Temperature and Hydrogen Concentration Limits for Delayed Hydride Cracking in Irradiated Zircaloy

2009 ◽  
pp. 339-339-19 ◽  
Author(s):  
JS Schofield ◽  
EC Darby ◽  
CF Gee
Keyword(s):  
Hydrogen Concentration ◽  
Delayed Hydride Cracking ◽  
Concentration Limits ◽  
Hydride Cracking

Critical Temperatures for Initiating and Arresting Delayed Hydride Cracking in a Zr-2.5Nb Pressure Tube

2005 ◽  
Vol 297-300 ◽  
pp. 1685-1690
Author(s):  
Young Suk Kim ◽  
Kyung Soo Im ◽  
Yong Moo Cheong
Keyword(s):  
Hydrogen Concentration ◽  
Solid Solubility ◽  
Driving Force ◽  
Stress Gradient ◽  
Zirconium Alloys ◽  
Concentration Limit ◽  
Critical Temperatures ◽  
Terminal Solid Solubility ◽  
Delayed Hydride Cracking ◽  
Hydride Cracking

The hydrogen concentration limit and critical temperatures for a delayed hydride cracking (DHC) in zirconium alloys have been reanalyzed using Kim’s DHC model that a driving force for DHC is not the stress gradient but the supersaturated hydrogen concentration or ∆C arising from a hysteresis of the terminal solid solubility on a heating and on a cooling. The DHC initiation occurs generally at the temperatures corresponding to the terminal solid solubility for precipititation (TSSP), demonstrating that the supercooling from the terminal solid solubility for dissolution (TSSD) is required to initiate the DHC. The DHC arrest temperatures correspond to the temperatures where the ∆C is reduced to zero. Therefore, we conclude that the ∆C is the driving force for the DHC and that the Kim’s DHC model is feasible.

Effect of Hydrogen Concentration on the Threshold Stress Intensity Factor for Delayed Hydride Cracking in Zr-2.5Nb Pressure Tubes

2011 ◽  
Author(s):  
Gordon K. Shek ◽  
Don R. Metzger
Keyword(s):  
Stress Intensity ◽  
Hydrogen Concentration ◽  
Threshold Stress ◽  
Crack Tip ◽  
Threshold Stress Intensity ◽  
Metallographic Examination ◽  
High Hydrogen ◽  
Pressure Tubes ◽  
Delayed Hydride Cracking ◽  
Hydride Cracking

The Zr-2.5Nb pressure tubes of CANDU reactors are susceptible to a crack initiation and growth mechanism known as Delayed Hydride Cracking (DHC), which is a repetitive process that involves hydrogen diffusion, hydride precipitation, hydrided region formation and fracture at a flaw or crack tip. The threshold stress intensity for DHC initiation from a crack, KIH, is an important material parameter for assessing DHC initiation from flaws in pressure tubes. KIH is used to determine whether DHC initiation may occur from flaws which are postulated as crack-like. It is also an input parameter in the engineering process-zone methodology to assess DHC initiation from blunt flaws. Tests were performed to determine the effect of hydrogen concentration in solution on KIH in unirradiated Zr-2.5 Nb material, subjected to different thermo-mechanical treatments to obtain different yield strength or hardness. Hydrogen concentration in solution represents the diffusible hydrogen available for the DHC process, and is different than the total hydrogen concentration which includes the immobile hydrogen in the zirconium hydride phase. For all material conditions, the KIH values at 250°C are significantly higher when the hydrogen concentration in solution is low. Post test metallographic examination indicates that the crack-tip hydride is large and has a taper shape when the hydrogen concentration in solution is high. This suggests that KIH is reached due to insufficient stress to crack the hydrides. When the hydrogen concentration in solution is low, the crack-tip hydride is small and KIH is reached due to limited hydride growth. Finite element diffusion analysis was performed to determine the crack tip hydride accumulation as a function of KI and hydrogen in solution. For high hydrogen concentration in solution, the model predicts a taper hydride shape and hydride lengths which are consistent with the trend observed in the experiments. Another set of KIH tests was performed at 200°C on unirradiated pressure tube material hydrided to 60 and 100 ppm hydrogen. The test results indicated that KIH is controlled by the hydrogen in solution and is not affected by the amount of hydrogen in bulk hydrides.

