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According to a recent study (Eastman-Seitz Committee, National Academy of Science) there is a need for a new generation of steady neutron sources with a thermal neutron flux peak between 5 to 10 times 1015/cm2 sec. Ideally the neutron source would have to operate continuously for several days (two weeks at least) with minimum time (2 to 3 days) for refueling and/or maintenance and it would also be used to irradiate materials and produce isotopes. This paper describes the preliminary design of the nuclear reactor for the proposed Center for Neutron Research (CNR). A duplication of existing designs (HFIR, (ORNL), ILL (Grenoble, France)) would imply high total power and small core life; the necessity of higher efficiencies (in terms of peak-flux-per-unit source or power) then becomes apparent. We have found analytical expressions for the efficiency in terms of a few parameters such as the volume of the source and the Fermi age and diffusion length of thermal neutrons in both the source and reflector regions. A single analytical expression can then be used for scoping the design and to intercompare radically different designs. Higher efficiencies can be achieved by reducing the volume and the moderation of a core immersed in a very low absorbing reflector; on the contrary a very long core life has a negative effect on the efficiency at beginning of life. Consequently, and after detailed calculations, we have found a candidate design with the following characteristics: core, U3Si2, 93% enriched, 18.1-kg 235U, metal fraction 50%, Al cladding, and 35-L volume; reflector and moderator, D2O; efficiency at end of life (EOL) with respect to the ILL reactor, 1.29; flux at EOL, 10 x 1015/cm2 sec (power in core 270. MW); core life, 14 days; burnup 28.4%.
A fast neutron irradiation facility has been designed, modeled, and constructed in the beam port 4 facility at The University of Texas at Austin’s TRIGA Mark-II Reactor. This facility targets the Watt-fission neutron spectrum in a controlled environment by reducing the present thermal and epithermal flux while preserving the fast neutron flux. The present facility will open new avenues in nuclear non-proliferation for fast-fission yields in addition to measuring radionuclide migration. The filter system was designed using MCNP and Solidworks and consists of a lead plug to stop gamma-rays, filter elements of natural boron and 96% enriched B10, collimation elements of borated polyethylene and natural boron, and an exit filter of boron nitride. A beam stop was constructed to reduce the ambient dose rate using borated paraffin wax, polyethylene, cadmium, and lead. Sensitivity studies were performed to configure an economic facility by optimizing the amounts and configurations of materials used in the filter. The filter is modular to allow for rearrangement of elements and the ability to change the materials used as needed should higher efficiencies be desired or a higher total flux. Initial results indicate the facility produces a 10 cm diameter beam with an integrated flux of 6.63x105 n/cm2/s at a reactor power of 950 kW and resembles the Watt-fission spectrum well with a slightly elevated epithermal neutron flux. The fast neutron flux above 0.1 MeV constitutes 98.77% of the total flux and the thermal neutron flux only 0.0014% of the total flux. STAYSL PNNL was used to unfold the neutron spectrum from 9 measurable reactions in 5 flux foils. Results suggest that the fast neutron flux is higher than anticipated in all STAYSL runs although the total flux is lower than anticipated.
Abstract: The objective of this research was to bring a thermal neutron beam facility to the Ohio State University Nuclear Reactor Laboratory for the purposes of neutron-based research. The neutron beam is extracted from the reactor core through a neutron collimator emplaced in Beam Port #2, the radial beam port facing the core at a 30° angle. The collimator is an aluminum tube containing components designed to filter and shape the neutron beam. The filters are poly-crystalline bismuth (10.16 cm thickness, 12.7 cm diameter) for significantly reducing gamma ray content and single-crystal sapphire (12.7 cm thickness, 10.16 cm diameter) for preferentially passing thermal neutrons while scattering more energetic neutrons out of the beam. The thermal neutron beam is defined by multiple 3.0 cm diameter apertures in borated aluminum. Apertures in polyethylene-based disks and in Pb disks provide shielding for fast neutrons and gamma rays, respectively, in the neutron collimator. Characterization of the beam was performed using foil activation analysis to find the neutron flux and a low-cost digital neutron imaging apparatus to "see" the beam profile. The neutron collimator delivers the filtered thermal neutron beam with a 3.5 cm diameter umbra and a thermal neutron equivalent flux of (8.55 +̲ 0.19) x 106 cm−2s−1 at 450 kW reactor power (90% of rated limit) to the sample location. The beam is highly thermalized with a cadmium ratio of 266 +̲ 13. The facility was designed for neutron depth profiling, a nondestructive analytical technique for finding the concentration versus depth in the near surface (tens of microns) for isotopes that undergo charged particle emitting reactions, such as 10B(n, 4He)7Li, 6Li (n, 3H)4He, and 3He (n, 1H)3H, to name a few.
The ability of the Ohio State University Research Reactor, OSURR, to conduct experiments from the generation of the neutron flux is important in conducting research by the University and external entities that require a flux of this magnitude. In particular, research involving a fast neutron flux is of interest due to the different interactions fast neutrons have as opposed to thermal neutrons. The OSURR is able to operate up to 500 kW, which creates a neutron flux in the order of 1013 n/cm2-s. Currently, Beam Port 2 provides a thermal neutron beam profile of 30 mm in diameter for experimentation such as neutron depth profiling, activation analysis, and evaluation of radiation damage to electronics. Beam Port 1 uses a sample area located adjacent and perpendicular to the fuel plates of the reactor for in-core irradiation. During experimentation, the remainder of Beam Port 1 must be plugged with removable concrete shielding to prevent radiation exposure that can be upwards of 1x104 rem/hr. The upgrade to Beam Port 1 consists of a collimator to shape the neutron flux from the reactor into a beam of fast neutrons, similar in diameter to Beam Port 2, in order to irradiate samples external to the reactor. In addition, mobile external shielding is designed to prevent exceeding the exposure limits of 5 rem/yr when the facility is in use. With this upgrade the research reactor has the ability to conduct simultaneous experiments with a fast and thermal neutron beam, external to the biological shielding, without releasing any harmful exposure.