Subgroup 29 U-235 Capture Cross Section in the keV to MeV Energy - - PowerPoint PPT Presentation

Subgroup 29

U-235 Capture Cross Section in the keV to MeV Energy Region

• O. Iwamoto (JAEA)

Progress report 2008

Introduction

• Fast-neuron critical experiments using U

fuels

– BFS (IPPE): underestimation of sodium voided reactivity – FCA (JAEA): large dependence of reactivity

• n neutron spectrum
• Capture cross sections of 235U in keV

region

sodium voided reactivity in BFS

MOX Sensitivity of 235U capture cross section sodium voided reactivity

FCA experiment (enriched U + C)

criticality of FCA IX assemblies Neutron spectra

Members

• ENDF: T. A. Bredeweg (LANL) T. Kawano

(LANL), R. McKnight (ANL, Monitor), L.C. Leal (ORNL), C. Lubitz (KAPL)

• JEFF: R. Jacqmin (CEA)
• JENDL: T. Nakagawa (JAEA), G. Chiba (JAEA),
• S. Okajima (JAEA), M. Ishikawa (JAEA), O.

Iwamoto (JAEA, Coordinator)

SG29 started on April 2007.

CONTROL ROD WORTH OF ZPPR-18A

• M. Ishikawa (JAEA)

Core Configuration of ZPPR-18A

Horizontal Vertical

C/E Values of Control Rod Worth

Nuclide- and Reaction-wise Contribution to C/E Changes of Control Rod Worth

FCA-IX BENCHMARK PROBLEM

• G. Chiba and S. Okajima (JAEA)

FCA-IX benchmark problem

• G. Chiba and S. Okajima (JAEA) have

prepared a FCA-IX benchmark problem

• Cores of the benchmark problem are

expressed as simple two-dimensional

• cylinder. Each core model is composed of

two material regions, i.e., a fuel region and a blanket region.

• Tables of the core specifications and the

region-wise number densities are given.

IMPACT OF JENDL-3.2 AND ENDF/B-VII.0 RESONANCE REGION IN BENCHMARK CALCULATIONS

• O. Iwamoto, G. Chiba, T. Nakagawa (JAEA), and L. Leal (ORNL)

Comparisons of cross sections in JENDL-3.2 and ENDF/B-VII.0

Comparisons of cross sections (continued)

capture/fission ratio

∫

+

− = > <

+

1

) ( ) ( ) ( 1

1

i i

E E f c i i i

dE E E E E σ σ α i f i c i

> < > < = > < σ σ α

JENDL-3.2 and ENDF/B-VII.0 resonance parameters combinations

Resonance Region JENDL-3.2 ENDF/B-VII.0 RRR 10-5 eV – 500 eV 10-5 eV – 2.25 keV URR 500 eV – 30 keV 2.25 keV – 25 keV Name Spectrum Handbook ID AVG (keV) ZEUS1 Intermediate HEU-MET-INTER-006, case1 5.05 ZEUS2 Intermediate HEU-MET-INTER-006, case2 10.33 ZEUS3 Intermediate HEU-MET-INTER-006, case3 24.02 ZEUS4 Intermediate HEU-MET-INTER-006, case4 FCA-IX-1 Intermediate 29.90 FCA-IX-2 Intermediate 116.52 FCA-IX-3 Intermediate 211.30

Resonance energy range Energy of the average lethargy causing fission (AVG)

JENDL-3.2 for 235U as reference

500 eV 2.25 kev 25 keV 30 keV RR( ENDF) RR (ENDF) (JENDL) (JENDL) 500 eV 2.25 kev 25 keV 30 keV RR( ENDF) (JENDL) (JENDL) (JENDL)

Case 1 Case 2

ENDF/B-VII.0 as reference

500 eV 2.25 kev 25 keV 30 keV RR( JENDL) (JENDL) (JENDL) ENDF FILE 3 500 eV 2.25 kev 25 keV 30 keV RR( JENDL) RR (ENDF) (ENDF) ENDF FILE 3

Case 3 Case 4 Case 5

500 eV 2.25 kev 25 keV 30 keV RR( ENDF) RR (JENDL) (ENDF) ENDF FILE 3

Effects on the reduction of the 235U gamma ( > 200 eV)

‐3 % ‐10 %

G1

Γγ −3 % (σγ -2%, σf +1%)

G2

Γγ −10 % (σγ ‐7%, σf +3%)

Sensitivity of ZEUS

ZEUS1 ZEUS2 ZEUS3 ZEUS4

Sensitivity of FCA

FCA‐IX‐1 FCA‐IX‐2 FCA‐IX‐3

Adjustment of U-235 cross section using integral data

Sensitivities of U‐235 capture to keff used in adjustment

Case 1: Uncertainties of experimental data are assumed to be 0.1%dk/kk’ correlation among the data of the same critical assembly is 0.8. Case 2: Uncertainties of experimental data are assumed to be 1.0%dk/kk’ correlation among the data of the same critical assembly is 0.95.

