NUREG/CR-0674 Benchmark Critical Experiments on Low-Enriched Uranium Oxide Systems With H/U = 0.77 Topical Report on Reference Critical Experiments Prepared by G. Tuck, I. Oh Rockwell International Energy Systems Group Prepared for U. s. Nuclear Regulatory Commission • la NOTICE This report was prepared as an account of work sponsored by an agency of the United States Government. Neither the United States Government nor any agency thereof, or any of their employees, makes any warranty, expressed or implied, or assumes any legal liability or responsibility for any third party’s use, or the results of such use, of any information, apparatus product or process disclosed in this report, or represents that its use by such third party would not infringe privately owned rights. Available from National Technical Information Service Springfield, Virginia 22161 DISCLAIMER This report was prepared as an account of work sponsored by an agency of the United States Government. 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NUREG/CR-0674 RFP-2895 Dist Code RC BENCHMARK CRITICAL EXPERIMENTS ON LOW-ENRICHED URANIUM OXIDE SYSTEMS WITH H/U = 0.77 Topical Report on U. S. Nuclear Regulatory Commission Reference Critical Experiments - NOTICE- This report was prepared as an account of work Grover Tuck sponsored by the United States Government. Neither the United States nor the United States Department of Inki Oh Energy, nor any of their employees, nor any of their contractors, subcontractors, or their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness or usefulness of any information, apparatus, product or process disclosed, or represents that its use would not infringe privately owned rights. Rockwell International Energy Systems Group Rocky Flats Plant P. O. Box 464 Golden. Colorado 80401 Date Published; August 1979 Prepared for Division of Safeguards, Fuel Cycle, & Environmental Research Office of Nuclear Regulatory Research U. S. Nuclear Regulatory Commission Washington, D.C. 20555 under U.S. Department of Energy Contract DE-ACO4-76DP03533 NRC FIN NO.A1036 NUREG/CR-0674 RFP-2895 (iii) ABSTRACT Ten benchmark experiments were performed at the Critical Mass Laboratory at Rockwell International’s Rocky Flats Plant, Golden, Colorado, for the U. S. Nuclear Regulatory Commission. They provide accurate criticality data for low-enriched damp uranium oxide (U3 0 g) systems. The core studied consisted of 152 mm cubical aluminum cans containing an average of 15,129 g of low-enriched (4.46% uranium oxide compacted to a density of 4.68 g/cm^ and with an H/U atomic ratio of 0.77. One hundred twenty five (125) of these cans were arranged in an ~ 770 mm cubical array. Since the oxide alone cannot be made critical in an array of this size, an enriched (~ 93% 235u) metal or solution "driver" was used to achieve criticality. Measurements are reported for systems having the least practical reflection and for systems reflected by 254-mm- thick concrete or plastic. Under the three reflection con­ ditions, the mass of the uranium metal driver ranged from 29.87 kg to 33.54 kg for an oxide core of 1864.6 kg. For an oxide core of 1824.9 kg, the weight of the high concen­ tration (351.2 kg U/m^) solution driver varied from 14.07 kg to 16.14 kg, and the weight of the low concentration (86.4 kg U/m^) solution driver from 12.4 kg to 14.0 kg. NUREG/CR-0674 RFP-2895 (v) SUMMARY Hie benclni^rk critical experiments program Is sponsored by the Nuclear Regulatory Commlss^ion to prorlde accurate criticality data for low-enriched damp uranium oxide CUgOg) systems. Sfeny appilcations of these data to nuclear criti­ cality safety questions are useful In the nuclear Industry. The benchmarfe critical parameters can provide a validated calculatlonal method to nuclear criticality safety engineers for use as a guide for specifying a criticality limit. The result of this experimental program Is the measure­ ment of ten critical configurations wherein both geometry and material parameters are specified. Uranium oxide used In this program was enriched to 4.46 wt-% 2^^U, compacted to a density of 4.68 g/cm^, and packed into 1.6-mm-thlck aluminum cans forming a 152 mm cube. Water was added to the oxide until an H/U atomic ratio of 0.77 was achieved. The experimental core studied was a 5 x 5 x 5 array of