Effect of Slit Shape on Joinability of Zircaloy - SiC/SiC Composite Tube Joint with Titanium Powder

As for the fuel cladding in the light-water reactor, silicon carbide fiber reinforced silicon carbide matrix (SiC/SiC) composite is one of the promising candidates as a replacement of Zircaloy due to many superiorities, where it is necessary to develop the end-cap seal of SiC/SiC composite cladding. In this research, the caulking method was employed as the method for sealing the end cap of SiC/SiC composite tube by Zircaloy tube where the titanium micro-powder was inserted between two tubes. The fiber laser was circumferentially irradiated on the outer surface of zircaloy tube, and the insert method of titanium powder was varied by changing the cutting method of SiC/SiC composite tube. The examinations about the slit shape effect for the flat cutting suggested that the hook slit is considered to be the best cutting method for holding the titanium powder during the laser irradiation where the width of slit should be narrower than that of laser irradiation line.

Development of Joining Method between Zircaloy and SiC/SiC Composite Tubes by Using Diode Laser

Silicon carbide fiber reinforced silicon carbide composite (SiC/SiC composite) is expected as a potential replacement of the current Zircaloy fuel cladding in the light-water reactor because of its many superiorities, where it is necessary to develop the end-cap seal of SiC/SiC composite cladding. In this research, the applicability of diode laser irradiation for joining between Zircaloy and SiC/SiC composite tubes was examined based on the caulking method. As the method for fitting two tubes, the screw cutting was conducted for the inner face of Zircaloy tube and the outer surface of SiC/SiC composite. In addition, as a method to improve the joinability of Zircaloy and SiC/SiC composite, titanium nanopowder was inserted into a gap between two tubes. The laser beam irradiation was circumferentially applied to the outer surface of Zircaloy tube. Although the mechanical caulking joint was successfully produced, the airtightness was insufficient for the end-caps of SiC/SiC composite cladding. However, the good adhesion between Zircaloy and SiC/SiC composite was partially produced as the result of the generation of all proportional solid solution between titanium and zirconium. Namely, it can be concluded that the circumferential diode laser irradiation with inserting titanium nanopowder between Zircaloy and SiC/SiC composite tubes would have a good potential for producing the caulking joint of two tubes.

Flow Instability and Critical Heat Flux for Flow Boiling of Water in a Vertical Annulus at Low Pressure

We investigated the critical heat flux (CHF) for flow boiling of water in a vertical annulus. The coaxial annulus has a diameter ratio of 1.37 and the inner zircaloy tube is heated directly over a length of 325 mm. CHF can occur prematurely due to flow instabilities. Therefore, we analyzed the flow stability at different heat input conditions using two types of pumps, a rotary and a gear type pump. The unstable CHF occurred at 61% and 90% of the stable value for the rotary and the gear type pump, respectively. Consequently, the following CHF experiments were conducted at stable flow conditions. The outlet pressure was constant at 120 kPa, the mass flux varied from 250 to 1000 kg/(m2s) and the inlet subcooling was at 102, 167, and 250 kJ/kg. The CHF results increase with mass flux from 0.67 to 2.62 MW/m2 and show similar trends compared to literature data. However, the experimental data for flow boiling in annuli at low pressure are limited. Additionally, we measured the dynamic contact angle between the zircaloy tube surface and water using the Wilhelmy method.