RADIATION EFFECTS CALCULATIONS

       R. K. Corzine, M. H. Barnett, Y. Zheng, D. J. Dudziak, and M. S. Wechsler

                             Department of Nuclear Engineering
                              North Carolina State University
                              Raleigh, NC 27695-7909 U.S.A.
                                 Email: dudziak@ncsu.edu

Two major projects, the Spallation Neutron Source (SNS) and the Accelerator Production of
Tritium (APT) facility, recently underwent conceptual designs in the U.S. In support of these
projects, calculations were performed of radiation effects in the form of production of atomic
displacements, helium, hydrogen, and transmutants. In the case of the SNS, emphasis was
placed on computing these radiation effects for 1-GeV proton irradiation of the 316 SS
containment vessel for the flowing mercury target. Additional calculations were performed to
determine the similitude of 150-MeV electrons to 1-GeV protons for PKA spectrum and
displacement cross section of 316 SS, in order to evaluate a possible experimental program.
Recent calculations for the APT have concentrated on modeling portions of an extensive
materials irradiation program performed with 800-MeV protons at the Los Alamos Neutron
Science Center, in order to correlate performance of various materials with predicted radiation
damage parameters. These parameters again included atomic displacements (dpa), and helium
and hydrogen production, all with associated proton and neutron fluences.            Materials
considered included W, Inconel-718, 316L SS, 304L SS, mod 9Cr-1Mo, Al 6061, and Al 5052.
All these calculations of radiation damage will be essential in predicting component lifetimes,
whether it be target or target containment materials and adjacent structural materials, or
accelerator beam windows. All calculations employed the LAHET Code System (LCS), using
LAHET2.82 for protons above 2 MeV and neutrons above 20 MeV, and MCNP4A for neutrons
below 20 MeV. Fairly large uncertainties still exist with respect to predicted radiation damage
values, mostly due to the theoretical models in LAHET. Studies have been performed of the
effect of various choices of physics parameters in LAHET, and comparisons have been made
with calculations using recent Los Alamos neutron and proton cross-section data from 20 to
150 MeV.      The calculational methods for predicting radiation effects are expected to be

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relevant for accelerator transmutation applications, and the correlations with experimental data
likewise should prove useful in predictions of component lifetimes for such applications.

Radiation effects calculations performed for the Spallation Neutron Source (SNS) facility1,2 and
the Accelerator Production of Tritium (APT) facility3,4 included proton and neutron fluences, and
production of atomic displacements, helium, hydrogen and transmutants in structural materials
including 316 SS, 316L SS, 304L SS, Inconel-718, and mod 9Cr-1Mo. These calculations were
for incident proton energies of 750 or 800 MeV in some cases, and 1 GeV in others. Several of
the materials considered may be of interest in designs of accelerator-driven transmutation
systems, including for beam windows and structural materials in target/blanket designs. There
are still scant experimental data for comparison with calculations, but the extant data for helium
production5 have provided some insight – but still large uncertainty – into selection of physics
models in the computer codes used for the computational analysis.          In addition, extensive
calculations have been performed for a major experimental program6 at the Los Alamos Neutron
Science Center (LANSCE). Ongoing analysis of the experimental results will provide additional
future benchmarks for computations, as well as correlations of computed radiation effects with
observed mechanical properties changes.

Comparisons of cross sections computed by the LAHET code system (LCS)7 with those
published by the Los Alamos National Laboratory nuclear data group T-2 for 20- to 150-MeV
neutrons8 show that the computed data in this energy range are of the order of 50% lower for
medium mass nuclei. The T-2 evaluated data, herein referred to as “LA150” data, agree well
with standard ENDF/B-6 data at 20 MeV, the conventional upper limit for ENDF/B evaluations.
Extensive parametric studies of physics options in LAHET2.82 have shown that the models
producing the greatest variation in displacement and helium production cross-section values were
the level density parameterization, the optional pre-equilibrium stage after the intranuclear
cascade, and, in the case of light nuclei, the Fermi breakup model.

Consideration was given to using a 150-MeV electron accelerator for simulation of radiation
damage effects of spallation irradiation of 316 SS. Initial analyses were performed9 to calculate

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the primary knock-on atom (PKA) spectra and displacement production cross sections for 150-
MeV electrons and 1-GeV protons to determine the degree to which the electron irradiation
would simulate the effects of the proton irradiation. Results showed that the PKA spectrum for
the 150-MeV electrons on 316 SS is much more weighted toward lower energy transfers than is
the case for 1-GeV protons, with a resulting average damage energy much lower for electrons
than protons.

