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 <[email protected]> Dr. Ronald H. Baney University of Florida 100 Rhines Hall P.O.Box 116400 Gainesville, Fl 32611-6400 Telephone: (352) 392 5167 Fax: (352) 392 6359 <rbane@mse .ufl. cdu> Ms Linda Pressley University of Florida 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.