Wettability of candidate Accident Tolerant Fuel (ATF) cladding materials in LWR conditions

Abstract

Since the 2011 Fukushima accident, substantial research has been dedicated to developing accident tolerant fuel (ATF) cladding materials. These analyses have mostly concentrated on the capability of potential ATF materials to withstand runaway steam oxidation and preserve their mechanical strength and structural integrity under thermal shocks. However, knowledge is relatively deficient for the thermal-hydraulic properties of these materials, particularly under light water reactor (LWR) operating conditions. The surface wettability is particularly important, as it affects the dynamics of the boiling heat transfer process, and consequently, the critical heat flux (CHF) and rewetting temperatures, which are important thermal limits for LWRs. Surface wettability determines nucleation site density, bubble departure diameter, and bubble departure frequency.

Therefore, it is essential to quantify the surface wettability of candidate ATF cladding materials to determine their thermal-hydraulic behavior compared to conventional Zircaloy claddings. The surface wettability is usually quantified through the sessile droplet contact angle, which is the angle formed between the liquid-vapor and the liquid-solid interface. The contact angle depends on the fluid, solid, surface finish, and operating conditions, i.e., temperature and pressure. However, most of the measurements available in the literature are performed at low pressure and in an inert atmosphere, which is quite different from the operating conditions of LWRs (i.e., in a steam-saturated atmosphere at a pressure as high as 15.5 MPa or 155 bars).

To close this gap, in this study, we designed and built an autoclave-type facility capable of measuring static, advancing, and receding contact angle in steam-saturated atmospheres, from sub-atmospheric conditions up to the critical point of water, i.e., 22.1 MPa (221 bar or 3200 psi) and 374°C. We measured the static contact angle of conventional Zircaloy-4 and candidate ATF cladding materials (e.g., Cr-coated Zr-4, FeCrAl, and SiC). The contact angle decreases with an increase in temperature for all the materials. Rough surfaces showed higher wettability, i.e., lower contact angle, compared to the smooth surfaces. These trends are expected from theory. All the materials showed different wettability under the same temperature and pressure conditions. Individual correlations for temperature dependence for each of them are proposed.