Safety/Relief Valve Quencher Loads:Evaluation for BWR Mark II and III Containments (Generic Letter No. 82-24)
UNITED STATES
NUCLEAR REGULATORY COMMISSION
WASHINGTON, D.C. 20555
November 4, 1982
TO BWR APPLICANTS WITH MARK II OR III CONTAINMENT (EXCEPT WPPSS II)
SUBJECT: SAFETY/RELIEF VALVE QUENCHER LOADS:
EVALUATION FOR BWR
MARK II AND III CONTAINMENTS
(Generic Letter No. 82-24)
Enclosed is a copy of NUREG-0802, "Safety/Relief Valve Quencher Loads:
Evaluation for BWR Mark II and III Containments. NUREG-0802 is being issued
to provide acceptance criteria for hydrodynamic loads on piping, equipment,
and containment structures resulting from SRV actuation. The NRC staff finds
that use of these acceptance criteria satisfy the requirements of General
Design Criteria 16 and 29 in Appendix A to 10 CFR Part 50. NUREG-0802,
however, is not a substitute for the regulations, and compliance with the
NUREG is not a requirement. An approach or method different from the
acceptance criteria contained herein will be accepted if the substitute
approach or method provides a basis for determining that the regulations
have been met.
The NRC had issued SRV load acceptance criteria for both Mark II
(NUREG-0487, Supplement No. 1, September 1980) and Mark III (SER for GESSAR,
July 1976). However, the staff, the Mark II Owners Group and GE recognized
that these criteria were very conservative because they were established at
the early stage of quencher development. Since then, extensive quencher test
programs were performed resulting in a sufficient data base to justify
re-evaluation the SRV load criteria. In response to the request by the Mark
II Owners Group and GE, the staff has re-evaluated the SRV loads and
established the new acceptance criteria in NUREG-0802. The staff also finds
the earlier criteria acceptable. The acceptance criteria in NUREG-0487
supplement No. 1 (for Mark II plants) or the acceptance criteria in an
attachment 2 (for Mark III plants) are conservative with respect to the
acceptance criteria proposed in Appendices A and B of NUREG-0802,
respectively and they are acceptable.
The reporting and/or recordkeeping requirements contained in this letter
affect fewer than ten respondents; therefore, OMB clearance is not required
under P.L. 96-511.
Darrell G. Eisenhut, Director
Division of Licensing
Office of Nuclear Reactor Regulation
Enclosure:
NUREG-0802
Attachments 1 & 2
8211080059
.
ATTACHMENT 2
ACCEPTANCE CRITERIA
FOR QUENCHER LOADS FOR
THE MARK III CONTAINMENT
I. INTRODUCTION
On September 2, 1975, the General Electric Company submitted topical
reports NEDO-11314-08 (nonproprietary) and NEDE-11314-08 (proprietary)
entitled, "Information Report Mark III Containment Dynamic Loading
Conditions," docketed as Appendix 3-B to the Amendment No. 37 for
GESSAR, Docket No. STN-50-447. As part of this report, a device called
a "quencher" would be used at the discharge end of safety/relief valve
(SRV) lines inside the suppression pool. Tests were performed in a
foreign country to obtain quencher load data that were used to
establish the Mark III data base. A statistical technique using the
test data to predict quencher loads for Mark III containment was also
presented. GE had submitted another topical report NEDE-21078 entitled,
"Test Results Employed by GE for BWR Containment and Vertical Vent
Loads," to substantiate their method to extrapolate the loads obtained
from the tests to the Mark III design.
We reviewed the above topical reports and had identified several areas
of concern. Meetings with GE were held to discuss these concerns. As a
result, GE presented a modified method during the April 2, 1976,
meeting held in Bethesda, Maryland. Subsequent to the meeting, this
modified method and proposed load criteria were reported in Amendment
No. 43, which was received on June 22, 1976. Our evaluation, therefore,
is based on the modified method and the load criteria calculated by
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this method.
II. SUMMARY OF THE METHOD OF QUENCHER LOAD PREDICTION
The statistical method proposed by GE to arrive at design quencher
loads for the Mark III containment consists of a series of steps.
Initially, a multiple linear regression analysis for the first
actuation event is performed with a data base taken from three tests
series: mini-scale (9 points), small scale (70 points) and large scale
(37 points).
