ML13038A440
| ML13038A440 | |
| Person / Time | |
|---|---|
| Site: | Palisades  |
| Issue date: | 02/07/2013 |
| From: | Northeast Technology Corp |
| To: | Entergy Nuclear Operations, Office of Nuclear Reactor Regulation |
| Chawla M | |
| References | |
| net-299-01 | |
| Download: ML13038A440 (72) | |
Text
NET-299-01 Abstract The Boron-10 Areal Density Gage for Evaluating Racks(BADGER) was developed by Northeast Technology Corporation(NETCO) for the Electric Power Research Institute (EPRI). BADGER is a device that allows the in-situ measurement of the boron-10 areal density of the neutron absorber material installed in spent fuel racks for the purpose of reactivity control.
The following report provides, an overview of the BADGER system, the test data from the Palisades campaign, an evaluation of test data including the minimum measured areal density (at the 95% probability/ 95% confidence level), and NETCO s conclusions with respect to the condition of the neutron absorber plates in the Palisades spent fuel racks.
BADGER test results indicate that the Palisades spent fuel pool neutron absorber plates have sustained significant in-service degradation. The areal density for all panels tested was analyzed under two conditions. All of the average panel areal densities were greater than the lowered calibration cell areal density of 0.0566 gm B10/cm2 at a confidence level of 95%. However, when analyzed to confirm that at least 95% of the individual measurements for a panel exceeded the calibration cell areal density of 0.0566 gm B10/cm2 with a confidence level of 95% (95/95), some panels failed to meet that criteria.
ii
NET-299-01 Table of Contents ABSTRACT .................................................................................................................... II TABLE OF CONTENTS................................................................................................. III LIST OF TABLES ..........................................................................................................IV LIST OF FIGURES..........................................................................................................V
1.0 INTRODUCTION
..................................................................................................... 1 2.0 OVERVIEW OF THE BADGER SYSTEM ............................................................... 2 2.1 BADGER EQUIPMENT DESCRIPTION ....................................................................... 2 2.2 TYPICAL OPERATION OF BADGER .......................................................................... 3 3.0 SCOPE OF THE TESTING AT PALISADES NUCLEAR PLANT ........................... 8 3.1 SPENT FUEL RACK DESCRIPTION ............................................................................. 8 3.2 NEUTRON ABSORBER PANELS SELECTED FOR TESTING ............................................. 8 4.0 BADGER TEST RESULTS.................................................................................... 12 4.1 PANEL COUNT RATES ........................................................................................... 12 4.2 PANEL MINIMUM AVERAGE AREAL DENSITY ............................................................ 13 5.0
SUMMARY
AND CONCLUSIONS ........................................................................ 21
6.0 REFERENCES
...................................................................................................... 22 APPENDIX A: BADGER Transmission Traces for Tested Panels APPENDIX B: Determination of Palisades Areal Density Standards APPENDIX C: Technical Appendix iii
NET-299-01 List of Tables Table Page 3-1 Palisades Neutron Absorber Panels Tested....................................................... 11 4-1 Palisades Neutron Absorber Panels 95/95 Count Rate Test per Detector ......... 18 4-2 Palisades Neutron Absorber Panel Count Rate Margin in Counts per Second ............................................................................. 19 iv
NET-299-01 List of Figures Figure Page 2-1 Typical Axial Cross Section of the Source and Detector Heads in Fuel Rack Cells ..................................................................................... 5 2-2 Lateral Cross Section of the Source and Detector Heads .................................... 6 2-3 Lateral Cross Section of the Palisades Calibration Cell........................................ 7 3-1 Palisades Spent Fuel Pool Layout........................................................................ 9 3-2 Palisades Storage Cell with Carbon Matrix Neutron Absorber Plates ............... 10 4-1 Count Rate Traces for Absorber Panel H-18 North ............................................ 14 4-2 Count Rate Traces for Absorber Panel H18 South............................................. 15 4-3 Normalized Transmission Traces for Absorber Panel I15 North......................... 16 4-4 95 by 95 Count Rate Difference from the 0.0566 gm B-10 /cm2 Calibration Standard. ......................................................................................... 17 v
NET-299-01
1.0 INTRODUCTION
Premature deterioration of neutron absorber materials has been observed in some spent fuel storage racks.[1,2,3] The Boron-10 Areal Density Gauge for Evaluating Racks (BADGER) was developed by Northeast Technology Corp. (NETCO) for the Electric Power Research Institute (EPRI) under research project WO-3907-01.[4,5] BADGER is a device which allows the in-situ measurement of the boron-10 areal density (10B areal density expressed as grams 10B/cm2) of the neutron absorber material installed in spent fuel racks for the purpose of reactivity control.
