NUCLEAR SCIENCE AND ENGINEERING: 178, 539–549 (2014)
Criticality Benchmarks for the Epithermal Test Assembly Experiments Emily M. Flora URS Professional Solutions 2131 South Centennial Drive, Aiken, South Carolina 29803
and Michael L. Zerkle* Bettis Atomic Power Laboratory P.O. Box 79, West Mifflin, Pennsylvania 15122 Received January 24, 2014 Accepted June 8, 2014 http://dx.doi.org/10.13182/NSE14-31
Abstract – The Epithermal Test Assembly (ETA) experiments were performed to test the adequacy of 233U, U, and 232Th cross sections in epithermal spectra in support of the Light Water Breeder Reactor (LWBR) Program. The ETA design contained a central heavy water–moderated test region surrounded by a light water– moderated annular driver region. Two series of experiments were performed: ETA-I with 235UO2-ThO2 fuel rods in the test region and ETA-II with 233UO2-ThO2 fuel rods in the test region. The dominant uncertainties in the critical configurations include the test-rod pitch, fuel density, and ThO2 mass for ETA-I; the test-region fuel-rod fuel density and 233U to (233U z Th) weight ratio for ETA-II; and the driver-region fuel-rod outer diameter, uranium enrichment, and pitch for both ETA experiments. Benchmark model results using MCNP5 are provided for ENDF/ B-V, ENDF/B-VI, ENDF/B-VII.0, and ENDF/B-VII.1 cross sections with only the ENDF/B-VII.0 results falling within three standard deviations of the benchmark model keff. The ETA-I and ETA-II benchmark evaluations have been included in the International Handbook of Evaluated Criticality Safety Benchmark Experiments and are replicated in the International Handbook of Evaluated Reactor Physics Benchmark Experiments. 235
I. INTRODUCTION
at the Bettis Atomic Power Laboratory in West Mifflin, Pennsylvania. Two series of experiments were performed: ETA-I with 235UO2-ThO2 fuel rods in the test region and ETA-II with 233UO2-ThO2 fuel rods in the test region. The initial core loading and approach to critical of the ETA-I experiment were performed from July to August 1969. The 235UO2-ThO2 fuel rods used in the ETA-I critical experiment test-region fuel rods were manufactured by Babcock & Wilcox (B&W) and were previously used in the B&W Thorium Uranium Physics Experiments (TUPE) and the Spectral Shift Control Reactor (SSCR) experiments.1,2 The initial core loading and approach to critical of the ETA-II experiment were performed from July to November 1970. The 233UO2-ThO2 fuel rods for ETA-II were obtained from Brookhaven National Laboratory (BNL) and were originally fabricated by Oak Ridge
The Epithermal Test Assembly (ETA) experiments were performed in support of the Light Water Breeder Reactor (LWBR) Program. The experiments were designed to provide a clean test of the adequacy of 233 U, 235U, and 232Th epithermal cross-section data. The ETA experimental program began in July 1969 and continued until March 1971. The ETA experiments consisted of a heavy water–moderated central test region surrounded by a light water–moderated annular driver region fueled by Two Region Experiment (TRX) highdensity UO2 fuel rods. The ETA experiments were performed in the Clean Critical Experiment (CCX) facility *E-mail:
[email protected] 539
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National Laboratory.3 These fuel rods were previously used in a series of exponential experiments4 at BNL. The complete benchmark evaluation reports5,6 for the ETA-I and ETA-II experiments are available in the International Handbook of Evaluated Criticality Safety Benchmark Experiments (ICSBEP Handbook) as HEUCOMP-THERM-018 and U233-COMP-THERM-004 and in the International Handbook of Evaluated Reactor Physics Benchmark Experiments (IRPhEP Handbook) as ETA-HWR-EXP-001 and ETA-HWR-EXP-002, respectively. A previous paper discussed in detail the parameter measurements and calculations that were performed for the ETA experiments.7 The LWBR demonstrated the ability of a light water– moderated reactor to breed using the 232Th-233U fuel cycle. Other critical experiments performed in support of the LWBR Program include a series of small 233U- and 235 U-fueled seed-and-blanket (SB) critical assemblies,8 a series of 233U-fueled (BMU) critical assemblies,9 and a series of single-module 233UO2-ThO2–fueled detailed cell experiments.10 The results of the beginning-of-life physics test program for the LWBR have also been documented.11 Benchmarks for the SB experiments are provided in the ICSBEP Handbook as U233-COMP-THERM-001 and HEU-COMP-THERM-015. II. DESCRIPTION OF ETA EXPERIMENTS
