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Plant Maintenance Special Section
Exelon’s Limerick Station Pioneers a Surveillance Frequency Control Program
Exelon’s Limerick station pioneers asurveillance frequency control programBY BETH RAPCZYNSKI, GENE Using a risk-informed process to determine how often to test equipment will lead to savings in A S PART OF an industry pilot pro- resources and costs, without compromising safety. gram, Exelon Nuclear’s LimerickGenerating Station last year be- came the first plant to receive approval fromthe U.S. Nuclear Regulatory Commissionto internally control the frequency of its sur-veillance testing using a risk-informedprocess. This groundbreaking initiative willlead to significant flexibility and resourcesavings for Exelon and, potentially, the in-dustry.
“Risk-informed” refers to incorporating insights from the plant’s probabilistic riskassessment (PRA) into a process that alsoconsiders equipment reliability and test/maintenance history to establish surveil-lance test frequencies.
Nuclear Energy Institute (NEI), Limerick,which has two 1191-MWe boiling waterreactors, piloted and licensed the NRC’sRisk Informed Technical Specification Ini-tiative 5b, the Surveillance Frequency Con-trol Program (SFCP), under the sponsor-ship of the BWR Owners’ Group. This Exelon’s Limerick Nuclear Generating Station (Photos: Exelon Corp.) initiative allows technical experts at Lim-erick to make risk-informed decisions re- tails an average of 4.3 person-hours to con- How it works
garding how often to test equipment and to duct. If all of these were to be converted via Essentially all surveillance tests required change the frequency of the tests accord- the SFCP to quarterly intervals, the annual by a plant’s technical specifications can po- ingly. Changes to technical specification savings would amount to roughly $100 000.
tentially be adjusted under the SFCP. There surveillance frequencies no longer require And, more important, the 2600 person hours lance tests at Limerick. The selection of candidate surveillance tests for priority at- backlog, and other station priorities.
tention—those tests considered to have the tervals results in reduced radiation expo- “The licensing and implementation of the highest benefits associated with interval ad- sure, less wear and tear on equipment, and SFCP represents a major milestone for the justment—are identified using input from industry, and is the first demonstration of various site organizations, including oper- risk, as well as savings of resources and significant regulatory improvement through costs, while the plant continues to operate the use of quality standards for risk analy- ment, radiation protection, and engineering.
sis,” said Biff Bradley, NEI’s manager of The criteria used to select candidate tests As a broad estimate of the savings, 76 in- risk assessment. “This adjustment will pre- are based on the potential for benefit in the strumentation and control surveillance tests serve or enhance safety while also provid- (at both Limerick units and in all divisions) For their efforts, the Exelon Nuclear em- ■ Maintaining dose as low as reasonably Beth Rapczynski is the community outreach man- pilot program received a Top Industry Prac- ager at Exelon Nuclear. Gene Kelly is senior ■ Outage impact (outage work control).
manager of engineering at the Limerick Gener- ating Station, and Phil Tarpinian is a senior en- Plant Maintenance Special Section
■ Past industry and plant-specific operat-ing experience.
■ Vendor-specified maintenance recom-mendations.
■ ANSI, IEEE, and other code-specifiedtesting standards.
■ Impact on defense-in-depth protection.
■ Impact of test interval change on the cal-culated core damage frequency and largeearly release frequency.
erick technical specification surveillancetests with fixed intervals (except those gov-erned by ASME Code requirements). Insome instances, intervals could conceivablybe extended more than once. The method-ology demonstrated at Limerick can also be The Limerick SFCP project team was awarded a Top Industry Practice Award at the 2007 Nuclear Energy Assembly. From left, Exelon Nuclear employees Vicki Warren, Phil Tarpinian, Mary Kowalski, Gene Kelly, and Glenn Stewart, and Exelon Corporation’s Examples of the SFCP in use
chairman, president, and chief executive officer, John Rowe.
Control rod drive notch tests: Limerick cialists who have experience in surveillance technical specifications had previously re- ■ Reducing wear and tear on safety equip- testing and system or component reliability. quired that the operability of all withdrawnment.
Each surveillance test considered for in- interval adjustment, the change is imple- each control rod at least one notch a mini- terval adjustment is comprehensively eval- mented and records of the panel’s deliber- mum of once every seven days, and so this 10). The evaluation is then reviewed by an audit purposes. Each evaluation addresses every week. The change that was evaluated allowed this interval to be extended from which is similar to an expert panel used for ■ Operation and maintenance history of seven days to 31 days. This was the first test to be adjusted following the licensing of plementation, but with the addition of spe- ■ Maintenance rule unavailability.
SFCP at Limerick, and both the IDP and the Exelon’s Limerick Station Pioneers a Surveillance Frequency Control Program
than a factor of four. Over the life of the part of the weekly exercise test for the review plant, this constitutes effective risk man- period examined. Also, the PRA impact was agement. And not to be discounted—it’s negligible, reinforcing the risk-informed ba- The benefit of adjusting this interval—in not just the licensed operators who like this sis for the control rod drive test adjustment.
the area of reactivity management—can be change—control room operators are posi- And so, there was no reduction in safety, seen in how the test is conducted. Each of impact on performance, or loss of critical test information as a result of conducting “notched” individually. Because of this, it this test less often. Therefore, station oper- takes over five hours to complete the sur- ating experience and the performance of the veillance, which is usually conducted on a control rod drive system provided the basis Sunday afternoon and involves three to four consideration, the control rod drive system at Limerick was found to be highly reliable, Scram discharge level switch calibra- ment event and human error throughout the performance over the past six years, the pe- tions: Scram discharge level switch calibra- tions are performed quarterly on each unit.
A total of eight channels are tested per unit, specifically calibrating level switches at the By extending the interval
scram discharge volume located in the reac- between tests from weekly to scram times to all tor enclosure. The switches are installed in a
relatively high-dose location. Shielding, hy- monthly, the potential for
drolasing, and other ALARA techniques are either impractical or have limited effect in reactivity errors is reduced
this instance. On an annual basis, the total dose incurred by I&C technicians who per- by more than a factor of four. addition, the scram form these calibrations for both Limerick
units has been on the order of 1 person-rem.
This surveillance is currently being eval- duration of the testing is always present, tion for all control rod drive mechanisms was and therefore no other activities are al- demonstrated to have substantial margin and lowed while this surveillance is being con- to be significantly less than the technical benefits will be seen in the form of radio- ducted. By extending the interval between specification scram insertion time limit.
logical risk reduction resulting from fewer tests from weekly to monthly, the potential Most important, there was no control rod that instances at the source point, as well as a re- the operators were unable to reposition as

Source: http://www2.ans.org/pubs/magazines/nn/docs/2007-10-2.pdf


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