Spectral Shift Control for Pressurized Water Reactors

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1 Spectral Shift Control for Pressurized Water Reactors Yigal Ronen and Gilad Raitses Department of Nuclear Engineering Ben Gurion University of the Negev Beer Sheva 84105, Israel Long term control in Pressurized Water Reactor (PWR) is obtained either by soluble poison or by burnable poison. From the neutronic utilization aspect, control by absorbing neutrons in a poison is a waste. Better utilization of the neutrons is achieved if the neutrons would be absorbed by fertile isotopes in order to produce fissile nuclei. These fissile nuclei would contribute to obtain more energy from the fuel. Control of a reactor by absorbing neutrons in the fissile nuclei can be obtained by the so-called spectral shift control. Which a spectral shift, the hard neutron spectrum is achieved at the Beginning Of Life (BOL) and the soft neutron spectrum at the End Of Life (EOL). Several suggestions for spectral shift control were suggested and implemented [1-11]. In this paper a spectral shift control for PWR will be presented. The control is obtained by cooling the reactor with a mixture of gas (helium or CO 2 ) and water. The amount of the moderator is determined by the amount of the gas. Since the volume is constant, introducing gas into the moderator causes the density of the water to change. This study is limited to the neutronic aspect and does not deal with all the thermohydraulic and cooling aspects of the suggested two phase flow. This study is only first step in order to determine the neutronic benefits of using such an approach. In case that, the neutronic benefits are large enough, it will stimulate an investigation related to the other aspects of this suggested control. This control has been applied to a PWR, the parameters of which are presented in Table 1 and Fig. 1. Calculations were performed by the BOXER [12], the code performs cell and twodimensional transport and depletion calculations. The cross sections library is based on ENDF/B (up to ENDF/B 5) and JEF-1 data. The structure of the present library based on 70 energy groups and contains 162 nuclides and mixtures. Most of them are taken from JEF-1. The regular PWR fuel rod geometry and composition were chosen for these calculations (see fig.1). The results obtained from BOXER are received from cell unit calculations. The number of energy group used in this work is 9, and 4 of them are thermal groups. During the calculations the buckling was taken in account in order to account for the leakage from the reactor core. The buckling is cm -2. 1

2 Parameter Table 1. Core and Fuel Assembly Parameters Total Power Output, MWth 3,400 Total Core Flow Rate, Mg/sec Core Inlet Temperature, o C Average Coolant Temperature, C 306 Average Coolant Pressure, bar 155 Fuel Assembly Size, cm 21.4 Number of Fuel Assembly 193 Fuel Material Composition UO 2 U enrichment 3.5% Number of Fuel Rods 264 Average Fuel Temperature, o C 650 Fuel Pellet Radius, cm Gas Gap thickness, cm Cladding thickness, cm Fuel Rod Radii, cm Fuel Cell Pitch, cm 1.26 Moderator/Fuel Volume Ratio cm (0.950 cm) Clad cm (0.836 cm) Gap cm (0.819 cm) Fuel 3.5 % of UO2 Fig. 1: Fuel Rod Design: Geometry and Material Composition 2

3 Regular PWR fuel (3.5 % enriched UO2) void case reference case k effective % of void % of void 50 % of void 37 % of void 22 % of void 0 % of void ,500 5,000 7,500 10,000 12,500 15,000 17,500 20,000 22,500 25,000 27,500 30,000 32,500 35,000 burnup, MWd/t Fig. 2: k effective as a function of the burnup. Table 2. EOL Material Composition (isotopes / barn cm) Isotope Reference Case Void Case U E E-04 U E E-04 U E E-02 Pu E E-06 Pu E E-04 Pu E E-05 Pu E E-05 Pu E E-05 Pu E E-09 Pu E E-10 Am E E-06 Am E E-06 Cm E E-07 Cm E E-07 Cm E E-08 3

4 The control was presented by a change in the density of the water starting with 60 % void (gas in volume) up to 0 % void at EOL. The results are presented in Fig.2 and Table 2 and compared to the regular change in reactivity (the reference case). From Fig. 2 it can be seen that for a one batch consideration, the burnup with spectral shift control is 34,500 MWd/t compare to an upper limit of 29,000 MWd/t in a poison control. Namely the spectral shift control increases the burnup by 19 %, namely from the same amount of fuel in the PWR, 19 % more energy can be obtained. This relatively large increase in the fuel utilization is a promising one which suggests that further investigations are needed. Reference: 1. D. MARS et al., Spectral Shift Control Reactor Design and Economic Study, BAW-1241, Badcock and Wilcox (1961). 2. M. C. EDLUND, Developments in Spectral Shift Reactors, Proc. 3 rd U.N. Conf. Peaceful Uses Atomic Energy, Vol. 6, p.314, United Nations, New York (1964). 3. J. STORRER and S. RIGG, The Vulcain Core Power Experiment,, Proc. 3 rd U.N. Conf. Peaceful Uses Atomic Energy, Vol. 6, United Nations, New York (1964). 4. R. L. HELLENS, R. A. MATZIE, G. MENZEL, and N. L. SHAPIRO, Reactor Design Based on the Spectral Shift Control Concept, Trans. Am. Nuc. Soc., 28, 574 (1978). 5. R. A. MATZIE and F. M. SIDER, Evaluation of Spectral Shift Controlled Reactors Operating on the Uranium Fuel Cycle, EPRI NP-1156, Electric Power Research Institute (1979). 6. Y. RONEN and A. GALPERIN, A Comparison between Spectral Shift Control Methods for Light Water Reactors, Ann. Nucl. Energy, 7, 1, 59 (1980). 7. F. CORREA, M. J. DRISCOLL, and D. D. LANNING, An Evaluation of Tight-Pitch PWR Cores, MITNE-227, Massachusetts Institute of Technology (1979). 8. A. GALPERIN and Y. RONEN, Application of the Variable Water Content Method of Reactivity Control for Pressurized Water Reactors, Trans. Am. Nucl. Soc., 43, 567 (1982). 9. A. GALPERIN and Y. RONEN, Modified Fuel Assembly Design for Pressurized Water Reactors with Improved Fuel Utilization, Nucl. Technol., 62, 238 (1983). 10. W. D. LEGGET, Advances in Nuclear Power, Proc. 2 nd Joint ASME-ANS Nuclear Engineering Conf., Portland, Oregon, July 26, Y. RONEN and Y. FAHIMA, Combination of Two Spectral Shift Control Methods for Pressurized Water Reactors with Improved Power Utilization, Nucl. Technol., 67, 46 (1984). 4

5 12. J. M. PARATTE, K. FOSKOLOS, P. GRIMM and C. MAEDER, Das PSI Codesystem ELCOS zur stationaren Berechnung von Leichtwasserreaktoren, Proc. Jahrestagung Kerntechnik, Travemunde, Germany, May 17-19, p.59,

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