The reactor tank at the CCX facility consisted of two regions for the ETA experiments: the test region and driver region. Inside the reactor tank, the test region moderated with heavy water was separated from the light water–moderated driver region through the use of a double-walled tank. The ETA-I and ETA-II experiment configurations were similar with the exception of the testregion fuel loading and the number of TRX fuel rods in the driver region. The driver-region fuel rods, tanks, lattice plates, and other support structures were the same for both experiments. A diagram of the core assembly for ETA-II is shown in Fig. 1. The TRX fuel rods used in the driver region of the experiments consisted of a 121.92-cm (48.00-in.) high column of 0.97282-cm (0.383-in.) diameter, 1.31 wt% 235 U enriched high-density UO2 fuel pellets clad in 0.07112-cm (0.028-in.) thick aluminum alloy 1100 tubing. A TRX fuel rod had an outer diameter of 1.15062 cm (0.453 in.) and a total fuel-rod length of 137.16 cm (54 in.). Figure 2 illustrates the design of the TRX fuel rod that was used in both the ETA-I and ETA-II configurations. The TRX fuel rods were arranged on top of an elevated base plate in the reactor tank with four aluminum alloy lattice plates placed at varying heights to maintain the fuel-rod configuration. There were six inner and six outer stainless steel support rods for the lattice structure with a 144.78-cm (57.00-in.) length and 1.27-cm (0.500in.) and 1.905-cm (0.750-in.) diameters, respectively.
The inner support rods were spaced on approximately a 73.914-cm (29.1-in.) diameter of the plate. The outer rods were spaced on a 139.7-cm (55.0-in.) diameter. A set of stainless steel spacer tubes was inserted around the support rods to set the axial spacing between the driver-region lattice plates. The inner spacer tubes were made of a 1.5875-cm (0.625-in.) outer diameter, 0.0889cm (0.035-in.) thick walled tube, and the outer spacer tubes were made of a 2.8575-cm (1.125-in.) outer diameter, 0.3048-cm (0.120-in.) thick walled tube. The lattice plates contained a total of 4240 1.16332-cm (0.458in.) diameter holes set on a 1.5367-cm (0.605-in.) square pitch. The ETA-I experiment used 2580 TRX fuel rods, and the ETA-II experiment used 2732 TRX fuel rods. The number of TRX fuel rods required to maintain criticality changed over the course of the ETA experiments as the light water impurity in the heavy water moderator increased. The configurations evaluated correspond to a light water impurity content of 1.0 wt% for ETA-I and 1.6 wt% for ETA-II. The test region for both the ETA-I and ETA-II experiments was contained within a double-walled tank, which was centered in the reactor tank. As in the driver region, four lattice plates were set at varying heights using support rods and spacers to maintain the fuel-rod configuration in the test region. Six welded spacer tubes on the bottom of the bottom lattice plate created a 1.27-cm (0.50-in.) gap between the base plate and bottom lattice plate for ETA-I. For ETA-II, six small stainless steel support rods, which were 2.54 cm (1.00 in.) long and 1.74498 cm (0.687 in.) in diameter, were also used to provide a 3.81-cm (1.50-in.) gap between the base plate and bottom lattice plate. There were six more stainless steel support rods, which were 1.27 cm (0.50 in.) in diameter; these connected the bottom lattice plate with the other three lattice plates. Three sets of aluminum alloy spacer tubes of different lengths and with a diameter of 1.5875 cm (0.625 in.) and wall thickness of 0.12446 cm (0.049 in.) were used to set the gaps between the lattice plates. Different lengths of spacer tubes were used for ETA-I and ETA-II. The lattice plates had a total of 256 2.6543-cm (1.045-in.) square holes on a 3.3782-cm (1.33in.) square pitch; four circular holes were also machined into the lattice and sublattice plates to accommodate special test equipment. The lattice plates also contained gaps in some areas to accommodate space for the safety rods, which were designed to ensure that the reactor remained shut down if the test region were inadvertently filled with light water instead of heavy water. Sublattice plates were used to position the fuel clusters in the center of the test lattice due to the spaces in the lattice plates for the safety rods. A sublattice plate had the same square hole dimensions and spacing as the lattice plates. The sublattice plates for ETA-II also used support rods to connect them. Two sets of eight stainless steel support rods were used with a 0.635-cm (0.250-in.) diameter. Figure 3 shows a test-region NUCLEAR SCIENCE AND ENGINEERING
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Fig. 1. ETA core assembly (based on ETA II test region).
