International Evaluation Co-operation

VOLUME 5
PLUTONIUM-239 FISSION CROSS-SECTION
BETWEEN 1 AND 100 keV

NUCLEAR ENERGY AGENCYNEA/WPEC–5

International Evaluation Co-operation

VOLUME 5
PLUTONIUM-239 FISSION CROSS-SECTION
BETWEEN 1 AND 100 keV

A report by the Working Partyon International Evaluation Co-operationof the NEA Nuclear Science Committee

CO-ORDINATORMONITORE. FortM. Salvatores

Comissariat à l’Énergie Atomique
C.E. Cadarache
FranceComissariat à l’Énergie Atomique
C.E. Cadarache
FranceNUCLEAR ENERGY AGENCY
ORGANISATION FOR ECONOMIC CO-OPERATION AND DEVELOPMENT

1

2

FOREWORD

A Working Party on International Evaluation Co-operation was establishedunder the sponsorship of the OECD/NEA Nuclear Science Committee (NSC)
to promote the exchange of information on nuclear data evaluations, validation,and related topics. Its aim is also to provide a framework for co-operative
activities between members of the major nuclear data evaluation projects.
This includes the possible exchange of scientists in order to encourage
co-operation. Requirements for experimental data resulting from this activity arecompiled. The Working Party determines common criteria for evaluated nuclear
data files with a view to assessing and improving the quality and completeness ofevaluated data.
The Parties to the project are: ENDF (United States), JEF/EFF (NEA Data
Bank Member countries), and JENDL (Japan). Co-operation with evaluation
projects of non-OECD countries are organised through the Nuclear Data Sectionof the International Atomic Energy Agency (IAEA).
The following report was issued by a Subgroup investigating the fission crosssection of Plutonium-239 in the energy range 1 to 100 keV. This cross section isof particular importance for fast reactor applications, such as keff , sodium void
reactivity coefficient and control rod worth. An analysis of recent experimental
data by L. Weston et.al. give significantly lower cross-section values that thesimultaneous evaluation performed by W. Poenitz for the ENDF/B-VI library.
The objective of the subgroup was to resolve this discrepancy.
The opinions expressed in this report are those of the authors only and do not
represent the position of any Member country or international organisation.
This report is published on the responsibility of the Secretary-General of
the OECD.

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Members of Subgroup 5

FRANCEUNITED STATES OF AMERICA

DERRIEN, H.
Commissariat à l’Énergie Atomique
C.E. Cadarache(retired)

WESTON, L.W.
Oak Ridge National Laboratory
YOUNG, P.G.
Los Alamos National Laboratory

FORT, E.
Commissariat à l’Énergie Atomique
C.E. Cadarache

INTERNATIONAL ORGANISATIONSLAGRANGE, Ch.
Commissariat à l’Énergie Atomique
C.E. Bruyères-le-Chatel

WAGEMANS, C.
Joint Research Centre
Geel, Belgium

SALVATORES, M.
Commissariat à l’Énergie Atomique
C.E. Cadarache

JAPAN
KAWAI, M.
Toshiba
Kawasaki-ku
NAKAJIMA, J.
JAERITokai-Mura
TAKANO, H.
JAERITokai-Mura

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CONTENTS

Summary....................................................................................................... 6
1. Introduction............................................................................................. 7
2. Experimental work .................................................................................. 7
2.1. Critical examination of the Weston and Todd experiment.................. 7
2.2. Experimental work performed in the framework of Subgroup 5......... 9
2.2.1. Geel experiment ................................................................... 9
2.2.2. Oak Ridge experiment.......................................................... 10
3. Examination of other sources of information ............................................ 123.1. Japanese evaluation work................................................................. 123.2. Theoretical and validation of work ................................................... 14
3.2.1. Theoretical work .................................................................. 14
3.2.2. Validation of JEF-2.2 against integral data ........................... 16
4. Conclusion .............................................................................................. 19
References..................................................................................................... 20

5SUMMARY

In order to understand the discrepancies between Weston’s data – NSE 88,
567, 1984 – and the major file evaluations, one experimental program and one
evaluation one have been agreed upon:
The experimental program which essentially aims at normalisation checking
has been performed in GEEL and Oak Ridge. It supports an upward
re-normalisation by ~3.1%. The evaluation program has not been completed andeven, as a consequence of the experimental results, loses a part of its justification.
But some acquired results are important and can be used for future 239Pu
evaluations. The JEF-2.2 benchmarking performed in another context supports
the results of the experimental program although it suggests a slightly larger renormalisation.

