High Temperature Corrosion Test of a Zirconium alloy

Abstract

The possibility of breakaway oxidation of zirconium alloy cladding at high temperatures around 1000ºC during small and large LOCA has been a concern in recent years. In March 2014, NRC proposed the revision of the ECCS acceptance criteria which breakaway oxidation was added to the previous criteria: the peak cladding temperature, cladding embrittlement, maximum hydrogen absorption, etc. in recent Federal Register. The standard, 10CFR50.46 which is proposed by NRC requires setting up the breakaway oxidation criterion based on actual performance of the fuel cladding and confirming the criterion by the periodic test of cladding tube which is used in the plant. High temperature oxidation of zirconium alloy has different trends with the low temperature oxidation because there are several differences between the low and high temperature oxidation. Because the oxidation depends on diffusion of oxygen, hydrogen, electron, and etc., oxidation rate is increased exponentially by increasing of oxidation temperature. Phase transformations of zirconium and their oxide are also concerned at high temperature; zirconium alloy can be received the distinctive phenomena by causing phase transformation. Zirconium alloy has a passivity of oxidation from the formation of dense oxide layer. The oxidation rate of zirconium alloy is reduced exponentially at first time. But when the zirconium oxide layer is changed into non-protective phase after some moment, oxidation kinetics is turned into linear or exponentially increased through the time. The transition of oxidation kinetics causes surface discoloration and this phenomenon is called as ‘breakaway oxidation’. As mentioned before, the transition of oxidation kinetics is usually faster at higher temperature. But breakaway oxidation can be easily observed at certain points which are 800ºC and 1000ºC not at higher temperature. In order to simulate LOCA condition which is represented by the high temperature steam oxidation of rapidly heated cladding and subsequent quenching by water, a radiant heating system with the steam and water supply system are used for the test. A radiant heating system that used in the most of LOCA simulation tests, adopts the 4 or 6 line bulbs in order to achieve uniform heating in a circumferential direction. The verification of test equipment should be performed with the sequence of draft regulatory guide which is published by U.S NRC. Axial and circumferential temperature distribution is measured by a specimen with the welded thermocouples and the results of measurement are satisfied with the recommendation in the guide. The steam supply equipment is one of the important systems for the high temperature oxidation test because the high temperature oxidation test should be conducted in steam ambient. But the initial steam supply equipment has a problem which is unstable steam flow, so the new steam supply equipment was installed on the system. The new steam supply equipment don’t show any unstable steam flow and the steam generation rates of new steam supply equipment were measured by plate-type heat exchanger. The measurement of weight gain was performed and the results of measurement are compared with the well-established data of ZIRLO which was tested by Westinghouse and ANL. The results of ZIRLO weight gain measurement are consistent with the well-established data and these results are verified the high temperature oxidation test successfully. The microstructure of ZIRLO after the high temperature oxidation test is confirmed. At the first time, the microstructure of ZIRLO has flat interface between metal and oxide, but wavy metal / oxide interface can be observed in the picture after some moment. Wavy interface between metal and oxide is estimated to the precursor to breakaway oxidation phenomenon and the microstructure after the high temperature oxidation test is shown similar as the most of previous research. Thus, the successful test results are shown in the microstructure.