Fig. 2. Schematic of a TRX high-density UO2 fuel rod. NUCLEAR SCIENCE AND ENGINEERING
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Fig. 3. Test-region lattice plate.
Fig. 6. Cross section of ETA-I fuel-rod bundle.
Fig. 4. Test-region sublattice plate.
lattice plate with the areas removed for the safety rods and sublattice plate. Figure 4 illustrates the design of the sublattice plate. The ETA-I test region was loaded with 2304 235UO2ThO2 fuel rods (6.7 wt% UO2 in UO2-ThO2, 93.2 wt% 235 U in U), which were placed in square aluminum alloy tubing in a loose 3|3 array. The 235UO2-ThO2 fuel rods were 157.48 cm (62 in.) long; contained a 152.4-cm (60-in.) high column of 0.6604-cm (0.260-in.) diameter fuel pellets; and were clad in 0.03556-cm (0.014-in.) thick, 0.78486-cm (0.309-in.) outer diameter aluminum alloy 1100 tubing. The designs of the 235UO2-ThO2 fuel
Fig. 5. Schematic of ETA-I test-region fuel rod. NUCLEAR SCIENCE AND ENGINEERING
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rods and fuel-rod bundles used in the ETA-I test region are illustrated in Figs. 5 and 6, respectively. The ETA-II test region was composed of 864 233UO2ThO2 fuel rods, which were arranged in 2|2 bundles using support blocks and spacers to set the fuel rods on a 1.3843-cm (0.545-in.) square pitch within the bundle. There were 216 fuel-rod bundles used in the ETA-II experiment leaving 40 empty lattice holes along the outside of the test-region configuration. The ETA-II testregion fuel rods were 119.6848 cm (47.12 in.) long and clad in Zircaloy-2 with a vibratory compacted 3 wt% 233 UO2–97 wt% ThO2 powder fuel column that was 1.0922 cm (0.43 in.) in diameter and 107.95 cm (42.5 in.) in length. The 233UO2-ThO2 powder contained roughly equal portions of grains with average sizes of 0.014 and 0.2 cm. The designs of the ETA-II 233UO2-ThO2 fuel rods and bundles are shown in Figs. 7 and 8, respectively. Figure 9 is an illustration of the cross section of the 233 UO2-ThO2 fuel bundle. In the experiment configuration, there are locations where control rods can be inserted between the doublewalled tank walls and where safety rods can be inserted into slots in the lattice plates in the test region. In the critical configuration, the control and safety rods were fully withdrawn; therefore, their effects are considered to be negligible and were not modeled in the benchmark evaluation. Several details of the critical configuration were not reported. These parameters include date of criticality, final driver-region fuel loading pattern, heavy water level, light water level, safety rod position, control rod position, and temperature.3,7,12 The range of these parameters was estimated and used to estimate the experimental uncertainty associated with these parameters based on established experimental requirements and procedures. The other physics parameters measured in the ETA-I and ETA-II experiments are summarized in Table I. Only the critical configuration has been evaluated in ETA benchmarks. III. SENSITIVITY ANALYSIS OF THE ETA EXPERIMENTS
The uncertainty calculations were performed using the MCNP5 (version 1.51) Monte Carlo code13 with the ENDF/B-VII.0 cross-section library on a Windows XP PC system. The evaluated uncertainties included testregion and driver-region fuel-rod parameters (e.g., fuel length, enrichment, and density), moderator waters for the test region and driver region, structural dimensions and material parameters (e.g., tank wall thicknesses, lattice plate thickness, and material densities), and temperature of the configuration. The dominant uncertainties in the critical configurations consist of the test-region fuel-rod fuel density, test-region fuel-rod pitch, and ThO2 mass for ETA-I; test-region fuel density and 233U to (233U z Th) NUCLEAR SCIENCE AND ENGINEERING
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Fig. 7. Schematic of ETA-II test-region fuel rod.
