Schedule For Implementation And Resolution Of Mark I Containment Long Term Program (Generic Letter 79-13)
GL79013
UNITED STATES
NUCLEAR REGULATORY COMMISSION
WASHINGTON, D. C. 20555
MARCH 12 1979
Docket Nos. 50-325 and 50-324
Mr. J. A. Jones
Executive Vice President
Carolina Power & Light Company
336 Fayetteville Street
Raleigh, North Carolina 27602
Dear Mr. Jones:
RE: SCHEDULE FOR THE IMPLEMENTATION AND RESOLUTION OF THE MARK I
CONTAINMENT LONG TERM PROGRAM
The generic aspects of the Mark I Containment Long Tem Program (LTP) are
nearing completion. We have concluded that it is appropriate at this time to
establish specific schedules for the implementation of the plant-unique
aspects of the LTP.
We have scheduled the completion of our review of the Load Definition Report
(LDR) and Plant Unique Analysis Applications Guide (PUAAG) for May 1979.
Upon the completion of our review of the LDR and PUAAG, we will advise the
Mark I Owners' Group of any specific exceptions to these documents that mist
be addressed for a satisfactory LTP plant-unique analysis. Your plant-unique
analysis should be submitted as soon after that time as possible. Following
our review of your plant-unique analysis, we will take appropriate licensing
action, including a license amendment, to assure the timely completion of
the LTP.
At this point in the program, you should be in a position to know the
majority of plant modifications that will be necessary to conform to the LTP
acceptance criteria. Therefore, we request that, within 60 days following
your receipt of this letter, you provide a bar-chart schedule showing the
time periods for the installation of specific plant modifications. Your
schedule should be directed toward the completion of the needed plant
modifications by December 1980.
Should you be unable to meet this targeted completion date for the
installation of the major plant modifications, your response should include
sufficient justification to demonstrate your best efforts to meet this goal.
7903270090
.
Mr. J. A. Jones - 2 -
An issue that relates to your LTP implementation schedule is the use of
"ramshead" devices for safety-relief valve discharge. The enclosed staff
evaluation discusses our conclusions regarding the basis for the definition
of the ramshead threshold temperature (i.e., stability limit). As discussed
in this report, the quencher discharge device has been shown to
significantly improve both the loading on the containment and the
condensation stability. However, we understand that you have requested
further discussions regarding the possible use of the ramshead discharge
device. We will arrange to discuss this issue with you In the very near
future.
Another aspect of the resolution of the LTP concerns the licensing fees
required by 10 CFR 170. The LTP constitutes a "special project" as defined
by that regulation. As such, we have determined that the fee associated with
the generic aspects of the LTP will be based on the manpower required to
review the LDR and PUAAG. The responsibility for this fee will be shared by
the Owners Group as a whole. In addition, a fee will also be imposed on each
Individual utility for the license amendment associated with the LTP. The
fee class for the license amendment will be based on the manpower required
to review the LTP plant-unique analysis and any related changes to the plant
Technical Specifications.
As discussed above, your detailed schedule for modifications should be
submitted within 60 days following your receipt of this letter. If you so
desire, we will meet with you to discuss your specific plant modification
schedules.
Sincerely,
V. Stello, Jr., Directors
Division of Operating Reactors
Office of Nuclear Reactor Regulation
Enclosure:
As stated
cc w/enclosure:
See next page
.
Carolina Power & Light Company -3-
cc: Richard E. Jones, Esquire
Carolina Power & Light Company
336 Fayetteville Street
Raleigh North Carolina 27602
George F. Trowbridge, Esquire
Shaw, Pittman, Potts & Trowbridge
1800 M Street, NW
Washington, D. C. 20036
John J. Burney, Jr., Esquire
Burney, Burney, Sperry & Barefoot
110 North Fifth Avenue
Wilmington, North Carolina 28401
Southport - Brunswick County Library
109 W. Moore Street
Southport, North Carolina 28461
.
EVALUATION BY THE
OFFICE OF NUCLEAR REACTOR REGULATION
OF
SUPPRESSION POOL
TEMPERATURE LIMITS
IN BWR FACILITIES
.
I. Introduction and Summary
Safety-relief valves (SRVs) in BWR plants are used for reactor vessel
pressure relief during either anticipated plant transients or accident
situations. These valves are installed on the main steam lines of the
reactor system with discharge lines from the valves routed to the
suppression pool. When the valves open, the steam is discharged through the
piping into the pool where it is condensed. A discharge device, which is
affixed to the end of the piping beneath the water level in the pool, serves
to mix the discharged air and steam with the pool water. The most common
discharge device in use today is the ramshead type, which consists of two
90-degree pipe elbows welded together, as shown in Figure 1.