The development of BADGER was prompted by the observed in-service deterioration of Boraflex - one type of neutron absorber material. This report describes the BADGER test conducted at Entergy Nuclear Operation s Palisades Nuclear Plant, July 14 July 24, 2008. In this case, BADGER has been applied to the spent fuel storage racks in the Palisades pool that utilize a borated graphite neutron absorber manufactured by the Carborundum Company. The test was conducted in accordance with NETCO Special Engineering Procedures, SEP-299-01, and SEP-299-02.
The following sections of this report provide an overview of the BADGER system, the test data from the Palisades campaign, an evaluation of test data including the measured count rates, and NETCO s conclusions with respect to the condition of the neutron absorber plates in the Palisades spent fuel racks.
1
NET-299-01 3.0 SCOPE OF THE TESTING AT PALISADES NUCLEAR PLANT 3.1 Spent Fuel Rack Description The Palisades spent fuel pool has spent fuel storage racks of two different designs; one design incorporates Boraflex neutron absorbers, the other a borated graphite neutron absorber manufactured by the Carborundum Company. The latter racks were subject to BADGER testing.
The spent fuel racks in the Palisades spent fuel pool with the Carborundum neutron absorber are shown in Figure 3-1. The racks consist of six modules of varying size.
The rack modules are formed by assembling an array of individual storage cells. The storage cells are separated by a 0.69" flux trap on a 10.25" pitch. The individual cells are constructed from two concentric square tubes as shown in Figure 3-2. The inner tube has a nominal inside dimension of 8.56" and is 0.125" thick. The outer tube has an outside dimension of 9.56" and is also 0.125" thick. Between the two concentric square tubes are plates of Carborundum neutron absorber on each face of the cell. The plates are nominally 0.210" thick, 8.26" wide and 33.5" long. Four such plates are stacked on each face to form a continuous neutron absorber plate 134.00 inches long. The overall length of the storage cells is 149.5 inches. When assembled into a rack module, each storage cell for a fuel assembly has two plates of Carborundum neutron absorber between each face.
3.2 Neutron Absorber Panels Selected for Testing Figure 3-1 shows the status of the Carborundum racks at the time of the BADGER test campaign. The location of vacant and occupied cells are indicated. Those cells highlighted in yellow contain panels that were initially selected for testing. A total of 60 panels were initially selected for testing. Due to time constraints, only 38 panels were scanned. The blue locations in Figure 3-1 indicate fuel assemblies that were discharged during the most recent refueling outage. Table 3-1 contains a list of the panels actually tested.
8
NET-299-01 Figure 3-2 Palisades Storage Cell With Carbon Matrix Neutron Absorber Plates 10
NET-299-01 Cell Panel H18E East H18N North H18S South H18W West I15E East I15N North I15S South I15W West O11E East O11N North O11S South O11W West O7E East O7N North O7S South O7W West O9E East O9N North O9W West P7E East P7N North P7S South P7W West Q6E East Q6N North Q6S South Q6W West R22E East R22N North R22S South R22W West S15N North S15S South S15W West T21E East T21N North T21S South T21W West Table 3-1 Palisades Neutron Absorber Panels Tested 11
NET-299-01 4.0 BADGER TEST RESULTS 4.1 Panel Count Rates The count rate traces for all panels tested are contained in Appendix A. The salient features of a few of these traces are described below to aid the reader in interpreting the traces in the Appendix.
