Design Criteria for Fracture Assessment of Pressurized Nuclear Components

This paper presents the design criteria adopted for fracture assessment of pressurized components of nuclear power plants. Although there are wide variety of components in a typical nuclear power plant, the thrust in this paper is on components which are part of the primary heat transport system. The paper presents an overview of design rules, practices adopted and experimental verification needed for ensuring the structural integrity of nuclear pressure vessels and piping

Multiaxial fatigue studies on carbon steel piping material of Indian PHWRs

The tests studies and analyses have been carried out in the area of “Multiaxial Fatigue” with an objective to improve the damage assessment methodologies and design rules. Nearly 50 numbers of fatigue tests were conducted on solid and tubular specimens of SA333Gr.6 material under pure axial, pure shear and combined axial-torsion in-phase/ out-of-phase loading combinations. A software has been developed for the evaluation of multiaxial fatigue damage for the analyses of tests data using different invariant fatigue models such as ASME Sec.III code procedures, von-Mises etc. The fatigue crack initiation life was predicted using the best fit axial fatigue life curve (without use of safety factors). These tests and their analyses have helped in understanding the fatigue failure behavior of piping material under complex cyclic loadings where the principal directions rotate during a loading cycle. The crack initiation angles have also been measured by analyzing the image of the tested specimens. The measured crack angles will help in validation of the critical plane based models

Predictors of Cardiac Mortality in the CCU: A Retrospective Study in a Tertiary Center

Background: Although prior studies have linked troponin I (TnI) elevation, serum sodium (Na) fluctuation, and reduced ejection fraction (EF) with an increased mortality in the medical/surgical critical care units, this has not been validated in the CCU. We aim to identify clinical and laboratory factors to predict cardiac related length of survival (LOS) in the CCU. Methods: We retrospectively analyzed 134 consecutive patients who were admitted to the CCU from December 2012 to March 2015, and who died during that admission. We used student T-test, correlation matrices, and Framingham risk factors adjusted multivariable logistic regression models to examine the role of TnI, serum Na, EF and other clinical covariates on LOS in cardiac death (CD) and non- cardiac death (NCD) group. Results: The average age of the study population was 70.0 ±14.3 (39.0% women). The prevalence of CD and NCD were 63% and 59%. LOS was statistically shorter in the CD vs. NCD group (5.3 days vs. 8.2 days, p=0.012). LOS negatively correlated with initial TnI (p= 0.05). LOS was not statistically affected by EF or Na level. Our regression models identified BMI and diabetes mellitus (DM) as strong predictors of CD (p= 0.04 and p=0.01). Conclusion: Our results validate prior studies showing that TnI, BMI, and DM are predictors of cardiac related mortality in the CCU. Patients with a cardiac etiology had a higher mortality rate and a shorter LOS. Future studies are needed to develop a scoring system specific for predicting mortality in the CCU

Low cycle fatigue and cyclic plasticity bahaviour of Indian PHWR / AHWR primary piping materials

The integrity assessment of the primary piping components needs to be demonstrated under normal operation cyclic loadings as well as under complex cycling loadings of extreme magnitude as may come during a severe earthquake event. In order to understand material's cyclic plasticity and fatigue ratcheting behaviour, systematic experimental and analytical investigations have been carried out on specimens of SA333Gr.6 carbon steel and SS304LN stainless steel. The materials specification of SA333Gr.6 is same as used in Primary Heat transport (PHT) piping of Pressurized Heavy Water Reactors (PHWRs) and materials specification of SS304LN steel is same as proposed for Indian Advanced Heavy Water Recactor (AHWRs) Main Heat Transport (MHT) piping. The test program included the properties and cyclic plasticity behaviour. The results of these tests have been investigated in detals using few popular finite element cyclic plasticity models to understand and quantify the materials' cyclic plasticity behaviour. The studies revealed the need to modify the Chaboche model to simulate the LCF/cyclic plasticity and ratcheting under different stress/strain amplitude loading conditions. On accounting for modification, the Chaboche model nicely predicted the LCF and ratcheting response for all the tests. The tests, finite element analyses results and their interpretations have been presented in this paper

Fatigue studies on stainless steel piping materials and components: Indian AHWR

As a part of component integrity test program at Bhabha Atomic Research Centre, fatigue tests on full scale pipe and pipe welds were conducted in addition to CT and TPB specimens. In this paper the outcome of this program is discussed. Specimen testing was conducted to determine the basic cyclic stress strain curve, LCF and FCGR properties. FCGR tests were conducted on CT and TPB specimens to understand the effect of different conditions: Specimen level tests result show that, for the present grade of material, the FCGR is not significantly affected by specimen type (CT and TPB), specimen thickness and notch orientation. FCGR resistance of the hot wire GTAW is superior compared to that of conventional SMAW. The effects of stress ratio are mildly significant at lower R-values for base metal but are significant for weld metal. Component tests were conducted to understand the effect of the following variables:(a)Component type and size: pipe and pipe weld, Pipe diameters 170 mm and 324 mm,(b)Pipe and pipe weld: initial notch in pipe base, and girth welded pipe, (c) Pipe welds: Conventional GTAW/SMAW and hot wire narrow gap GTAW, (d)Environment: Air and water, (e) Type of loading: Constant amplitude cyclic, vibration, Block, overload and underload. Results indicated that the fatigue life of the component is reduced under water environment compared to air environment. Fatigue life of the pipe subjected to block loading (increasing stress ratio followed by decreasing stress ratio), intermittent overloading and underloading is also decreased compared to that of constant amplitude loading. Vibration loading reduces fatigue life significantly. Crack growth in thickness direction is more compared to circumferential direction for all types of loading which is desirable for demonstration of LBB criteria. Fatigue life of the notched component has also been predicted using the Paris constants data from the specimen level tests. Fatigue crack growth and the crack shape of the growing crack have been evaluated for regular interval of loading cycles. The predictions compares well with those of experiments

