Publications

Abstract not provided.

In the Nuclear Security Enterprise (NSE), many high reliability components must be stored for long periods of time before being called on to function a single time. During dormant storage, changes in the performance of these components may occur due to environmental exposures. These exposures may enhance the natural degradation of materials or result in shifts in the performance of electronics. Ongoing assessment of these components is necessary to inform the need for upgrades or replacements to ensure high reliability requirements are being maintained. This paper presents several assessment methodologies that are used and have been proposed for this problem. We also present methods that we believe to be most appropriate for the assessment of nuclear weapons components subjected to dormant storage.

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We present an approach to the development and evaluation of environmental stress screening (ESS) for a dormant-storage, multi-shot component. The ESS is developed to precipitate and detect latent manufacturing defects without significantly degrading the component's probability of successful function under normal operating environments. The evaluation of the ESS is achieved by using an additional strength of screen (SOS) operation to test for escapes from the screen. The resulting data are pass/fail data only, because the characteristics of this type of component do not allow a standard 'time to failure' analysis. The calculated SOS efficiency f is then used to estimate initial field 'reliability.' We illustrate the use of the methodology with a case study involving an electrical component manufactured within the Nuclear Security Enterprise (NSE). In development and qualification, twelve failures were detected by the ESS, and the SOS operations detected one escape. The resulting analysis showed the SOS efficiency to be approximately 92%, adequate for the component reliability goal. The resulting initial field reliability was estimated to be 99.3%, acceptable for this electrical component. Failure investigations were conducted to determine the root cause of each of these failures. Information from these investigations resulted in changes to the manufacturing process to eliminate or minimize the reoccurrence of these failures. The number of ESS failures have been reduced, and no additional failures have been observed at the SOS operation.

We present an approach to the development and evaluation of environmental stress screening (ESS) for a dormant-storage, multi-shot component. The ESS is developed to precipitate and detect latent manufacturing defects without significantly degrading the component's probability of successful function under normal operating environments. The evaluation of the ESS is achieved by using an additional strength of screen (SOS) operation to test for escapes from the screen. The resulting data are pass/fail data only, because the characteristics of this type of component do not allow a standard 'time to failure' analysis. The calculated SOS efficiency f is then used to estimate initial field 'reliability.' We illustrate the use of the methodology with a case study involving an electrical component manufactured within the Nuclear Security Enterprise (NSE). In development and qualification, twelve failures were detected by the ESS, and the SOS operations detected one escape. The resulting analysis showed the SOS efficiency to be approximately 92%, adequate for the component reliability goal. The resulting initial field reliability was estimated to be 99.3%, acceptable for this electrical component. Failure investigations were conducted to determine the root cause of each of these failures. Information from these investigations resulted in changes to the manufacturing process to eliminate or minimize the reoccurrence of these failures. The number of ESS failures have been reduced, and no additional failures have been observed at the SOS operation.

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A reliability and availability model has been developed for a portion of the 4.6 megawatt (MWdc) photovoltaic system operated by Tucson Electric Power (TEP) at Springerville, Arizona using a commercially available software tool, GoldSim{trademark}. This reliability model has been populated with life distributions and repair distributions derived from data accumulated during five years of operation of this system. This reliability and availability model was incorporated into another model that simulated daily and seasonal solar irradiance and photovoltaic module performance. The resulting combined model allows prediction of kilowatt hour (kWh) energy output of the system based on availability of components of the system, solar irradiance, and module and inverter performance. This model was then used to study the sensitivity of energy output as a function of photovoltaic (PV) module degradation at different rates and the effect of location (solar irradiance). Plots of cumulative energy output versus time for a 30 year period are provided for each of these cases.

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To determine whether a component is meeting its reliability requirement during production, acceptance sampling is employed in which selected units coming off the production line are subjected to additional environmental and/or destructive tests that are within the normal environment space to which the component is expected to be exposed throughout its life in the Stockpile. This report describes what these tests are and how they are scored for reliability purposes. The roles of screens, Engineering Use Only tests, and next assembly product acceptance testing are also discussed, along with both the advantages and disadvantages of environmental and destructive testing.