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PLUTONIUM-239 FISSION CROSS-SECTION
BETWEEN 1 AND 100 keV

1. Introduction
The fission cross-section measured by Weston – NSE 88, 567, 1984 –in excellent resolution conditions is about 5% lower than almost all recent
measurements and all major evaluations in the range 1–100 keV. Expressedin terms of critical mass, control rod worth, and void coefficient,
this 5%-difference is of importance for fast reactor calculations.
In order to solve this problem Subgroup.5 of the NEA/NSC Working Party on
International Evaluation Co-operation adopted the following two-part plan:
1. Critical examination of Weston’s experiment with possibly additional
experiments to check questionable points;
2. Examination of other sources of information relating to fission crosssection.

2. Experimental work
2.1. Critical examination of Weston’s and Todd’s experimentThis is a T.O.F. experiment performed at ORELA using a multiparallel platefission chamber; the shape of the neutron flux was measured relative toa 10BF3 chamber up to a neutron energy of 1 keV, and relative to a 6Li glass
scintillator at higher energies. The flux inter-normalisation was made in
the energy interval between 100 and 1000 eV with a statistical uncertainty of lessthan 0.1%. Furthermore the σf-curve was normalised at thermal energy;
this method of normalisation is unsuitable for experiments aimed at obtaining data
at high energy, because of sample thickness self-absorption, and dead-time
effects. The energy range of flux intercomparison in Weston’s experiment has
7been chosen to intercompare the various sets of data through fission integral
values:If =∫1000 eV

100eV

σ f ( E ) dE

For the Weston’s 84 data the fission integral is If = 8996 b•eV, which is known tohave rather significant uncertainties: 1.9% for normalisation, 1% systematic,
0.15% statistical. When considering Gwin’s measurements performed in similar
conditions, we obtain If = 9268 b•eV [1] and If = 9286 b•eV [2].
The 4%-difference suggested the possibility of a normalisation error in Weston’s
experiment. But since Gwin’s experiments are affected with a large uncertainty(11%) in the 10B-content of the neutron flux counter, they have not been
considered as absolutely reliable references.
In addition when looking at Figure 1, and considering the Poenitz standard[16] as a good average representation of major evaluations, we observe a
difference of ~3% between 0.1 and 10 keV and ~5% between 10 and 100 keV.
This situation of energy-dependent discrepancy raises the questions ofa normalisation error and of a possible energy-dependent effect in Weston’s
experiment.
Therefore it has been decided to plan experimental programs both at
Oak Ridge and Geel in order to check this normalisation point.

Figure 1 Ratio of Weston’s data (1984) to simultaneous fit of Poenitz
8

2.2. Experimental work performed within the framework of Subgroup 5
2.2.1. Geel experiment
In Geel (1992) two different geometries have been used by C. Wagemans [3]:1. Measurement with double ionisation chamber in a ‘2-π geometry’.
The characteristics were as follows:− 239Pu sample:
186 µg/cm 2
10− B sample:
10 µg/cm 2− Flight path length:
8.5 cmThe normalisation has been made in two ways, all based on the thermalvalue of the 239Pu fission cross-section.
i. via the fission integral between 20 and 60 meV,
60 meV∫20meV σ f dE = 25.36 b • eV , from which an average value of 634 b is
obtained and is to be compared with 631.4 b, which is the average ofseveral measurements selected by H. Derrien from his most recent
resonance parameter evaluation [4];ii. via a linear least square fit of the σ E data between 20 and
30 meV, from which a value of 784.25 b was derived for the thermal
cross-section.
Within these conditions of normalisation Wagemans derived
If = 9190 ± 110 b•eV. The 1.2%-uncertainty results from 0.2% of counting
statistics, 0.5% of uncertainty on the background correction, and 0.6% dueto the normalisation.
2. Measurement in ‘low geometry’ using surface barrier detectors, with:− 239Pu sample:− 10B sample:− Neutron flight path length:

106 µg/cm 2
10 µg/cm 2
8.3 cmFor this measurement the fission integral value is (9450±200) b•eV.
The components of the total uncertainty are: 0.8%, 0.5% and 1%

9respectively, due to counting statistics, background and normalisation
corrections.
Considering both data, a weighted average value is obtained for If:
Wa

If = 9250±96 b•eV.

