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Analysis of the Fourth Zeus Critical Experiment

Russell D . MostellerPeter J . JaegersRoger W. Brewer

2004 Winter Meeting of the American Nuclear SocietyWashington, D CNovember 14 - 18, 2004

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Analysis of the Fourth Zeus Critical Experimen t

Russell D . Mosteller,* Peter J. Jaegers,t and Roger W . Brewer *

*Diagnostics Applications Group (X-5), Los Alamos National LaboratorytAdvanced Nuclear Technology Group (N-2), Los Alamos National Laboratory

To Be Presented at the 2004 Winter Meeting of the American Nuclear SocietyWashington, DC November 13 - 18, 2004

The first four Zeus experiments were designed to test the adequacy of 235U cross sections inthe intermediate-energy range . Detailed descriptions of the first three experiments in the serieshave been presented previously. This presentation focuses primarily upon the fourth experimentin the series, which has a harder spectrum than any of the previous three .

The MCNP5 Monte Carlo code was used to construct a detailed model of the experiment .Results from calculations with a number of different MCNP nuclear data libraries are presented .These results are discussed in conjunction with those from the analyses of the previous threeexperiments . It is shown that ENDF/B-V produces small, nearly constant biases for all fourcases. In contrast, ENDF/B-VI and preliminary ENDF/B-VII libraries produce a consistentupward trend as the spectra of the experiments harden . This pattern suggests that theENDFB-VI and preliminary ENDF/B-VII cross sections contain a small energy-dependent biasin the intermediate-energy range .

MCNP is a trademark of the Regents of the University of California, Los Alamos National Laboratory

Analysis of the FourthZeus Critical Experimen t

Russell D . Mosteller*Peter J . JaegersRoger W . Brewe r

Los Alamos National Laboratory

*e-mail : mosteller@lanl . gov

Presented at the 2004 Winter Meeting of the American Nuclear SocietyNovember 14 - 18, 2004 Washington, DC

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OVERVIEW OF PRESENTATIO N

Description of the fourth Zeus experimen t

Comparison of MCNP5 results using ENDF/B-V, ENDF/B-VI, andpreliminary ENDF/B-VII nuclear data librarie s

Comparison with results from previous Zeus experiment s

Conclusions

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The Fourth Zeus Critical Experimen t

The Zeus experiments were designed to test the adequacy of 235U crosssections in the intermediate-energy rang e

The cylindrical core of the Zeus experiments contains alternating regionsof highly enriched uranium (HEU) and graphite

The core is enclosed inside a copper reflector with rectangular externalside s

The Zeus experiments were constructed on the Comet vertical assemblymachine at the Los Alamos Critical Experiments Facility (LACEF )

The fourth Zeus experiment achieved criticality on November 15, 200 1

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SCHEMATIC OF THE ZEUS DESIG N

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DETAILS OF THE FOURTH ZEUS EXPERIMEN T

Core contains 9 nearly identical units, each of which has a central HEUplatter (0 .118 inch thick) sandwiched between an upper and lowe rgraphite platter (each 0 . 3966 inch thick , on average )

Outer diameter of the platters is 21 inches , except for the bottom HEUpla tter which has an outer diameter of 15 inche s

Distance from one side of the reflector to the other is 34 . 76 inche s

Top and bo ttom reflectors are 5 . 68 inches thick

A period of 17 . 328 seconds was measured(-29 . 8¢ supercritical =* keff = 1 . 0019 )

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VERTICAL SLICE THROUG HFOURTH ZEUS CORE

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MCNP5 CALCULATION S

Each calculation employed 650 generations with 10 , 000 neutrons pergeneratio n

Results from first 50 generations were excluded from the statistic s

Results therefore are based on 6 , 000 , 000 active histories for each cas e

Calculations were performed with each of three sets of continuous-energy nuclear data libraries :

ENDF/B-V ( .50c, .55c )

ENDF/B-Vl (ENDF66 + ACTT )

Preliminary ENDF/B-VII (T16_2003 + ORNL resonance parametersfor uranium isotopes, ENDF/B-Vl for all others )

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RESULTS FOR keff AN D FISSION FRACTION

Source keff Ak

Measurement 1 . 0019 ± 0 . 0008 -

Prelimina ry ENDF/B -VII 1 . 0038 ± 0 . 0003 0 .0019 ± 0 .000 9

ENDF/B-VI 1 . 0050 ± 0 . 0003 0 .0031 ± 0 .000 9

ENDF/B-V 1 . 0011 ± 0 .0003 -0 . 0008 f 0 .00091 1

Fission Fraction (% )

Source Fast Intermediate Therma l

Prelimina ry ENDF/B -VII 50 .63 49 . 37 0

EN DF/B-VI 50 .45 49 . 55 0

E N D F /B -V 52 . 16 47 . 84 0

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CONCLUSIONS FO RFOURTH ZEUS EXPERIMENT

The fourth Zeus experiment satisfies its design objective by producing avery hard intermediate / soft fast spectru m

ENDF/B-V, ENDF/B-VI, and preliminary ENDF/B-VII nuclear datalibraries all produce acceptable results for the fourth Zeus experiment,although both the ENDF/B-VI and ENDF/B-VII results differ from themeasured value by more than two standard deviation s

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CHARACTERISTICS OF TH EFIRST FOUR ZEUS EXPERIMENT S

HEU Graphite Critical Mas sExperiment Pla tters Platters (kg U) C 235 0

1 10 79 . 5** 126 .5 51 . 2 : 1

2 9 54 112 .8 38 . 6 : 1

3 9* 36 106 .6 27 . 4 : 1

4 9* 18 106 .8 13 . 6 : 1

*Bottom platter has outer radius of 15 inches rather than 2 1

**Core contained 79 full-height graphite pla tters , as well as a half-heightgraphite pla tter and a very thin stainless steel platter used as a shi m

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RESULTS FOR keff

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Inte rmediate - En e rg y F ission Fraction

f ENDF/B-VO ENDF/B-V Iv P rel i mina ry ENDF/B -V I I

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CONCLUSIONS FOR ZEUS EXPERIMENTS

ENDF/B-V produces a small, essentially constant bias relative to themeasured values of k eff

ENDF/B-VI and preliminary ENDF/B-Vll exhibit sizeable swings in theirbias (0.010 Ak for ENDF/B-VI, 0 .0075 Ok for ENDF/B-VII )

Biases for ENDF/B-Vl and preliminary ENDF/B-Vll become more positiveas their spectra harde n

This behavior suggests a spectrum-dependent bias in the ENDF/B-Vland preliminary ENDF/B-Vll cross sections within the upper intermediate-energy rang e

Detailed evaluation of the four Zeus experiments moderated by graphiteappears as HEU-MET-INTER-006 in the 2004 edition of the InternationalHandbook of Evaluated Criticality Safety Benchmark Experiments

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