Skip to main content
Open Access Publications from the University of California

Recent Work

Welcome to the McClellan Nuclear Radiation Center (MNRC). At the heart of the MNRC is the newest research reactor in the United States. The nuclear reactor at the MNRC attained first operation in 1990, and has over 30 years of productive service remaining.

The custom designed TRIGA (Training, Research, and Isotope Production General Atomics) reactor can operate at a steady state power of up to 2 MW or pulse to approximately 1000 MW for 20 milliseconds. Our staff of 20 reactor operators, health physics technicians, scientists and engineers has over 100 years of experience with the MNRC. Normal operations are 16 hours per day, five days a week, with two shifts. We have the flexibility to change operating schedules to meet customer requirements.

The MNRC was originally developed by the USAF to detect low-level corrosion and hidden defects in aircraft structures using neutron radiography. Since then, MNRC service has expanded to include computer tomography (three-dimensional neutron radiography), silicon doping, isotope production, neutron activation analysis, and radiation effects testing. We have the capability of moving materials and parts into the central core facility and locations adjacent to the core while the reactor is operating.

Cover page of Phenotypic and genomic analyses of a fast neutron mutant population resource in soybean

Phenotypic and genomic analyses of a fast neutron mutant population resource in soybean


Mutagenized populations have become indispensable resources for introducing variation and studying gene function in plant genomics research. In this study, fast neutron (FN) radiation was used to induce deletion mutations in the soybean (Glycine max) genome. Approximately 120,000 soybean seeds were exposed to FN radiation doses of up to 32 Gray units to develop over 23,000 independent M2 lines. Here, we demonstrate the utility of this population for phenotypic screening and associated genomic characterization of striking and agronomically important traits. Plant variation was cataloged for seed composition, maturity, morphology, pigmentation, and nodulation traits. Mutants that showed significant increases or decreases in seed protein and oil content across multiple generations and environments were identified. The application of comparative genomic hybridization (CGH) to lesion-induced mutants for deletion mapping was validated on a midoleate x-ray mutant, M23, with a known FAD2-1A (for fatty acid desaturase) gene deletion. Using CGH, a subset of mutants was characterized, revealing deletion regions and candidate genes associated with phenotypes of interest. Exome resequencing and sequencing of PCR products confirmed FN-induced deletions detected by CGH. Beyond characterization of soybean FN mutants, this study demonstrates the utility of CGH, exome sequence capture, and next-generation sequencing approaches for analyses of mutant plant genomes. We present this FN mutant soybean population as a valuable public resource for future genetic screens and functional genomics research.

Cover page of Periodic magnetic fieldas a polarized and focusing thermal neutron spectrometer and monochromator

Periodic magnetic fieldas a polarized and focusing thermal neutron spectrometer and monochromator


A novel periodic magnetic field PMF optic is shown to act as a prism, lens, and polarizer for neutrons and particles with a magnetic dipole moment. The PMF has a two-dimensional field in the axial direction of neutron propagation. The PMF alternating magnetic field polarity provides strong gradients that cause separation of neutrons by wavelength axially and by spin state transversely. The spin-up neutrons exit the PMF with their magnetic spins aligned parallel to the PMF magnetic field, and are deflected upward and line focus at a fixed vertical height, proportional to the PMF period, at a downstream focal distance that increases with neutron energy. The PMF has no attenuation by absorption or scatter, as with material prisms or crystal monochromators. Embodiments of the PMF include neutron spectrometer or monochromator, and applications include neutron small angle scattering, crystallography, residual stress analysis, cross section measurements, and reflectometry. Presented are theory, experimental results, computer simulation, applications of the PMF, and comparison of its performance to Stern–Gerlach gradient devices and compound material and magnetic refractive prisms.

