United States Nuclear Regulatory Commission - Protecting People and the Environment

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. 
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