For the SNS, the highest displacement rates computed in the 316 SS target structure are 11.47
dpa/yr, of which 6.24 dpa/yr are due to neutrons (90% from neutrons below 20 MeV), and the
remaining due to protons (90% from the incident protons at 1 GeV).10 An assessment of this
target’s structural lifetime based upon uniform elongation criteria proposed for the ITER fusion
reactor11 predicts a useful lifetime of 10 to 12 months. These values for an early 1997 target
design and will likely change as the design evolves.

Early calculations for the APT materials involved benchmarking 3He and 4He production
calculations using the LCS7 against experiments conducted by Green et al.5 for 750-MeV protons
at the Los Alamos LAMPF facility. * The He production cross section is given by
                                                      n/n 0
                                             ó He =                                     (1)

where n/n0 is the number of 3He or 4He atoms produced per incident particle, and Nvx is the atom
density times the target thickness; in this case, x = 1 cm was chosen. Detailed results are given in
references 12 and 13, where the experimental results are compared with calculations using three
different level-density models in LAHET2.82; viz., the mass- and isospin-dependent HETC
model, the energy-dependent GCCI model, and the mass-dependent Jülich model. No model was
consistently best for 750-MeV protons, with the HETC model best for Al, a light nucleus; the
Jülich model best for the medium mass nuclei Fe, Ni, and Cu; and the GCCI model best for the
heavy mass nuclei Mo, W, and Au. Major underestimates of 4He production would be made by
using other than the Jülich model for medium mass nuclei, but major overestimates would result
from using the Jülich model for heavy mass nuclei. Best agreement was always when using the

    LAMPF is the Los Alamos meson factory incarnation of LANSCE prior to 1995.

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pre-equilibrium model14 after the intranuclear cascade stage, where less energy is available in the
evaporation-fission stage where most helium nuclei are produced. Without the pre-equilibrium
model, calculated values with the best level-density model are roughly a factor of two higher than
experimental values. Similar results were found for 3He, but with production cross sections
roughly an order of magnitude lower than for 4He.

Detailed comparisons of 4He production cross sections were made between those calculated by
LAHET and those evaluated in the LA150 data files for medium mass nuclei Fe, Ni, Cr, and Nb.
The latter data are derived from nuclear model calculations using the GNASH reaction theory
code, benchmarked against experimental data when available.8 Because the nuclear interactions
below 150 MeV become more sensitive to nuclear structure and quantum effects in scattering, the
intranuclear cascade models become less reliable. In general, the LA150 4He production cross-
section data not only match the standard ENDF/B-6 data best at 20 MeV, but are considerably
higher than those from LAHET2.82 for medium mass nuclei over the entire range 20-150 MeV,
with rare exceptions where the pre-equilibrium model is not used in LAHET2.82. Thus, the
LA150 data were used whenever they were available for an element.             Figure 1 presents a
representative comparison, but in this case for Cr the discontinuity of the LA150 data with
standard ENDF/B-6 data at 20 MeV is greater than for the other nuclei studied.

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Fig. 1: 4He production cross section from LA150 data and computed with various level-density
models. Note: pre-equilibrium off except for case of GCCI model comparison.

Because of a paucity of experimental data for radiation effects of protons and neutrons of
energies ~ 1 GeV, an extensive program of experiments was undertaken at LANSCE, using an
800-MeV beam at 1 mA. In support of this program, extensive calculations were performed13 for
representative samples in the irradiation facility. Water-cooled sample holders containing the
candidate material specimens were placed at various positions relative to the proton beam, and
the beam profile was modeled. While proton fluences were reasonably representative of those
expected in APT for a full-power year, prototypic neutron fluences were not achieved for many
of the samples. Candidate window materials were directly exposed to the incident proton beam,
while candidate target/blanket structural materials were exposed to a more representative mixed
neutron and spallation-produced proton flux. The irradiations occurred over two campaigns, in
fall of 1996 and spring of 1997. Standardized LAHET2.82 calculations employed the Bertini

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intranuclear cascade,15 GCCI level-density, and pre-equilibrium models.       Maximum proton
fluences in the sample tubes varied from a high of 4.2 x 1021 p/cm2 for an Inconel-718 sample
which receives the majority of the incident proton beam (producing 12.5 dpa, 990 appm He, and
11,000 appm H), to about 0.5 x 1021 for a 304L SS sample at a position that receives a mixed
neutron and spallation-produced proton flux.       Neutron fluences were almost an order of
magnitude lower, with a maximum value of 7.6 x 1020 n/cm2, and the higher neutron fluences
generally appeared in samples with the higher proton fluences, but not necessarily in the same
order because of reflection from downstream tungsten samples. Also computed were the He, H,
and dpa production values, which were dominated by the proton flux in the case of the window
materials directly in the incident proton beam. Figure 2 shows the calculated dpa in various
samples for distances X along the proton beam transverse profile, which is a circular Gaussian
with 2σ = 3.5 cm and 3.0 cm for the first and second irradiation periods, respectively. Profiles
for He and H production are essentially the same as for dpa.

