor will receive a steady-state high heat flux in excess of 20 MW/m2, which exceeds heat loads applied to general engineered components. We have proposed applying a screw tube heat exchanger using pressurized water flow for divertor cooling. In the screw tube design, the cooling surface has threads like a nut, which enhance heat transfer by promoting continuous mixing of flowing water near the heat-transfer surface. Experimental results show a screw tube heat exchanger has higher heat removal capability than does a swirl tube with an inserted twisted tape, which is the type adopted for the ITER divertor. During service, the screw tube heat exchanger is exposed to cyclic heat loads; however, it has been noted that the screw thread roots potentially act as crack initiators. Therefore, the thermomechanical features of the screw tube have been studied experimentally and numerically (Fig. 2-12). In the experiments, repetitive heat loads of 20 and 30 MW/m2 were applied to the screw tube mock-up formed from a copper (Cu)-alloy, CuCrZr, (a candidate material of the ITER divertor cooling tube). The experiments continued until a water leak due to thermal fatigue occurred in the cooling channel. Using finite element analyses, the thermomechanical behavior of the screw tubes under the applied experimental conditions were investigated. The screw tube fatigue lifetimes based on the mechanical strain in the tube walls were predicted. In Fig. 2-13, the fatigue lifetime prediction of the screw tube (solid line) and the experimental results (cycle number to failure or occurrence of water leak) are plotted. Both results are in good agreement. This indicates that the fatigue lifetime for a cooling channel with complex geometry like a screw tube can be predicted with the aid of numerical analyses. Fractographic observations at the water-leaked wall (Fig. 2-14) show that the fatigue crack started from the heated outer surface and propagated toward the cooled inner surface. This indicates that the screw geometry did not act as a crack initiator under the single-sided high heat load imposed on divertors. From the results of the thermomechanical tests performed so far, the applicability of a screw tube heat exchanger for divertor cooling has been demonstrated. Further studies that include the electromagnetic effects on a cooling channel are planed to examine the suitability of the screw tube cooling technology on fusion machines.

Reference K. Ezato et al., Thermal Fatigue Experiment of Screw Cooling Tube under One-Sided Heating Condition, J. Nucl. Mater., 329-333(1), 820 (2004).

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