ML20211N668
| ML20211N668 | |
| Person / Time | |
|---|---|
| Site: | Fort Calhoun  |
| Issue date: | 02/12/1987 |
| From: | Office of Nuclear Reactor Regulation |
| To: | |
| Shared Package | |
| ML20211N666 | List: |
| References | |
| NUDOCS 8703020177 | |
| Download: ML20211N668 (8) | |
Text
nc
$4 UNITED STATES 8_ o NUCLEAR REGULATORY COMMISSION
$ ; WASHINGTON, D. C. 20555
% /
SAFETY EVALUATION BY THE OFFICE OF NUCLEAR REACTOR REGULATION OMAHA PUBLIC POWER DISTRICT DOCKET NO. 50-285 FORT CALHOUN STATION, UNIT NO. 1 THERMAL SHIELD INSPECTION DEFERRAL
- 1. 0 INTRODUCTION By letter dated April 4, 1984 (Reference 1), Omaha Public Power District (OPPD) committed to perform an inspection to determine the condition of the reactor vessel thermal shield at the Fort Calhoun Station. The purpose of this inspection was to assure that the thermal shield and thermal shield suppart system were not degrading as observed at other Combustion Enginee.-ing (CE) plants. Since that time, OPPD has performed comprehensive research and analysis of the thermal shield degradation phenomena which has resulted in new information and monitoring techniques which were not available in 1984. Based upon the results of this new information, OPPD proposes to change the commitment for a 1987 inspection to a commitment to conduct an ongoing thermal shield monitoring program capable of detecting precursors to internals degradation. Further, should precursors to degradation be detected, OPPD will conduct an inspection and/or repairs as needed. In addition, at the latest, an inspection of the reactor internals will be conducted as required by the 10 year In-service Inspection during the spring outage in 1993. OPPD's submittal of August 28, 1986 (Reference 2) seeks NRC approval for this change in .
commitment.
- 2. 0 BACKGROUND The thermal shield support system degradation problem was first observed at Maine Yankee during the September-October 1982 outage. It was also observed at S+. Lucie Unit 1 and Millstone Unit 2 during 1983. As a precautionary measure, Combustion Engineering informed OPPD about this potential problem. As a result, OPPD expanded its 10-Year In-service
! Inspection (ISI) program, performed in January 1983, to include a thorough l visual inspection of all accessible portions of the thermal shield positioning pins. Fort Calhoun's thermal shield was found to be in excellent condition. The ISI results were submitted for staff review on April 26, 1983 and were found to be satisfactory. OPPD pursued the thermal shield (TS) issue by contracting with CE to perform an additional safety analysis to evaluate the impact of a postulated thermal shield failure at the Fort Calhoun Station. The results, transmitted to the NRC on August 2, 1983 (Reference 3), concluded that failure of the thermal l shield support system was not a safety concern. The staff recommended to
- OPPD that an appropriate time for the next thermal shield inspection would l be in 1987. This recommendation was based on the assumption that the t B703o20377 870212 gDR ADOCK 05000285 PDR
i
-2 - -
Loose Parts Monitoring System would not provide the information necessary to identify a thermal shield problem prior to failure. However, as a result of analyses by the licensee and CE, it was determined that the Internal Vibration Monitoring (IVM) System could be used to detect a loss of effectiveness of the positioning pins prior to any significant damage occurring. The use of this monitoring system is discussed by the licensee in detail in the present submittal and forms the basis for justifying their request to defer the planned inspection of the thermal shield from 1987 to 1993.
3.0 DISCUSSION 3.1 Failure Mechanism The failure of the thermal shield support systems at other Combustion Engineering NSSS facilities has been analyzed extensively. The initiating event was the loss of preload in the positioning pins.
Several factors have been identified as potential mechanisms for reducing the preload in the positioning pins, but the exact cause has not been ascertained. Regardless of the cause for loss of preload, it has been established that degradation to the thermal shield support system is a slow process. The precursor to significant thermal shield degradation is the gradual loss of the effectiveness of the positioning pins.
The process begins with a gradual loss of preload in the positioning pins. Some of the factors identified as potential contributors to
! the loss of preload are radiation induced stress relaxation, fluctuating hydraulic loads, deformation of the core support barrel
~
i (CSB) during assembly (as the weight of the thermal shield is i transferred to the support lugt), and possible installation errors.
A combination of these factors seems to be the best explanation available. Plastic deformation of the positioning pins because of underdesign has also been postulated as a possible contributor to the problem, but this seems unlikely since such deformation would have taken place early in the operating life of the station. The fact
. that.the Fort Calhoun thermal shield is still intact and properly supported suggests that this degradation mechanism probably will not
- be operative at this plant.
The plants that have experienced failures have locking bars installed
! to keep the pins in place; when contact is lost between the pin and the CSB, movement between the pin and thermal shield is possible.
