FR0202094
Development of a Highly Efficient Burnable Poison Matrix Material for Cycle Lifetime Extension
James S. Tulenko University of Florida 202 Nuclear Science Center P.O. Box 118300 Gainesville, Fl 32611-8300 Phone: 352-392-1401 Fax: 352-392-3380
3/30 Paper Abstract
The University of Florida (UF) is carrying out basic research on a new class of thermally stable boron containing materials that from early indications appear to have special properties that will greatly enhance the performance of Burnable Poison Rod Assemblies (BPRA's) and address one of the major disadvantages of the use of boron shims. The new class of polymer materials, polyacetylenic carboranylsiloxane, termed "Carborane", were developed by Dr. T. Keller of the Naval Research Laboratory (NRL).. Dr. T. Keller is cooperating in this research effort. Other classes of boron containing polymer materials are also under review. Displacement of water by the boron shims incurs an "end of cycle reactivity penalty" since at the end of cycle the moderator coefficient is strongly negative. "Carborane" has the property of being able to contain a tailored amount of boron while maintaining an extremely high hydrogen content, and at the same time being extremely stable to high temperatures and to neutron irradiation. Tests run by the NRL have shown that "Carborane" is stable to about 1000 °C. The high hydrogen and carbon content contained in the "Carborane" Polymer offsets the large fuel cycle reactivity penalty which occurs with current generation BPRA's, as a result of the reactivity loss resulting from the BPRA's displacement of moderator water in the guide tubes of Pressurized Water Reactor (PWR) assemblies. Current generation BPRA's utilize B4C in an AI2O3 matrix. In an attempt to minimize the reactivity penalty from water displacement, Westinghouse has developed a costly annular BPRA, called the Wet Annular Burnable Absorber (WABA) assembly. This burnable poison rod design reduces the moderator displacement by 22% by the use of a central annular water hole. The "Carborane" matrix proposed by the University of Florida reduces the water displacement penalty by 59%, utilizing the hydrogen and carbon present in the "Carborane." In addition to increasing margins, a cost benefit of approximately $500,000 per two-year cycle is projected from reduced enrichments gained from the elimination of much of the water displacement reactivity penalty. 1. Introduction The University of Florida (UF) is carrying out basic research on a new class of thermally stable boron containing materials that appear to have special properties, which will greatly enhance the performance of Burnable Poison Rod Assemblies (BPRA) The new class of polymer material, containing acetylene moieties, siloxane moieties, and carborane moieties termed "Carborane", was developed by Dr. T. Keller of the Naval Research Laboratory (NRL). "Carborane" has the special properties of: 1) containing a tailored amount of boron, 2) an extremely high hydrogen content, and 3) being extremely stable to high temperatures. Development of this material was a major break through in the development of high temperature elastomers. Tests run by the NRL1 have shown that "Carborane" is thermally stable to almost 1000 C. Additionally a similar family member, carborane polysiloxanes, is used as an adhesive for in-reactor bonding of transducers because of its stability with regard to reactor coolant, reactor temperature and reactor radiation levels. Other related boron-containing ceramic precursors have been under development for many years and are being included in our research. Partial ceramification of these classes of preceramic polymers should result in materials with similar properties to the "Carborane" materials. The driving impetus for this research program comes from the large fuel cycle penalty that occurs with the use of current generation BPRA's as a result of an end of cycle reactivity loss due to the BPRA's displacement of moderator water in the guide tubes of Pressurized Water Reactor (PWR) assemblies. Current generation BPRA's, both Westinghouse and Framatome, utilize BPRA's containing B4C in an AI2O3 matrix. In an attempt to minimize the reactivity penalty resulting from the water displacement, Westinghouse has developed a high cost annular BPRA, called the Wet Annular Burnable Absorber (WABA) assembly, shown in Figure 1. Cross Section A-A
. 'JPPES BID PUG ANNULAR PLQUK
OSEUOYTOKS 150.0 TYPICAL HOLMHNN DEVICE
134.0 TYPICAL ANNUUUI MUTTS ABSORBER L£KGTuH
BURNABLE POISON ROD (WABA) CROSS SECTION t CATAWBA NUCLEAR STATION Figure 4.2.2-13 1988 Update
Figure 1 - Westinghouse Wet Annular Burnable Absorber (WABA) assembly. This annular rod reduces the moderator displacement by 22% by the use of a central annular hole shown in Figure 1. However, this is a high cost component because of the double wall, complex welding, and intricate flow channels. UF believes that the use of "Carborane" as a matrix material is much less costly and more neutron effective that the WABA and has even greater advantages over the BPRA's of Framatome. The "Carborane" matrix reduces the water displacement penalty by 59% by utilizing the hydrogen and carbon present in "Carborane." as a moderator material. Initial studies2 have showed that the use of "Carborane" as a burnable poison material: (1) increased fuel cycle length, (2) decreased initial boron chemical shim concentration, and (3) reduced maximum assembly power peaking.