During SRV operation, when air and steam are discharged into the suppression
pool, vibratory loads (due to bubble formation and subsequent collapse) are
imposed on the containment structure and components within the pool. The
characteristics and magnitude of the load profile are dependent upon the
type of discharge device, the temperature of the pool, and the mass and
energy discharge rate.
For the ramshead device, the two most significant loads occur during vent
clearing and subsequent steam condensation. When the latter loading
condition occurs at elevated pool temperatures, condensation becomes
unstable and significantly higher loads result. Because of this phenomenon,
General Electric (GE) has proposed a pool temperature limit for all plants
using ramshead devices to avoid operation in this unstable condensation
zone. GE's proposed threshold for unstable condensation is 150F for the
bulk pool temperature and 160F locally. Justification for the limit was
supplied by GE to the staff in the form of topical reports (References 1 and
2). These reports contain the experimental data base used by GE to establish
the temperature threshold. The initial concern arose from an event that
occurred at a foreign plant, that caused damage to the containment and
subsequent leakage from the wetwell.
We have recently completed our review of the GE supplied justification for
the pool temperature limit. We and our consultants (from BNL and MIT) have
concluded (Reference 3) that the data base alone is not sufficient to
support the GE proposed temperature limit because of a lack of full-scale
SRV ramshead discharge load data. First, the data base consisted of
small-scale elbow and straight pipe data as well as small-scale ramshead
tests, with no scaling analysis provided to show the direct applicability of
such tests. Second, the results showed substantial data scatter.
.
-2-
Limited plant operational data were also provided, indicating that local
pool temperatures of approximately 165F have been experienced during a
stuck-open SRV event without any evidence of structural damage. This
experience can be considered as supporting data for the limited-mass
flow-pool temperature zone that occurred. However, it cannot be considered
as the operational basis for all potential events.
We have, therefore, concluded that the GE bulk suppression pool temperature
threshold of 150F cannot be adequately supported with the existing data
base for the ramshead discharge device. We can, however, conclude that the
actual temperature threshold is in the vicinity of the GE proposed limit
(i.e., about 150F). In light of our current understanding of the
ramshead device and since actual plant pool temperatures could approach the
GE-proposed limit, we believe that the ramshead device should be replaced to
preclude the unstable condensation phenomena. The basis for this conclusion
follows in Section II of this report.
A "quencher" type of device has been used for several years in foreign-based
plants. This device was developed to improve the performance of SRV
discharge at elevated pool temperatures as well as to reduce the air
clearing loads. The principle behind the quencher-type device is to promote
the creation of large surface areas of air and steam bubbles for rapid
mixing and diffusion rather than the jet type of discharge mixing provided
by the ramshead device. Thus, the quencher consists of pipe sections that
contain many small holes, either uniform or graduated along the surface to
promote and enhance diffusion and condensation in the pool. The quenchers
art typically referred to as either the "cross" or "T" types, depending upon
their geometrical configuration.
The data base for several quencher-type design has demonstrated superior
performance at elevated pool temperatures. Characteristically, a quencher-
type device has not exhibited the temperature threshold phenomenon that has
been observed for the ramshead device, based on the test data generated to
date. Pool temperatures have approached the boiling point (i.e., greater
than 97C) without any noticeable load increases. Hydrodynamic loads on
structures during vent clearing art also reduced, due to the inherently
better distribution of the steam/air mixture in the pool. The use of the
quencher device would therefore load to larger safety margins.
.
-3-
Based on the available data, we conclude that a design basis suppression
pool temperature limit has not been adequately established for the ramshead
device. Furthermore, we believe that, even if full-scale ramshead testing
were performed, it is likely that a temperature limit would be established
so that operator action would be required during SRV, discharge transients
to ensure that the pool temperature limit would not be exceeded. (Note:
Full-scale ramshead testing at elevated pool temperatures to establish a
design basis pool temperature limit has not been proposed). Therefore, in
the absence of any further information on the ramshead, we conclude that it
should not be used. We also conclude that the quencher-type device provides
improved safety margins and can be used in all BWR plants with water
suppression containments. The comparative benefits are given in the
following table:
Table 1
SRV DISCHARGE DEVICE EVALUATION SUMMARY
Local
Temperature Air Clearing
Device Limit Remarks Loads
Ramshead 160F 1. Test data do not +21 psi
support the pro-
posed limit. -10 psi
2. Severe vibration
occurs if the limit
is exceeded.
Quencher 200F 1. Test data show no +6 psi
severe vibration for
tank water tempera- -5 psi
tures approaching
boiling.
2. Steam condensation
loads are about
+-2.2 psi.
Minimum temperature limit for onset of condensation instability.
Peak positive and negative torus shell loads observed in the
Monticello in-plant tests.
.
- 4 -
We have considered the bases for interim operation of the Mark I plants
currently using ramshead devices. The SRV loads are cyclical in nature,
thereby creating the potential for fatigue degradation of the containment.
