External Glass Peening of Zircaloy Calandria Tubes to Increase the Critical Heat Flux

Abstract

Glass-peening the outside surfaces of Zircaloy calandria tubes increases the nucleation sites available for boiling heat transfer and has been demonstrated to enhance the critical heat flux (CHF) in pool-boiling experiments. The objective of this study is to optimise the heat transfer enhancement by glass peening while ensuring that the microstructure of the peened tube is acceptable for reactor use. Pool boiling tests were done using small Zircaloy tubes with as-received (smooth) surfaces and variously-peened surfaces, to evaluate two peening parameters, glass-bead size and the coverage of peened surface. Our results showed that the maximum enhancement of CHF (by 60% compared with as received tubes) was obtained using a glass-bead size of 90-125 um with a coverage of 100%. The CHF enhancement was found to be insensitive to glass-bead size over a widerange (from 60-90 um to 125-180 um). Using a fixed glass-bead size of 125-180 um toevaluate the influence of peening coverage, the maximum effect on the CHF response wasobtained with a coverage of 100%. The microstructures of the peened tubes wereevaluated using light microscopy, X-ray and neutron diffraction, and mechanical tests. After peening, the microstructure in the sub-surface layer (~30 mm) consisted of deformeda-Zr grains and the crystallographic texture of the grains changed slightly. After stressrelievingat 500 degrees C for 1 h, some re-crystallisation had occurred and the residual strains remaining in the tube were low. The tensile and burst properties of glass-peened and stress-relieved tubes were similar to as-received tubes. The microstructures introduced by peening and stress-relieving were judged to have little effect on creep and growth behaviour. Since there are no deleterious consequences of the glass-peening treatment, the peened and stress-relieved tubes are found to be acceptable for reactor use.