Physical and Chemical Phenomena Associated with the Dissolution of Solid UO2 by Molten Zircaloy-4

Abstract

Laboratory experiments were performed in the temperature range 1800 to 2000°C in inert gas to study the chemical interaction between solid uranium dioxide (UO2) and liquid Zircaloy-4, and the wettability of UO2 by molten Zircaloy as functions of time and Zircaloy oxygen concentration. The experiments were interrupted after various reaction times to determine the extent of the chemical interaction and of the UO2 dissolution. The results show that the dissolution of UO2 by molten Zircaloy is primarily a chemical process, and that the extent of the interaction depends on the wettability of UO2 by molten Zircaloy. The wettability, however, depends strongly on the oxygen content of the Zircaloy and therefore on the time of UO2/Zircaloy contact. The wettability improves with increasing oxygen content. Zircaloy reduces UO2 to form a homogeneous [uranium, zirconium, oxygen (U,Zr,O)] melt at low oxygen concentrations or a heterogeneous (U,Zr,O) melt which contains (U,Zr)O2 particles at high oxygen concentrations. During cooling, the (U,Zr,O) melt decomposes into a (U,Zr) alloy with high uranium content and oxygen-stabilized α-Zr(O). The amount of fuel which can be liquefied by molten cladding depends on the initial oxygen content of the melt. Oxygen-free liquid Zircaloy can dissolve considerably more UO2 than Zircaloy rich in oxygen. The significance of these experiments is that UO2 fuel can be “liquefied” by molten Zircaloy far below the melting point of UO2.

New determination of the UO2/Zircaloy reaction kinetics and calculation of the oxygen diffusion coefficients

“…From the results of the MONA experiments [1,2], the U02/Zry reaction kinetics seem to obey an Arrhenius relationship. Therefore, this assumption is probably appropriate.…”

“…During the interaction between U02 and Zry the following reaction layers were formed on the c1adding inside and outside surface in a particular sequence at all examined temperatures and reaction times [1,2,3,12]:…”

“…(d) The final stage of these procedures is to correct the nominal test temperatures of the MONA experiments under inert gas conditions [1,2]. In these series of experiments Zones IV and V did not form.…”

“…Extensive out-of-pile U02/Zry interaction experiments have been performed at temperatures ranging from 900 to 1700 oe in the high temperature/high pressure equipment MONA under inert gas (Ar) conditions [1,2] and oxidizing conditions (Ar + 25 vol. % 02) [3,4,12].…”

“…This can be readily discerned from the difference in the data between two series of tests (A) and (8) performed at the same nominal temperature of 1600 oe in the MONA equipment under inert gas conditions described in [1,2]. In the MONA apparatus [1,2,3,4,12] non-contact temperature measurements were performed using an infrared and an emissivityinsensitive dual-wavelength pyrometer. In general, it is difficult to perform accurate temperature measurements at high temperatures.…”