weight ratio for ETA-II; and TRX fuel-rod enrichment, outer diameter, and pitch for both ETA experiments. As previously stated, several of the configuration details were not recorded when the experiments were performed. However, those parameters and their associated uncertainties were estimated based on standard practices. The evaluated uncertainties for the ETA-I and ETA-II experiments are given in Tables II and III, respectively. Parameters that were evaluated and have negligible uncertainties are not included in the tables. IV. BENCHMARK MODEL
Simplified benchmark models were developed to represent the critical configuration of the ETA-I and
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Fig. 8. ETA-II test-region fuel-rod bundles. Fig. 9. Cross section of the ETA-II fuel-rod bundle.
ETA-II experiments. The dimensions and material compositions used in the benchmark models are described in detail in the benchmark evaluation reports.5,6 To further simplify the models, several structures and details were removed. For the ETA-I benchmark model, these simplifications included the following: the four circular holes in the test-region lattice plates were removed and modeled as square holes; the support rods and spacers for the lattice plates were modeled as solid cylinders by
removing the water gap between them; support structures below the test region, driver region, and double-walled tank were removed and replaced with moderator; and the smaller, upper end plugs of the 235UO2-ThO2 fuel rods were replaced with moderator. These simplifications introduced a bias of 0.00049 Dk and an additional 0.00015 Dk uncertainty into the benchmark model keff, which is listed in Table IV. Vertical and cross-sectional
TABLE I Supplemental ETA-I and ETA-II Measurements Parameter d02 CR* r02 d23 d25 232 Th Doppler Fast spectrum comparison
ETA-I
ETA-II
232
232
232
Th fission/235U fission Th capture/235U fission Epithermal/thermal 232Th capture ratio — Epithermal/thermal 235U fission ratio 232 Th capture in a heated rod
232
Th fission/233U fission Th capture/233U fission Epithermal/thermal 232Th capture ratio Epithermal/thermal 233U fission ratio — —
Measurement of activation with 197Au(n,c), 55Mn(n,c), 235U(n, f ), 238U(n, f ), 27Al(n,a), and 96Zr(n,c) relative to the 4/1 TRX experiment
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TABLE II
TABLE III
Summary of Evaluated Uncertainties for ETA-I
Summary of Evaluated Uncertainties for ETA-II
Dkeff (1s)
Parameter
Parameter
Dkeff (1s)
233
UO2-ThO2 fuel Fuel diameter (cm) Enrichment (wt% 235U in U) 234 U in U (wt%) ThO2 mass (g) O as ThO2, in fuel (wt%) O as UO2, in fuel (wt%) Fuel length (cm) Fuel density (g/cm3) Boron equivalent impurities (included/not included) Rod pitch (cm) Bundle pitch (cm) Outer diameter/clad thickness (cm) Al-1100 clad density (g/cm3) Al-6061 bundle tube wall thickness (cm) Al-6061 bundle tube length (cm) Al-6061 bundle tube density (g/cm3)
0.00006 0.00011 0.00003 0.00051 0.00006 0.00006 0.00007 0.00050 0.00042 0.00055 0.00010 0.00031 0.00004 0.00022 0.00002 0.00009