2.2.2. Oak Ridge experiment
In Oak Ridge a new fission cross-section measurement was performedby Weston et al. [9]. The experimental technique was similar as previously but
the detectors were different: A parallel plate 10B ionisation chamber with solid
coating of Boron on the centre plate was used rather than the 10BF3 gas.
The fission chamber was also different as the active plates were loaded with 239Pu
only, so that better counting statistics is obtained for the same irradiation time.
The flight path length was 19 m, and the measurement was continuous frombelow 0.025 eV up to 10 keV. Careful attention was paid to the normalisation
which was more precise than in the previous measurement –0.5% against 1.9%[8] –, while the overall systematic uncertainty was estimated to be 0.6% – to becompared with 1%. The normalisation was done on the integral σ f = E over theinterval 0.02 to 0.03 eV referring to the ENDF/B-VI evaluation:
0.03 eV∫0.02 eV σ f E dE = 118.98 b • eV

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The If value obtained in these conditions is:We

If = 9302±102 b•eVThe total uncertainty of this value is estimated from the just above-mentioned
values on normalisation and systematic uncertainty, including
a 0.1%-component due to statistical uncertainty.
This normalisation operation is consistent with the set of normalisation
operations used by Wagemans et al.
Although obtained in somewhat different conditions of detection, Wagemans’and Weston’s If values based on thermal fission data are in excellent agreement.
All these values have been obtained from data normalised on the ENDF/B-VI

10thermal value of the 239Pu fission cross-section as a primary standard, andadapting the ENDF/B-VI values for the 10B(n,α) 7Li cross-section.
The final If value which results from the experimental work on normalisation
is obtained from a weighted average of Wagemans’ and Weston’s data, which are
shown in Table 1.

Table 1 Wagemans’ and Weston’s data
WAGEMANS1

9190±110

92

WAGEMANS2

92

WESTON9450±200929302±102

STANDARD If = 9275±85 b•eVThis average value which results from particularly careful measurements
should be recommended as a secondary standard in addition to the thermal value,
to be used in the case of two-step measurements or any measurement subject toenergy-dependent effects. In the following, we will refer to it as the Standard If.
The Normalisation Factor to be applied to Weston’s data (1984) is derived as:
NF =

9275
= 1031.
± 0.0098996

The If values obtained from the few measurements performed in the thermal
range up to 1000 eV (see Table 2), are consistent with the Standard If, sincea mean value of 9278 b•eV is derived, affected with a 61 b•eV standard deviation.

Table 2 If value in b•eV related to experimental data
GWIN

71

9268

GWIN

76

WAGEMANS

9286* This re-calculated using the ENDF/B-VI values for the11809280 *
10

B(n,α) 7Li cross-section.

Note that all above-mentioned fission integrals are normalised on
σf = 748.0 b at thermal energy.

3. Examination of other sources of information
Below we describe the work performed within the framework of Subgroup 5 inorder to explore other sources of information than the experimental one. It brings
an indirect support to the normalisation and shows that many of the major
evaluations could be improved, although they perform satisfactory in several
applications. The following provides data, information or tools which should be
used for improving these evaluations over large energy ranges.

3.1. Japanese evaluation work
A specific study, made in Japan (Nakagawa, Nikai,...), has shown that othertypes of data such as α data or competitive reaction data like capture, non elastic,
inelastic,.etc., are too inaccurate to be used as references.
In the resolved range, H. Derrien in CEA-France and Nakagawa in
JAERI-Japan have produced a new resonance parameter set extended up to
2.5 keV.
This evaluation is a simultaneous fit [4] of various experimental data setsaccording to a Bayesian method using the SAMMY code allowing backgroundand normalisation coefficient adjustment. The following experimental data basewas used:− Absorption and fission data from Gwin et al. [1,2],− Fission data from Gwin et al. [5,6], Blons [7], Weston and Todd [8,9];− Transmission data from Spencer [10], Harvey [11].

