Helium production in HFIR-irradiated pure elements

Journal of Nuclear Materials 141- 143 (I%%)X24-828 824

North-Holland, Amsterdam

HELIUM PRODUCTION IN HFIR-IRRADIATED PURE ELEMENTS D.W. KNEFF

I, L.R. GREENWOOD

‘, B.M. OLIVER’

and R.P. SKOWRONSKI

1

’ Rockweft~~t~~~atlonuf Corporutwn, Rocketdytte Division, 6633 Cunogu Awnue, Cmogu Furk, C..4 9I 303, USA .’ Argonne National Laborutary,

Chemical

Technology Diuision. 9700 South Cuss Avenue. Argonne,

IL 60439, USA

Helium measurements have been made for Ni, Fe, Ti, Nb, Cr. and Cu samples from several different fusion materials irradiations in the mixed-spectrum High Flux Isotope Reactor (HFIR). The results are compared with helium predictions based on neutron energy spectrum me~urements and the ENDF/B-V Gas Production File. The comparisons indicate good agreement for Ni, Fe, and Cr, but cross-section discrepancies were observed for helium production by fast neutrons in Ti, Nb, and Cu. Nickel and copper exhibit enhanced helium production at high neutron flucnces, and there may also be a small enhancement for iron. This work demonstrates the importance of measuring helium production in materials of interest over the full range of neutron fluences used in fusion materials tests. The combination of measurements and predictions from well-characterized neutron fields can then be used to provide accurate helium estimates for materials irradiations.

1. Introduction

Mixed-spectrum (comparable thermal and fast neutron flux) reactors constitute an important resource for the irradiation testing of fusion reactor materials. The high neutron fluences can produce high displacement damage and, for nickel and stainless steel, large heliumto-displacement ratios. The interpretation of these high-fluence experiments often depends on calculated displacement damage and helium production rates. Both displacement damage and helium production are being calculated routinely for fusion reactor materials after mixed-spectrum reactor irradiations [l], and the helium generation rates have been measured for a number of materials from several of these irradiations [2,3]. Comparisons between the calculations and the measurements provide a means for testing the cross sections used in making the helium predictions, and provide a measure of confidence in the use of these predictions. The present paper summarizes the results of helium measurements and predictions for Ni, Fe, Ti, Nb, Cr, and Cu samples that were irradiated in the mixed-spectrum High Flux Isotope Reactor (HFIR) at the Oak Ridge National Laboratory. Comparisons between helium calculations and experimental measurements are used to demonstrate the accuracy of our predictions for some elements and to indicate errors in the nuclear data file ENDF/B-V for other elements. The results also show the effects of multiple-step nuclear reactions and sample bumup by neutron reactions at high fluences, and their potentially large effect on helium production. 2. Irradiations and helium measurements The measurements reported in this paper were made for samples from six different fusion materials experiments in HFIR, covering a total neutron fluence range of - 1 x 10zl to 2 x 1O23 n/cm*. Most of the highfluence data are from experiments CTR32 (10863

0022-3115/86/$03.50 0 Elsevier (North-Holland Physics ~blis~n8

Science Publishers Division)

MWd), CTR31 (21853 MWd), and CTR30 (43316 MWd) that were performed in the HFIR-PTP (Peripheral Target Positions). The lower-fluence data are from the RB-HF 2350-MWd spectral characterization irradiation, performed at the out-of-core RB (Removable Be~llium) position, and which included both hafniumcovered and bare irradiation samples. Each irradiation included dosimetry capsules at several different vertical positions in the experimental volume. These capsules contained sets of radiometric dosimetry wires for neutron fluence and energy spectrum characterization of the irradiation environment. Several capsules included bare and encapsulated pure element samples for helium generation measurements. The pure element materials had been previously characterized to verify that they contained no background helium. Following irradiation, the radioactivity of the capsules was allowed to decay for several months before disassembly. The activation rates of all samples were then measured using gamma spectroscopy with germanium detectors. The measurements were generally accurate to f 1.5%. After activation counting, the helium generation samples were etched to eliminate possible surface effects of alpha recoil and analyzed by isotope-dilution gas mass spectrometry [4] for reactor-generated 4He. The irradiation locations of the samples and their measured helium concentrations are summarized in tables 1 and 2. Multiple samples were analyzed at most locations with excellent repr~ucibi~ty and absolute uncertainties of *l-2%. The nickel and copper data are presented to update and to provide comparisons with HFIR results that have been reported previously [2,3]. Selected samples of each element from each irradiation were also analyzed for 3He, which is formed from the decay of tritium that is often found in reactor environments. The 3He concentrations were generally found to be less than 1 appb (10m9 atom fraction) for the Ni, Fe, Cr, and Cu samples, but were somewhat higher ( - 2 to 170 appb) for the CTR30-32 Ti and Nb samples. B.V.