Fig. 2: dpa values computed by LCS for samples in two different irradiation locations (17A and
18C). Cf. Ref. 13.

                                                                                   Page 6 (of 8)
1. “Conceptual Design Report, National Spallation Neutron Source,” Vol. 1, Oak Ridge
     National Laboratory report NSNS/CDR-2/V1, Oak Ridge, TN, May 1997.
2. T. A. Gabriel, J. M. Barnes, L. A. Charlton, J. DiStefano, K. Farrell, J. Haines, J. O. Johnson,
     L. K. Mansur, S. J. Pawel, M. Siman-Tov, R. Taleyarkhan, M.W. Wendel, T. J. McManamy,
     and M. J. Rennich, “Target Systems Overview for the Spallation Neutron Source,” Nuclear
     Technology, to be published.
3.   “APT Conceptual Design Report,” Los Alamos National Laboratory report LA-UR-97-1329,
     April 1997.
4. S. A. Maloy, W. F. Sommer, R. D. Brown, J. D. Eddlemen, E. Zimmerman, and G. Willcutt,
     “Progress Report on the Accelerator Production of Tritium Materials Irradiation Program,”
     Materials for Spallation Neutron Sources, The Minerals, Metals, and Materials Society
     (TMS), 1998, pp. 131-138.
5. S. L. Green, W. V. Green, F. H. Hegedus, M. Victoria, W. F. Sommer, and B. M. Oliver,
     “Production of Helium by Medium Energy (600-800 MeV) Protons,” Journal of Nuclear
     Materials, Vol. 155-157, 1988, pp. 1350-1353.
6. S. A. Maloy and W. F. Sommer, “Spallation Source Materials Test Program,” Proc. of the
     Topical Meeting on Nuclear Applications of Accelerator Technology, Am. Nucl. Soc., 1997,
     pp. 58-61.
7. R. E. Prael and H. Lichtenstein, “User Guide to LCS: The LAHET Code System,” Los
     Alamos National Laboratory report LA-UR-89-3014, September 1989.
8. M. B. Chadwick, S. C. Frankle, R. C. Little, and P. G. Young, “Nuclear Data Libraries for
     Incident Neutrons and Protons to 150 MeV in ENDF-6 Format,” Proc. Of the Topical
     Meeting on Nuclear Applications of Accelerator Technology, Am. Nucl. Soc., 1997, pp. 175-
9. Y. Zheng, M. S. Wechsler, M. H. Barnett, D. J. Dudziak, and L. K. Mansur, “PKA Spectrum
     and Radiation Damage for 150-MeV Electrons on 316 Stainless Steel,” Trans. Am. Nucl.
     Soc., Vol. 80, 1999.
10. Marvin Hugh Barnett, III, Calculations of Radiation Damage to the SNS Stainless Steel
     Target Vessel, Master of Science thesis, North Carolina State University, April 1999.

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11. S. Majumdar, “Design Standard Issues for ITER In-vessel Components,” Fusion Engineering
   and Design, Vol. 29, 1994, pp. 158-163.
12. L. S. Charlton, L. K. Mansur, M. H. Barnett, R. K. Corzine, D. J. Dudziak, and M. S.
   Wechsler, “Calculations of Helium Production in Materials at Spallation Neutron Sources,”
   Proc. Of the AccApp’98, Am. Nucl. Soc., Gatlinburg, TN, 20-23 September 1998.
13. Rhonda Karen Corzine, Radiation Damage Calculations for the Accelerator Production of
   Tritium Program, Master of Science thesis, North Carolina State University, April 1999.
14. R. E. Prael, “A Review of Physics Models in the LAHET Code,” Intermediate Energy
   Nuclear Data: Models and Codes, Proc. of a Specialists’ Meeting, OECD, France, 1994.
15. H. W. Bertini, “Intranuclear-Cascade Calculation of the Secondary Nucleon Spectra from
   Nucleon-Nucleus Interactions in the Energy Range 340 to 2900 MeV and Comparisons with
   Experiment,” Phys. Rev. C, Vol. 20, No. 7, 1979, p. 2227.

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