Performance evaluation of UO2-Zr fuel in power-ramp tests

In power reactors using UO2-Zr fuel, rapid power increases may lead to failures in fuel pins that have been irradiated at steady or decreasing heat loads. The paper presents results which extend the experience with power-ramp performance of high burnup fuel pins. A test fuel element containing both pellet and Vipac UO2-Zr fuel pins was irradiated in the HBWR at Halden for effectively 2 1/2 years to an average burnup of 20,600MWd/t UO2 at gradually decreasing power levels. The subsequent non-destructive characterization revealed formation of transverse cracks in the Vipac fuel columns. After the HBWR irradiation, five of the fuel pins were power-ramp tested individually in the DR 3 Reactor at Risoe. The ramp rates in this test series ranged from 7-60W/cmmin. The maximum local heat loads seen in the ramp tests were 20-120% above the highest levels experienced at the same axial positions during the HBWR irradiation. Three pellets and one Vipac fuel pin failed, whereas another Vipac pin gave no indication of clad penetration. Profilometry after the ramp testing indicated the formation of small ridges for both types of fuel pin. For Vipac fuel, the ridges were less regularly distributed along the pin length than for pellet fuel. Neutron radiography revealed the formation of additional transverse and longitudinal fuel cracks during the power ramps for both types of fuel pin. The cases were analysed by means of the Danish fuel performance codes. The calculations are discussed in the paper. The results of the investigations indicate qualitative similarities in over-power performance of the two fuel types. (author)