825

D. W. Knefj et ai. / H&WI production in HFIR-irradiared pure elenlents Table 1 Helium production measurements for HFIR-irradiated pure elements iMaterial

Experiment

Core height a> (cm)

Nickel

T2 RBI CTR32

CTR32

Iron

CTR32

CTR31

CTR30

Titanium

RB-HF @if-covered) RB-HF (bare)

CTR32 CTR33.

CTR30

Niobium

CTR32 CTR31

CTR30

Chromium

RB-HF (Hf-covered)

Total neutron fiuence (10z2 n/cmz)

Meas.

Calc.

(appm) b,

(wm)

1386 2089 2815 4397 1416 1929 3568 3694 3025 1515 4011 7286

1292 20.50 2954 3674 1371 2015 3230 3219 2965 1337 4041 6899

17.0 7.2 10.7 2.0 20.8 16.7 4.2 - 4.3 -8.3 - 20.8 20.8 - 12.5

2.31 3.04 3.98 4.58 2.78 3.50 4.70 4.69 4.45 2.74 5.44 7.89

17.61 4.41 - 7.38 21.07 4.41 - 12.26 21.07 4.41 - 12.26

3.48 4.81 4.60 5.57 9.67 8.30 11.0.5 19.16 16.45

- 0.08 - 12.78 - 19.76 20.72 - 0.07 - 7.69 - 18.81 4.57 21.23 4.57 - 12.10 21.23 4.57 - 12.10

0.174 0.136 0.086 0.233 0.446 (I.408 0.241 4.80 5.41 9.65 8.34 IO.92 19.14 16.53

0.086 0.064 0.046 0.068 0.106 0.096 0.067 2.17 2.52 4.29 3.90 5.27 8.49 7.67

4.73 23.39 4.73 - 11.94 21.39 4.73 - 11.94

4.79 5.44 9.64 8.38 10.79 19.11 16.60

- 8.89

0.155

&alc./Expt.

4He concentration

2.24 3.12 2.99 3.66 6.76 5.73 8.17 14.69 12.36

(C/E) b’ 0.93 0.9x 1.05 0.84 0.97 1.04 0.92 0.87 0.98 0.88 1.01 0.95

2.37 3.25 3.14 3.68 6.39 5.64 7.64 12.95 IO.99

1.06 1.04 1.0s 1.01 0.95 0.98 0.94 0.88 O-89

0.210 0.160 0.114 0.132 0.234 0.209 0.130 5.52 6.04 10.49 9.46 12.27 21.44 18.67

2.44 2.50 2,SQ 1.93 2.21 2.17 1.93 2.54 2.40 2.45 2.42 2.33 2.52 2.43

0.78 0.91 1.54 1.43 1.93 3.09 2.69

0.59 0.66 1.18 1.02 X.32 2.33 2.03

0.15 0.73 0.76 0.7X 0.68 0.75 0.75

0.049

0.052

1.06

a) Distance above core midplane. b, Atomic parts per million (10e6 atom fraction).

