Characterization of the Neutron Spectra in Various Oregon State University TRIGA® Reactor Irradiation Facilities

E.D. Ashbaker1; S.R. Reese1; and L.R. Greenwood2 (1Oregon State University; 2Pacific Northwest National Laboratory)

The Oregon State University TRIGA® Reactor (OSTR) has a number of irradiation facilities providing a wide range of neutron energies and fluxes. These facilities include a pneumatic transfer tube, a rotating rack, a thermal column, four beam ports, in-core irradiation tubes, a cryogenic gas transfer system and a large vault radiography facility. Until now, the neutron energy spectra in these various facilities were not precisely known. The spectra were, depending on location, assumed to resemble a thermal Maxwellian distribution or a Watt fission spectrum. Two recent changes have made knowledge of the neutron energy spectrum in the irradiation facilities a necessity. First, the 1 MeV equivalent neutron damage factor has become increasingly important to researchers interested in using the OSTR for materials science/radiation damage testing. Precise knowledge of the spectrum is needed in order to calculate this factor. Second, changes to the configuration of beam port shielding and our increased use of neutron beams makes it prudent to have a better knowledge of the neutron energy spectrum for calculating dose in the event of an accidental overexposure. To resolve this, the OSTR was modeled using the Monte-Carlo N-Particle (MCNP) transport code. Activation foils, with activation cross sections that covered a wide and overlapping range of energies, were also employed. Data from the foils were used to adjust the calculated MCNP spectrum using the dosimetry unfolding code STAY'SL. STAY'SL is a least-squares adjustment code system which provides the best fit to the neutron flux spectrum as well as uncertainties in the group fluxes due to the uncertainties in the activation data, dosimetry cross sections, and input group fluxes. Results indicate that the MCNP model may underestimate the thermal neutron contribution by as much as a factor of four. However, the calculations are in excellent agreement with the activation data for the epithermal and fast neutrons.

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