تعيين چقرمگي شكست و طول بحراني ترك در ميله سوخت Zr-1Nb

One of the main components of fuel assemblies is the cladding tube, which is subjected to special thermal and loading conditions in the reactor. On the other side, presence of cracks in cladding tubes are unavoidable and in order to ensure the correct operation of the fuel rod, it is necessary to determine the permissible dimensions of the crack under its working conditions in the reactor and temperature ranges between 25°C to 300°C. Also, the presence of hydrogen atoms in the reactor water and the penetration of them into the structure of the fuel rod has caused the formation of hydrides, which are a place for initiation and growth of cracks. So, the fracture toughness of cladding tube should be eva‎luated as a function of temperature and hydrogen embrittlement in order to determine allowable crack length. In this research, fracture toughness of Zr-1Nb cladding tube for different temperature is determined with extracting sample test from zirconium billets according to requirements of ASTM E1820 standard and doing tension tests for as-received and hydrided samples. According to the results, with increasing in temperature test and facilitating in formation of plastic zone at the crack tip, the fracture toughness of as-received cladding tube increases from 249 KJ⁄m^2 at ambient temperature to 342 KJ⁄m^2 at 300°C. Also, at ambient temperature with penetration of hydrogen to cladding tube structure and formation of hydrides, the behavior of material alters from a ductile material to a brittle one and the fracture toughness decreases from 249 KJ⁄m^2 for as-received samples to 28 KJ⁄m^2 for hydride ones. Also, in this research, the critical crack length of cladding tube is determined using analytical and finite element method. According to the results of both methods, the critical crack length for hydrided material is lower than the corresponding value for as-received material at all test temperatures. As an example, according to results of analytical method the critical crack length for as-received cladding tube is 19.0 mm at ambient temperature, which with formation of hydrides this value decreases to 13.7 mm.

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