ML20006E558
| ML20006E558 | |
| Person / Time | |
|---|---|
| Site: | Point Beach  |
| Issue date: | 02/21/1990 |
| From: | Office of Nuclear Reactor Regulation |
| To: | |
| Shared Package | |
| ML20006E557 | List: |
| References | |
| NUDOCS 9002230559 | |
| Download: ML20006E558 (5) | |
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UNITED S TATES i ~.
E' NUCLEAR REGULATORY COMMISSION
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SAFETY EVALUATION BY THE OFFICE OF NUCLEAR REACTOR REGULATION t
RELATED TO THE PP0P05ED BORAFLEK SURVEILLANCE PROGRAM l
WISCONSIN ELECTRIC POWER COMPANY POINT BEACH NUCLEAR PLANT, UNIT N05. 1 AND 2 DOCKET N05. 50-266 AND 50-301
- 1. INTRODUCTION On April 4, 1979, the staff issued a safety evaluation report approving the surveillance program for Boraflex to be used as a neutron attenuation material.in the spent fuel storage pools of the Point Beach Nuclear Plant, Unit Nos. I and 2.
The surveillance program would verify the continued integrity of the Boraflex material in the fuel storage racks.
In this program, freshly discharged fuel assenblies are placed next to 2-inch by 2-inch Boraflex coupons to provide maximum irradiation to the Boraflex.
l The Boraflex coupons will be examined at intervals of 2, 5, and 10 years.
Changes to the examination intervals may be made depending on the results of the early examinations. The examinations include neutron radiography, neutron attenuation test, weight and thickness measurements, hardness test, and isotopic analysis of the Boraflex samples.
On February 11, 1987, the Wisconsin Electric Power Company (the licensee) reported to NRC the results of the Boraflex examinations that were performed in 1985 and 1986. Ten Botaflex coupons were found to have significant decreases in semple thickness, width, and weight, and they were fragile and easily broken. The cumulative gamma radiation doses that these Boraflex coupons received range from 1.0E10 (1.0 times 10 raised to the tenth power) to 1.6E10 rads. However, the neutron attenuation capability of these coupons was not significantly reduced. Two full-length Boraflex inserts, one of which received no radiation while the other received a cumulative gan:na radiation dose of 1.0'i10 rads, showed no degradation other that son
' discoloration along the edges of the irradiated insert.
The licensee explained the different results between the small Boraflex coupons and the fulblength Boraflex inserts by the differences in the encapsulation methods of Boraflex, sample geometry, and sample handling.
In any case, the licensee stated that the small coupons are not representative of the full-length Boraflex sheets.
As a result of the observed degradations in the Boraflex coupons, the licensee proposed, on April 13 and November 1, 1989, to replace the existing survelliance procedure, REl-25 " Spent Fuel Rack Neutron Absorbing Material Surveillance Specimen Program," with a new surveillance program. The new program uses neutron attenuation measurements to examine 10 full-length Boraflex panels selected from those that have been exposed to the greatest number of freshly discharged fuel assemblies at the time of the surveillance.
These 10 panels include 4 panels with accelerated exposures and 6 panels selected at random. This surveillance will be repeated at 5-year intervals.
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2-If degraded Boraflex is found, new fuel assemblies or spent fuels with a burnup less than 38,400 MWD /MT will be stored in a designated area in the fuel storage pool in a checkerboard pattern.
- 2. EVALUATION Nuclear reactor plants provide storage facilities nr pools for the wet i
storag2 of new and spent fuel assemblies. The fuel storage pools must i
mainttiin the stored fuels in a subcritical array during all credible storage conditions.
In the fuel pools where the fuel assemblies are closely arranged (the so-called high-density fuel storage pools), neutron absorbing materials i
1 (poisons)areusedtomaintainsubtriticality. Sheets of Boraflex a methylated polysiloxane elastomer uniformly filled with finely divided boron carbide powder, are often used as poisons in the fuel storage pools. Since 1980, 23 commercial nuclear power plants in the United States have been using Boraflex as the neutron absorbing material in the fuel storage pools.
