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- Efsing, Pål, 1965-
(författare)
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Delayed Hydride Cracking in Irradiated Zircaloy
- 1998
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Under some circumstances nuclear fuel cladding tubes made from zirconium based alloys may develop long axial cracks. The formation of these cracks is mainly thought to be connected with the oxiditian and hydriding of the cladding which takes plage after the the formation of a small primary defect. One mechanism proposed to be responsible for the propagation of the axial cracks is delayed hydride cracking, DHC. DHC is a process where hydrogen diffuses upwards the tensile stress gradient tbat exisur in front of an existing crack or flaw. This cancentrates the hydrogen solved in the matrix to the area ahead of the growing crack. When ihe solubility limit is passed in front of the crack, hydrides are precipitated. The hydrides are assumed to be brittle in their behaviour at temperatums up to 300°ree;C. When a certain critical size is passed, the hydrides or hydride-package fracture in a brittle matmer if the lotal stress intensity leve1 is above a threshold value. allowing the crack to grow the distance of the hydridel hydride-package. The process then repeats itself at the new location of the crack tip. The aim of the thesis was to determine if regular BWR Zircaloy-2 cladding was susceptible to crack growth due to DHC or a mechanism similar to DHC in its axial direction. To enable testing on actual spent fuel cladding, a test tcchnique was developed and applied both to unirradiated and irradiated material. The specimen is similar to a normal centre cracked tension, CCT-, specimen. The test program has included investigations on the crack growth rates at 200" and 300°ree;C, the threshold stress intensity level, KIH below which no crack growth occurs and the intubation period bcfore cracking starts. The experimental work has focused on hydrogen tontents above 5OOppmH.In the unirradiated case the maximum crack growth was found to be in the vicinity of 6.10-7 m/s, while the irradiated case demonstrated crack growth rates close to 10-6 rn/s. The threshold stress intensity leve1 was found to be strongly dependent on the yield strength of the material. such that higher yield strength resulted in lower Km. The intubation period was found to be fairly constant, regardless of the hydrogen tontent and yield strength but dependent on the temperature at which the specitic experiment was conducted.The obtained crack growth rates indicates that the growth of long axial cracks in nuclear fuel cladding can be described by a mechanism similar to delayed hydride cracking at hydragen levels above 500 ppm lotally. Whether the mechanism of crack growth is DHC as described in the case high strength Zr-2.5 wt% Nb and low hydrogen tontents or a process similar to that can however not be verified experimentally sinte most of the evidente is indirect. The temperature dependence is consistent with an activation energy for crack growth close the tbeoretically derived value of 69.5 Idlmole for crack growth controlled by hydrogen diffusion in a stress induced potential gradient. Thus the crack growth can be described by an Arrhenius relationship in the steady state region with reference to applied K, stage II.
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| 2. |
- Efsing, Pål, 1965-, et al.
(författare)
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The influence of temperature and yield strength on delayed hydride cracking in hydrided Zircaloy-2
- 1996
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Ingår i: Zirconium in the Nuclear Industry: Eleventh International Symposium. - West Conshohocken : ASTM International. - 9780803153431 ; , s. 394-403
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Bokkapitel (refereegranskat)abstract
- To determine if delayed hydride cracking (DHC) can be the cause of the long axial cracks occasionally found in BWR fuel cladding, a systematic study of DHC in Zircaloy cladding has begun. In the initial stage of the project, a test technique was developed and applied to unirradiated samples of Zircaloy. The present study includes an investigation of the influence of the yield strength and temperature on the crack growth rate and the threshold stress intensity that must be exceeded before cracking begins.Recrystallized (RXA) Zircaloy-2 has been compared to stress relief annealed (SRA) Zircaloy-2 with similar texture and composition. The results show that the crack propagation rate increases with increasing yield strength at similar stress intensity levels by as much as a decade when the yield strength is tripled. The maximum crack propagation rate measured in this study is ∼6 × 10-7 m/s. The threshold stress intensity, KIH, was found to decrease with increasing yield stress. The measured threshold values are in the range of 13.5 to 7.5 MPa. These figures are close to theoretically derived values using a critical fracture stress criterion of the hydrides as the limiting factor. The incubation period before cracking begins is found to be longer at 200°C than it is at 300°C.
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| 3. |
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| 4. |
- Pettersson, Kjell, et al.
(författare)
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Studies on Delayed Hydride Cracking of Zircaloy Cladding
- 1999
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Ingår i: Ninth International Symposium on Environmental Degradation of Materials in Nuclear Power Systems—Water Reactors. - Hoboken, NJ, USA : John Wiley & Sons, Inc.. - 9781118787618
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Konferensbidrag (refereegranskat)abstract
- Occasionally Zircaloy fuel cladding fail by forming long axial splits which lead to substantial releases of fission products. Postirradiation examinations of failed fuel suggested that Delayed Hydride Cracking (DHC) could be the mechanism. Tests on unirradiated cladding were used in order to develop a test method for irradiated cladding. These tests showed that unirradiated cladding was sensitive to DHC and that the threshold stress intensity factor for crack growth decreased and the maximum crack growth rate increased with increasing yield strength. The subsequent tests on irradiated cladding showed that it had about the same properties as unirradiated cladding with a similar yield strength. The cracking mechanism was studied with metallographic and fractographic examinations. The observations suggest that the cause of cracking is re-orientation of hydride plates in the vicinity of the crack tip from a plane perpendicular to the plane of the crack to an orientation parallel with the crack. This reduces the local fracture toughness to a value below the applied K and the crack grows until it is arrested in material where the re-orientation has not yet taken place. The driving force for re-orientation is the stress field at the crack tip, and as expected from such a mechanism the observed temperature dependence of cracking is consistent with the combined activation energies for hydrogen diffusion and solubility in Zircaloy. A recent study of the effect of hydride plates oriented perpendicular to the stress in uniaxial tests indicates however that these have no effect on ductility at temperatures above 100°C, while the DHC tests were conducted at 200 and 300CC. The exact mechanism of cracking is therefore still something of a mystery.
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