3. Reactor characterization

and Mium

gwefbtims

The neutron flux and energy spectra were characterized for each irradiation using the activation data

and the multiple-foil activation dosimetry method fl]. The data were used to adjust an initial flux spectrum for each irradiation using the computer code STAYSL [5] and ENDF/B-V activation cross sections [6]. STAYSL

826

D. W. Kneff et al. / He&urn ~r~d~et~~n in HFIR-i~r~dlated

Table 2 Helium production measurements for HFIR-irradiated Experiment

Core height a)

(cm)

pure elements

copper

Thermal neutron fluence b’

‘He concentration

(102’ n/cm2)

Calc./Fxpt.

Measured

Calculated (appm) Cl

(appm)

fast

”

(C/E)

thermal

RB-HF (Hf-covered)

20.24 - 3.88 - 13.41

0.0091 0.0162 0.0127

0.072

0.039

< 0.001

0.117 0.096

0.070 0.055

i 0.001

RB-HF (bare)

11.51 - 3.73 - 12.62

0.139 0.158 0.127

0.113 0.136 0.110

0.067 0.076 0.061

0.002 0.003 0.002

0.61 0.58 0.57

CTR32

21.86 5.20 - 11.47

0.84 1.58 1.24

2.50 5.77 4.54

1.58 2.90 2.57

0.39 2.10 1.18

0.79 0.87 0.83

CTR31

21.86 5.20 - 11.47

1.91 3.19 2.57

6.78 21.08 14.95

3.30 5.97 5.33

2.65 13.08 7.58

0.88 0.90 0.86

CTR30

21.86 5.20 - 11.47

3.36 6.12 4.96

24.1 81.3 52.4

6.17 11.32 10.01

14.90 66.00 39.92

0.87 0.95 0.95

a) Distance above core midplane. b, 2200 m/s value, including thermal-neutron self-shielding corrections; for RB-HF ”

(Hf-covered)

and 3 times the thermal

fluence

total incident neutron for the other experiments.

< 0.001

0.54 0.60 0.57

fluence is 10 times the thermal

fluence

Atomic parts per million (low6 atom fraction).

performs a generalized least-squares adjustment of the cross sections, flux spectrum, and measured activation data, using their uncertainties and covariances. The initial flux spectrum is determined from a spectral characterization irradiation, using a calculated flux spectrum [7] and a more extensive dosimetry set. A neutron energy spectrum for the HFIR-FTP is given in ref. [l]. Final spectrum uncertainties are typically +- 10%. The adjusted neutron spectra and measured flux gradients were then used to predict helium concentrations for the helium generation samples using

ENDF/B-V [6,8-131 and the computer code SPECTER [14]. The predictions are given in tables 1 (column 6) and 2 (columns 5,6). The results for nickel and copper include additional calculations for contributions from thermal and epithermal neutrons, as described further below. 4. Comparisons between measurements and predictions Comparisons between the measured and predicted helium generation in the analyzed samples are given in

column 7 of tables 1 and 2, where the comparisons are expressed as ratios of the calculated to experimental values (C/E). These ratios provide a test of the spectrum-integrated ENDF/B-V cross sections applicable to the neutron energy spectra incident upon the samples and an indication of high-fluence effects on helium generation rates. The ENDF/B-V cross sections, COU-

pled with the unfolded neutron energy spectra, provide generally good (within f 10%) agreement for Ni, Fe, and Cr. The results for Ti, Nb, and Cu, however, indicate discrepancies in the ENDF/B-V cross sections for helium production by fast neutrons. Ni, Cu, and perhaps Fe also exhibit enhanced helium production at high neutron fluences, because of bumup and multiplestep nuclear reaction effects. Helium is produced in nickel in HFIR predominantly by the two-stage reaction ‘*Ni(n, ~)‘~Ni(n, cw) 56Fe. The large measured helium concentrations (table 1) demonstrate the large helium production rates for thermal neutrons, and the importance of this reaction in simulating fusion reactor irradiation effects in a mixedspectrum fission reactor. The two-stage calculations for nickel were based on the 58Ni(n, y) cross section from the ENDF/B-V activation library [6], plus 5qNi(n, ar) and 59Ni(n abso~tion) cross-section evaluations by F.M. Mann (21. The latter cross section includes the competition from 59Ni(n, y) and 59Ni(n, p) reactions at thermal energies. The calculation of total helium concentrations also includes helium production by epithermal and fast neutrons, with the latter based on Ni(n, a) cross sections from the ENDF/B-V Gas Production File [S]. The average C/E value for nickel from table 2 is 0.95 t: 0.07. This demonstrates agreement between the measurements and predictions to within the uncertainties of the calculations (+ 10%) and helium mea-

surements ( f 2%).

