ANALYSIS OF SODIUM VALVE RELIABILITY DATA AT CREDO*
T. F. Bott and P. M. Haas
Oak Ridge National Laboratory
Oak Ridge, Tennessee
Abstract
The Centralized Reliability Data Organization (CREDO) has been established
at Oak Ridge National Laboratory (0RNL7 by the Department of Energy to provide
a centralized source of data for reliability/maintainability analysis of
advanced reactor systems. The current schedule calls for development of the
data system at a moderate pace, with the first major distribution of data
in late FY-1980. Continuous long-term collection of engineering, operating,
and event data has been initiated at EBR-II and FFTF.
While the system is being developed and initial data are being obtained
from the two reactor sites, historic data from U. S. sodium test loops and
sodium-cooled experimental reactors have been collected and analyzed. Engineering,
operating and event data have been collected on sodium valves, pumps, flow
meters, rupture discs, heat exchangers and cold traps from more than a dozen
sites. By far, the most abundant data available are on sodium valves. This
paper summarizes results of statistical analyses of sodium valve data collected
to date. Numerical (point and interval) estimates of failure rates were first
made assuming an exponential distribution. Graphical techniques were then
employed to investigate time-dependent failure behavior. The majority of the
data sets were found to be well described by Wei bull distributions with shape
factors less than unity (decreasing failure rate).
The effects of key design characteristics (e.g., seal type, activator type)
and operational factors (such as sodium chemistry) on failure behavior have been
investigated, and limited data available on cycling rates have made it possible
to estimate the probability of failure on demand for selected data sets. Repair
data were also available for two sites, and quantitative estimates of repair-
time distributions are made.
Results of the analysis show that although there is a rather strong site-
to-site variation, the estimated failure rates for sodium valves are comparable
to those reported for water valves.
Research sponsored by the Nuclear Power Development Division and the Reactor
Research and Technology Division, U. S. Department of Energy under contract
W-7405-eng-26 with the Union Carbide Corporation.
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