Non-linearities are introduced where necessary by using-quadratic
variables and formed straight line segments. The regression
coefficients are estimated from the appropriate data set. The resulting
equation contains a constant term plus corrective terms that take into
account the influence of all key parameters.
In the second step, the subsequent actuation effect is determined by
postulating a direct proportionality between the observed maximum
subsequent actuation pressure and the predicted first actuation
pressure. The proportionality constant is found by considering the
large scale data.
In the third step, the total variance of the predicted future SRV
subsequent actuation is found by noting that the total variance is the
sum of three terms: (1) a term due to the uncertainty in the
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first actuation prediction which is calculated from standard (normal
variate) formulas, (2) a term due to the uncertainty in the
proportionality factor as was calculated in the second step above, and
(3) a term due to the variance of the residual maximum subsequent
pressure. It is now assumed that this variance is proportional to the
square of predicted maximum-subsequent actuation pressure. The
proportionality constant is found from the large scale subsequent
actuation data (10 values).
In the fourth step, design values for Mark III are determined from the
estimated (i.e., predicted) values of maximum subsequent actuation
pressure and its standard deviation by employing standard tables of
so-called "tolerance factors." These tables are entered with three
quantities: (1) n, the number of sample data points from which the
estimate of the mean and standard deviations are obtained. GE has set
n x 10, based on 10 maximum subsequent actuation points used in the
third step, (2) the probability value, and (3) the confidence level.
The design value is then simply the predicted value plus the tolerance
factor times the estimated standard deviation.
The approach as outlined above is used to calculate the positive
pressures for a single SRV considering multiple actuations which
represents the most severe SRV operation condition. For the single
actuation case, the calculational procedures are similar with the
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method mentioned above with the following exceptions:
1. The calculation which involves subsequent actuations is
eliminated; and,
2. Thirty-seven data points were selected for establishing the
tolerance factor since these data points in the large-scale tests
relate to single value actuation.
For negative pressure calculation, a correlation of peak positive and
negative pressures is developed. The correlation is based on the
principle of conservation of energy and verified by the small-scale and
large-scale test results.
Based on the method outlined above, GE has calculated the SRV quencher
loads for the Mark III and established the load criteria for six cases
of SRV operation. The calculated load criteria based on 95-95%
confidence level are given on Table 1 which is attached.
III. EVALUATION SUMMARY
As a result of our review, we have concluded that the statistical
method proposed by GE and the load criteria shown on Table 1 are
acceptable. This conclusion is based on the following:
1. The method has properly treated all available test data and is
based essentially on the large-scale data with correction terms
that take into account the influence of non-large-scale variables.
Since the large-scale tests were performed in an actual reactor
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with a suppression containment conceptually similar with GE
containments extrapolation from the large-scale by statistical
technique, therefore, is appropriate and acceptable.
2. The method has been conducted in a conservative manner. The
primary conservatisms are:
a. The calculation is based on the most severe parameters. For
example, the maximum air volume initially stored in the line,
the maximum initial pool temperature and the highest primary
system pressure were selected to establish quencher load
criteria.
b. For the cases of multiple valve actuation, the load criteria
are based on the assumption that the maximum pressures
resulting from each valve will occur simultaneously. We
believe that the assumption is conservative since different
lengths of line and SRV pressure set points will result in
the occurrence of maximum pressures at different times and
consequently lower loads.
3. The proposed load criteria, which are provided on the attached
Table 1, are acceptable. The criteria were established by using
95-95% confidence limit. Our consultant, the Brookhaven National
Laboratory, has performed an analysis for the effect of confidence
limit. The result of this analysis indicates that for 95-95%
confidence limit, approximately 1% of the number of RSV actuations
may result in containment loads above the design value. We believe
that
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this low probability is acceptable considering the conservatism of
the method of prediction, i.e., the actual loads should not exceed
the design value.
