The thermo-hydraulic and ignition behavior of full-scale Pressurized Water Reactor (PWR) fuel assembly mock-ups during hypothetic spent fuel pool Loss of Coolant Accidents (LOCA) will be investigated in the frame of the OECD-NEA Sandia Fuel Program (SFP).
The choice of the cladding alloy has been intensively discussed in the initial phase of the project. The PWR cladding alloys Zircaloy-4 or ZIRLO have been originally foreseen to be used in these tests. Due to problems regarding availability and proprietary rights of these materials, finally the Boiling Water Reactor (BWR) material Zircaloy-2 was selected as heater rod material. The main arguments for Zry-2 were availability of material and all data, the positive experience with the material during the BWR test series and the very similar properties of Zry-2 and Zry-4. Some participants asked for a proof of the latter statement, especially because the database on Zry-2 is scarce in comparison with Zry-4. In this context KIT performed a series of separate-effects tests (SETs) to confirm the similarity of the two alloys and to deliver corresponding data. The following report present ... mehrs the results of isothermal and transient oxidation tests of Zry-2 and Zry-4 in air atmosphere, including simulations of the planned pre-ignition and ignition tests, as well as results of mechanical tests.
Isothermal and transient experiments were conducted in a commercial thermal balance coupled with a mass spectrometer up to temperatures of 1600 °C. Ring compression and tensile tests were performed with a universal testing machine from Instron.
The results of the extensive experimental program revealed very similar oxidation behaviour of the two alloys Zircaloy-2 and Zircaloy-4 in the temperature range investigated. So, there are no concerns regarding the use of the BWR alloy Zircaloy-2 in the PWR SFP test series at Sandia National Laboratories from this point of view. During the planned pre-ignition tests only very low pre-oxidation of the cladding and so insignificant effect on the main ignition test is expected. The point in time of the ignition of the bundle strongly influences the starting conditions for the further progression of the test, especially the degree of pre-oxidation and therewith the amount of remaining metal.
The mechanical properties of the two alloys in the as-received state and the heater cladding are significantly different. The manufacturing process of the heater rods caused increase of strength and reduction of ductility. Further strong changes of mechanical properties are expected during course of the tests due to oxidation. This has to be taken into account for the analyses of the tests.