Figure 4-1 shows the count rate for the four BADGER detectors versus axial cell elevation for the north panels in cell H18. With the BADGER head resting at the bottom of the cell, the detector elevation corresponds to almost the bottom of the absorber plate. As the detectors move up the cell the count rates are fairly uniform in the neutron absorber plate region. At the top of the trace a sharp increase in count rate is experienced as the detectors exit the top of the neutron absorber plate region of the cell. The uniform nature of all four detectors in this cell indicates a uniform boron-10 areal density of this panel.
Also shown on each of the detector scans are the 95% probability/95% confidence level lower limit count rate for the 0.0566 gm B-10/cm2 calibration cell. The data for this count rate limit is taken from scan calscan056-3 and is detailed subsequently. The 95%/95%
lower limit is defined as:
N95/95 = -
where: = average count rate in the 0.0566 gm B-10/cm2 calibration cell
= one sided tolerance factor
= standard deviation A one sided tolerance factor is defined as the number of standard deviations necessary to include from a sample population such that a given portion of the population (95%) is captured with a given level of certainty (95%). The following provides a more explicit definition and is taken from, Lieberman, Gerald J. "Tables for One-Sided Statistical Tolerance Limits". Industrial Quality Control. April, 1958 12
NET-299-01 Appendix C shows some sample calculations and further details on this and other parts of the analysis.
Figure 4-1 shows that all count rates on panel H18N fall below the lower limit (N95/95) in the calibration cell. It is therefore concluded that all measured points on panel H18N have an areal density greater than 0.0566 gm B-10/cm2 with a probability of 95% and with a confidence level of 95%.
Figure 4-2 contains a plot of the detector count rates as a function of axial elevation for the scan of the South panel of cell H18. Since the overall absorber panels are comprised of four short plates, change in the count rate at approximately 34, 68, and 102 inches would indicate the absorber plate interfaces. This panel shows some regions of thinning (Detector 2) and discontinuities (Detectors 3 and 4) at approximately these elevations. As in the case of panel H18N, all detector count rates lie below the 95/95 lower limit calibration cell count rate. It is therefore concluded that all measured points on H18S have a B-10 areal density greater than 0.0566 gm B-10/cm2 at a 95%
probability with a 95% confidence level.
The trace of the absorber plates in I15 North is shown in Figure 4-3. These plates appear to be characterized as having two axial zones with slightly different areal densities. One zone from 0 to ~40 inches (near the interface of the first and second plates) and the second zone above ~40 inches elevation. Below 40 inches all detector count rates are below the 95/95 lower limit count rate while above 40 inches, 3 of the 4 detectors have count rates in excess of the limit.
Several of the panels tested had axial zones that had count rates above the N95/95 lower limit. Table 4-1 summarizes the results of all 38 scans for each of the four detectors. A "pass" for a given detector on a stack of plates means that 95% of the measured count rates are below the N95/95 count in the calibration cell. A "fail" means that less than 95%
of the measured count rates meet the N95/95 lower limit. Figure 4-4 contains a plot of the count rate difference between the average count rate of the tested panels and the average count rate in the calibration cell (including the 95/95 bias). Each point on the plot represents this difference for each detector in every scan. For example, the highest value (approximately 15 cps) indicates that the count rate for detector 2 on the scan of H18 West is approximately 15 counts per second higher (including the 95/95 bias) than the average count rate for detector 2 in the calibration cell.
4.2 Panel Minimum Average Areal Density The manufacturing drawings[6] for the storage racks were reviewed to determine the minimum certified areal density of the Palisades neutron absorber plates. The minimum certified areal density of the absorber plates based on Reference 6 is specified as 0.0863 gm B-10/cm2. This is slightly lower than the areal density (0.0894 gm B-10/cm2) of the first NETCO calibration standard.
During the initial course of testing at Palisades, the BADGER traces were subjected to preliminary analysis. This preliminary analysis showed the tested panels had areal densities below the 0.0894 gm B-10/cm2 calibration standard. The calibration cell was 13
NET-299-01 removed from the pool and refitted with a second standard that had an areal density of 0.0566 gm B-10/cm2.