Initiation and Arrest of Delayed Hydride Cracking in Zr–2.5Nb Tubes

2008 ◽  
Vol 131 (1) ◽  
Author(s):  
Young S. Kim
Keyword(s):  
Hydrogen Concentration ◽  
Solid Solubility ◽  
Thermal Cycle ◽  
Bulk Region ◽  
Critical Temperatures ◽  
Concentration Difference ◽  
Terminal Solid Solubility ◽  
Delayed Hydride Cracking ◽  
Increasing Temperature ◽  
Hydride Cracking

Using Kim’s delayed hydride cracking (DHC) model, this study reanalyzes the critical temperatures for DHC initiation and arrest in Zr–2.5Nb tubes that had previously been investigated with the previous DHC models. At the test temperatures above 180°C, DHC initiation temperatures fell near the terminal solid solubility for precipitation temperatures, requiring some undercooling or ΔT from the terminal solid solubility for dissolution (TSSD) temperatures, and increased toward TSSD with the number of thermal cycles. At the test temperatures below 180°C, DHC initiation occurred at temperatures near TSSD with little ΔT. DHC arrest occurred on heating toward TSSD where the hydrogen concentration difference between the bulk region and a crack tip ΔC decreased to a minimum ΔCmin, under which nucleation of the hydrides was restrained. ΔCmin after the first thermal cycle increased with increasing temperature, demonstrating that nucleation of the hydrides becomes more difficult with increasing temperatures. Different DHC initiation and arrest temperatures with the test temperatures or hydrogen concentrations are discussed in view of a supersaturation of hydrogen (ΔC) for nucleation of hydrides in the zirconium matrix.

Initiation and Arrest of Delayed Hydride Cracking in Zr-2.5Nb Tubes

2006 ◽  
Author(s):  
Young S. Kim ◽  
Sang B. Ahn ◽  
Yong M. Cheong
Keyword(s):  
Hydrogen Concentration ◽  
Solid Solubility ◽  
Thermal Cycle ◽  
Driving Force ◽  
Critical Temperatures ◽  
Terminal Solid Solubility ◽  
Delayed Hydride Cracking ◽  
Hydride Cracking

Using Kim’s delayed hydride cracking (DHC) model, this study reanalyzes the critical temperatures for DHC initiation and arrest in Zr-2.5Nb tubes that had previously been investigated with Dutton and Puls’s DHC model. At temperatures over 180 °C along with a hydrogen concentration of over 15 ppm H, the DHC initiation in a CANDU Zr-2.5Nb tube was suppressed, which required a cooling or ΔT from the terminal solid solubility for dissolution (TSSD) temperatures. With the number of the thermal cycle increasing, the DHC initiation temperatures or Tcs gradually shifted towards the TSSD. At a hydrogen concentration as low as 7 ppm H and temperatures lower than 180 °C, a DHC initiation occurred at temperatures near the TSSD with little ΔT. Different DHC initiation temperatures with hydrogen concentrations are discussed in view of precipitation of hydrides in the zirconium matrix either by a cooling or by a stress-induced γ- to δ-hydride transformation. The DHC arrest temperatures were governed by the critical supersaturated hydrogen concentration or ΔC regardless of the thermal cycle treatment. By correlating the DHC cracking and arrest temperatures with the supersaturated hydrogen concentration or ΔC for the DHC cracking and arrest, we conclude that the ΔC arising from the hysteresis of the terminal solid solubility of hydrogen on a heat-up and on a cool-down is the driving force for the DHC.