Result of adjustment

C/E values before and after adjustment Relative change in U‐235 capture

Conclusions from the impact of the 235U capture cross sections in benchmark calculations

• It can be seen that ENDF/B-VII.0 capture cross section is much

larger than JENDL-3.2 and statistical model calculations in the range of about 100eV to 3keV. Discrepancies are found also in the elastic scattering cross sections below 30 keV. The total and fission cross sections have no such noticeable discrepancies.

• The ENDF/B-VII.0 α results are in the upper part of the experimental
• results. However, JENDL-3.2 agrees well with the experimental

results.

• Difference between ENDF/B-VII.0 and JENDL-3.2 from 500eV to

2.25keV has a large impact on both the FCA-IX and Zeus calculations while difference below 500eV has an impact only on the Zeus calculations.

• Replacement of the ENDF/B-VII.0 resonance parameters by

JENDL-3.2 for uranium-235 makes the C/E dependence on spectrum hardness smaller. However, this also results in unacceptable high C/E values. This can be an indication that there exists other energy region which also contributes to the multiplication factor. These cases need further investigation.

Conclusions (continued)

• A simple decrease in the capture cross section, i.e. in ,

reduce the C/E dependence on spectrum hardness. However, it scales the up. These results may indicate that other cross section or combinations of effects may be causing the biases.

• Detailed neutron balance analyses indicate that the

leakages for the ZEUS benchmark experiments are on the average of 28 % whereas in the FCA it is about 4 %. The contribution of each component to for the ZEUS and FCA benchmark experiments are different from each

• ther.
• Through cross section adjustment with the integral data

and the covariance information for uranium-235 cross sections, it is indicated that capture cross section in resonance range is overestimated.

Suggestion for the WPEC

• Study on the impact of the 235U capture cross

sections in benchmark calculations indicates that there is a problem with the capture cross section.

• However, the magnitude of the change on the

capture cross section is not well understood.

• Existing capture cross section measurements in

the energy region from 50 eV to 30 keV shows differences in the average capture cross sections that can be more than 10 %.

• It will be helpful to perform new capture cross

section measurements in the keV region to improve the resonance parameter evaluation.

Suggestion for the WPEC (continued)

• Suggested measurements

– Measure high-resolution capture cross section. Existing measurements were done on neutron flight-path of 60-m length or less. Capture cross section measurements done in a flight-path such as 100 meter would be excellent for a better understanding of the data. – Capture-to-fission ratio (alpha measurements) could be essential.

• The R-matrix analysis of the new data will

lead to a better determination of the 235U resonance parameters in the keV region.

1 DEN/DER/SPRC WPEC meeting Tokai-mura, June 3-4-5, 2008

U-235 capture cross section around 1 keV – Is there a problem? (Ref. JEF/DOC-1250)

• J. Yoo, KAERI
• R. Jacqmin, CEA

1. Introduction 2. Benchmark description and calculation 3. Analysis and discussion 4. Summary and conclusion

2 DEN/DER/SPRC WPEC meeting Tokai-mura, June 3-4-5, 2008

“Inconsistencies” reported by JAEA

• Observed in very specific cases, in C/E values for several fast U-fuelled

cores: FCA IX-1, XI-2, XI-3 reactivity measurement BFS-62-3A sodium void measurement ZPPR-18A control rod measurement

• Tests show discrepant behaviour with JENDL-3.3 vs. JENDL-3.2
• Suspected reason: U-235 capture in the 1-2 keV region in JENDL-3.3

JAEA proposed to investigate the problem under WPEC → SG29

• SG29, started in April 2007
• FCA core models proposed as benchmarks

Studied by CEA/KAERI as a contribution to SG29 Introduction

3 DEN/DER/SPRC WPEC meeting Tokai-mura, June 3-4-5, 2008

FCA IX-1, IX-2, XI-3 benchmark

• Small cylindrical U-metal fuelled cores + graphite
• Depleted U metal radial and axial blankets, ~ 35 cm thick
• 93% U-235
• Stainless steel