This movement results in wear between the threads of the pin and the thermal shield, thereby further reducing the effectiveness of the i pins and allowing the amplitude of relative motion to increase. ,
L Fluctuating hydraulic loads also cause wear to initiate in the thermal shield support lug region. These support lugs provide the
- primary component of the stiffness coefficient in the upper region of i the thermal shield. Once the thermal shield support lugs begin to wear, the support system approaches an unstable condition and severe damage can occur.
i
Initially, loss of preload hill only cause wear to occur at low power conditions when thermal loading of the pins is lowest. Oparation at low power conditions will continue the degradation process through wear. If not corrected, the positioning pins eventually will lose their effectiveness at full power conditions as well. The reason is ;
at full power conditions the differential expansion of the core support barrel relative to the thermal shield (the CSB being at a higher temperature) increases the load on the positioning pins.
This effectively provides support to a thermal shield that has lost some of its initial preload. At low power the thermal shield is near isothermal conditions and these additional loads are absent. Thus, a loss of preload should be detectable at low power long before it will be seen at full power conditions.
3.2 Detection of Thermal Shield Support Degradation As part of their analysis of the St. Lucie 1 thermal shield failure, Combustion Engineering determined that the Internals 'ilbration Monitoring System (IVM) is an effective indicator of t.he adequacy of the support of the thermal shield. The IVM system monitors the excore neutron flux frequency and infers a vibration frequency and mode of vibration of the core support barrels (CSB). The CSB vibra-tion frequency remains at the same value as long as the thermal shield is supported adequately. A shift in certain well defined frequencics of the CSB vibration is indicative of a change in the relative motion of the thermal shield to the core support barrel.
The loss of effectiveness of the positioning pins is initially detectable during low power near isothermal conditions. Analysis of the IVM data taken at low power gives the first indication of this effect taking place. This analysis will provide a sufficiently early indication of the problem to enable OPPD to schedule examination and/or corrective action during a normally planned refueling outage.
On July 3, 1986, the licensee recorded IVM data at low power, near isothermal conditions. These data were analyzed and showed no indication of loss of effectiveness of the positioning pins. The licensees conclude that the thermal shield is adequately supported.
3.3 Monitoring Program There are three separate elements of the Fort Calhoun Loose Parts Monitoring Program that are currently in place. The first element consists of recording the eight accelerometer channels onto magnetic tape on a monthly basis. This data is kept for historical reference purposes. Audio monitoring by the Shift Technical Advisors (STA's) is the second element of the program. The STA's listen to each channel once per 8-hour shift for any sounds of impacting occurring in the primary system.
The third and new element of the program will be to use a modified amplitude probability distribution (MAPD) function on the eight recorded accelerometer channels. The MAPD is the Amplitude Probability Density function with the probability density plotted on a semi log scale versus the amplitude of the signal. The MAPD is used to quantify changes in the Loose Parts Monitoring signals. The plots can be used to give an indication of the root mean square "g" value of the impacts as well as the rate of impact. The magnetic taping of the accelerometers will continue on a monthly interval with the MAPD plots being generated on a quarterly basis. The STA monitoring will continue on a once per-shift cycle.
The Internals Vibration Monitoring Program at Fort Calhoun utilizes the excore detectors located around the reactor vessel. Presently, a magnetic tape recording is made of the excore detectors' signals on a monthly basis. The licensee will evaluate the excore signals on a quarterly basis to monitor for early signs of any reactor internals degradation. The monthly taping and power spectral densities will continue at the same rate along with collecting, reducing, and evaluating low power data at least once per fuel cycle.
3.4 Pertinent Design Differences The pertinent design differences between the Fort Calhoun thermal shield system and the three plants that experienced thermal shield system degradation are summarized below. The effects of those design differences on the possible loss of preload in the positioning pins are discussed in the next section.
(1) The Fo;t Calhoun thermal shield is smaller in diameter (inside diameter of 127 in. versus 156-3/4 in.), but longer in length (164 in. versus 152 in, for Maine Yankee and 137-3/4 in, for the other two); the dry weights of the thermal shields are approximately the same for all four plants.
(2) The Fort Calhoun thermal shield has 8 support lugs versus 9 for the other three plants.
(3) The Fort Calhoun thermal shield has 8 upper pins versus 9 and 16 lower pins versus 17 compared to the other three I plants.
(4) The CSB/TS annular gap is 1-9/16 in. for the Fort Calhoun plant versus 2-5/8 in. for the other three plants.
- (5) Because the Fort Calhoun thermal shield is longer in i length, the lower positioning pins are located 11 in, below the core support plate versus 6 in. for Maine Yankee. In the St. Lucie 1 and Millstone 2 plants the pins are within I
the active core region.
i i
. _ , _ - ,_m ,._ __._____ _,_- __ _ __-__-..___ ,____,_,m__ _ , , _ _ _ _ _ . - , - _ _ _ . . _ _ . _ _ _ _ _ . , _
(6) The average total core design flow rate in the Fort Calhoun plant is 190,000 gpm versus approximately 324,000 gpm for the other three plants; the average coolant flow velocity in the annular gap is approximately 8% lower than for the other three plants.
(7) The positioning pins in the Fort Calhoun plant are secured by a locking collar that is welded around the inside and outside circumferences to the positioning pin and thermal shield, respectively. The other three plants have locking bars which fit into a slot at the threaded end of the positioning pin and are tack welded only to the thermal shield.