TRX fuel Diameter (cm) Enrichment (wt% 235U) 234 U in fuel (wt%) Fuel length (cm) Density (g/cm3) Outer diameter/clad thickness (cm) Al-1100 clad density (g/cm3) Rod pitch (cm) TRX rod configuration TRX rod addition/removal
0.00005 0.00081 0.00030 0.00004 0.00036 0.00094 0.00007 0.00075 0.00013 0.00014
Moderator Moderator level above fuel Heavy water impurities (wt% H)
0.00022 0.00026
Structures and materials Test-region lattice plate thickness (cm) Driver-region lattice plate thickness (cm) Test-region inner tank wall thickness (inner radius) (cm) Test-region outer tank wall thickness (outer radius) (cm) Aluminum alloy 6061 density (g/cm3) Temperature and moderator density Temperature (uC)
0.00005 0.00007 0.00038 0.00014 0.00006 0.00003
Benchmark model simplifications Uncertainty associated with simplifications
0.00015
Combined uncertainty
0.00197
layouts of the benchmark model for the ETA-I experiment are shown in Figs. 10 and 11. Similar model simplifications were made for the ETA-II benchmark model. These simplifications NUCLEAR SCIENCE AND ENGINEERING
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UO2-ThO2 fuel U/(233U z Th) weight ratio 233 U wt% in uranium Fuel length (cm) Fuel density (g/cm3) Boron equivalent impurities (included/not included) Bundle pitch (cm) Outer diameter/clad thickness (cm) 233
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TRX fuel Diameter (cm) Enrichment (wt% 235U) Fuel impurities (included/not included) Fuel length (cm) Density (g/cm3) Outer diameter/clad thickness (cm) Aluminum clad density (g/cm3) Rod pitch (cm) TRX rod configuration Structures and materials Driver-region lattice plate thickness (cm) Test-region inner tank wall thickness (inner radius) (cm) Test-region outer tank wall thickness (cm) 304 stainless steel density (g/cm3) Aluminum alloy 6061 density (g/cm3)
0.00057 0.00003 0.00011 0.00062 0.00020 0.00011 0.00041 0.00015 0.00084 0.00024 0.00002 0.00031 0.00085 0.00008 0.00067 0.00045 0.00019 0.00035 0.00016 0.00002 0.00001
Temperature and moderator density Temperature (uC)
0.00013
Benchmark model simplifications Uncertainty associated with simplifications
0.00011
Combined uncertainty
0.00185
included the following: the support structures beneath the test region, driver region, and double-walled tank were removed and replaced with moderator; the top spacer blocks on the fuel bundles (shown in Fig. 8) were removed and replaced with moderator; the four circular holes in the test-region lattice plates were modeled as square holes; the spring inside the test-region fuel rod was modeled as void; and the water gaps between the support rods and spacers for the lattice plates were removed and the support rods modeled as solid cylinders. These simplifications resulted in a 0.00084 Dk bias with an associated uncertainty of 0.00011 Dk. An additional bias of 0.00085 Dk associated with impurities in the 233UO2-ThO2 fuel was calculated for the ETA-II experiment. The combination of the two biases was applied to the benchmark keff, which is listed in Table IV. Vertical and cross-sectional layouts of the ETA-II benchmark model are shown in Figs. 12 and 13.
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Fig. 10. Vertical layout of ETA-I configuration.
Fig. 11. Quarter of the ETA-I core configuration.