Homogeneous core characteristics Benchmark description and calculations

7.77 12.5 26.8 Graphite/Uranium 0.20 0.39 1.00 (ref)

• Rel. core volume

0.62 0.46 0.30 Spectral hardness r 0.87 0.93 1.00 (ref) Core C-12 rel. content 1.00 1.00 1.00 (ref) Core rel. steel content 3.00 2.00 1.00 (ref) Core rel. U-238 content 3.00 2.00 1.00 (ref) Core rel. U-235 content 35.56 40.64 60.96 Core height (cm) 17.89 23.10 30.35 Core outer radius (cm) IX-3 IX-2 IX-1

4 DEN/DER/SPRC WPEC meeting Tokai-mura, June 3-4-5, 2008

JAEA results using JENDL-3.2 and JENDL-3.3 + JAEA code system Benchmark description and calculations

5 DEN/DER/SPRC WPEC meeting Tokai-mura, June 3-4-5, 2008

KAERI/CEA results using JEF-2.2 and JEFF-3.1 + ERANOS code Benchmark description and calculations

0.99000 0.99500 1.00000 1.00500 1.01000 1.01500 FCA-IX-1 FCA-IX-2 FCA-IX-3

Experiment no. C/E of k-effective

JEFF-3.1 JEF-2.2

• 0.018
• 0.016
• 0.014
• 0.012
• 0.01
• 0.008
• 0.006
• 0.004
• 0.002

0.1 1 10 100 1000 10000 100000 1e+06 1e+07 1e+08

Relative sensitivity Energy [eV] FCA-IX-1 FCA-IX-2 FCA-IX-3

k-effective C/E Sensitivity of k-eff to the U-235 Capture XS

6 DEN/DER/SPRC WPEC meeting Tokai-mura, June 3-4-5, 2008

Breakdown of the calculated JEFF-3.1 – JEF-2.2 reactivity difference (in pcm) Analysis and discussion

• 32
• 417
• 1353

Total

• 206
• 182
• 136

Fission spectrum

• 392
• 422
• 418

Other nuclides 871 759 577 U-238

• 305
• 572
• 1376

U-235 IX-3 IX-2 IX-1

7 DEN/DER/SPRC WPEC meeting Tokai-mura, June 3-4-5, 2008

Breakdown of JEFF-3.1 – JEF-2.2 reactivity difference by nuclide and region Analysis and discussion

U235 U238 H1 C0 O16 CR50 CR52 CR53 CR54 MN55 FE54 FE56 FE57 FE58 NI58 NI60 NI61 NI62 NI64

• 1.50E-02
• 1.25E-02
• 1.00E-02
• 7.50E-03
• 5.00E-03
• 2.50E-03

2.60E-18 2.50E-03 5.00E-03 7.50E-03 1.00E-02

FCA-IX-1 FCA-IX-2 FCA-IX-3

Nuclide Contribution to reactivity change

Core Blanket

8 DEN/DER/SPRC WPEC meeting Tokai-mura, June 3-4-5, 2008

Breakdown of JEFF-3.1 – JEF-2.2 reactivity difference by nuclide and reaction Analysis and discussion

• 1.5E-02
• 1.0E-02
• 5.0E-03

0.0E+00 5.0E-03 1.0E-02

U235 U238 H1 C0 O16 CR50 CR52 CR53 CR54 MN55 FE54 FE56 FE57 FE58 NI58 NI60 NI61 NI62 NI64 Contribution to reactivity change

TRANSPORT N,XN INELASTIC ELASTIC FISSION CAPTURE

FCA-IX-1

• 8.0E-03
• 6.0E-03
• 4.0E-03
• 2.0E-03

0.0E+00 2.0E-03 4.0E-03 6.0E-03 8.0E-03 1.0E-02

U235 U238 H1 C0 O16 CR50 CR52 CR53 CR54 MN55 FE54 FE56 FE57 FE58 NI58 NI60 NI61 NI62 NI64 Contribution to reactivity change

TRANSPORT N,XN INELASTIC ELASTIC FISSION CAPTURE

• 4.0E-03
• 2.0E-03

0.0E+00 2.0E-03 4.0E-03 6.0E-03 8.0E-03 1.0E-02 1.2E-02

U235 U238 H1 C0 O16 CR50 CR52 CR53 CR54 MN5 FE54 FE56 FE57 FE58 NI58 NI60 NI61 NI62 NI64 Contribution to reactivity change