These are all extremely positive results, which will improve both the cost and operational performance of the current generation pressure water reactors. The use of BPRA's are becoming more important as the reactor fuel cycles are being switched over to longer two year cycles that require that burnable poison be used to reduce power peaking and to control reactivity so as to maintain beginning of cycle coolant chemical shim levels at acceptable levels. The new matrix material proposed by the University of Florida, in addition to increasing margins, should generate a cost benefit of approximately $500,000 for a two-year fuel cycle. This cost savings results from the reduced enrichments needed in the fuel. The extra enrichment is no longer necessary to offset the water displacement reactivity penalty. The University of Florida is being funded by the U.S. DOE to research the behavior of Carborane and related boron-containing polymeric ceramic precursors in order to: 1) determine the thermal and irradiation stability of the selected matrix, 2) determine the key physical properties (thermal conductivity, density, coefficient of thermal expansion, etc. 3) determine the cost and ease of manufacture of this material for nuclear use, and 4) perform detailed nuclear and thermal analyses and carry out cost analysis of the use of the selected polymer material as a burnable poison matrix material in the fuel cycle. 2. Review of Boron-Containing Polymers 2.1 Background Over the years, various classes of boron-containing polymers have been reported in the literature. Many of these have been developed for high temperature elastomer or resin composite structural applications. Others have been developed as shapeable or spinable precursors to boron- containing ceramics3. Chief among the high temperature polymer types are the "Carborane"-containing polymers. Olin Mathieson researchers developed a class of polymers based upon "Carboranes" combined with the thermally stable siloxanes4 or silicones early in the sixties under government funding. These polymers were produced by reacting decaborane with acetylene to produce orthocarborane. The orthocarborane was thermally rearranged into meta and para isomers. These isomers were then lithiated and then reacted with chlorosilane endblocked siloxane oligomers to produce monomers with the formula:
Cl(CH3)2SiOCB1oH1oC((CH3)2SiO)nCl.
Polymerization of these monomers led to some of the most thermally stable and robust elastomeric systems ever reported. Elastomeric materials prepared from these polymers were reported to be stable enough to withstand molten aluminum metal. These materials have been developed for adhesives for ultrasonic transducers used to monitor vibrations in nuclear reactors at temperatures above 600 F and in high radiation environments.5 These systems required incorporation of vinyl functionality in the polymer to facilitate cross-linking and cure. Keller and his associates at the Naval Research Laboratories (NRL)6>7'8'9'10 have developed thermoset variations on these polymer systems, which are shown in Figures 2 and 3. NRL has incorporated polyacetylenic functionality into the carboranylsiloxane polymers by reacting dilithiated polyacetylenes with the chlorosilane end blocked monomers shown above. These polymers have the general formula of:
[(B10H10C2)a((CH3)2SiO)b(C2)c]n. The residual acetylenic functionality can be easily cured by heat or radiation to produce very thermally and hydrolytically stable resins. The value of "b" and "c" can be easily varied. This allows for control of the elemental composition of the polymer and the amount of boron in the polymer without loss of thermal stability. This capability to vary the composition is key to the flexibility of the use of these polymers. Other variations on this theme have been reported by Keller and his associates. In these other variations, the decaborane moiety has been replaced with a phenylboron moiety and Dilithiopolyacetylenes were co-reacted with phenyldichloroborane and chlorine endblocked polydimethylsiloxane oligomers to produce polymers with the formula:
(C2)a[(CH3)2SiO)]b(C6H5B)c.
