Axial cracking phenomena in burnable poison rods. Final report

Technical Report · Mon May 01 04:00:00 EDT 1995

OSTI ID:73015

Fuhrman, N; Scott, D B ^[1]

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1. ABB Combustion Engineering Nuclear Operations, Windsor, CT (United States)

A hot cell examination of Al{sub 2}O{sub 3}-B{sub 4}C burnable poison rods irradiated in a PWR was conducted to determine the cause of axial cladding cracks observed during inspection at the spent fuel pool. The underlying mechanism established for the cracking phenomenon was the secondary hydriding of the Zircaloy-4 cladding following water ingress through a perforated primary hydride blister. The major source of hydrogen for the secondary hydriding was the chemical reaction between the B{sub 4}C particles in the burnable poison pellets and the incoming water. Two types of hydride zones were found: (1) longitudinally-oriented hydride zones at the cladding inside surface which were believed to result from the preferential growth of the localized hydride phase in the longitudinal direction, and (2) a uniform, heavily hydrided circumferential rim adjacent to the inner surface of the cladding. The latter type was relatively rare and probably resulted when the hydrogen partial pressure generated in the rod was high and maintained for some time. In both cases, radially-oriented hydride platelets were precipitated in the region between the inside hydride phase and the external surface of the cladding. The observed cladding cracks probably occurred when cladding tensile stresses in the circumferential direction were intensified by pellet-cladding mechanical interaction (PCMI) resulting in brittle fracture. PCMI was enhanced by precipitation of boron compounds in the pellet-cladding gap and in the pellet pore structure. The cracks propagated along the radially-oriented hydride platelets until arrested by running into the ductile metal phase.

• Research Organization: Electric Power Research Inst., Palo Alto, CA (United States); ABB Combustion Engineering Nuclear Operations, Windsor, CT (United States)
• Sponsoring Organization: Electric Power Research Inst., Palo Alto, CA (United States)
• OSTI ID: 73015
• Report Number(s): EPRI-TR--104979
• Country of Publication: United States
• Language: English

Related Subjects

21 SPECIFIC NUCLEAR REACTORS AND ASSOCIATED PLANTS
22 GENERAL STUDIES OF NUCLEAR REACTORS
ALUMINIUM OXIDES
BORON CARBIDES
BURNABLE POISONS
CONTROL ELEMENTS
CRACK PROPAGATION
CRACKS
FRACTURE PROPERTIES
FUEL ASSEMBLIES
FUEL CANS
HYDRIDATION
POST-IRRADIATION EXAMINATION
PWR TYPE REACTORS