Histone methyltransferase Dot1 and Rad9 inhibit single-stranded DNA accumulation at DSBs and uncapped telomeres

Cells respond to DNA double-strand breaks (DSBs) and uncapped telomeres by recruiting checkpoint and repair factors to the site of lesions. Single-stranded DNA (ssDNA) is an important intermediate in the repair of DSBs and is produced also at uncapped telomeres. Here, we provide evidence that binding of the checkpoint protein Rad9, through its Tudor domain, to methylated histone H3-K79 inhibits resection at DSBs and uncapped telomeres. Loss of DOT1 or mutations in RAD9 influence a Rad50-dependent nuclease, leading to more rapid accumulation of ssDNA, and faster activation of the critical checkpoint kinase, Mec1. Moreover, deletion of RAD9 or DOT1 partially bypasses the requirement for CDK1 in DSB resection. Interestingly, Dot1 contributes to checkpoint activation in response to low levels of telomere uncapping but is not essential with high levels of uncapping. We suggest that both Rad9 and histone H3 methylation allow transmission of the damage signal to checkpoint kinases, and keep resection of damaged DNA under control influencing, both positively and negatively, checkpoint cascades and contributing to a tightly controlled response to DNA damage

The CDK-Activating Kinase (CAK) Csk1 Is Required for Normal Levels of Homologous Recombination and Resistance to DNA Damage in Fission Yeast

BACKGROUND: Cyclin-dependent kinases (CDKs) perform essential roles in cell division and gene expression in all eukaryotes. The requirement for an upstream CDK-activating kinase (CAK) is also universally conserved, but the fission yeast Schizosaccharomyces pombe appears to be unique in having two CAKs with both overlapping and specialized functions that can be dissected genetically. The Mcs6 complex--orthologous to metazoan Cdk7/cyclin H/Mat1--activates the cell-cycle CDK, Cdk1, but its non-redundant essential function appears to be in regulation of gene expression, as part of transcription factor TFIIH. The other CAK is Csk1, an ortholog of budding yeast Cak1, which activates all three essential CDKs in S. pombe--Cdk1, Mcs6 and Cdk9, the catalytic subunit of positive transcription elongation factor b (P-TEFb)--but is not itself essential. METHODOLOGY/PRINCIPAL FINDINGS: Cells lacking csk1(+) are viable but hypersensitive to agents that damage DNA or block replication. Csk1 is required for normal levels of homologous recombination (HR), and interacts genetically with components of the HR pathway. Tests of damage sensitivity in csk1, mcs6 and cdk9 mutants indicate that Csk1 acts pleiotropically, through Cdk9 and at least one other target (but not through Mcs6) to preserve genomic integrity. CONCLUSIONS/SIGNIFICANCE: The two CAKs in fission yeast, which differ with respect to their substrate range and preferences for monomeric CDKs versus CDK/cyclin complexes as substrates, also support different functions of the CDK network in vivo. Csk1 plays a non-redundant role in safeguarding genomic integrity. We propose that specialized activation pathways dependent on different CAKs might insulate CDK functions important in DNA damage responses from those capable of triggering mitosis

Defective Resection at DNA Double-Strand Breaks Leads to De Novo Telomere Formation and Enhances Gene Targeting

The formation of single-stranded DNA (ssDNA) at double-strand break (DSB) ends is essential in repair by homologous recombination and is mediated by DNA helicases and nucleases. Here we estimated the length of ssDNA generated during DSB repair and analyzed the consequences of elimination of processive resection pathways mediated by Sgs1 helicase and Exo1 nuclease on DSB repair fidelity. In wild-type cells during allelic gene conversion, an average of 2–4 kb of ssDNA accumulates at each side of the break. Longer ssDNA is formed during ectopic recombination or break-induced replication (BIR), reflecting much slower repair kinetics. This relatively extensive resection may help determine sequences involved in homology search and prevent recombination within short DNA repeats next to the break. In sgs1Δ exo1Δ mutants that form only very short ssDNA, allelic gene conversion decreases 5-fold and DSBs are repaired by BIR or de novo telomere formation resulting in loss of heterozygosity. The absence of the telomerase inhibitor, PIF1, increases de novo telomere pathway usage to about 50%. Accumulation of Cdc13, a protein recruiting telomerase, at the break site increases in sgs1Δ exo1Δ, and the requirement of the Ku complex for new telomere formation is partially bypassed. In contrast to this decreased and alternative DSB repair, the efficiency and accuracy of gene targeting increases dramatically in sgs1Δ exo1Δ cells, suggesting that transformed DNA is very stable in these mutants. Altogether these data establish a new role for processive resection in the fidelity of DSB repair