ML20235N473

Guidelines for Neutron Radioassay Measurements at Quad-Cities Unit One Spent Fuel Storage Pool

ML20235N473
Person / Time
Site:	Quad Cities
Issue date:	01/31/1989
From:	COMMONWEALTH EDISON CO.
To:
Shared Package
ML20235N471	List: ML20235N467 ML20235N473
References
PROC-890131, NUDOCS 8903010236
Download: ML20235N473 (6)

Text

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s GUIDELIN2S FOR NEUTRON RADI0 ASSAY MEASUREMENTS i ,

AT QUAD-CITIES UNIT ONE SPENT FUEL STORAGE POOL

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INTRODUCTION In late July, 1986 Quad-Cities Nuclear Power Station learned that Pt. Beach Nuclear Power Station was observing Boraflex (neutron absorber) degradation.

This was discovered during surveillance testing on their High Density Fuel Racks. It was further learned that the degradation was oggurring in locations that had reached high exposure levels (approximately 1x10 rads). The high exposure levels were attributed to placing the freshly discharged fuel in the same location each outage.

Because of the similarity in operating practice, Quad-Cities was concerned about the integrity of the racks and criticality of the pool. This prompted

. Quad-Cities to examine the condition of the Reload High Density Fuel Racks in the Unit one fuel pool. The reload racks are used to accommodate the freshly discharged fuel each refueling outage as compared to the discharge racks which are used for long term storage. Therefore, these reload racks will reach higher exposures and see more thermal cycles than the discharge racks. The testing showed that Anomalies (gaps) had formed in the High Density Fuel Racks and, as expected, the reload racks contained the most and largest gaps. In addition, planned future testing will be performed in the Unit One fuel racks because they have reached higher exposure than the Unit Two racks.

This data was used with theoretical behavior data to formulate a conservative Gap Growth Model. This model, along with conservative assumptions, was used in a criticality analysis of the fuel pool. The results of the analysis showed that the criticality of the spent fuel pool would never reach or exceed the 0.95 as long as the Gap Growth Model Technical was bounding Specification Limit and each fuel of K,gg bundle was= limited to a maximum K of less than 1.26. Therefore,CommonwealthEdisonmustcontinuetomSnkEo!Ythe gap growth in the High Density Fuel Racks until such time that sufficient data exista (f rom CECO or othtr utilities) to prove the Cap Growth Model conserva-tively predicts the behavior of the Boraflex in the racks.

The purpose of this document is to provida guidelines that will form the basis of a surveillance program, in addition to the coupon surveillance, to verify Technical Specification compliance.

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SURVEILLANCE OBJECTIVE:

i The objective of this surveillance is to verify the Technical Speci-fication Lf 7, K of the spent fuel pool is less than 0.95 now and at any timt th$I future.

To meet this objective, the surveillance program should include the  !

following processes:

1. Select sample size to examine and set surveillance interval.

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2. Collect gap size and location data on the sample.

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3. Trend the gap and location data.

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4. Verify the Gap Growth Model is bounding.

SAMPLE SIZE SELECTION AND SURVEILLANCE INTERVAL:

Commonwealth Edison evaluated two testing methodologies. The first test method will examine a minimum of 25 percent of reload cell panels prior to the next 4 Unit One outages. This testing will involve the Standard Test and Special Test Methods. The second method involves a statistical approach and the use of the Special Test Method only.

The first method is based on verifying the continued validity of the Gap Growth Model and the safety evaluation that is based on this model.

Beginning in the second quarter of 1989 (tentatively scheduled for May, 1989) and after the next three refueling outages, the Standard Test will be employed to examine a minimum of 25 percent of reload cell panels.

This will continue until all panels in the reload racks are examined once, or the conservative Gap Growth Model is shown to bound the gap distribution. This data will be used to verify the largest gaps are being trended and will provide information to evaluate the number of additional gaps to be trended. <

The Special Test Method will be used to trend a minimum of 50 to 75 gaps.

This number will vary depending the number observed in the Standard Test Method. These trends will provide the necessary information to show the Gap Growth Model conservatively predicts gap growth.