These experimental data sets are characterised by high-energy resolution
and/or low background. With respect to the data sets from which previous
resolved resonance parameters – included in JEF-2.2 and ENDF/B-VI – havebeen derived, additional sets are included, in particular the high-resolution
transmission data by Harvey et al. [11] and the high-resolution cross-section
measurement by Weston and Todd (1988) [14]. If we use this Japanese
evaluation, we obtain for the range 100-1000 eV a fission integral equal to12

9304 b•eV, which is to be compared with the If values obtained from major
evaluations as quoted in Table 3.

Table 3 If value in b•eV related to evaluated data
ENDF/B-VI

JEF-2.2

JENDL-3

POENITZ
standard

DERRIEN

9017

9040

9040

9377

9304

92

The If values have been calculated from averaged fission cross-sections
according to the energy scheme given in Table 4 below.

Table 4 Averaged fission cross-section
DERRIENJENDL-3

JEF-2

80.12
94.74
17.52
50.64
54.42
18.63
17.85
8.31
9.59

91.87
16.98
49.1
53.23
18.14
13.31
8.083
9.391

200-500

11.92

500-600
600-700
700-800
800-900
900-1000
500-1000

ENERGYeV

NAKAGAWA
1992

WESTON
1984 -renorm.

POENITZ
standard

0.010-10
9-20
20-40
40-60
60-100100-200
200-300
300-400
400-500

80.25
91.87
17.09
49.16
52.94
18.14
17.32
8.083
9.391

17.97
50.87
54.33
18.56
17.89
8.34
9.58

18.66
17.88
8.43
9.57

11.59

11.60

11.93

11.96

15.39
4.37
5.51
4.84
8.33

15.06
4.131
5.324
4.730
8.230

15.06
4.131
5.324
4.730
8.230

15.57
4.30
5.53
4.89
8.38

15.86
4.46
5.63
4.98
8.30

7.69

7.496

7.496

7.73

7.79

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3.2. Theoretical and validation work
To start with, it was decided to perform two model calculations in order todescribe the fission data, the first one based on Weston’s ‘84 Œ, and the second
on ENDF/B-VI •. Both would be based on the same neutron channel description
and validated against clean integral experiments.

3.2.1. Theoretical work
High resolution transmission data have been obtained at ORELA by J. Harvey
et al. [11] using three samples, cooled at liquid-nitrogen temperature,
the thicknesses of which were chosen so as to be a good compromise between
getting accurate data and moderate self-screening effects. From these transmission
data, a total cross-section has been derived. It is in agreement with theexperimental data of Poenitz et al. [12] but significantly lower – by 3 to 4% –than JEF-2 or ENDF/B-VI. In our opinion the difference results from the selfscreening correction and also from the better quality of raw transmission data.
The data base formed by Poenitz’ and Derrien’s data have been considered as
total cross-section reference data in the range 1 to 500 keV.
The scattering radius and Sl=0 and Sl=1 neutron strength function values in
the resonance range were extracted from the simultaneous fit by H. Derrien to
derive the JEF-2 resonance parameter sets.
These were all ingredients needed to derive an optical model parametrizationthat was obtained by Ch. Lagrange in 1990 (OMP-90). Compared withthe previous OMP used in JEF-2 (OMP-86) [13], real and imaginary parts ofthe potential were re-normalised; the ranges of the effective interactions andthe spin-orbit potential remained unchanged. With this new optical model
parametrization, an overall improved fit to the experimental data is obtained:
neutron scattering angular distributions (see Figure 2) and the total cross-section
above 0.5 MeV, which is an energy range rich in data (see Figure 3).

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Figure 2
Comparison of experimental inelastic scattering cross-sections at 3.4 MeV
with data calculated with OMP-86 and OMP-90

Figure 3
Experimental total cross-sections comparedwith data calculated with OMP-86 and OMP-90

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In addition there is a significant decrease (10-15%) in the compound crosssection below 1.5 MeV (See Figure 4).
This is an important feature from which direct conclusions can be drawn.

