Igor Remec

Effects of gamma-induced displacements on HFIR pressure vessel materials

Abstract Accelerated radiation-induced embrittlement of the High Flux Isotope Reactor (HFIR) surveillance materials has been investigated since its discovery in 1986. Recent comprehensive dosimetry experiments revealed the presence of an intense gamma field at the HFIR surveillance locations near the pressure vessel. Gamma-induced reactions were found to dominate the response of several dosimeters and were crucial for the explanation of dosimetry results. This finding precipitated an assessment of the gamma-induced displacements-per-atom (dpa) rate, which was found to exceed the neutron-induced dpa rate at all locations analyzed. When the sum of neutron and gamma dpa is used for the interpretation of the HFIR surveillance results, the HFIR data are consistent with data from the Oak Ridge Research Reactor (ORR) and other test reactors. The accelerated embrittlement is therefore explained in terms of hitherto uncounted dpa induced by gamma rays.

Estimated Limits on Uncontrolled Beam Losses of Heavy Ions for Allowing Hands-On Maintenance at an Exotic Beam Facility Linac

Abstract The purpose of the study is to obtain estimates of limits on uncontrolled beam losses of heavy ions for allowing hands-on maintenance at a heavy-ion linac for a rare isotope beam facility. Semiempirical formulas are used to estimate dose equivalent rates from activated accelerator components for 1 W/m uncontrolled losses of protons up to 1 GeV. The estimated dose rates after a 100-day irradiation time, 4-h postshutdown cooling time are compared to a hands-on maintenance limit of 1 mSv/h (100 mrem/h) at 30 cm. The transport codes PHITS and MCNP5 and activation code DCHAIN-SP 2001 are used to verify the estimate for proton losses and to obtain limits on heavy-ion beam losses that will satisfy the hands-on maintenance dose rate limit.

Proton Induced Activation in Mercury: Comparison of Measurements and Calculations

Measurements and simulations of the proton beam interaction with the mercury target were performed to support Spallation Neutron Source design. Due to the abundance of isotopes produced in mercury, the long delay between the irradiation and the measurements, and the self-shielding of the mercury sample, the measurements were difficult to perform and the activities of several isotopes have large uncertainties. Calculations predicted the activities of the most reliably measured isotopes within 20%–40%; however, some large discrepancies were observed for some isotopes for which the measurements were considered less reliable. Predicted dose rates were in very good agreement with the measurements.

Radiation Simulations and Development of Concepts for High Power Beam Dumps, Catchers and Pre-Separator Area Layouts for the Fragment Separators for RIA

The development of high-power beam dumps and catchers, and pre-separator layouts for proposed fragment separators of the Rare-Isotope Accelerator (RIA) Facility are important in realizing how to handle the 400 kW in the primary beam. Examples of pre-conceptual designs of the pre-separator area and components, along with examples of ongoing radiation simulations with results characterizing the secondary radiation are given. These initial studies will yield insight into the impact of the high-power dissipation on fragment separator design, remote handling concepts, nuclear safety and potential facility hazard classification, shielding design, civil construction design, component design, and material choices. Furthermore, they will provide guidance on detailed radiation analyses as designs mature.

Benchmarking of Neutron Production of Heavy-Ion Transport Codes

Accurate prediction of radiation fields generated by heavy ion interactions is important in medical applications, space missions, and in design and operation of rare isotope research facilities. In recent years, several well-established computer codes in widespread use for particle and radiation transport calculations have been equipped with the capability to simulate heavy ion transport and interactions. To assess and validate these capabilities, we performed simulations of a series of benchmark-quality heavy ion experiments with the computer codes FLUKA, MARS15, MCNPX, and PHITS. We focus on the comparisons of secondary neutron production. Results are encouraging; however, further improvements in models and codes and additional benchmarking are required.

