抄録
For nuclear reactor systems, the critical heat flux (CHF) data is very important because it limits reactor efficiency. Improvement of CHF requires that the cooling liquid can contact the heating surface, or a high-wettability, highly hydrophilic heating surface, even if a vapor bubble layer is generated on the surface. In our previous study, we confirmed that the surface wettability changed significantly or that highly hydrophilic conditions were achieved, after irradiation of ^<60>Co gamma ray, by the Radiation Induced Surface Activation (RISA) phenomenon. To delineate the effect of RISA on boiling phenomena, surface wettability in a high-temperature environment and critical heat flux (CHF) of metal oxides irradiated by gamma rays were investigated. The CHF experiment in pool boiling condition was carried out. The pressure is atmospheric pressure, resulting in the boiling point to be 100 degree C. The test piece had been hold horizontally on the electrode. The heating was conducted using the Joule heating by CD supply. Two kinds of the test pieces were used in this experiment. Type (A) is the plate type test piece, whose geometries are 0.2mm in thickness, 3mm in height and 60mm in length. Type (B) is the wire type test piece, whose diameter is 0.5mm. These test pieces were made of titanium. To generate the oxidized surface, the test pieces had been oxidized using plasma jetting for 40 seconds. The test section was irradiated by ^<60>Co gamma ray with predetermined radiation intensity and period. The CHF of oxidized titanium was improved up to 100 percent after 800kGy ^<60>Co gamma ray irradiation. And the oxidized layer enhanced the boiling transfer. For the gamma ray irradiation effects, the boiling curves of the oxidized titanium showed the same tendency. The gamma ray irradiation does not affects the boiling phenomena, however, it does affect on the critical heat flux. The point of this result is what increases the CHF. We call this effect Radiation Induced Boiling Enhancement (RIBE). Before we conducted the CHF experiment, contact angles of the test pieces were measured to show the relationship between wettability and CHF. The CHF in the present experiment increases with surface wettability in the same manner as shown by Liaw and Dhir's results. The mechanism of the CHF improvement will be modeled future using the present experimental results.