4. With regard to the subsequent actuation, the load criteria are
based upon a single SRV actuation, G.E. has established this basis
by regrouping the SRV's in each group of pressure set points. As
indicated in Amendment 43, there are three groups of pressure set
points for the 19 SRV's for the 238-732 standard plant, namely,
one SRV at a pressure set point of 1103 psig, 9 SRV's at 1113
psig, and the remaining 9 SRV's at 1123 psig. Only one SRV is now
set at the lowest pressure set point. Based on this pressure set
point arrangement for the 19 SRV's, GE has analyzed the most
severe primary pressure transient, i.e., a turbine trip without
bypass. Results of the analysis shows that initiation of reactor
isolation will activate all or a portion of the 19 SRV's which
will release put the stored energy in the primary system.
Following the initial blowdown, the energy generated in the
primary system consists primarily of decay heat which will cause
the lowest set SRV to reopen and reclose (subsequent actuation).
The time duration between subsequent actuation was calculated to
be a minimum of 62 seconds and increasing with each actuation. The
time duration of each blowdown decreases from 51 seconds for the
initial blowdown and decreases to 3 seconds at the end of the
period of subsequent actuations which is 30 minutes after
initiation of
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reactor isolation.
The staff finds the result of the GE analysis reasonable.
Therefore, the assumption of only the lowest set SRV operating in
subsequent actuation is justified and acceptable.
The acceptance of the quencher load criteria is based on the test data
available to us. We realize, however, that the tests lack exact dynamic
or geometric similarity with the quencher system for the Mark III
containment. The test results, therefore, could not be applied
directly. Though the quencher loads for the Mark III appear
conservative in comparison with the test data, some degree of
uncertainty is acknowledged. The uncertainty is primarily due to a
substantial degree of scatter of all test data. We therefore will
require in-plant testing.
IV. REGULATORY POSITION
It is our position that applicants for Mark III containments using the
criteria specified below:
1. The structures affected by the SRV operation should be designed to
withstand the maximum loads specified in Table 1. For the cases
of multiple valve actuation, the quencher loads from each line
shall be assumed to reach the peak pressure simultaneously and
oscillate in phase.
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2. The quencher loads as specified in Item 1 above are for a
particular quencher configuration shown in the topical reports
NEDO-11314-08 and NEDE-11314-08. Since the quencher loads are
sensitive to and dependent upon the parameters of quencher
configuration, the following requirements should be met:
a. the sparger configuration and hole pattern should be
identical with that specified in Section A7.2.2.4 of
NEDE-11314-08.
b. The value of key parameters should be equal to or less than
that specified below:
Total air volume in each SRV line (ft3) 56.13
Distance from the center of quencher
to the pool surface at high water
level 13'-11"
Maximum pool temperature during
normal plant operation (F) 100
c. The value of those key parameters should be equal to or
larger than that specified below:
Water surface area per quencher (ft2) 295
SRV opening time (sec) 0.020
3. The spatial variation of the quencher loads should be calculated
by the methods shown in Section 2.4 of the topical report
NEDE-21078.
4. The load profile and associated time histories specified in Figure
A5.11 of NEDO-113/4-08 should be used with a quencher load
frequency of 5 to 11 Hz.
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5. For the 40 year plant life, the number of fatigue cycles for the
design of the structures affected by the quencher loads should not
be less than that specified in Section A9.0 of NEDO-11314-08.
6. In-plant testing of the quencher should be conducted to verify the
quencher design loads and oscillatory frequency. The in-plant
tests should include the following:
a. single valve actuation;
b. consecutive actuation of the same valve; and,
c. actuation of multiple valves.
Included should be measurements of pressure load, stress, and
strain of affected structures. A prototypical plant should be,
selected for each type of containment structure. For example, the
pressure responses from a concrete containment should not be used
for a free-standing steel containment and vice versa. Tests should
be conducted as soon as operational conditions allow and should be
performed prior to full power operation.
7. Based on the in-plant test results, reanalyses should be performed
to ensure the safety margin for the structures, which include the
containment wall, basemat, drywell walls, submerged structures
inside the suppression pool, quencher supports and components
influenced by S/R loads. If the analysis indicates that the safety
margin for the structures will be reduced because of the
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new loads identified from the test, modification or strengthening
of the structures should be made in order to maintain the safety
margin for which the structures were originally designed. The
applicants for the Mark III containment with quenchers for S/R
valves should submit a licensing topical report for approval. This
report should present a test program and identify the feasibility
of modification or strengthening of the structures.
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