Oxidation Behavior of Fuel Cladding Tube in Spent Fuel Pool Accident Condition

Abstract

In spent fuel pool SFP under loss-of-cooling or loss-of-coolant severe accident condition, the spent fuels would be exposed to air and heated by their own residual decay heat. Integrity of fuel cladding tube is crucial for SFP safety therefore study on oxidation of fuel cladding tube in air at high temperature is thought important. Zircaloy-2 Zry2 and Zircaloy-4 Zry4 were used for thermogravimetric analyses TGA in different temperatures in air at different flow rates to evaluate oxidation behavior. Oxidation rate increased with testing temperature. In a range of flow rate of air which is predictable in spent fuel lack during a hypothetical SFP accident, influence of flow rate was not clearly observed below 950°C for the Zry2, or below 1050°C for the Zry4. In higher temperature, oxidation rate was higher in high flow rate condition, and this trend was seen clearer when temperature increased. Oxide layers were carefully examined after the TGA analyses and compared with mass gain data to investigate detail of oxidation process in air. It was revealed that the mass gain data in pre-breakaway regime reflects growth of dense oxide film on specimen surface, meanwhile in post-breakaway regime, it reflects growth of porous oxide layer beneath fracture of the dense oxide film.

Investigation of Zircaloy-2 oxidation model for SFP accident analysis

“…The plug was 5 mm in length and 11.2 mm in diameter. The Zr liner on inner surface of the Zry2 cladding was removed before TG analyses in previous works [24][25][26], but on the contrary, it was not removed in this work because the cladding sample was closed by the plug and thus the inner surface was not oxidized in the oxidation tests.…”

“…The authors adopted basic equations for oxidation model which were previously proposed by Coindreau et al for Zircaloy-4 (Zry4) in the temperature range between 600 and 1000 °C [19]. Necessary coefficients for the equation were calculated from the isothermal TG data which were previously obtained on Zry2 in high air flow rate oxidation tests by the authors [24][25][26]. In this case the applicability of the equation for Zry2 The authors previously conducted the TG analyses on Zry2 sample 20 mm in length at air flow rate 250 mℓ/min, and reported that oxidation kinetics were not different from that at air flow rate 144 mℓ/min when oxidation tests were conducted below 1000 °C [26].…”

“…Necessary coefficients for the equation were calculated from the isothermal TG data which were previously obtained on Zry2 in high air flow rate oxidation tests by the authors [24][25][26]. In this case the applicability of the equation for Zry2 The authors previously conducted the TG analyses on Zry2 sample 20 mm in length at air flow rate 250 mℓ/min, and reported that oxidation kinetics were not different from that at air flow rate 144 mℓ/min when oxidation tests were conducted below 1000 °C [26]. Therefore the oxidation model construction in this work was exclusively conducted by using the TG data obtained at high air flow rate (144 mℓ/min).…”

“…where,Δ m is the mass gain (g), S is the surface area (m 2 ) of specimen, t is the oxidation time (s), and k p is the parabolic rate constant (g 2 /m 4 applied accelerated rate kinetics for this post-breakaway or post-transition regime, respectively [9,14,19]. In this work, applicability of these kinetics were investigated for the TG data on Zry2 oxidation in air which were previously obtained by the authors [26]. post-breakaway or post-transition phase, accelerated rate constant which can be calculated by using the following equation was thought most adequate to be applied;…”

“…The authors have previously conducted isothermal oxidation tests in air on short samples of Zry2 cladding by using TG analysis apparatus, and reported in detail the oxidation behavior [24][25][26]. In this work, a model for Zry2 oxidation in air environment was constructed by using these basic data.…”