Figure 4
Comparison of the coumpound nucleus formation cross-section
obtained by using OMP-86 (curve 1) and OMP-90 (curve 2)

The model evaluations Œ is almost completed and shows that a fission crosssection based on Weston ‘84 is low with respect to all other evaluations up to
1 MeV, i.e., in the full sensitivity range of fast reactors (see Figure 5). We also
observe, with respect to the JEF-2.2 evaluation, a 10%-lowering of the inelastic
cross-section. This suggests that a model based on data from Poenitz,
for example, would result in even lower values for the inelastic cross-section.
Concerning the impact on integral data of k-eff, we note that a lowering of σn,n’
compensates to some extent the effect of a lowering of σn,f because of the η-curve
shape. Figure 5 also shows the amplitude of discrepancies between major values
concerning this cross-section.

3.2.2. Validation of JEF-2.2 against integral data
The model evaluation • has not been performed. The model evaluation Œ has
not been tested yet against chosen clean integral data, but the JEF-2.2

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benchmarking, which is being performed allows some conclusions to be drawn.
The conditions of this benchmarking are fully described in reference [15].
They can be summarised as follows:

Figure 5
Experimental fission cross-section data compared withthe model calculations (OMP-90) based on Weston’s data

The integral data base was made of a large number of data (∼200) of different
types – critical masses, Bucklings, spectral indices, neutron deep penetration
data, ... – for a large range of spectral hardness.
The JEF-2.2 library has been processed into a 1968-energy-group library
containing infinite dilute cross-section and probability tables, which have been
produced by using the NJOY-THEMIS and CALENDF codes, respectively.
A great effort has been devoted to Quality Assurance, in order to ensure that dataprocessing would preserve the information quality and integrity. The integral data
have been re-calculated with this fine group library. Cross-section modifications,
to minimise discrepancies between experimental and re-calculated integral data,
have been obtained by a statistical adjustment according to the “general leastsquares” method. Since there is no uncertainty information in JEF-2, a previous
systematic study had defined the covariance data needed for obtaining the best
conditions for this adjustment procedure.

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The quality of the data adjustment has been measured by the a posteriori
χ value which according to the theory should be equal to N ± 2 N , N being
the number of degree of freedom – number of integral data in the present case.
A technique based on the comparison of the practical χ2 distribution withthe theoretical one was used to identify in the integral data those data (∼10%)
giving spurions information in order to obtain χ2/N = 0.997.
2

The conclusions of this adjustment, published at the Gatlinburg Conference[18] have been carefully compared nucleus by nucleus with external independent
information – experimental, critical analysis – obtained in the meantime andrelated to 235U, 56Fe, 58Ni and 23Na.
The agreement is excellent. The conclusions of this global adjustment cantherefore be considered as perfectly reliable for 239Pu. They are as follows:− For σn,n:decrease by about 10%,
This conclusion is perfectly consistent with the abovementioned model calculation.

− For σn,f:if the following modifications are made, namely:+3.1% renormalisation of Weston’s ‘84 data, on the one
hand, and JEF2 σf-adjustment, on the other hand, JEF2− 1 in % Weston 

then, the discrepancies – expressed by –are significantly reduced except for energy intervals of
3.355 – 9.119 keV and 9.119 – 24.79 keV located in
the unresolved range.
This is explained as follows: The calculations for evaluation purposes are
performed in Cadarache, CEA-France, with the FISINGA code [17] with a crosssection calculation formalism which is different from the one recommended by the
ENDF/B prescriptions and used in the data processing code NJOY. In particular
this last one uses a single level formalism for the unresolved range and integer
values for the degree of freedom for the χ2 distribution for fission widths. So,
the evaluated average parameters have been translated into NJOY-type data insuch a way that they respect the infinite dilute values but not the self-shielding
factors, which are calculated with the CALENDF code from generated resonanceladders. Finally all effects result in too low effective fission cross-sections andconsequently in anomalously high adjustment corrections.