For currently operating Mark I plants, we have determined that there is
sufficient fatigue margin to permit continued plant operation while a new
discharge device is being developed and installed. Although some damage to
the torus internals has been observed due to apparent SRV operation, there
has not been a loss of containment integrity or function in any case.
II. Evaluation of Supporting Data for Ramshead Device
In late 1975, GE submitted a topical report (Reference 1) to support the
temperature limit for the suppression pool when using a ramshead device. The
report, however, contained test data for SRVs having a straight down pipe
discharge device and no test data for the ramshead device. As a result of
our evaluation, we conclude that the data base did not support the proposed
limit.
In response to our request, GE provided additional data (Reference 2) that
contained three sources of test data: subscale test data of ramshead and
elbow devices, small-scale test data of straight-down pipes, and plant
operational data. Results of our evaluation of this report are discussed
below.
A. Local and Bulk Temperature Differences
Local temperature is referred to as the water temperature that is in
the vicinity of the discharge device but not in contact with the steam
bubble. Bulk temperature, on the other hand, is a calculated
temperature that assumes a uniform pool temperature. Bulk temperature
is normally used for pool temperature transient analyses. Because the
test facilities are confined pools, the measured temperatures are
considered to be local temperatures. This has been confirmed through
evaluation of the test data. Generally, the test results show less than
a 2- to 3-degree variation within the test pool.
.
- 5 -
To allow proper interpretation of the test data, GE performed a test at
the Quad Cities plant. The pool was instrumented with 18 thermocouples,
6 of which were located in the vicinity of the discharge device to
determine local pool temperatures. The test was conducted by
continuously discharging an SRV into the suppression pool for 27
minutes. Throughout the transient, the results showed that the measured
local temperature did not deviate from the calculated bulk temperature
by more than 10F. Based on this result, GE has suggested that a
difference of 10F between local and bulk conditions be used. We
concur with this evaluation of the test data.
Based on the temperature difference, therefore, the GE-proposed
150F bulk temperature limit is equivalent to a 160F local
temperature. Test results then represent local temperature conditions.
The following data evaluation is based on this assumption.
With respect to the quencher device, the magnitude of the difference
between the local and bulk temperatures has not been established due to
the lack of an adequate data base. However, recently performed in-plant
tests are expected to provide the necessary data base. We will continue
our review of this matter.
B. Sub-scale Ramshead and Elbow Data
Sub-scale tests were performed at Moss Landing Test Facility and in a
separate test facility in San Jose, California. These consisted of
seven tests using a ramshead and 37 tests using a 90-degree elbow. The
mass flux ranged from 50 to 195 lbm/sq ft-sec. The local threshold
temperature for steam condensation instability calculated by GE for
each of these tests ranged from 152 to 176F for the ramshead
and 146 to 172F for the elbow.
Based on the following specific concerns, we conclude that the
applicability of the sub-scale test data has not been adequately
demonstrated and cannot be supported without additional testing.
.
- 6 -
1. Scaling Law Application: We know from our experience with the Mark
I pool swell phenomenon, and from the work that has been done by
the Mark II Owners' Group on steam condensation chugging, that
small-scale modeling laws are complex and must be established from
fundamental principles and carefully applied in model testing. No
such modeling laws have been derived for the SRV discharge
phenomenon. Test facilities were not scaled to simulate an actual
plant. Therefore, neither dynamic nor geometrical similarities can
be established by the tests. Furthermore, GE has not justified the
assumption that scaling has no effect on the temperature
threshold.
2. Data Scattering: Substantial data scattering appears in the
sub-scale test results. As noted previously, the temperature
threshold ranges from 146 to 176F. With such a wide
scattering, the probability for the temperature threshold to be
below the GE proposed 160F is relatively high (16% of the
sub-scale data points fall below the limit).
C. Small Scale Straight Down Pipe Data
This data set was obtained from foreign tests (Reference I). The tests
used a straight-down pipe and yielded 12 data points. The threshold was
defined as the pool temperature at which the peak-to-peak pressure
oscillation first reached 2 bar (29 psig) outside a circular projection
with twice the pipe diameter on the floor of the tank. Results of the
tests show that all data points fall below the 160F limit.
However, the straight-pipe discharge is phenomenologically different
from that of the ramshead device and therefore this data is not
applicable.
D. Plant Operational
Data The GE memorandum report (Reference 2) provides actual in- plant
data. Five plants have experienced SRV discharge into the suppression
pools where temperatures in excess of 100F were reached with no
reported instabilities. Specifically, the highest pool temperature from
those events ranged from 122 to 165F. However, the report
only provides detailed data for two plants identified as Plant A and
Plant C.
.
- 7 -
Data indicate that Plant A was manually scrammed before the suppression
pool temperature reached 100F following a stuck-open event. The
suppression pool temperature increased rapidly and reached 165F
when the reactor pressure was 184 psig. Plant C reached 146F only
because the reactor was scrammed at a lower pool temperature.
