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Probabilistic Evaluation of Baffle-Former Bolt Cracking in PWRs

A methodology for evaluating the probability of baffle-former bolt cracking was developed for applicability to presurized water reactors.

Product Number: 51317--8857-SG
ISBN: 8857 2017 CP
Author: George Licina
Publication Date: 2017
Industry: Water/Wastewater
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$20.00
$20.00

A methodology for evaluating the probability of baffle-former bolt cracking was developed for applicability to PWRs. The methodology is based upon observed IASCC test results for the stainless steels most commonly used for baffle-former bolts (e.g. Type 304 SA Type 347 SA and Type 316 CW) and predictions of the representative stress patterns in those bolts that were developed as inputs to the model. The predictive methodology for IASCC develops a single parameter that incorporates dose and stress ratio (applied stress divided by yield strength where the yield strength includes irradiation hardening) then defines the statistical distribution of that parameter using a Weibull distribution.Baffle-former bolt cracking has been observed in a number of PWRs and these incidents have raised concerns about the likelihood of future cracking or failures. In this damage model IASCC “failure” is defined when the component becomes fully susceptible to stress corrosion cracking; that is after a certain level of irradiation and sustained stress. The length of time required for the material to become fully susceptible depends on the level of stress and irradiation dose. IASCC crack initiation which is defined to be equivalent to failure as noted above is defined by an IASCC susceptibility curve that relates applied stress and cumulative neutron dose. The dose duration under constant stress is interpreted as the incubation time needed to make the material susceptible to stress corrosion cracking after which crack initiation and propagation to full rupture under constant stress will occur within a relatively short time (hundreds of hours). The IASCC failure model uses the calculated stress in a material such as the baffle-former bolt and calculates a damage index as the ratio of the current applied stress to the allowable stress as a function of irradiation dose.To benchmark the model IASCC initiation data for various irradiated bolting materials was obtained in the form of IASCC initiation stress as a function of irradiation dose and the modeled stress both of which change with exposure. The stress level from IASCC tests was plotted as percent of irradiated yield strength and this method was used to determine the probability of IASCC occurring at various stress levels using a Weibull fit of the cumulative failure probability vs. IASCC ratio. By taking the calculated bolt-by-bolt stresses in a typical PWR and the accumulated fluence or dose level the model was used to predict the probabilities (or numbers) of baffle-former bolt failures due to IASCC over time (i.e. at various Effective Full Power Years). The model provides a probabilistic estimate of the number of cracked bolts that might be expected to be found during any future refueling outage with inspections of the baffle-former bolts. Such a priori knowledge is important because the plan for inspection of the baffle-former bolts may require additional contingencies depending on the likely outcome.

Key words: baffle-former bolts, pressurized water reactors, IASCC, stainless steel

A methodology for evaluating the probability of baffle-former bolt cracking was developed for applicability to PWRs. The methodology is based upon observed IASCC test results for the stainless steels most commonly used for baffle-former bolts (e.g. Type 304 SA Type 347 SA and Type 316 CW) and predictions of the representative stress patterns in those bolts that were developed as inputs to the model. The predictive methodology for IASCC develops a single parameter that incorporates dose and stress ratio (applied stress divided by yield strength where the yield strength includes irradiation hardening) then defines the statistical distribution of that parameter using a Weibull distribution.Baffle-former bolt cracking has been observed in a number of PWRs and these incidents have raised concerns about the likelihood of future cracking or failures. In this damage model IASCC “failure” is defined when the component becomes fully susceptible to stress corrosion cracking; that is after a certain level of irradiation and sustained stress. The length of time required for the material to become fully susceptible depends on the level of stress and irradiation dose. IASCC crack initiation which is defined to be equivalent to failure as noted above is defined by an IASCC susceptibility curve that relates applied stress and cumulative neutron dose. The dose duration under constant stress is interpreted as the incubation time needed to make the material susceptible to stress corrosion cracking after which crack initiation and propagation to full rupture under constant stress will occur within a relatively short time (hundreds of hours). The IASCC failure model uses the calculated stress in a material such as the baffle-former bolt and calculates a damage index as the ratio of the current applied stress to the allowable stress as a function of irradiation dose.To benchmark the model IASCC initiation data for various irradiated bolting materials was obtained in the form of IASCC initiation stress as a function of irradiation dose and the modeled stress both of which change with exposure. The stress level from IASCC tests was plotted as percent of irradiated yield strength and this method was used to determine the probability of IASCC occurring at various stress levels using a Weibull fit of the cumulative failure probability vs. IASCC ratio. By taking the calculated bolt-by-bolt stresses in a typical PWR and the accumulated fluence or dose level the model was used to predict the probabilities (or numbers) of baffle-former bolt failures due to IASCC over time (i.e. at various Effective Full Power Years). The model provides a probabilistic estimate of the number of cracked bolts that might be expected to be found during any future refueling outage with inspections of the baffle-former bolts. Such a priori knowledge is important because the plan for inspection of the baffle-former bolts may require additional contingencies depending on the likely outcome.

Key words: baffle-former bolts, pressurized water reactors, IASCC, stainless steel

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