Cover page of Rocky Flats CAAS System Recalibrated, Retested, and Analyzed to Install in the Criticality Experiments Facility at the Nevada Test Site

Rocky Flats CAAS System Recalibrated, Retested, and Analyzed to Install in the Criticality Experiments Facility at the Nevada Test Site


Neutron detectors and control panels transferred from Rocky Flats Plant (RFP) were recalibrated and retested for redeployment to the CEF. Testing and calibration were successful with no failure to any equipment. Detector sensitivity was tested at the TRIGA reactor, and the response to thermal neutron flux was satisfactory. MCNP calculated minimum fission yield (~ 2 × 1015 fissions) was applied to determine the thermal flux at selected detector positions at the CEF. Thermal flux levels were greater than 6.39 × 106 (n/cm2-sec), which was about four orders of magnitude greater than the minimum alarm flux. Calculations of detector survivable distances indicate that, to be out of lethal area, detector needs to be placed greater than 15 ft away from the source. MCNP calculated flux/dose results were independently verified by COG. CAAS calibration and the testing confirmed that the RFP CAAS system is performing its functions as expected. New criteria for the CAAS detector placement and 12-rad zone boundaries at the CEF are established. All of the CAAS related documents and hardware are transferred from LLNL to NSTec for installation at the CEF high bay areas.

Cover page of Development of a Large-Scale Iodine-125 Production System at UC Davis/MNRC

Development of a Large-Scale Iodine-125 Production System at UC Davis/MNRC


The demand for iodine-125 (125I) as a medical radioisotope for use in the treatment of prostate cancer continues to increase. However, due to uncertainties with current commercial production facilities, potential supply issues have emerged prompting several reactors worldwide to consider the development and installation of large-scale 125I production facilities. In 2002, MNRC installed and operated successfully for ~ 1.5 years, a closed loop system using aluminum material for containment of the enriched 124Xe (99%) target during irradiation. However, problems with design features and restrictions on serviceability and repairs ultimately forced MNRC to abandon it as a first target failure resulted in high contamination levels in the whole system which further restricted personnel accessibility. Today, a new target and a multi-compartment (modular) transport and decay system with automatic operation and dispensing of high-level batches of 125I and with ready access for maintenance and repairs have been designed to resume 125I production activities. The new operating conditions provide ample opportunities for increased production with shorter irradiation times.

Cover page of Thermal Neutron Computed Tomography of Soil Water and Plant Roots

Thermal Neutron Computed Tomography of Soil Water and Plant Roots


Neutron radiography is a noninvasive imaging technique that measures the attenuation of thermal neutrons, as is done with x-ray and γ-ray radiography, to characterize the internal composition of materials. Neutron and x-ray imaging are complementary techniques, with neutron imaging especially well suited for materials containing H atoms and other low-atomic-weight attenuating materials. Although neutron computed tomography (NCT) techniques are routinely used in engineering, relatively little is known about their application to soils. We developed new techniques that use thermal neutron attenuation to measure the spatial and temporal distribution of water in soils and near roots at near 0.5-mm spatial resolution or higher. The neutron source was a Mark II Triga Reactor at McClellan Nuclear Radiation Center in Sacramento, CA. After calibration using both deuterated and regular water, the effects of beam hardening and neutron scattering could be corrected for, provided that the total path length for a soil–water mixture does not exceed 1.0 cm, limiting soil sample thickness to about 2.5 cm. Using regular water, for a wide range of soil water content values, experiments demonstrated that NCT is sensitive to small changes in soil volumetric water content, allowing estimation of the spatial distribution of soil water, roots, and root water uptake. Although the spatial resolution of the applied NCT system was 80 μm, an error analysis showed that the averaging measurement volume should be not less than about 0.5 mm for the uncertainty in volumetric water content to be minimized to near 0.01 m3 m−3. A single root water uptake experiment with a corn (Zea mays L.) seedling demonstrated the successful application of NCT, with images showing spatially variable soil water content gradients in the rhizosphere and bulk soil.