18On a qualitative level the benchmarking confirms first the need fora re-normalisation upwards by 3 to 4% but also the “general” shape of the fission
cross-section as measured by Weston which comes out to be lower than mostevaluations in the range 10 to 100 keV. More refined conclusions would haverequired a benchmark testing of the model evaluation Œ, as previously planned.

Table 5 Discrepancies between JEF-2 and Weston’s data
ENERGY GROUP

JEF-2

WESTON-84

BEFOREAFTER

STRUCTURE eV

barn

barn

MODIFICATIONMODIFICATION

4.5400E+02 - 1.2340E+03
1.2340E+03 - 3.3550E+03
3.3550E+03 - 9.1190E+03
9.1190E+03 - 2.4790E+04
2.4790E+04 - 6.7380E+04
6.7380E+04 - 1.1110E+05

7.091
3.653
2.246
1.722
1.578
1.544

6.99
3.50
2.18
1.67
1.48
1.42

1.44
4.38
3.05
3.1
6.6
8.7

-1±1.71.4±1.3
5.6±3.1
3.5±2.2
1.7±1.4
0.5±1.3

4. Conclusion
The experimental program performed in Geel and Oak Ridge has solved
the problem of the normalisation in Weston’s experiment. It ended up in
the definition of a standard value for If – the fission integral between 100 and1000 eV, equal to 9275±±85 b•eV. Concerning the possible energy-dependent
experimental effect, the exhaustive JEF-2 benchmarking tends to prove its nonexistence although the demonstration requires more accuracy.
Due to low background and good energy resolution of the experiment,
Weston’s ‘84 re-normalised cross-section values are proposed as reference data
for any future 239Pu evaluation, which should also take benefit from some otherconclusions derived in the course of this work, such as the competitive data or thebest presently avaible OMP for 239Pu.

19

References[1]

R. Gwin, L.W. Weston, G. de Saussure, R.W. Ingle, J.H. Tood,
F.E. Gillespie, R.W. Hockenbury and R.C. Block, N.S.E. 45, 25 (1971).

[2]

R. Gwin, E.G. Silver, R.W. Ingle and H. Weaver, N.S.E. 59, 79 (1976).

[3]

C. Wagemans P. Van Uffelen, A. Deruytter, R. Barthelemy and J.Vangils,
N.S.E. 115, 173(1993).

[4]

H. Derrien and J. Nakagawa, Journal Nuclear Science and Technology
30, 845 (1993).

[5]

R. Gwin et al., N.S.E. 61, 116 (1976).

[6]

R. Gwin et al., N.S.E. 88, 37 (1984).

[7]

J. Blons, N.S.E. 51, 130 (1973).

[8]

L.W. Weston and J. Tood, N.S.E. 88, 567 (1984).

[9]

L.W. Weston, J. Todd and H. Derrien, N.S.E. 115, 164 (1993).

[10] R.R. Spencer et al., N.S.E. 96, 318 (1987).
[11] J.A. Harvey, Proceedings of the International Conference Nuclear Data
for Science and Technology, MITO, May 1988, p.195.
[12] W.P. Poenitz, J.F. Whalen and A.B. Smith, N.S.E. 78, 333 (1981).
[13] Ch. Lagrange and D.G. Madland, Physical Review C. Vol.33, N°5,
p.1616.
[14] L.W. Weston and T.H. Todd, Unpublished work, Private communication to
H. Derrien.
20[15] E. Fort and M. Salvatores “Fast Reactor Benchmarks and Integral Data
Testing and Feedback into JEF-2”, Symposium on Nuclear Data
Evaluation Methodology, Brookhaven, U.S.A., October 1992.
[16] W.P. Poenitz, “Data Interpretation Objective Evaluation Procedure andMathematical Techniques for the Evaluation of Energy-dependent Ratio,
Shape and Cross-Section Data”, Workshop Nuclear Data Evaluation
Methods Procedures, Upton, New York, U.S.A., September 1980,
BNL-NCS-51363, p.249.
[17] E. Fort and D. Lafond, “The FISINGA Code”, unpublished.
[18] E.Fort and M. Salvatores, “JEF-2 Validation, Methodology - Present
Results -Future plans”, International Conference on Nuclear Data for
Science an Technology, May 1994, Gatlinburg, U.S.A.

21