aluminum cans, each containing an average of 15,129 g of uranium oxide. Since the oxide alone cannot be made critical in this size array, a high-enriched 93% 23 5u) uranium driver, which replaced one to four cans near the core’s center, was used to achieve criti­ cality. The drivers were an enriched uranium metal sphere, high concentration (351.2 kg U/m^) solution, and low con­ centration (86.4 kg U/m^) solution. NUREG/CR-0674 RFP-2895 (vi) Three reflector conditions were studied. The miniaally- reflected case had the least practical reflector, made of 6 .35-mm-thick steel plate to contain the oxide cans. The other two reflector conditions had the critical configuration within thick-walled cubical shells composed of concrete or plastic, both conmH!>n materials used in the nuclear industry- For both reflectors, interior dimensions of the shell were ~ 770 nun, and the reflector thicknesses were ~ 254 mm for the two materials. All experiments were performed on a horizontal split table. Each half of the table supported a portion of the experimental assembly, and the critical approach was made by decreasing the separation between the two portions of the core. Under the three reflection conditions, the critical core separation ranged from 6.3 mm to 14.6 mm for the following oxide and driver masses: the mass of the uranium metal driver ranged from 29.87 kg to 33.54 kg for an oxide mass of 1864.6 kg; the mass of the high concentration solution driver ranged from 14.07 kg to 16.14 kg for an oxide mass of 1824.9 kg; and the mass of the low concentration solu­ tion driver ranged from 12.4 kg to 14 kg for an oxide mass of 1824.9 kg. NUREG/CR-0674 RFP-2895 (vii) TABLE OF CONTENTS Page ABSTRACT ----------------------- iii SUMMARY ------------------------------------ V LIST OF F I G U R E S -------------------- ix LIST OF TABLES --------------------------- xi ACKNOWLEDGMENTS ---------------------------- xiii PREVIOUS REPORTS -------------------------------- xv INTRODUCTION -------------------------------------------- 1 EXPERIMENTAL PROCEDURE ------------------ 7 OXIDE C A N S --------------------------------------- 13 Oxide Compaction ----------------- 13 Aluminum Cans ------------------ 13 Packing of Oxide and Water into Cans ------- 17 Material Description --------------- 26 Determination of H/U Value ------------ 32 METAL AND SOLUTION DRIVERS ---------------------------------39 Metal Driver ------------------- 3 9 Solution Driver ----------------- 39 HORIZONTAL SPLIT TABLE --------------------------------- 53 REFLECTORS ------------------------------------------ 61 Concrete Reflector ---------------- 61 Plastic Reflector ---------------- 64 Steel and Environmental Reflectors -------- 70 CORE DIMENSIONS ------------------------------- 83 CRITICAL RESULTS ------------------------------------------- 93 Critical Parameters --------------- 93 Concrete-Reflected Experiments ---------- 95 Plastic-Reflected Experiments ---------- 96 Minimally-Reflected Experiments --------- 98 DISCUSSION OF UNCERTAINTIES ----------------------------- 103 REFERENCES -------------------------------------------------- 105 NUREG/CR-0674 RFP-2895 (ix) LIST OF FIGURES Figure Number Title 1 Experimental assembly of low-enriched oxide cans on the north and south halves of the horizontal split table in the concrete shells with the end reflectors removed. 2 Reciprocal multiplication with the table closed versus the mass of high concentration (351.2 kg U/m^) solution driver. 3 A compacted block of uranium oxide resting on a plastic sheet. 4 Sketch of the aluminxm cans which contain the uranium oxide, 5 A padked and sealed oxide can, 6 A photograph of the special oxide can packed and sealed. 7 Graph of oxide weight versus the number of days elapsed for each weighing since March 1, 1978. 8 Typical metal driver assembly and its supporting equipment. 9 An assembled uranium metal driver which will be positioned in the empty space in the oxide cans when the horizontal table is closed. 10 One pair of solution driver cans: one for the south half-table and the other for the north half-table. 11 The horizontal split table loaded for an experi­ ment but fully open. 12 The cross section of the south half of the horizontal split table. 13 The cross section of the north half of the horizontal split table. 14 Concrete