The most significant design differences between the Fort Calhoun reactor internals and the other Combustion Engineering plants are (1) the location of the lower positioning pins relative to the active core region, and (2) the use of a locking collar to secure the positioning pin instead of a locking bar. The lower positioning pins are located at an elevation below the active core region, lower in elevation than at the other CE plants.
. This location makes the pins less susceptible to the effects of radiation induced stress relaxation, which is one of the factors identified as contributing to a loss of preload.
4.0 EVALUATION From the analysis and data submitted by the licensee', the failure mechanism is shown to be a slow process with a gradual loss of preload in the positioning pins as a precursor. From the standpoint of design differences reducing the potential for failure of the thermal shield support system, the most significant are the locking collar, the smaller annular gap between the CSB and thermal shield, and the lower coolant flow velocities. The fact that the lower positioning pins are located below the active core region for the Fort Calhoun plant is an important factor.
The licensee and CE have asserted that loss of preload is less likely in the case of the Fort Calhoun plant. In particular, they call attention to the fact that radiation induced stress relaxation (believed to be a major contributor to loss of preload) will be less for the lower pins of the l- Fort Calhoun thermal shield because the pins are located below the active I core. This argument does not apply to the upper pins. Furthermore, the effect cannot be quantified. Therefore, it must be assumed that some pins will lose preload although the number and timing of the loss of preload in the pins cannot be predicted.
t I
r
Once preload is lost, the mechanism initiating positioning pin thread wear is turbulent buffeting and vortex-shedding, which give rise to both random and periodic fluid forces acting on the pins. The fluid annulus, hence exposed pin length, is 40 percent smaller for the Fort Calhoun plant and the coolant flow velocity is 8 percent less. As a result, the magnitude of the fluid forces acting on a pin in the Fort Calhoun plant will be approximately one-half the force acting on the pins at other plants. At full. power the fluid force associated with vortex shedding is estimated to be approximately 5 lbs for the other plants and only 2.5 lbs in the case of the Fort Calhoun plant. An additional, important factor is that the effectiveness of the fluid forces will be significantly reduced for the Fort Calhoun plant because of the shorter pin which is much stiffer.
(
Once preload is lost, the locking bars used on the other three plants do not secure the pins in the sense of maintaining preload on the threads and preventing axial motion. On the other hand, the Fort Calhoun locking collars are torqued to 50 ft-lb, thereby preloading the locking collars and positioning pins to the thermal shield. The locking collars are then welded a.round the circumference to.both the positioning pin and thermal shield. As a result of the preload on the threads, relative motion between the two threaded surfaces is inhibited. This, in combination with fluid forces one half the magnitude of the forces in other plants, and forces which are less effective because they are acting on stiffer pins, can be expected to lead to significantly less (possibly negligible) thread wear.
The circumferential welds on the locking coilar can be expected to remain intact as the pins are not subjectel to large stresses in the, weld regions. Consequently, the locking collars can be expected to remain effective in providing preload on tte threads and inhibiting wear.
5.0 CONCLUSION
Based on a review of the analyses and data submitted by the licensee, the staff concurs with the assessment that the failure mechanism is a gradual process with a loss of the preload in the positioning pins as a precursor.
The design differences by themselves appear to be sufficient to support the proposal to defer inspection. However, since many of the effects cannot be quantified, for example, thread wear due to unsteady fluid loads, it is important that a surveillance system be in place. OPPD has suggested such a system consisting of loose parts monitoring and IVM using neutron detectors. The fact that support system degradation has been shown to be a slow process makes detection using such schemes feasible.
The IVM system, in conjunction with the l'oose part monitoring system, is likely to detect an initial loss of the effectiveness of the support system in sufficient time to allow for a planned inspection and repair program to be implemented before significant damage is incurred However, additional understanding of the IVM system, the analysis and interpretation of the IVM system signals, and the establishment of threshold levels must be developed. This level must include an evaluation of the potential errors in the system's detectors, instrumentation and
. )
indication subsystems. Once it is determined that these threshold levels have been exceeded, indicating the initiation of the slow degradation process, the licensee is committed to developing and implementing a timely i program of inspection and/or corrective action. The staff concludes that the deferral of the thermal shield support system inspection during the 1987 refueling outage is acceptable contingent upon the licensee's commitment to submit the additional analyses of the IVM system for staff review and approval. This submittal should include the licensee's intended inspection and corrective action plans. This information should be submitted within 6 months of the date of this letter, i
e
~
~ ~
~
References
- 1. Letter No. LIC-84-090 to R. A. Clark (NRC) from W. C. Jones (OPPD) dated April 4, 1984 Commitment to perform an inspection of the thermal shield at the Fort Calhoun Station.
- 2. Letter No. LIC-86-421 to A. C. Thadani (NRC) from R. L. Andrews (OPPD) dated August 28, 1986 requesting deferral of the Fort Calhoun Thermal Shield Support System.
- 3. Letter No. LIC-83-189 to R. A. Clark (NRC) from W. C. Jones (OPPD) dated August 2, 1983. Impact of a postulated thermal shield failure at the Fort Calhoun Station.
Date: February 12, 1987 Principal Contributor:
J. Rajan