Table IV compares the benchmark model keff and MCNP5 V1.51 sample calculations using the three different cross-section libraries for the ETA-I and ETA-II experiments. As shown in Table IV, there is not good agreement between the benchmark model keff and the MCNP5 calculated results, particularly for the ENDF/ B-V and ENDF/B-VI cross-section libraries. The ENDF/ B-VII.0 results are within three standard deviations of the benchmark model keff. Additional calculations were performed using MCNP5 V1.60 with ENDF/B-VII.0 and ENDF/B-VII.1 cross sections after the evaluation was published. As shown in Table V, both the ENDF/B-VII.0 and ENDF/B-VII.1 results for ETA-I are within three standard deviations of the benchmark model keff; however, for ETA-II only the ENDF/B-VII.0 results are within three standard deviations of the benchmark model keff. Table VI provides the fraction of fissions in the thermal, intermediate, and fast energy ranges for the NUCLEAR SCIENCE AND ENGINEERING
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Fig. 12. Vertical layout of ETA-II configuration.
ETA-I and ETA-II benchmark models. The fractions of fissions in the test and TRX driver fuel regions are also provided for each energy range. The fission fractions for the two ETA experiments were fairly similar. For ETA-I, 78% of fissions occurred in the thermal energy range while 17% of fissions were in the intermediate energy range and 5% were in the fast energy range. About 76% of the fissions in the ETA-II model occurred in the thermal energy range with 19% of fissions in the intermediate energy range and 5% in the fast energy range. In the intermediate energy range, the majority of fissions are concentrated in the heavy water–moderated test region for both ETA experiments. V. CONCLUSIONS
Benchmark models of the ETA critical experiments have been developed. These critical experiments were designed to test the adequacy of the epithermal crosssection data for 235U, 233U, and 232Th. The experiments consisted of a heavy water–moderated test region containing
Fig. 13. Quarter of the ETA-II core configuration. NUCLEAR SCIENCE AND ENGINEERING
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TABLE IV Comparison of Benchmark Model keff and Sample Calculations
Experiment
Benchmark Model
MCNP5 V1.51 (ENDF/B-V)
MCNP5 V1.51 (ENDF/B-VI)
MCNP5 V1.51 (ENDF/B-VII.0)
ETA-I ETA-II
0.9995 + 0.0020 1.0017 + 0.0019
0.9909 + 0.0001 0.9933 + 0.0001
0.9866 + 0.0001 0.9877 + 0.0001
0.9949 + 0.0001 0.9964 + 0.0001
TABLE V Results from Sample Calculations Using MCNP5 V1.60
Experiment
MCNP5 V1.60 (ENDF/B-VII.0)
MCNP5 V1.60 (ENDF/B-VII.1)
ETA-I ETA-II
0.9947 + 0.0001 0.9966 + 0.0001
0.9947 + 0.0001 0.9959 + 0.0001
TABLE VI Fission Fraction in ETA-I and ETA-II Fuel Regions ETA-I
ETA-II Fraction In
Energy Range Thermal (E v 0.625 eV) Intermediate (0.625 eV v E v 0.1 MeV) Fast (E w 1 MeV)
Fraction In
Fission Fraction
Test Fuel
TRX Fuel
Fission Fraction
Test Fuel
TRX Fuel
78% 17%
22% 79%
78% 21%
76% 19%
17% 81%
83% 19%
5%
22%
78%
5%
20%
80%
fuel rods surrounded by a light water–moderated driver region fueled by TRX UO2 fuel rods. Two configurations were evaluated and published in the ICSBEP Handbook; HEU-COMP-THERM-018 evaluates the ETA-I experiment with 235UO2-ThO2 fuel rods in the test region, and U233-COMP-THERM-004 evaluates the ETA-II experiment with 233UO2-ThO2 fuel rods in the test region. These evaluations have been replicated in the IRPhEP Handbook as ETA-HWR-EXP-001 and ETA-HWREXP-002, respectively. The evaluations include an uncertainty analysis performed on the experiment parameters, which included the fuel rods in the driver and test regions, the support structures, the temperature, and the moderator. Modeling simplifications of the critical configuration for both experiments resulted in the application of a bias to the benchmark model keff. Sample calculations were performed using MCNP5 with ENDF/B-V, ENDF/B-VI, ENDF/B-VII.0, and
ENDF/B-VII.1 cross sections. For the ETA-I experiment, the ENDF/B-VII.0 and ENDF/B-VII.1 results were within three standard deviations of the benchmark model keff. Only the ENDF/B-VII.0 results were within three standard deviations of the benchmark model keff for the ETA-II experiment.
