Evaluation of missing pellet surface geometry on cladding stress distribution and magnitude
Abstract Missing pellet surface (MPS) defects are local geometric defects in nuclear fuel pellets that result from pellet mishandling or manufacturing. The presence of MPS defects can cause significant clad stress concentrations that can lead to through-wall cladding failure for certain combinations of fuel burnup, and reactor power level or power change. Consequently, the impact of MPS defects has limited the rate of power increase, or ramp rate, in both pressurized and boiling water reactors (PWRs and BWRs, respectively). Improved three-dimensional (3-D) fuel performance models of MPS defect geometry can provide better understanding of the probability for pellet clad mechanical interaction (PCMI), and correspondingly the available margin against cladding failure by stress corrosion cracking (SCC). The Consortium of Advanced Simulations of Light Water Reactors (CASL) has been developing the Bison-CASL fuel performance code to consider the inherently multi-physics and multi-dimensional mechanisms that control fuel behavior, including cladding stress concentrations resulting from MPS defects. This paper evaluates the cladding hoop stress distributions as a function of MPS defect geometry with discrete pellet radial cracks for a set of typical operating conditions in a PWR fuel rod. The results provide a first step toward a probabilistic approach to assess cladding failure during power maneuvers. This analysis provides insight into how varying pellet defect geometries affect the distribution of the cladding stress, as well as the temperature distributions within the fuel and clad; and are used to develop stress concentration factors for comparing 2-D and 3-D models.
Cracking and relocation behavior of nuclear fuel pellets during rise to power
Abstract Out-of-pile experiments and analyses of pellet thermal deformation and in-pile analyses of fuel rod gaps were carried out to elucidate pellet relocation behavior during fuel rod operation. Behavior of cracked pellets, subjected to cladding restriction, was evaluated by analogy to compaction of granular materials. From this work a gap model was proposed to express gap closure during rise to power. The concept of a critical void ratio was introduced for determination of the hard PCI (pellet-cladding interaction) onset condition. The following results were obtained within the range of the design parameters and operating conditions of typical BWR fuel rods. 1. (1) When the rod power, P, reached P = 60 W/cm, pellets simultaneously cracked and increased their diameters (pellet jump). The amount of pellet jump, ΔDj, could be given by ΔDj (μm) = 3.6 D, where D is pellet diameter (mm). 2. (2) The gap G(B)(μm) could be given from the following relocation, assuming that K is the gap change (μm) caused by phenomena which do not directly participate in pellet cracking (e.q. swelling, creep); G(B)=C ′ 1 (2−C 3 ) e −C 2 A−C ′ 1 (1−C 3 ) e −(C 2 A+C 4 B) where A = (P−60), B is the burnup (GWD/T), C1 = G0 − (ΔDj + K), G0 is the initial gap (μm), C2 = 0.0039, C3 = 1.41 and C4 = 0.95. 3. (3) The hard PCI occurred when the void ratio of the sectional area inside the cladding decreased to about 0.25%.
Chemical thermodynamics of the system Cs--U--Zr--H--I--O in the light water reactor fuel-cladding gap
Equilibrium thermodynamic calculations were performed on the Cs-U-Zr-H-I-O system that is assumed to exist in the fuel-cladding gap of light water reactor fuel under in-reactor, steam, and 50% steam--50% air conditions. The in-reactor oxygen potential is assumed to be controlled by either UO/sub 2+x/ + Cs/sub 2/UO/sub 4/ or Zr + ZrO/sub 2/. The important condensed phases in-reactor are UO/sub 2+x/, Cs/sub 2/UO/sub 4/, and CsI, and the major gaseous species are Cs, Cs/sub 2/, CsI, and Cs/sub 2/I/sub 2/. The presence of steam does not alter these species, although CsOH also becomes a major gaseous species. In a 50% steam--50% air mixture, the equilibrium condensed phases are U/sub 3/O/sub 8/ or UO/sub 3/ and Cs/sub 2/U/sub 15/O/sub 46/. Under a nonequilibrium situation where zirconium metal can react with iodine, ZrO/sub 3/ or liquid ZrI/sub 2/ is present, and the gaseous species ZrI/sub 3/ and ZrI/sub 4/ have large partial pressures.