Thermal expansion of corium prepared from UO 2 and Zircaloy-2
- PDF / 237,001 Bytes
- 5 Pages / 432 x 648 pts Page_size
- 72 Downloads / 163 Views
Thermal expansion of corium prepared from UO 2 and Zircaloy-2 Shun Hirooka, Masatoshi Akashi, Teppei Uchida, Kyoichi Morimoto and Masato Kato Japan Atomic Energy Agency, 319-1194 Muramatsu, Tokai-mura, Naka-gun, Ibaraki-ken, Japan
ABSTRACT In this study, sintered pellets were prepared from Zircaloy-2 oxide and UO 2 as a parameter of content ratio (Zr contents were 0, 24.3, 49.0, 73.4, and 97.9 at% in metal). The sintered pellets were heated in 5%H 2 /Ar gas. UO 2 pellets underwent simple thermal expansion caused by thermal vibration while Zircaloy-2 oxide pellets underwent thermal expansion and volume change with phase transformation. Finally, the 24.3, 49.0, and 73.5 at%Zr-UO 2 pellet specimens showed both phenomena. However, phase transformation temperatures were lower than that of Zircaloy-2 oxide, and volume changes were much smaller. X-ray diffraction patterns obtained after thermal expansion measurements showed that the 24.3 at%Zr-UO 2 specimen contained tetragonal and cubic (Zr, U)O 2 while the 73.5 at%Zr-UO 2 specimen contained mainly monoclinic ZrO 2 . INTRODUCTION Fuel pellets of UO 2 and Zircaloy-2 (Zry-2) claddings form corium by meltdown of the reactor core. In a meltdown accident, the Zry-2 is considered to be well oxidized in vapor and the rapid oxidation of Zry-2 to form ZrO 2 starts at 1200 ࡈC [1]; this oxidation is a significant heat source. For thermal analysis of the corium, the density, thermal diffusivity and specific heat are required. However, the density change is quite complicated because of the phase transformation and thermal expansion. Thermal properties of UO 2 have been well studied [2-5]. It is reported that stoichiometric UO 2 undergoes simple thermal expansion and its crystal structure does not change. Differing from UO 2 , ZrO 2 undergoes thermal expansion and phase transformation which causes a volume change during heat treatment [6]. Below 1000 ࡈC, ZrO 2 exists as a low density monoclinic phase. During heating, ZrO 2 shows a phase transformation to the tetragonal phase which causes contraction. Concerning the UO 2 -ZrO 2 system, the phase diagram shows that c(cubic)-(U, Zr)O 2 and t(tetragonal)-(U, Zr)O 2 are formed from UO 2 and m(monoclinic)-ZrO 2 above 1115 ࡈC [1]. At higher temperatures, the c-(U, Zr)O 2 phase dominates. However, this phase may occur after the UO 2 -ZrO 2 system is rapidly cooled and solidified. Therefore, a reinvestigation of behaviors is necessary. In this study, UO 2 and Zry-2 oxide powders were mixed, pelletized and sintered. Thermal expansion of the prepared specimens were measured with a dilatometer under heat treatment to 1700 ࡈC in 5%H 2 /Ar gas.
97
EXPERIMENT Specimens were prepared from UO 2 and Zry-2 powders. The Zry-2 powder contained 98.4 at% Zr, 1.12 at% Sn, 0.23 at% Fe, 0.13 at% Cr, and 0.08 at% Ni. The UO 2 and Zry-2 powders were mixed to give Zr contents of 24.3, 49.0, 73.5, and 97.9 at% in metal and then they were pelletized (Fig. 1) and sintered for 3 hours in 0.05%H 2 /Ar + 10000ppmH 2 O (Fig. 2). In the sintering, Zry-2 was oxidized t
Data Loading...
