Implementation of vented fuel assemblies in the supercritical CO₂-cooled fast reactor
Author(s)McKee, Stephanie A
Massachusetts Institute of Technology. Dept. of Nuclear Science and Engineering.
Michael J. Driscoll.
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Analysis has been undertaken to investigate the utilization of fuel assembly venting in the reference design of the gas-cooled fast reactor under study as part of the larger research effort at MIT under Gen-IV NERI Project No. 05-044, "Optimized Competitive Supercritical-CO₂ Cycle GFR for Gen-IV Service." The focus of this work is on selection and optimization of a fuel assembly configuration best suited for venting, assessment of the radionuclide release of such an assembly design, and identification of plant systems which must be altered in order to support fuel venting. The innovative tube-in-duct design of the reference GFR fuel assembly is particularly well suited to venting, allowing fission products to diffuse into a common header before being routed along a vent path and eventually being released to the primary coolant system. A set of equations were developed which model the transport of fission products from fuel to vent path to primary coolant inventory and then into the containment atmosphere, with emphasis on conservatism in calculations of coolant impurity levels. Using these equations in a computer code, the lengthy list of radioactive and chemically volatile fission products for study was reduced to only fifteen species of any great concern. Of these, 85Kr and 137Cs were considered conservative bounding cases on the behavior of the other nuclides. The chemistry of the fission products released to the coolant was explored. In particular, reactions between fission products and their surroundings were identified, and estimates of deposition of both compounds and free atoms on steel surfaces were made. Investigation of reactions between fission products and CO₂ revealed that the formation of stable oxides would lead to deposition of most volatile species within the fuel assembly's debris trap.(cont.) The radioactive volatiles which remain in the primary comprise only tritium, two isotopes of iodine, two isotopes of tellurium, and three isotopes of cesium. These species deposit on primary surfaces to a great enough degree to preclude hands-on primary system maintenance. In addition, techniques for removal of volatile and radioactive species from the primary coolant were identified, and a scheme for purification of the primary coolant volume was selected. The analysis confirms that primary coolant activity can be maintained at acceptable levels when purification of the primary coolant volume occurs once per hour. The response of the vent system to changes in primary system pressure was also investigated. In particular, the small periodic pressure transients known as breathing were studied, as were loss-of-coolant type scenarios. Both cases were investigated using hand calculations and computational techniques, and the radiological consequences of each were addressed. The analysis demonstrated the ability of the vented fuel assembly to quickly equalize pressure in the event of a sudden drop in operating pressure, and also showed the ability of a succession of plena to prevent the expulsion of much of the activity inventory of the plena during a breathing transient. Conversely, the use of a single plenum results in reduced activity release during a LOCA. After consideration of both results, a two-plenum approach, with a large lower annular plenum following a small upper axial plenum, was selected as the final design.
Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Nuclear Science and Engineering, 2008."April 2008."Includes bibliographical references (p. 144-147).
DepartmentMassachusetts Institute of Technology. Dept. of Nuclear Science and Engineering.
Massachusetts Institute of Technology
Nuclear Science and Engineering.