821

D. W. Kneff et al. / Helium production in HFIR-irradiated pure elements

The iron results from table 1 give an average C/E value of 0.98 * 0.07, which also indicates consistency between the helium measurements and the ENDF/B-V Gas Production File [9]. However, the iron C/E values exhibit a decreasing trend with increasing neutron fluence. This suggests a small, nonlinear increase in helium production with increasing neutron fluence because of bumup and multiple-step reactions. The lower-fluence C/E values thus provide a better comparison for testing ENDF/B-V fast-neutron cross sections. The helium concentration measurements for titanium and niobium (table 1) differ significantly from the helium predictions, with average C/E values of 2.34 f 0.20 and 0.73 f 0.03, respectively. These comparisons indicate large discrepancies with the ENDF/B-V files [lO,ll]. The small but systematic differences between the C/E values for bare and hafnium-covered titanium from RB-HF are attributed to spectral differences that are not reflected in the calculations. The chromium data (table l), which represent only one reactor location, have a C/E value (1.06) which suggests consistency with the ENDF/B-V cross section [12]. However, a preliminary measurement from the Oak Ridge Research Reactor (ORR) is inconsistently higher (C/E = 1.37), indicating a need for further measurements. Additional work is in progress. For copper, helium is produced by fast neutrons and, at high fluences (> lo** n/cm*), by a three-stage thermal-neutron reaction mechanism [3]: 63Cu(n,

y)64Cu( P-)64Zn(n,

y)65Zn(n,

a)62Ni.

Table 2 summarizes our copper measurements for four HFIR irradiations, and gives the helium predictions separately for the fast and thermal neutron contributions. Both fast and thermal neutrons contribute significantly to helium production in the CTR (CTR30-32) samples, with the three-stage mechanism dominating at the higher fluences. From the CTR data we deduced three thermal-neutron reaction cross sections (including effects from a 7% epithermal HFIR flux) which allow us to calculate explicitly the three-stage helium contribution in HFIRirradiated copper [3]. The cross sections were derived from a combination of post-irradiation 65Cu/63Cu isotopic measurements, 65Zn reaction rate measurements, total helium production measurements, and known neutron capture cross sections, with the C/E ratio for the fast-neutron helium production an additional parameter used in fitting the data. These calculations yielded 270 + 170 b for the 64Cu(n, Y)~~CU reaction, 66 & 8 b for the 65Zn(n absorption) [(n, y) + (n, p) + (n, a)] reaction, and 4.7 k 0.5 b for the 65Zn(n, ar)62Ni reaction. The CTR data were also used to derive an empirical expression that estimates the enhancement in helium production in HFIR-irradiated copper by thermal neutrons:

Table 3 Summary

of calculation/experiment

Material

C/E value

Comments

Nickel

0.95 f 0.07

Iron

0.98 i 0.07

Chromium Titanium Niobium Copper

1.06 2.34kO.20 0.73 f 0.03 0.58 f 0.02 0.76 + 0.05

Prediction includes thermal effects Some fluence dependence observed Single comparison only

Enhanced

4He (appm)

(C/E)

