Environmental Engineering Reference
In-Depth Information
for lack of recovery (as bad as old) as a result of repair or TM. Here
t
N
is
the time the last repair or TM.
NUREG/CR-6508
40
gives more detailed formulae for calculating
the unavailability of components in the standby mode. They take into
account the frequency of testing and the frequency and type of preventive
maintenance.
So, for the equipment in standby mode, the periodic tests and the type
of recovery 'as good as new', the average unavailability over the interval
between preventive maintenance/inspection will be:
T a
TL T
−
d
q
= 1
−
exp
−l +
Ta
2
+
+
,
[2.17]
0
26
22
Ld
+
λ
0
is the failure rate at the initial time (λ
0
=
a
in [2.11]);
L
is the interval
between preventive maintenance/inspections;
T
is the interval between
periodic tests;
d
is the average time of unavailability during the preventive
maintenance/inspection.
Accordingly, the average unavailability for equipment in standby mode
without periodic testing and the type of recovery 'as good as new' is
expressed by:
Ta
d
2
q
= −
1
exp
−l
−
L
+
.
[2.18]
0
26
Ld
+
For the elements of the safety system with the restoration 'as bad as old'
(preventive maintenance/inspection control only the readiness of equipment
and, from this point of view, are similar to periodic tests), the average
unavailability over the life
S
is calculated as:
T a
TS
d
2
q
= −
1
exp
− −l
−
La
+
+
.
[2.19]
0
2
6
22
Ld
+
In the absence of periodic testing, unavailability is defined as:
T a
LS
d
2
q
= −
1
exp
− −l
−
La
+
+
.
[2.20]
0
2
6
22
Ld
+
The equations [2.17]-[2.20] are the first-order approximation for the
exponential dependence (Taylor series). They give fairly accurate results for
values less than 0.1 and a conservative estimate for higher values.
A more complicated case of partial recovery of equipment is also
considered.
In addition, NUREG/CR-6508
40
includes formulas for calculating the
unavailability in similar cases using the Weibull model. For example, for
equipment in standby mode, the periodic tests and the type of recovery'as
good as new', the average unavailability over the interval between preventive
maintenance/inspection will be:
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