In pressurized water reactor plants, boron is also added as a neutron absorbing material to the water in the fuel storage pools, i
In the fuel storage pools of the Point Beach Nuclear Plant, Unit Nos. 1 and 2, the fuel storage racks consist of individual square cross-section, rectangular cylinders with an outer dimension of 9.98 inches and a length of about 14 feet. The rectangular cylinders are edge-welded to form a honeycomb structure.
Two contiguous poison compartments are formed inside each of the storage cylinders by welding in five pieces of right-angle stock. These angle pieces form two 1.53-inch by 8.29-inch spaces, perpendicular to each other, into which two poison assemblies are inserted.
Each poison assembly supports two Boraflex sandwiches.
Each Boraflex sandwich consists of a Boraflex sheet (8 inches wide, 145 inches long, and O.11 inch thick) sandwiched between two stainless steel plates. The sandwich is seam welded at all edges except for the venting provisior.s. The venting provisions allow escape of any gas which may be generated from the polymer binders in Boraflex during heating and irradiation, thus preventing possible bulging or swelling of the Boraflex sandwiches. The two Boraflex sandwiches are separated from each cther by 1.06 inch of water in the fuel storage pool. Thus, there are two Boraflex sandwiches between any two stored fuel assemblies in the fuel storage pool.
The water in the fuel storage pools contains 1800 parts per million of boron.
The pool water temperature is maintained so as not to exceed 145 degrees Farenheit.
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Boraflex has undergone extensive testing to determine the effects of gama and neutron irradiation in various environments and to verify its structural integrity and suitability as a neutron absorbing material (Bisco Products, Inc., Technical Report No. NS-1-001, " Irradiation Study of Boraflex Neutron 1
Shielding Materials," Revision 1. August 12,1981). The evaluation tests have shown that Boraflex is unaffected by the environment of fuel storage pools and will not be degraded by corrosion.
Boraflex in a solution of 2000 parts per million of boron was exposed to a cumulative gamma radiation dose of 1.03E11 rads with a concurrent neutron flux of 8.3E13 neutrons / centimeter square /second. The results show that Boraflex maintains its neutron attenuation capabilities in the environments of borated water and gamma and neutron radiation. However, irradiation caused some loss of flexibility and shrinkage of the Boraflex material.
Long-term soaking of Boraflex in borated water at high temperatures was also performed (Bisco Products Inc., Technical Report No. NS-1-002, "Boraflex NeutronShieldingMaterialProductPerformanceData." August 25,1981). The test results show that Boraflex withstands a temperature of 240 degrees Farenheit in a solution of 3000 parts per million of boron for 251 days witht,ut visible distortion or softening. The Boraflex showed no evidence of swelling or loss of ability to maintain a uniform distribution of boron carbide. The temperature of the fuel storage pool water at the Point Beach Nuclear Plant under normal operating conditions is 120 degrees Farenheit and will not exceed 145 degrees Farenheit under credible conditions. These temperatures are well below the 240 degrees Farenheit of the test temperature.
In general, the rate of a chemical reaction decreases exponentially with decreasing temperatures. Therefore, the staff does not anticipate any significant deterioration of the Boraflex due to high temperatures and borated water at the Point Beach Nuclear Plant under normal operating conditions over the design life of the fuel storage racks.
The tests (ibid) also show that neither irradiation, environment, nor l
Boraflex composition has a discernible effect on the neutron attenuation of l
the Bore? lex material. The tests show that Boraflex does not possess leachable halogens that might be released into the pool envirenment in the presence of L
l radiation. Similar conclusions are reached regarding the leaching of element boron from the Boraflex.
Boron carbide of the grade ncrmally present in the Boraflex typically contains 0.1 weight percent of soluble boron. The test results have confirmed the encapsulation capability of the silicone polymer matrix to prevent the leaching of soluble species from the boron carbide.
Recently, anomalies and degradations of Boraflex were observed in two nuclear power plants that use Boraflex in the fuel storage pools.
