Effect of Hydrogen Concentration on the Threshold Stress Intensity Factor for Delayed Hydride Cracking in Zr-2.5Nb Pressure Tubes
Bibliographic record
Abstract
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.
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How this classification was reachedexpand
Full frame distilled prediction
Teacher imitationNot calibrated prevalence, not ground truth. Human validation pending. Learned from the 10,348 direct Codex labels and 10,348 direct Gemma labels. Candidate is the union of thresholded teacher heads; consensus is their intersection. These outputs are machine_predicted_unvalidated and are not human labels or direct frontier model labels.
Codex and Gemma teacher scores by category
| Category | Codex | Gemma |
|---|---|---|
| Metaresearch | 0.001 | 0.000 |
| Meta-epidemiology (narrow) | 0.000 | 0.000 |
| Meta-epidemiology (broad) | 0.000 | 0.000 |
| Bibliometrics | 0.000 | 0.000 |
| Science and technology studies | 0.000 | 0.000 |
| Scholarly communication | 0.000 | 0.000 |
| Open science | 0.000 | 0.000 |
| Research integrity | 0.000 | 0.000 |
| Insufficient payload (model declined to judge) | 0.001 | 0.000 |
Machine scores (provisional)
The two teacher heads of the student model, read on this work. A score orders the frame for review; it never asserts a category, and the validation status ships verbatim with every row.
Baseline scores from an immature model (maturity gate not passed, 7 training rounds). Scores rank; they never assert a category.
score_only:v0-immature-baseline · verbatim from the scoring run: score_only means the number may rank works, and no category label ships from itClassification
machine, unvalidatedMachine predicted; a candidate call from one teacher head, not a consensus.
How this classification was reached, model by model and score by score, is at the end of the page under "How this classification was reached".