TRANSPORT N,XN INELASTIC ELASTIC FISSION CAPTURE

FCA-IX-3 FCA-IX-2

9 DEN/DER/SPRC WPEC meeting Tokai-mura, June 3-4-5, 2008

U-235 contributions to the JEFF-3.1 – JEF-2.2 reactivity difference Analysis and discussion

• 300.00
• 250.00
• 200.00
• 150.00
• 100.00
• 50.00

0.00 50.00 1e-01 1e+00 1e+01 1e+02 1e+03 1e+04 1e+05 1e+06 1e+07 1e+08 Energy [eV] Capture Fission Elastic Inelastic n,xn Transport Sum Contribution to reactivity change [pcm]

• 300.00
• 250.00
• 200.00
• 150.00
• 100.00
• 50.00

0.00 50.00 1e-01 1e+00 1e+01 1e+02 1e+03 1e+04 1e+05 1e+06 1e+07 1e+08 Energy [eV] Capture Fission Elastic Inelastic n,xn Transport Sum Contribution to reactivity change [pcm]

FCA-IX-3 FCA-IX-2

• 300.00
• 250.00
• 200.00
• 150.00
• 100.00
• 50.00

0.00 50.00 1e-01 1e+00 1e+01 1e+02 1e+03 1e+04 1e+05 1e+06 1e+07 1e+08 Energy [eV] Capture Fission Elastic Inelastic n,xn Transport Sum Contribution to reactivity change [pcm]

FCA-IX-1

10 10 DEN/DER/SPRC WPEC meeting Tokai-mura, June 3-4-5, 2008

U-238 contributions to the JEFF-3.1 – JEF-2.2 reactivity difference Analysis and discussion

• 150.00
• 100.00
• 50.00

0.00 50.00 100.00 150.00 200.00 250.00 300.00 1e-01 1e+00 1e+01 1e+02 1e+03 1e+04 1e+05 1e+06 1e+07 1e+08 Energy [eV] Capture Fission Elastic Inelastic n,xn Transport Sum Contribution to reactivity change [pcm]

• 150.00
• 100.00
• 50.00

0.00 50.00 100.00 150.00 200.00 250.00 300.00 1e-01 1e+00 1e+01 1e+02 1e+03 1e+04 1e+05 1e+06 1e+07 1e+08 Energy [eV] Capture Fission Elastic Inelastic n,xn Transport Sum Contribution to reactivity change [pcm]

• 150.00
• 100.00
• 50.00

0.00 50.00 100.00 150.00 200.00 250.00 300.00 1e-01 1e+00 1e+01 1e+02 1e+03 1e+04 1e+05 1e+06 1e+07 1e+08 Energy [eV] Capture Fission Elastic Inelastic n,xn Transport Sum Contribution to reactivity change [pcm]

FCA-IX-1 FCA-IX-3 FCA-IX-2

11 11 DEN/DER/SPRC WPEC meeting Tokai-mura, June 3-4-5, 2008

Summary

• “Inconsistencies” found by JAEA with JENDL files have been investigated

by CEA/KAERI using JEFF files

• Models used are small FCA IX-1, IX-2, IX-3 U+C fast cores, 93% U-235,

depleted U blanket provided by JAEA under WPEC/SG29

• ERANOS calculations and perturbation analyses
• CEA/KAERI results are consistent with those of JAEA
• Reasons for the observed reactivity variations are mainly differences

between the JEF-2.2 and JEFF-3.1 U-235 capture & fission cross sections in the core → net <0 term U-238 transport cross section (l > 0) in the blanket → net >0 term

• For the lowest leakage core (FCA IX-1) which is most sensitive to the U-

235 capture in the keV range, a fairly good C/E agreement appears to be

• btained with JEFF-3.1

Summary and conclusion

12 12 DEN/DER/SPRC WPEC meeting Tokai-mura, June 3-4-5, 2008

Conclusion

• CEA/KAERI findings are consistent with those of JAEA, but…
• Reactivity “trend” does not come from U-235 capture in the keV range

alone, but from positive and negative contributions of various magnitude mainly caused by U-235 capture: negative, keV range U-235 fission: negative 100 keV range, positive MeV range U-238 transport: positive, MeV range

• With JEF(F) files, there is no clear evidence of an “inconsistency”,

and one cannot immediately conclude that there is a problem with the latest evaluated U-235 capture cross section around 1 keV

• There is probably room for small changes/improvements in the JEFF-3.1

(and JENDL-3.3) U-235 capture cross section in the keV region, but the FCA IX-1, IX-2, IX3 benchmarks alone do not provide enough evidence to motivate changes

Summary and conclusion

• G. Chiba (JAEA)

P1 elastic scattering cross section of U-238 JEFF-2.2, JENDL-3.3 >> JEFF-3.1, JENDL/AC

Sensitivity of P1 elastic scattering cross section of U-238 to keff

C/E of criticality of small fast systems `Reflector bias’ observed in the JENDL-3.3 results disappears in JENDL/AC due to the revision of P1 elastic of U-238.