In addition to the minor physical changes in color, size, hardness, and brittle-l ness reported by the licensee, gaps or physical separations of up to 4 inches were discovered in the Quad Cities Station, Unit Nos. 1 and 2 (North-east Technology Corp, Report No. NET-042-01, " Preliminary Assessment of I
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w r Boraflex Performance in the Quad Cities Spent Fuel Storage Racks," April 10, 1987). Gaps up to 1.4 inch were observed in the Grand Gulf Nuclear Station, Unit No.1 (Letter from John G. Cesare, Jr., System Energy Resources, Inc.,
to Lester L. Kintner, U. S. NRC, dated November 21,1988).
The exact mechanisms that caused the observed physical degradations of Boraflex have not been confirmed. The staff postulates that gamma radiation from the spent fuel assemblies initially induced cross-linking of the polymer in Boraflex, producing shrinkage of the Boraflex material. When cross-linking became satu-rated, scissioning (a process in which bonds between atoms are broken) of the i
polymer pre M N ted as the accumulated radiation dose increased. Scissioning produced pore 1V which allowed the water in the fuel storage pools to permeate the Boraflex matrix. Scissioning and water permention could embrittle the Bora-flex material.
In short, gama radiation from spent fuels is the most probable cause of the minor physical degradations of Boraflex, such as the changes in color, size, hardness, and brittleness that were observed in the licensee's surveillance coupons. The staff does not have sufficient information to deter-mine conclusively what caused the gap formation in some Boraflex panels of the other two nuclear reactor plants. However, it is conceivable that if the two ends of a full-length Boraflex panel are physically restrained, then shrinkage caused by gamma radiation can break up the panel and lead to gap formation.
Subsequent to the reports on degraded Boraflex by the licensee and the Quad Cities Station, NRC issued Information Notice No. 87-43, "Gops in Neutron-absorbing Material in High-density Spent Fuel Storage Racks," on September 8, 1987, notifying nuclear power plant licensees of the potential gap formation l
of Boraflex. The Electric Power Research Institute published the report. EPRI NP-6159, "An Assessment of Boraflex Performance in Spent-nuclear-fuel Storage Racks," in December 1988, summarizing the behavior of Boraflex under various test conditions and in nuclear power plants under field conditions.
The Bisco Products, Inc. published additional test data on Boraflex in Technical Report No NS-1-050 (Interim), " Irradiation Study of Boraflex Neutron Absorber Interim Test Data," Revision 1, Nnvember 25, 1987. The interim test data show that shrinkage of Boraflex of up to 3.4 percent in length and up to 4.5 percent in width appears to have saturated at an accumulated gamma radiation dose of IE11 rads. The licen!,ee states that the average total exposures of Boraflex panels in the Point Beach Nuclear Plant range from 1.8E10 to 2E10 reds in the l
design plant life of 40 y:ars.
The staff determined that reasonable assurance exists that physical restraints are absent in the Boraflex panels of the Point Beach Nuclear Plant, because the Boraflex sheets are not physically fastened to or permanently glued onto any structure. Although shrinkage of the Boraflex sheets may occur, it is not likely that gaps will form in any significant extent during the projected life of the Boraflex assemblies. The fact that the full-length Boraflex insert, which had received an exposure of IE10 rads of gamma radiation, showed no gaps j
supports this finding.
Minor physical degradations can take place in the i
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-s-Boraflex from irradiation of the spent fuel assemblies. However, these minor degradations should not lead to significant loss of neutron absorbing capability of the Boraflex panels.
In the unlikely event of gap formation in the Boraflex panels that would affect the neutron attenuation capability, the surveillance program should detect such degraded Boraflex panels. Hence, the licensee should have sufficient time to perform the corrective actions.
- 3. CONCt.USION On the basis of the above evaluation, the staff concludes that the proposed surveillance' program for monitoring the physical integrity of the full-length Boraflex panels in the fuel storage racks can reveal deterioration that might lead to loss of neutron attenuation capability during the design life of the fuel storage racks.
If significant loss of neutron attenuation is found in any Boraflex panel, the licensee can take the corrective actions to ensure subtriticality of the stored fuel assemblies. The staff, therefore, concludes that the proposed surveillance program and corrective actions are acceptable.
Principal Contributor: Dr. J. Wing Dated: February 21, 1990 l
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