Studies of Limits on Uncontrolled Heavy Ion Beam Losses for Allowing Hands-On Maintenance

Dose rates from accelerator components activated by 1 W/m beam losses are obtained semiempirically for a 1 GeV proton beam and by use of Monte Carlo transport codes for the proton beam and for 777 MeV/u 3He, 500 MeV/u 48Ca, 86Kr, 136Xe, and 400 MeV/u 238U ions. The dose rate obtained by the semi-empirical method, 0.99 mSv/h (99 mrem/h) at 30 cm, 4 h after 100 d irradiation by a 1-GeV proton beam, is consistent with studies at several accelerator facilities and with adopted hands-on maintenance dose rate limits. Monte Carlo simulations verify this result for protons and extend studies to heavy ion beam losses in drift-tube linac and superconducting linac accelerating structures. The studies indicate that the 1 W/m limit imposed on uncontrolled beam losses for high-energy proton beams might be relaxed for heavy ion beams. These studies further suggest that using the ratio of neutrons produced by a heavy ion beam to neutrons produced by a proton beam along with the dose rate from the proton beam (for thin-target scenarios) should allow an estimate of the dose rates expected from heavy ion beam losses.

Neutronic Analyses in Support of the HFIR Beamline Modifications and Lifetime Extension

At the High Flux Isotope Reactor, in operation since 1966 at the Oak Ridge National Laboratory, a larger HB-2 beam tube was installed to enhance capabilities for neutron science research. Neutronic analyses, including dosimetry measurements, radiation transport simulations, and simultaneous neutron and gamma spectrum adjustment calculations, performed to assess the impact of modifications on the PV lifetime are presented.

Analysis of HFIR Dosimetry Experiments Performed in Cycles 400 and 401

The High Flux Isotope Reactor (HFIR) has been in operation at Oak Ridge National Laboratory since 1966. To upgrade and enhance capabilities for neutron science research at the reactor, a larger HB-2 beam tube was installed in April of 2002. To assess, experimentally, the impact of this larger beam tube on radiation damage rates [i.e., displacement-per-atom (dpa) rates] used in vessel life extension studies, dosimetry experiments were performed from April to August 2004 during fuel cycles 400 and 401. This report documents the analysis of the dosimetry experiments and the determination of best-estimate dpa rates. These dpa rates are obtained by performing a least-squares adjustment of calculated neutron and gamma-ray fluxes and the measured responses of radiometric monitors and beryllium helium accumulation fluence monitors. The best-estimate dpa rates provided here will be used to update HFIR pressure vessel life extension studies, which determine the pressure/temperature limits for reactor operation and the HFIR pressure vessels remaining life. All irradiation parameters given in this report correspond to a reactor power of 85 MW.

Radiation environment at ISOL target station of rare isotope facility

Next-generation exotic beam facilities will offer several approaches to produce rare isotopes far from stability. One approach is Isotope Separation On-line, ISOL, which is the isotope production by interactions of light ion beams with heavy nuclei of targets. A pre-conceptual design of an ISOL target station was done as part of the research and development work for the Rare Isotope Accelerator, RIA. This report summarizes results of radiation calculations for the RIA ISOL target station. This includes radiation effects such as prompt radiation at the target station and from neutron sky-shine, activation of ground water, air, and components.

Determination of component activation and radiation environment in the second stripping region of a highpower heavy-ion linear accelerator

In supporting pre-conceptual research and development of the Rare-Isotope Accelerator (RIA) facility or similar next-generation exotic beam facilities, one critical focus area is to estimate the level of activation and radiation in the linear accelerator second stripping region and to determine if remote handling is necessary in this area. A basic geometric layout of the second stripping region having beamline magnets, beam pipes and boxes, a stripper foil, beam slits, and surrounding concrete shielding was constructed for Monte Carlo simulations. Beam characteristics were provided within the stripping region based on this layout. Radiation fields, radioactive inventories, levels of activation, heat loads on surrounding components, and prompt and delayed radiation dose rates were simulated using Monte-Carlo radiation transport code PHITS[1]. Results from simulations using a simplified geometry show that remote handling of foils and slits will be necessary. Simulations using a more realistic geometry were performed and the results agree well with the estimation by the simple model.