Figure 2 shows the loci of the Plant A and C events on a plot of pool
temperature versus SRV steam moss flux during blowdown. It also shows
the GE-proposed pool temperature limit. It is clear that the plants
experience SRV discharges far below the GE proposed pool temperature
limit at virtually all mass fluxes except the lowest. Thus, their
experience does not provide support for the higher mass flux at the
GE-proposed limit of 160F.
III. Discussion of SRV Quencher Discharge Device Designs
In 1972, a foreign BWR plant with water pressure suppression containment
experienced severe vibratory loads on the containment structure during
extended SRV operation at high pool temperatures. The loads were severe
enough to cause damage to the containment shell and components and to result
in water leakage from the suppression pool.
Following this incident, extensive experiments were conducted to investigate
various alternate discharge configurations. The objective of the
investigation was to develop a device that would reduce the hydrodynamic
loads during SRV air clearing and provide stable steam condensation. Varied
configurations of the discharge device considering more than 20 design
parameters were investigated. Results of the investigation concluded that
the quencher-type device yielded superior performance. Some of the test
results are provided in a GE topical report (Reference 1).
Figure 1 shows the configuration of a typical cross quencher, which is
currently used by all Mark III containments. The cross quencher has four
arms with each arm perforated by several rows of small holes. The tip of
each arm is plugged and the device measures approximately 10 feet long from
tip to tip. Steam flows through the hub is distributed among the four arms,
and is discharged into the pool. The T-quencher device presently being
developed for the Mark I plants is similar to the cross quencher except that
it has only two arms that are approximately 20 foot long from tip to tip.
The quencher device produces a cloud of air or steam mist, whereas the
ramshead produces large bubbles.
.
- 8 -
Because of the quencher configuration, the magnitude of the quencher air
clearing load is reduced by a factor of two to four. In addition, steam
condensation instability does not occur although the pool temperature
approaches boiling point.
Figure 3 shows the comparison of structural loading functions for quencher
and ramshead devices for a 238 GESSAR Mark III plant. Although these loading
functions are not applicable for the Mark I design, they demonstrate that
the quencher device, in general, substantially reduces the loads on the
containment structure with the magnitude of the load reduction being
dependent on the quencher configuration and its relative location to the
adjacent structures.
Foreign large-scale testing and in-plant tests from the United States
(Monticello) have verified the reduction in hydrodynamic loads when using
the quencher-type discharge device. Additional testing on a small scale has
also shown the temperature threshold for unstable condensation to increase
to about 200F using the quencher-type device. GE is presently
conducting full-scale confirmatory testing of the cross-type quencher device
at the Caroso plant in Italy. Additional testing on a full-scale plant has
been performed in Japan at the Tokai 2 facility.
IV. Conclusion
The suppression pool temperature limit proposed by GE to preclude unstable
condensation during SRV discharge through a ramshead device has not been
adequately demonstrated. Furthermore, we believe that, even if sufficient
full-scale testing of the ramshead device were to be performed to adequately
define the suppression pool temperature limit, it is likely that the
resulting limit would require several operator actions and perhaps an
additional margin in the allowable pool temperature during normal plant
operation to preclude unstable condensation.
The test data that has been generated to date for the quencher devices have
not exhibited the unstable condensation observed in the ramshead tests at
elevated pool temperatures. These data also demonstrate that the quencher
air clearing loads on the containment are substantially lower than the loads
resulting from discharge through a ramshead device. Furthermore, based on
the limited number of suppression pool temperature
.
- 9 -
transient analyses that we have received for Mark I plants, it appears that
a lesser amount of operator action would be required.
Based on the improved performance demonstrated for the quencher discharge
devices and the uncertainty associated with the definition of the pool
temperature limits for ramshead discharge devices, we conclude that the use
of ramshead devices in BWR water suppression containment systems is not
acceptable for long-term operation. We also conclude that the quencher-type
devices provide a satisfactory resolution to the condensation stability
concerns and is, therefore, an acceptable replacement.
.
References
1. General Electric Company, "Test Results Employed by GE for BWR
Containment and Vertical Vent Loads," GE Topical Report NEDE-21078-P,
October 1975.
2. General Electric Company Memorandum Report, "170F Pool Temperature
Limit for SRV Ramshead Condensation Stability," September 1977.
3. Ain A. Sonin and C. Tung, "Comments on the Pool Temperature Limit for
Avoiding Pulsating Condensation with Ramshead SRVs," Brookhaven
National Laboratory, February 1978.
4. General Electric Company, "Information Report Mark III Containment
Dynamic Loading Conditions (Final)," GE Topical Report NEDO-11314-08,
July 1975.
Page Last Reviewed/Updated Tuesday, March 09, 2021