ACKNOWLEDGMENTS The authors wish to extend their thanks to several individuals who helped with the evaluation of the ETA experiments and provided a majority of the figures used in this paper. R. L. Reed of URS Professional Solutions provided much guidance and constructive analysis as part of an internal assessment during the evaluation process and assisted with the running of the numerous MCNP5 models. C. White of Idaho National Laboratory produced many of the figures in great detail. NUCLEAR SCIENCE AND ENGINEERING
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REFERENCES 1. N. L. SNIDOW et al., ‘‘Thorium Uranium Physics Experiments Final Report,’’ BAW-1191, Babcock & Wilcox (May 1960). 2. T. C. ENGELDER et al., ‘‘Spectral Shift Control Reactor Basic Physics Program Critical Experiments on Lattices Moderated by D2O-H2O Mixtures,’’ BAW-1231, Babcock & Wilcox (Dec. 1961). 3. M. L. ZERKLE, ‘‘Design of the ETA-I and ETA-II Critical Experiments,’’ B-TM-1640, Bettis Atomic Power Laboratory (Sep. 2009). 4. H. H. WINDSOR, W. J. TUNNEY, and G. A. PRICE, ‘‘Exponential Experiments with Lattices of Uranium-233 Oxide and Thorium Oxide in Light and Heavy Water,’’ Nucl. Sci. Eng., 42, 150 (1970); http://dx.doi.org/10.13182/NSE70-2. 5. E. M. FLORA, ‘‘ETA-I: D2O Moderated Lattice of UO2ThO2,’’ HEU-COMP-THERM-018, International Handbook of Evaluated Criticality Safety Benchmark Experiments, Vol. II, NEA/NSC/DOC/(95)03, Organisation for Economic Co-operation and Development/Nuclear Energy Agency. 6. E. M. FLORA and A. H. BRIDGES, ‘‘D2O Moderated Lattice of 233UO2-232ThO2,’’ U233-COMP-THERM-004, International Handbook of Evaluated Criticality Safety Benchmark Experiments, Vol. V, NEA/NSC/DOC/(95)03, Organisation for Economic Co-operation and Development/ Nuclear Energy Agency.
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7. J. HARDY, JR., J. J. VOLPE, and D. KLEIN, ‘‘Measurement and Analysis of Parameters in Tight 232 Th-235U and 232Th-233U Lattices Moderated with Heavy Water,’’ Nucl. Sci. Eng., 55, 401 (1974); http://dx.doi.org/ 10.13182/NSE74-2. 8. S. MILANI and S. H. WEISS, ‘‘Small Uranium-233 Fueled Seed-and-Blanket Critical Experiments,’’ WAPD-TM-614, Bettis Atomic Power Laboratory (Nov. 1967). 9. ‘‘BMU Series of 233U Fueled Critical Experiments,’’ WAPD-TM-1117, J. A. MITCHELL, Ed., Bettis Atomic Power Laboratory (Jan. 1975). 10. ‘‘U-233 Oxide – Thorium Oxide Detailed Cell Critical Experiments,’’ WAPD-TM-1101, G. G. SMITH, J. P. SEMANS, and J. A. MITCHELL, Eds., Bettis Atomic Power Laboratory (Oct. 1974). 11. ‘‘Results of Initial Nuclear Tests on LWBR,’’ WAPD-TM1336, W. K. SARBER, Ed., Bettis Atomic Power Laboratory (June 1979). 12. J. HARDY, JR., J. J. VOLPE, and D. KLEIN, ‘‘Measurement and Analysis of Parameters in Tight 232 Th-235U and 232Th-233U Lattices Moderated with D2O,’’ WAPD-TM-1089, Bettis Atomic Power Laboratory (Jan. 1974). 13. ‘‘MCNP—A General Monte Carlo N-Particle Transport Code, Version 5,’’ LA-UR-03-1987, Los Alamos National Laboratory (Apr. 2003).