The second method involves a rigorous statistical approach. This test will not directly verify that the Gap Growth Model bounds the gap distri-bution and will require a large amount of full panel testing. In each panel there are 14 - 10 inch segments each 10 inch segment would take 200 seconds to test. Therefore, this type of testing would take approximately 4 to 5 months to complete as compared to 3 weeks for the method one testing.

i Commonwealth Edison believes that both methods listed above are acceptable ways of providing Technical Specification compliance. However, the first method will provide a direct method of verifying the conservative Gap Growth Model. Therefore, Commonwealth Edison will use the first method in selecting a sample size for testing.

s DATA COLLECTION:

Onc'e the sample size is determined, the data will be collected employing the same non destructive testing methods used in the past (Standard Test Method and Special Test Method).

The non destructive testing is based on the physical principle known as backscattering. This principle involves highly energetic neutrons that are emitted from a neutron point source. The fast neutrons are trans-mitted through the neutron detectors and the adjacent cell walls without interaction. Once the fast neutrons enter the water in the adjacent cells they lose energy due to collisions with the water molecules (thermalization). As these neutrons collide and lose energy, they are also reflected back towards the detectors. The thermal neutrons reflected back will be absorbed by the Boraflex which significantly atter.uates thermal neutrons. However, if Boraflex is absent from a portion of the cell wall the thermalized neutrons will pass through the wall and interact with the detectors, yielding a noticeable increase in detector count rate.

1. STANDARD TEST METHOD This testing equipment consists of a Stainless Steel housing which hold an energetic neutron source and four thermal neutron detectors.

Each one of the detectors are located adjacent to one of the four cell walls and have an active length of 8 inches. The detectors are moved at a continuous rate from the top of the cell to the bottom then oack to the top. The count rate of each of the detectors are recorded on a chart recorder. This method provides an estimate of the size and location of the gaps. The results will be used to estimate gap size and location and provide input = to which gaps will be trended using the Special Test Method.

2. SPECIAL TEST METHOD (CRA ACTERIZATION)

This testing method is based on the same Backscattering principle as the standard test method. However, this method provides a statistically accurate representation of the anomaly. The increase in accuracy is attributed to the geometry of the two Helium 3 pro-portional counters (detectors) used in this test. The detectors are placed equal distance apart on the face of the detector housing.

These detectors are wrapped in lead and Cadmium (Cd). The lead is used to reduce the possibility of gamma interaction from the spent fuel in the pool. The cadmium sheet (which absorbs thermal neutrons) contains a one-half inch high window which is located at the front center of the detector. This window essentially reduced the active length of the detector to one-half inch.

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3. BENCHMARKING:

The purpose of Benchmarking is to verify the accuracy of the testing equipment while obtaining data that could nelp identify the anomaly sizes better. This test involves performing the above tests in a special simulated high density fuel cell. This cell, known as the Benchmark cell, contained known defects to which the detectors accuracy could be tested. At the beginning of the test, the Benchmark cell is used to verify that the detectors will be able to accurately characterize the known defects. At the completion of the test, it is used to verify the data in the Special Test Method was in agree-ment with the initial data. Once the Benchmarking and the testing is complete, the test daia will be analyzed.

TRENDING THE DATA:

Once the gap size and location data is received, the data will be placed in a data base. The data base will provide a record of gap growth and exposure. Commonwealth Edison will then use this data to compare to the Gap Growth Model to verify the validity of the safety evaluation and continued Technical Specification compliance.

EVALUATION OF DATA: n The trend data will be evaluated against Figure 1, Gap Size vs. Exposure, to verify the Gap Growth Model bounds the gap distribution. This project will involve Nuclear Fuel Services and station personnel. Once complete the surveillance interval will be re-evaluated to reflect the current data. ,

FIGURE 1 5.0 :  ; .g

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l 108 108 102 10" Gamma Exposure, Rads MAXIMUM CUMULATIVE GAP SIZE VERSUS EXPOSURE

REFERENCES:

"1. " Criticality Safety Evaluation of Boraflex Degradation in the Quad-Cities Spent Fuel Storage Racks", Southern Science Report SS-167, June, 1987.

2. "An Assessment of Boraflex Performance in Spent-Nuclear-Fuel Storage Racks", EPRI Report NP-6159, December, 1988.

3. " Preliminary Assessment of Boraflex Performance in the Quad-Cities Spent Fuel Storage Racks", Northeast Technology Report-NET-0420-01, April 10, 1987.

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