values for HFIR

RB position PTP position; fast-neutron comparison only

= 0.669@*.*‘,

where @ is the thermal neutron fluence in units of lo** n/cm’. The CTR data gave a fast-neutron C/E ratio for copper of 0.76 for the HFIR-PTP. (The C/E values in table 2 combine the fast and thermal effects.) This fast C/E ratio yields a spectrum-integrated, fast-neutron Cu(n, a) cross section of 313 k 20 pb. The RB-HF measurements, representing lower fluences, negligible thermal neutron effects, and a differing energy spectrum, have a lower average C/E value of 0.58 f 0.02. This difference may be due in part to spectral uncertainties, but may also reflect inconsistencies in the energy dependence of the helium production cross section for copper. 5. Conclusions The results are summarized in table 3. They demonstrate our ability to use energy-spectrum and helium generation measurements to test spectrum-integrated cross-section evaluations for helium production. The C/E values indicate the presence of discrepancies in ENDF/B-V cross sections and the presence of unexpected high-fluence effects that can contribute significantly to helium production. It is thus important that helium measurements be performed for all materials of interest in fusion test environments, over the full range of applicable neutron fluences. The measurements provide direct data for the interpretation of materials effects and for tests of helium predictions. The combination of measurements and verified predictions can then be used to provide accurate helium estimates for materials irradiations. Support in this work by J.R. Szekeres and H. Farrar IV (Rockwell) is gratefully acknowledged. This work was funded by the US Department of Energy’s Office of Fusion Energy.

828

D. W. Kneff et ~1. / Helium

productron

References [l] L.R. Greenwood, in: Proc. Fourth ASTM-EURATOM Symp. on Reactor Dosimetry, Gaithersburg, MD, March 22-26, 1982, Ed. F.B.K. Kam, NUREG/CP-0029 (US Nuclear Regulatory Commission, 1982) p. 783. [2] L.R. Greenwood, D.W. Kneff, R.P. Skowronski and F.M. Mann, J. Nucl. Mater. 122 & 123 (1984) 1002. [3] D.W. Kneff, L.R. Greenwood. B.M. Oliver, R.P. Skowronski and E.L. Callis, in: Proc. Int. Conf. on Nuclear Data for Basic and Applied Science, Santa Fe, NM, May 1985, Radiat. Eff. (in press); see also D.W. Kneff, L.R. Greenwood and E.L. Callis, in: Damage Analysis and Fundamental Studies, Quarterly Progress Report, April-June 1985, DOE/ER-#46/22, US Department of Energy (1985) p. 8. [4] B.M. Oliver, J.G. Bradley and H. Farrar IV, Geocbim. Cosmochim. Acta 48 (1984) 1759. [5] F.G. Perey, ORNL/TM-6062, Oak Ridge National Laboratory (1977); modified by L.R. Greenwood (1979). [6] Evaluated Nuclear Data File ENDF/B-V, Special Purpose Activation File, National Nuclear Data Center, Brookhaven National Laboratory (evaluations by several authors). [7] R.A. Lillie. Oak Ridge National Laboratory, personal communications.

in HFIR-irradiated

pure elements

[8] M. Divadeenam, ENDF/B-V Gas Production File for Ni, Mat 5328, Brookhaven National Laboratory (1977); see BNL-NCS-51346, Brookhaven National Laboratory (1979). [9] C.Y. Fu, ENDF/B-V Gas Production File for Fe, Mat 5326, Oak Ridge National Laboratory (1979); see ORNL/TM-7523, Oak Ridge National Laboratory (1980). [IO] ENDF/B-V Gas Production File for Ti, Mat 5322, C.A. Philis (Centre d’Etudes de Bruyeres-le-Chfitel), A.B. Smith (Argonne National Laboratory), and R. Howerton (Lawrence Livermore National Laboratory) (1977): see ANL/NDM-28, Argonne National Laboratory (1977). [II] ENDF/B-V General Purpose File for 93Nb, Mat 1189, R. Howerton (Lawrence Livermore National Laboratory) and A. Smith et aL (Argonne National Laboratory) (1974); see ANL/NDM-6, Argonne National Laboratory (1974). [12] A. Prince and T.W. Burrows, ENDF/B-V Gas Production File for Cr, Mat 5324, Brookhaven National Laboratory (1977); see BNL-NCS-51152, Brookhaven National Laboratory (1979). [13] C.Y. Fu, ENDF/B-V Gas Production File for Cu, Mat 5329, Oak Ridge National Laboratory (1979); see ORNL/TM-8283, Oak Ridge National Laboratory (1982). 1141 L.R. Greenwood and R.K. Smither, ANL/FPP/TM-197, Argonne National Laboratory (198.5).