During one of the scans of the 0.0566 gm B-10/cm2 standard, lower than expected count rates were observed. It is believed during positioning of the probes, that the probe heads may have been out of plumb; resulting in misalignment of the source or detector heads. This would lead to a reduction in the measured count rates. This hypothesis was verified using a simplified MCNP model to determine the effect of misaligned source/detector heads on detector count rates. The results confirmed that an offset to either head could result in a reduced count rate of at least 47%.
The discussion in Section 4.1 has indicated that half the panels tested had areal densities greater than 0.0566 gm B-10/cm2 at 95% probability/95% confidence level.
The other half had a lower areal density. The data was also evaluated to determine whether the panel average values of the tested plates exceed the areal density of the material in the calibration cell.
Table 4-2 contains a list of the tested panels and the minimum mean count rate difference from the 0.0566 gm B-10/cm2 calibration standard used in the Palisades test
(= mean calibration count rate mean test panel count rate). This evaluation differs from the 95 by 95 analysis, but shows that, with 95% confidence, the mean count rate of all panels tested is less than the mean count rate of the calibration standard. These average values are compared at a 95% confidence level but do not represent an N95/95 analysis. This provides an alternative comparison scheme that focuses on average rather than point-to-point variations. Under this scheme positive values fail the test and negative values pass. This leads to the conclusion that, on a panel average basis, all of the panels tested had an areal density greater than 0.0566 gm B-10/cm2 with a confidence level of 95%.
14
NET-299-01 Figure 4-1 Count Rate Traces for Absorber Panel H-18 North 15
NET-299-01 Figure 4-2 Count Rate Traces for Absorber Panel H-18 South 16
NET-299-01 Figure 4-3 Normalized Transmission Traces for Absorber Panel I15 North 17
NET-299-01 Figure 4-4 95 by 95 Count Rate Difference from the 0.0566 gm B-10/cm2 Calibration Standard.
(Values are for individual detector averages among all panels scanned) 18
NET-299-01 Panel Detector-1 Detector-2 Detector-3 Detector-4 H18E Pass Pass Pass Pass H18N Pass Pass Pass Pass H18S Pass Pass Pass Pass H18W Fail Fail Fail Pass I15E Fail Fail Fail Pass I15N Fail Fail Fail Pass I15S Pass Pass Pass Pass I15W Fail Fail Fail Pass O11E Fail Fail Fail Pass O11N Fail Fail Fail Pass O11S Pass Pass Pass Pass O11W Fail Fail Fail Pass O7E Pass Pass Pass Pass O7N Pass Pass Pass Pass O7S Pass Pass Pass Pass O7W Pass Fail Pass Pass O9E Pass Pass Pass Pass O9N Pass Fail Fail Pass O9W Fail Fail Fail Pass P7E Fail Fail Fail Fail P7N Fail Fail Fail Pass P7S Pass Pass Pass Pass P7W Pass Pass Pass Pass Q6E Pass Pass Pass Pass Q6N Pass Pass Pass Fail Q6S Pass Pass Pass Pass Q6W Pass Pass Pass Pass R22E Fail Fail Fail Pass R22N Fail Pass Pass Pass R22S Pass Pass Pass Pass R22W Pass Pass Pass Pass S15N Fail Fail Fail Pass S15S Pass Pass Pass Pass S15W Fail Fail Fail Pass T21E Pass Pass Fail Pass T21N Fail Fail Fail Pass T21S Pass Pass Pass Pass T21W Pass Pass Pass Pass Table 4-1 Palisades Neutron Absorber Panel 95/95 Count Rate Test per Detector.
