Volume 7, Issue 7 (July 2010)
Optimization of Zry-2 for High Burnups
Two major high burnup boiling water reactor (BWR) issues today are (1) shadow corrosion (hydrogen) induced fuel channel bowing and (2) accelerated hydrogen pickup fraction, HPUF of fuel claddings. Both these issues occur only at high burnups. Several factors impact the tendency for fuel channel bowing, the most recent one that is responsible for the current issue of difficulties to insert control rods is due to differences in hydrogen pickup of the different facing surfaces of the fuel channel. Accelerated HPUF of fuel cladding at high burnups may result in reducing fuel clad integrity during reactivity initiated accidents and cask drop accident during dry storage conditions. Some fuel vendors are now shifting from Zry-2 to Zry-4 in structural components to mitigate the fuel channel bowing issue. All BWR fuel vendors are also looking into new alloys (other than Zry-2 and Zry-4) for structural materials. However, one should keep in mind that it is imperative that a mechanistic understanding of the accelerated HPUF is developed to understand the accelerated HPUF tendency in Zry-2. Without such an understanding, it is not possible to rule out that accelerated HPUF can occur also for the new alloys under certain in-pile conditions. This paper summarizes the state of knowledge on the accelerated HPUF and provides suggestions on (1) the mechanism responsible for the accelerated HPUF of Zry-2 at high burnups and (2) how the currently used Zry-2 material for fuel rods and structural materials in BWRs can be modified to delay the accelerated HPUF at high burnups.