Delayed Hydride Cracking Velocity of Irradiated Zr-2.5Nb Tubes after a 30-Year Operation in the Wolsong Unit 1

2005 ◽  
Vol 475-479 ◽  
pp. 1409-1414
Author(s):  
Young Suk Kim ◽  
Sang Bok Ahn ◽  
Kyung Soo Im ◽  
Wan H. Oh
Keyword(s):  
Yield Strength ◽  
Fast Neutron ◽  
Neutron Fluence ◽  
Compact Tension ◽  
Round Robin ◽  
Velocity Dependence ◽  
Research Project ◽  
Round Robin Test ◽  
Delayed Hydride Cracking ◽  
Hydride Cracking

The aim of this study is to investigate a change in delayed hydride cracking (DHC) velocity of Zr-2.5Nb tubes with fast neutron fluence (E>1MeV) and predict the DHC velocity of the irradiated Wolsong 1 Zr-2.5Nb tubes at a neutron fluence corresponding to the 30 year design lifetime. To this end, the DHC velocity were determined at temperatures ranging from 100 to 280 oC on unirradiated Zr-2.5Nb tubes and the irradiated Zr-2.5Nb tubes in the Wolsong Unit-1 to the neutron fluence of 8.9x1025 n/m2 (E>1MeV). DHC tests were conducted on the compact tension specimens charged with 34 to 100 ppm hydrogen in accordance with the KAERI DHC procedures that have been validated through a round robin test on DHC velocity of Zr-2.5Nb tubes as an IAEA coordinated research project. Irradiated Zr-2.5Nb tubes had 3 to 5 times higher DHC velocity than that of unirradiated Zr-2.5Nb tubes while the inlet region of the irradiated Zr-2.5Nb tube with the highest yield strength had a slightly higher DHC velocity compared to that of the outlet region with the lowest yield strength. From a normalized correlation of yield strength and DHC velocity of the Zr-2.5Nb tubes, the yield strength was found to govern the DHC velocity of the Zr-2.5Nb tubes irrespective of the neutron fluence and operating temperatures. The DHC velocity of the irradiated Zr-2.5Nb tubes is predicted after a 30 year operation in the Wolsong Unit 1 on the basis of an increase in the yield strength with neutron fluence and a DHC velocity dependence on the yield strength of Zr-2.5Nb tubes.

A Probabilistic Integrity Assessment of Flaw in Zirconium Alloy Pressure Tube Considering Delayed Hydride Cracking

2003 ◽  
Vol 17 (08n09) ◽  
pp. 1587-1593 ◽  
Author(s):  
Sang Log Kwak ◽  
Joon Seong Lee ◽  
Young Jin Kim ◽  
Youn Won Park
Keyword(s):  
Probabilistic Analysis ◽  
Nuclear Reactor ◽  
Failure Criteria ◽  
Pressure Tube ◽  
Initial Surface ◽  
Tube Material ◽  
Integrity Assessment ◽  
Pressure Tubes ◽  
Delayed Hydride Cracking ◽  
Hydride Cracking

In the CANDU nuclear reactor, pressure tubes of cold-worked Zr-2.5Nb material are used in the reactor core to contain the nuclear fuel bundles and heavy water coolant. Pressure tubes are major component of nuclear reactor, but only selected samples are periodically examined due to numerous numbers of tubes. Pressure tube material gradually pick up deuterium, as such are susceptible to a crack initiation and propagation process called delayed hydride cracking (DHC), which is the characteristic of pressure tube integrity evaluation. If cracks are not detected, such a cracking mechanism could lead to unstable rupture of the pressure tube. Up to this time, integrity evaluations are performed using conventional deterministic approaches. So it is expected that the results obtained are too conservative to perform a rational evaluation of lifetime. In this respect, a probabilistic safety assessment method is more appropriate for the assessment of overall pressure tube safety. This paper describes failure criteria for probabilistic analysis and fracture mechanics analyses of the pressure tubes in consideration of DHC. Major input parameters such as initial hydrogen concentration, the depth and aspect ratio of an initial surface crack, DHC velocity and fracture toughness are considered as probabilistic variables. Failure assessment diagram of pressure tube material is proposed and applied in the probabilistic analysis. In all the analyses, failure probabilities are calculated using the Monte Carlo simulation. As a result of analysis, conservatism of deterministic failure criteria is showed.

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