This type of chemistry allows for still further control of the boron content. Keller et al. have patented a process for conversion of acetyleniccarboranolsiloxane polymers, into Si-C-B ceramics. Numerous other patents and publications describe other boron-containing polymer systems that can be converted into boron-containing ceramics. Work presented by Dr. Baney11'12 reviews the chemistry of polymer precursors to ceramics, often called preceramic polymers. These materials are polymers, which can be formed into shapes and then converted into ceramics by pyrolysis. Among these polymeric reactions are Lewis acid adducts of decaborane to form spinnable polymer precursors to B-C-N ceramics. These polymer systems offer potential as a nuclear flux modifier, though their hydrolitic stability in a partially pyrolized state containing hydrogen atoms remains to be determined. Another such class of boron containing polymeric precursors to ceramics is based upon polycarbodi-imides13.. The cured precursors are highly crosslinked and are very thermally stable. Their hydrolytic stability under the conditions of a reactor environment has not been studied. Japanese researchers have very recently reported another boron-containing precursor to ceramics, which would appear, from the structure, to possess the properties beneficial as a nuclear flux moderator polymer. These are polymers prepared by hydroboration polymerization of diynes with mesitylborane. These materials have been supplied to us by their Japanese developers14 for us to evaluate for in-reactor use. Published thermogravimetric analysis (TGA) indicates that the Japanese material behaves as a partially pyrolized residue that should be thermally and hydrolytically stable. Boron containing ceramic precursor materials, along with the "Carborane" class of materials, are being evaluated as potential burnable poison matrix materials. Figure 2 - Poly(Carborane-Siloxane-Acetylene) Thermoset Polymer under Development at the Naval Research Laboratory
200 400 600 800 1000 TEMPERATURE°(C) Fig. 3 - Thermal TGA for Poly (Carborane-Siloxane-Acetylene) 2.2 Choice of Polymer Burnable Poison Matrix Polymers to be utilized, as burnable poison matrix materials must possess the following properties: They need a significant boron content i.e. -10 weight %. They must have a hydrogen atom content of ~ 7 wt % They must be shapeable or moldable to conform to the shape of their containment. The polymers should be capable of curing into a thermoset type of resin. They should have thermal stability of > 1000 F. They must not dissolve or decompose if exposed to the hydrothermal conditions of a reactor. They must be stable against breakdown into volatile species in the radiation environment of a reactor core. They must possess adequate thermal transport properties. The thermoset polyacetylenic carbosiloxanes described by Keller along with the partially ceramified boron containing polysilylcarbodi-imides described by the German researchers and the organoboron polymers described by the Japanese researchers1 offer the most promise of meeting the above list of requirements. The research team is preparing elemental compositional variations, (C:B:H), of these polymers system as described in the literature and evaluating them as potential candidates for nuclear flux moderators. At least three variations in carbon to boron to hydrogen ratios are being prepared for each system. The polymers are being characterized for chemical composition by elemental analysis using X-Ray Photoelectron Spectroscopy (XPS) and Energy Dispersive X-Ray (EDX). Chemical structures are being determined by Fourier transform infrared (FTIR) and solid-state proton nuclear magnetic resonance (!H NMR). Thermal stability is being assessed by TGA. Hydrolytic stability will be determined by heating in a water environment in an autoclave at 650 F and reexamining the specimens by !H NMR, FTIR, XPS and EDS. The polymers will be molded into thin films to demonstrate their ability to be processed. The thin film samples will be used to measure their thermal conductivity properties. Selected polymers will be