INEL-9670250 EXPERIMENTAL CRITICAL PARAMETERS of PLUTONIUM METAL CYLINDERS FLOODED WITH WATER Dr. Robert E. Rothe Consultant September 30,1994 Prepared for LMITCO under subcontract C94-170466 and for the US DOE under DOE Idaho Ops office contract # DE-AC07-94ID13223 DISCLAIMER Portions of this document may be illegible in electronic image products. Images are produced from the best available original document. DISCLAIMER 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, nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or use- fulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any spe- cific commercial product, process, or service by trade name, trademark, manufac- turer, or otherwise does not necessarily constitute or imply its endorsement, recom- mendation, or favoring by the United States Government or any agency thereof. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof. ABSTRACT Forty-nine critical configurations are reported for experiments involving arrays of 3 kg plutonium metal cylinders moderated and reflected by water. Thirty-four of these describe systems assembled in the laboratory, while 15 others are derived critical parameters inferred from 46 subcritical cases. The arrays included 2x2xN, N = 2, 3, 4, and 5, in one program and 3x3x3 configurations in a later study. All were three-dimensional, nearly square arrays with equal horizontal lattice spacings but a different vertical lattice spacing. Horizontal spacings ranged from units in contact to 180 mm center-to-center; and vertical spacings ranged from about 80 mm to almost 400 mm center-to-center. Several nearly-equilateral 3x3x3 arrays exhibit an extremely sensitive dependence upon horizontal separation for identical vertical spacings. A line array of unreflected and essentially unmoderated canned plutonium metal units appeared to be well subcritical based on measurements made to assure safety during the manual assembly operations. All experiments were performed at two widely separated times in the mid-1970s and early 1980s under two programs at the Rocky Flats Plant's Critical Mass Laboratory. ui CONTENTS ABSTRACT iii TABLE OF CONTENTS v INTRODUCTION 1 THEORY 5 PROCEDURE 20 Water Flow 20 Manual Assembly 24 First Program 27 Assembly 27 Reciprocal Multiplication 27 Critical Approach 35 Second Program. 38 Assembly 38 Reciprocal Multiplication 47 Critical Approach 52 TEMPERATURE 54 FISSILE MATERIAL 57 WATER REFLECTOR/MODERATOR 73 APPARATUS 78 Reservoir 79 Water Distribution 80 First Program 82 Tank 82 Trays 85 Hardware 90 Sumary 98 Second Program 100 Tank 100 Sleeves.... 102 Apparatus 106 Summary ENVIRONMENT 114 Room 115 Experimental Tank 119 Possible Neutronic Influences 121 RESULTS 123 First Program 123 Second Program 141 UNCERTAINTIES 150 A HYPOTHETICAL SITUATION 155 THE INCIDENT 159 LESSONS LEARNED 169 FUTURE EXPERIMENTS 174 ACKNOWLEDGMENTS 182 FIGURES Figure 1. Examples of extrapolated critical water heights with good confidence in the results. For the left hand curve, criticality occurred midway between two layers of plutonium; but it occurred just below the top of the upper layer for the other 10 Figure 2. Some extrapolated critical heights were less certain than others. The left hand curve (128.3) was confidently critical at 351 + 0.5 mm; and the right hand one was just as confidently subcritical. The middle curve (133.4) may have been critical at about 460 mm or, possibly, it would have been subcritical but very reactive if more water had been added '. 11 Figure 3. Critical heights quoted for the second experimental program were obtained from an interpolation technique using one very slightly supercritical height and one very slightly subcritical one , 15 Figure 4. One of two critical heights from the second experimental program obtained by extrapolation (but not because of administrative controls). The interesting "structure" is discussed in the text 17 vi Figure 5. Reciprocal multiplication curves begun with no water in the tank exhibited dramatic decreases in detected neutrons as these neutrons were moderated and absorbed. The shaded regions at the top show qualitatively how these effects diminished and the true impact of reactivity additions manifested itself. 