19
NET-299-01 Panel Detector-1 Detector-2 Detector-3 Detector-4 H18E 15.8 17.5 12.7 11.9 H18N 18.0 21.1 17.7 14.6 H18S 16.0 15.7 13.1 11.2 H18W 7.8 1.2 4.0 6.8 I15E 7.9 4.5 5.5 7.8 I15N 7.7 5.1 5.7 8.9 I15S 15.6 17.8 13.4 12.9 I15W 8.6 9.4 4.6 6.9 O11E 6.0 6.2 2.1 6.2 O11N 6.0 6.0 2.1 7.8 O11S 16.4 18.7 14.9 13.8 O11W 6.9 7.9 3.2 7.3 O7E 18.1 21.4 21.0 17.1 O7N 16.8 19.1 18.7 16.0 O7S 16.6 16.3 17.8 14.5 O7W 11.3 7.5 11.3 8.6 O9E 15.9 15.1 16.5 13.3 O9N 8.7 5.7 7.5 6.0 O9W 7.6 5.0 5.9 7.5 P7E 6.8 6.8 2.1 3.8 P7N 6.3 7.3 4.6 7.1 P7S 15.6 19.0 16.2 14.6 P7W 13.7 15.9 11.4 10.1 Q6E 10.6 11.6 7.8 7.9 Q6N 8.8 9.5 6.5 7.1 Q6SS2 9.3 10.2 6.9 7.3 Q6W 10.5 10.8 6.7 7.1 R22E 4.3 3.5 4.7 7.7 R22N 8.9 8.0 10.1 8.7 R22S 11.3 8.6 11.6 8.8 R22W 17.3 17.9 19.0 15.3 S15N 4.9 6.3 2.4 5.8 S15S 16.7 18.8 12.9 11.0 S15W 3.7 4.2 0.6 5.7 T21E 8.1 8.1 3.3 4.7 T21N 8.0 8.7 4.8 6.9 T21S 15.5 12.2 14.8 9.3 T21W 17.6 20.5 16.4 13.6 Table 4-2 Palisades Neutron Absorber Panel Count Rate Margin in Counts per Second.
20
NET-299-01 5.0
SUMMARY
AND CONCLUSIONS A series of 38 neutron absorber panels in the Palisades Nuclear Station spent fuel storage racks were subjected to non-destructive BADGER testing to determine the condition of the neutron absorber material. This test campaign was conducted over a three week period in July of 2008 and was intended to assess the condition of the Carborundum neutron absorber plates in the Palisades spent fuel racks. A special calibration cell was custom fabricated to replicate the geometry of the Palisades racks.
The calibration cell ultimately contained neutron absorbers with an areal density of 0.0566 gm B-10/cm2.
The detector count rate data for all scans of the Carborundum plates was subsequently analyzed and subjected to evaluation on an individual count rate basis for each detector at all axial elevations on the plates and on an average count rate basis for each detector on each panel. In the individual count rate analysis the detector count rates were compared to the lower limit detector count rates in the calibration cell at a 95%
probability with a 95% confidence level.
Using the individual detector count rate analysis, it was concluded that approximately half of the panels had a boron-10 areal density of 0.0566 gm B-10/cm2 or greater at a 95% probability with a 95% confidence level. The other half of the population of panels tested had an areal density less than 0.0566 gm B-10/cm2 at the same probability and with the same confidence level.
However, considering the average detector count rate for each detector and comparing it to the average count rate in the calibration cell, it is concluded that all panels had an average areal density greater than 0.0566 gm B-10/cm2 with a confidence level of 95%.
21
NET-299-01
6.0 REFERENCES
- 1. White Paper: Neutron Absorber Performance in Spent-Nuclear-Fuel Storage Racks , Electric Power Research Institute: Palo Alto, California; August 1996.
- 2. Radiation Induced Changes in the Physical Properties of Neutron Absorber , a Neutron Absorber Material for Nuclear Applications , K. Lindquist, D. Kline, and R. Lambert, Journal of Nuclear Materials: Vol. 217, pp. 223-228; 1994.
- 3. Neutron Absorber Test Results and Evaluation , TR-101986, Electric Power Research Institute: Palo Alto, California; February 1993.
- 4. BADGER, a Probe for Nondestructive Testing of Residual Boron-10 Absorber Density in Spent-Fuel Storage Racks: Development and Demonstration , TR-107335, Electric Power Research Institute: Palo Alto, California; October 1997.
Palo Alto, California; May 1998.
- 6. NUSCO Drawing Number 5097M2005, Revision 5.
22
Appendix A BADGER Transmission Traces for Tested Plates
Appendix B Determination of Palisades Areal Density Standards
Appendix C Technical Appendix