molded into pellets and cured to demonstrate processability. . The radiation performance characteristics are critical because the boron atoms will be converted by neutron capture to helium and lithium atoms. The effect of the helium atoms on the stability of the polymer matrix is important. The University of Florida Reactor will be utilized to irradiate the selected samples. The samples will be placed in the reactor for a extended period and then removed and examined to determine performance characteristics of the polymer after irradiation taking into consideration helium gas release and matrix swelling. The radial temperature profile of the BPRA rods will be determined considering gap conductance, thermal expansion, thermal conductivity, and neutron and gamma heating of the BPRA matrix material. The thermal performance of the matrix material under these conditions will be evaluated. The nuclear behavior of the chosen matrix materials has been and will be determined using the CASMO nuclear analysis code. Cross-sections were generated in CASMO for use in the fuel management nodal code "EASCYC." The core performance of fuel containing the proposed burnable poison rod assemblies has been determined using the EASCYC nodal fuel management code. The fuel cycle cost benefits of the proposed material versus current generation BPRA was assessed for the Crystal River (177) class of pressurized water reactor. The EASCYC results have confirmed the anaylses extrapolated from CASMO results. Dr. R. Baney has developed a rule of thumb estimate to develop the selling price (cost) of a new polymer material. This prescription is based upon his more than thirty-five years experience in the chemical industry. This formulation has been shown to be extremely good in giving a ballpark estimate of expected costs. The cost formula is S = 3.5Cn, where S is the selling price, C is the raw materials cost and n is the number of processing steps required to get the finished product. Based upon this formula and an analysis of economic benefits a first cut cost /benefit analysis has been performed. The initial costs developed compared very favorablely to current generation cost for BPRA's.
10 Literature References
1 T. Keller et al, US Patent 5780569, Linear Carborane - (Siloxane or Silane) - Acetylene Based Co- Polymers, July 14, 1998. 2 J.S. Tulenko et al, "An Innovative Spectral Shift Burnable Poison Rod Assembly Design," ANS Transaction, Washington, D.C., November 2000. 3 D. Seyferth, Preceramic Polymers - Past, Present and Future, Materials Chemistry, 245: 131-160 (1995). T. Heying, S. Papetti, and O. Schaffling, US patent 3388091 Resins and Elastomers from Siloxy Carboranyl Polymers, Ju. 11 (1968) 5 D. Palmer, US patent 4069083, Bonding Material and Method, Dec. (1975). 6 L. Henderson, T. Keller, Synthesis and Characterization of Poly (carborane-siloxane-acetylene), Macromoleules, 27(1994) 1660 7 D. Son and T. Keller, Oxidatively Stable Carborane-Siloxane-Diacetlyene Copolymers, J. Poly. Sci., Part A, Polymer Chemistry,33, 2969, (1995) 8 T. Keller and D. Son, US patent 5563181, Siloxane UnsaturatedHydrocarbon Based Thermosetting Polymers, Oct. (1996) 9 T. Keller and L. Henderson, US patent 5272237, Carborane-(Siloxane or Silane) Unsaturated Hydrocarbon Based Polymers, Sept. (1994) 10 R. Sunder and T. Keller, Synthesis and Characterization of Linear Boron-Silicon-Diacetylene Copolymers, Macromolecules, 29 (1996)3647. 11 R. Baney, Chemtech, Designing preceramic polymers, 18 (1988) 738. 12 R. Baney and G. Schandra, Encyclopedia of polymer science and engineering, Preceramic polymers, 13(1988)312. 13 M. Weinmann, R. Haung,, B. Joachim, F. Aldinger, J. Schuhmacher and K. Miller, Boron-containing polysilylcarbodi-imides: a new class of molecular precursors for Si-B-C-N ceramics, J. Organometallic Chem., 541, 345, (1997). 14 (a)Y. Chuoj, Y. Sasaki, N. Kinomura andN Matsumi, Polymer, Stable organoboron polymers prepared by hydroboration polymerization ofdiynes with mesitylborane, 41, (2000) 5047.
14 (b) Ube Chemical Company 'Tyrano Python Product Catalog, 2001
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