C/E for criticality of small fast systems

EALF (keV) Fuel Reflector BFS-62-1 59.7 IEU+Na U Zeus-4 110 HEU+C Cu HMF030 216 HEU+Be U ZPPR-21F 284 HEU+SS C FCA-IX-3 287 HEU+C U ZPR-3/23 381 HEU+Al Al BigTen 425 IEU U Flattop-25 751 HEU U Godiva 974 HEU

C/E of criticality of uranium-fueled fast systems

0.990 0.995 1.000 1.005 1.010 1.015 1.020 B F S

• 6

2

• 1

Z e u s

• 4

H M F 3 Z P P R

• 2

1 F F C A

• I

X

• 3

Z P R

• 3

/ 2 3 B i g T e n F l a t t

• p
• 2

5 G

• d

i v a C/E ENDF/B-VII.0 JENDL/AC

Sensitivity of U5 capture to keff

04/June/2008 WPEC SG29 Meeting 1

Proposal of New Integral Experiment at FCA for Evaluation of U-235 Capture Cross Sections

• Results of Sensitivity Analysis -
• T. Yamane, G. Chiba, S. Okajima

Reactor Physics Group, JAEA

04/June/2008 WPEC SG29 Meeting 2

Basic concept to select a new experimental core

To evaluate U-235 capture cross section, a new experimental core was considered. From view point of effectiveness of U-235 capture cross section evaluation in kev energy range, the previous results were taken into account;

• Na void reactivity worth measured at BFS,
• Systematic change of neutron spectra at FCA experiments.

To check the effectiveness, the following sensitivity was considered;

Σ ΔΣ ⋅ = Σ ΔΣ ⋅ Σ ΔΣ Δ ⇒ − = → Δ S J J J J J J ρ ρ ρ ρ ρ ρ ρ ) 33 ( ) 33 ( ) 32 ( ) 33 ( ) 32 33 (

04/June/2008 WPEC SG29 Meeting 3

Fuel Plates Arrangements of Experimental Core

C(1/4") C(1/4") C(1/16") C(1/16") HEU(1/16") HEU(1/16") C(1/4") C(1/4") C(1/4") C(1/4") C(1/4") C(1/4") C(1/4") C(1/4") C(1/4") EU(1/8") EU(1/16") C(1/4") HEU(1/16") EU(1/16") EU(1/8") C(1/4") EU(1/16")

Reference fuel cell (FCA-IX-1 core) Fuel cell for New Core （Averaged Enrichment=32wt%）

04/June/2008 WPEC SG29 Meeting 4

Plates Arrangements of Test Region

Na cell Void cell

NA NA SUS NA NA HEU(1/16") HEU(1/16") NA NA NA SUS SSV SSV SUS SSV HEU(1/16") SSV SSV HEU(1/16") SSV SUS SSV

FCA-IX-1 core

04/June/2008 WPEC SG29 Meeting 5

Plates Arrangements of Test Region

Na cell Void cell

NA EU(1/8") NA NA NA NA HEU(1/16") EU(1/16") EU(1/16") EU(1/8") EU(1/16") NA EU(1/8") SSV HEU(1/16") EU(1/16") SSV EU(1/16") SSV SSV EU(1/8") SSV EU(1/16") SSV

New Core (Case6)

04/June/2008 WPEC SG29 Meeting 6

Reference core (FCA-IX-1 core)

X X X X

Test zone Radial blanket(DUB) Core Test zone Test zone

Core Radial blanket(DUB) Radial blanket(DUB) Axial blanket (DUB) Axial blanket (DUB)

Test zone

r~30cm H~60cm

04/June/2008 WPEC SG29 Meeting 7

New Experimental Core（Surrounded by SB Regions)

