DOE 2000. - Waste Isolation Pilot Plant - U.S. Department of Energy

WIPP RH PSAR DOE/WIPP-03-3174 CHAPTER 5
failure may be due to loss of off-site power or coincident with the Design Basis Tornado (DBT) or
earthquake (DBE). An evaluation of the off-site power loss frequency is conducted in Table D-3 of
Appendix D. Comparing the frequency of the DBE (RH6) and DBT with the frequency of off-site power
loss indicates that the most likely scenario is loss of off-site power.
Regardless of the initiating event, the hoist brake system functions to prevent the uncontrolled movement
of the hoist, and prevents the resultant waste canister breach accident scenario. Due to the importance of
this system, a fault tree analysis 42 on the waste hoist brake system was conducted: (1) to quantify the
failure frequency on demand, (2) to verify system reliability, and (3) to identify system improvements or
controls. The fault tree analysis of the current hoist configuration quantifies the frequency of failure as
1.3E-07/demand.
The no-mitigation frequency for the waste material release from the failure of the waste hoist is
9.05E-10/yr as shown in Figure D-2 of Appendix D.
An analysis of the frequency of hoist brake system failure has been performed by the Environmental
Evaluation Group (EEG) 43 . The extensive uncertainty analysis performed in EEG-59, indicates that the
mean frequency of 1.3E-07 corresponds to an 82 percent confidence level. At the 95 percent confidence
level, the analysis indicates that the annual failure rate is 4.5E-07. The mean value of 1.3E-07 is used in
the event tree in Figure D-2 for the failure probability of the brake system. The EEG analysis confirms,
that the no-mitigation accident scenario frequency is beyond extremely unlikely (10 -6 /yr $frequency).
The input data and assumptions used for determining the failure probability of the brake system of the
waste hoist are:
C
Maintain configuration of the waste hoist on which fault tree was based (for details see
WIPP/WID-96 -2178 40 ).
C
C
C
C
C
C
C
C
Maintenance program including post-maintenance functional testing - stroke testing is assumed
for all valves following a maintenance operation
Sensors and related components are regularly tested and calibrated. No attempt was made in
this analysis to evaluate the possible consequences of faulty signals due to the miscalibration of
the sensors
Mission time for waste hoist is 1000 hours/yr (i.e, 7,000 round trips per yr) which includes
transfer of RH and CH waste to the Underground
Maximum test time for standby components is 24 hours
Waste hoist is operated in automatic mode only when transferring RH waste because manual
mode is not modeled in the fault tree analysis
Assumed that neither inadvertent braking nor speed decrease would cause any safety concerns
Assumed that neither inadvertent movement or speed increase would result in conditions
beyond the design capabilities of the detection and protective features of the brake system
Waste hoist system is subjected to a series of thorough "pre-operational check" test at the start
of each eight hour shift. Operation of the waste hoist system does not begin until the tests are
successfully completed. In the fault tree analysis this test interval is conservatively assumed to
be three times greater or 24 hours.
5.2-31 January 22, 2003