18 Figure 6. A generalized schematic view of the equipment used in both experimental programs 23 Figure 7. The manual assembly of arrays for the first experimental program took place inside a three-sided radiation shield 28 Figure 8. A three-layered array is shown being transferred from the manual assembly shield toward the open-topped experimental tank of the first program 32 Figure 9. Top and bottom (right) slotted components for setting AX and AY in the second experimental program 41 Figure 10. Detail of the top of an assembled 3x3x3 array of the second experimental program 44 Figure 11. The three-sided shield used for loading plutonium cylinders into sleeves in the second experimental program 46 Figure 12. Loaded sleeves were transported by crane using a scissor device to grip the sleeve in the second experimental program 48 Figure 13. Plutonium metal (shaded) fit closely within aluminum inner cans which had rolled steel lids (both: single cross hatch). These slipped rather loosely inside stainless steel cans (double cross hatch), composed of two parts glued together at a stepped joint 58 Figure 14. Four stages of the double containment of plutonium metal. The aluminum cans seen in two views actually contain a lead simulation of the plutonium cylinder (note hole in lid) 59 Figure 15. The calculated component (solid line below shading) of the isotope M1Am between the year the cylinders were machined and their return to production (vertical arrow). The shading suggests the uncertainty because the date the plutonium was last cleansed of Americium is uncertain 62 vu Figure 16. Detailed dimensions (mm) of the aluminum inner and stainless steel outer can, shown only in one side section. Both are to the same scale; but the separation between the two is exaggerated for clarity 67 Figure 17. The tank for the first experimental program is shown wrapped in a thermal insulation blanket for a future study that never materialized. The cover was not present during experiments. The insulated line at the right is part of that same future study 84 Figure 18. Examples of the types of trays used in the first program. The bottom tray is to the left 86 Figure 19. The full compliment of trays fabricated for the first experimental program. Three sets for arrays in contact (76 mm) are seen to the left. The bottom tray, different from all the others, rests on top of each set. Only the two sets to the far left were not used. One 2x2 tray is missing from the photograph 89 Figure 20. Some combination of these spacer shims on each threaded rod plus the 82.55 mm thickness of a tray defined AZ for the first experimental program. The thinnest shim is not shown 94 Figure 21. Hold-down tubes transmitted force from the lifting cross to the top of the array. Shims of Fig. 20 are seen to the left center (smallest one missing) 96 Figure 22. Cross section of the apparatus used in the first experimental program. Components named are described in detail in the text. Water (shaded) is shown at some point early in an experiment. The tank and plutonium cans were round, but the array and its hardware were square. In this illustration, a 2x2xN array uses trays intended for a 3x3 pattern 99 Figure 23. The plastic tank of the second experimental program contained up to 81 kg of plutonium metal in an array flooded with water. Water was removed for this three-quarter overhead photograph 103 Figure 24. A canned cylinder is about ready to be inserted into an aluminum sleeve in this staged photograph. Many details of sleeve fabrication are visible 105 Figure 25. Slots in the framework at top and bottom allowed infinite adjustment in one direction. Slots in orthogonal bars (only one shown) at the top and at the bottom allowed the same in the other 108 vui Figure 26. Cross section of the apparatus used in the second experimental program. Components named are described in detail in the text. The central column shows a sleeve in place. The one to the right shows the plutonium cylinders with the sleeve not shown. Only sleeve positioning disks are shown to the left .113 Figure 27. The open-topped experimental tank for the first experimental program is shown in the upper center of this aerial view. East is to the top of the picture. The equipment immediately to the right of the tank was designed for another program but never used 120 Figure 28. Two critical heights (dots) are plotted to the left of the vertical asymptote (dashed line). This asymptote is determined from subcritical arrays at wider AZ. Both combine to produce the heavy solid curve of critical configurations to the left 133 Figure 29. Subcritical data can be used to derive critical values (ordinate = zero) of AZ for fully-reflected arrays. Dots are data from the last columns of Tables IX to XII. Arrows are lower bounds to the curve to the right because they correspond to critical configurations 134 Figure 30. The 362.0 mm experiment appeared to be critical; but experiments with both greater and smaller AZ were subcritical. This anomalous critical result is questionable 137 Figure 31. Inferred asymptotic vertical lattice spacings are graphed against horizontal spacings. The curves generate other fully water-reflected arrays to be evaluated.