X X X X

Radial blanket(ＳＢ) Radial blanket(DUB) Core Test zone

Core Radial blanket(SB) Radial blanket(SB) Axial blanket (SB) Radial blanket(DUB) Radial blanket(DUB) Axial blanket (SB)

Test zone

r~30cm H~60cm

04/June/2008 WPEC SG29 Meeting 8

Isotope and Reaction Breakdown of Sensitivity ( )

• 0.1

0.0 0.1 0.2 0.3 0.4 U235 Cap. U235 Fiss. U238 Cap. C Elas. C Mu Na Cap. Na Elas. Na Inelas. Cr Elas. Fe Cap. Fe Elas. Total All Ref(L=20.3cm) Ref(L=40.6cm)

Δρ(J33->J32)/ ρ(J33)

• 0.1

0.0 0.1 0.2 0.3 0.4 U235 Cap. U235 Fiss. U238 Cap. C Elas. C Mu Na Cap. Na Elas. Na Inelas. Cr Elas. Fe Cap. Fe Elas. Total All SB(L=20.3cm)J33 SB(L=40.6cm)J33

Δρ(J33->J32)/ ρ(J33)

Ref core (FCA-IX-1)

ρ(J33)=-34pcm(L=20.3cm) =-83pcm(L=40.3cm)

New core(with SB regions)

ρ(J33)=- 67pcm(L=20.3cm) =-180pcm(L=40.3cm)

（L : Axial length of perturbed region)

) 33 ( ) 32 33 ( J J J ρ ρ → Δ

04/June/2008 WPEC SG29 Meeting 9

Energy Breakdown of Sensitivity of Na Void Reactivity Worth ( ) for U-235 Capture Cross Sections

（注：Lは摂動領域長を表す。）

• 0.02

0.00 0.02 0.04 0.06 0.08 0.10 0.12 0.14 101 102 103 104 105 106 107

Ref(L=20.3cm) Case6(L=20.3cm) Case6SB(L=20.3cm) SB(L=20.3cm)

Δρ

(J33->J32)/ρ (J33)

En(eV)

(void region:-2Ｚ to +2Ｚ)

) 33 ( ) 32 33 ( J J J ρ ρ → Δ

04/June/2008 WPEC SG29 Meeting 10

Energy Breakdown of Sensitivity of Na Void Reactivity Worth ( ) for U-235 Capture Cross Sections

（注：Lは摂動領域長を表す。）

• 0.02

0.00 0.02 0.04 0.06 0.08 0.10 0.12 0.14 101 102 103 104 105 106 107

Ref(L=40.6cm) Case6(L=40.6cm) Case6SB(L=40.6cm) SB(L=40.6cm)

Δρ

(J33->J32)/ρ (J33)

En(eV)

(void region:-4Ｚ to +4Ｚ)

) 33 ( ) 32 33 ( J J J ρ ρ → Δ

04/June/2008 WPEC SG29 Meeting 11

Conclusion and Schedule

A new experimental core was selected to evaluate U-235 capture cross section in kev energy range. Na void reactivity worth measurement is effective in the evaluation of U-235 capture cross section. The experiment was delay since FCA operation stopped in July, 2007. The experimental core will be constructed in the end of this FY and measurements will carried out in the beginning of the next FY. (FCA operation will restart in November, 2008.)

Conclusions

• SG29 has investigated several benchmarks having a

high sensitivity to the U‐235 capture cross section in the keV range.

• Sensitivity studies confirm the possible

underestimation of this cross section in JENDL‐3.3, ENDF/B‐VII.0, JEFF‐3.1, although there is no compelling evidence to rule out other possible causes

• The uncertainties in alpha (= capture/fission ratio) and

sigma‐c measurements are compatible with a ‐5% adjustment in the U‐235 capture cross section around 1 keV

Future actions

• For the various benchmarks, compute sensitivity

coefficients to the cross sections, and not just the variations in the JENDL files. For the specific case of FCA‐IX‐3, try to identify the reasons for the strong reactivity overestimation with all recent evaluations

• Explain the inconsistencies in the alpha and capture
• measurements. Investigate possible new experiments to

get more accurate alpha and capture data in the keV range

• Consider a dedicated sodium void experiment in FCA, with

high sensitivity to U‐235 capture in the keV region, as soon as FCA is allowed to resume operation( > October 2008)

• A revised U‐235 evaluation based on a renormalized

capture will be prepared by ORNL for testing purposes

Proposal

• An extension of the SG29, by approximately

10‐12 months, would be desirable to complete the above tasks, particularly to include the findings of the FCA experiments