Cover page of Neutron Tomography and Space

Neutron Tomography and Space


The University of California/Davis McClellan Nuclear Radiation Center (UCD/MNRC) was originally constructed by the U.S. Air Force as a nondestructive testing tool to detect moisture and corrosion in large honeycomb filled structures of aircrafts. The MNRC was transferred to UCD in February of 2000 as part of the Base Realignment and Closure (BRAC) process of McClellan Air Force Base. UCD MNRC has a sound base of research and industrial partnerships. Jet Propulsion Laboratory, Pasadena, CA. approached UCD MNRC with the need to image the condition of brushes contained in motors used in their space related projects. JPL explained that they were unable to see what they needed to see with X-rays. They wanted to know if we could see the carbon brushes in these motors. Using their samples, we initially performed two computed radiography (CR) shots 90 degrees apart through the diameter. Furthermore, another shot along the rotational axis was taken. The brushes could not be seen in the shot along the axis. The exposures through the diameter showed an inconsistency between the two motors’ brushes. We then performed neutron computed tomography (CT) on both motors with one degree projection. Reconstruction clearly showed that the motor with the inconsistent shape brushes had the brushes installed incorrectly. There is a significant difference in both time and cost between CR and CT of the motors. CR took about 30 minutes from beginning of the set up to the complete evaluation, while CT took about five hours to finish. UCD/MNRC was not completely satisfied with just saying that there is an inconsistency between the two motors. Through working outside the envelope, a complete picture of the brushes condition was seen with CT in about 30 minutes.

Cover page of Simple microscope using a compound refractive lens and a wide-bandwidth thermal neutron beam

Simple microscope using a compound refractive lens and a wide-bandwidth thermal neutron beam


The results of imaging experiments using biconcave, spherical compound refractive lenses (CRLs) and a wide-bandwidth thermal neutron beam are presented. Two CRLs were used, consisting of 155 beryllium and 120 copper lenses. The experiments were performed using a thermal neutron beam line at McClellan Nuclear Radiation Center reactor. The authors obtained micrographs of cadmium slits with up to 5× magnification and 0.3 mm resolution. The CRL resolution was superior to a pinhole camera with the same aperture diameter. The modulation transfer function (MTF) of the CRL was calculated and compared with the measured MTF at five spatial frequencies, showing good agreement. ©2007 American Institute of Physics

Cover page of Fast Neutron Radioactivity and Damage Studies on Materials

Fast Neutron Radioactivity and Damage Studies on Materials


Many materials and electronics need to be tested for the radiation environment expected at linear colliders (LC) to improve reliability and longevity since both accelerator and detectors will be subjected to large fluences of hadrons, leptons and gammas. Examples include NdFeB magnets, considered for the damping rings, injection and extraction lines and final focus, electronic and electro-optic devices to be utilized in detector readout, accelerator controls and the CCDs required for the vertex detector, as well as high and low temperature superconducting materials (LTSMs) because some magnets will be superconducting. Our first measurements of fast neutron, stepped doses at the UC Davis McClellan Nuclear Reactor Center (UCD MNRC) were presented for NdFeB materials at EPAC04 where the damage appeared proportional to the distances between the effective operating point and Hc. We have extended those doses, included other manufacturer's samples and measured induced radioactivities. We have also added L and HTSMs as well as a variety of relevant semiconductor and electro-optic materials including PBG fiber that we studied previously only with gamma rays.

Cover page of Application of Neutron-Absorbing Structural-Amorphous metal (SAM) Coatings for Spent Nuclear Fuel (SNF) Container to Enhance Criticality Safety Controls

Application of Neutron-Absorbing Structural-Amorphous metal (SAM) Coatings for Spent Nuclear Fuel (SNF) Container to Enhance Criticality Safety Controls


Spent nuclear fuel contains fissionable materials (235U, 239Pu, 241Pu, etc.). To prevent nuclear criticality in spent fuel storage, transportation, and during disposal, neutron-absorbing materials (or neutron poisons, such as borated stainless steel, BoralTM, MetamicTM, Ni-Gd, and others) would have to be applied. The success in demonstrating that the High-Performance Corrosion- Resistant Material (HPCRM)1 can be thermally applied as coating onto base metal to provide for corrosion resistance for many naval applications raises the interest in applying the HPCRM to USDOE/OCRWM spent fuel management program. The fact that the HPCRM relies on the high content of boron to make the material amorphous – an essential property for corrosion resistance – and that the boron has to be homogenously distributed in the HPCRM qualify the material to be a neutron poison.

Cover page of Radiation Hardness Testing of Materials at the UC Davis/ McClellan Nuclear Radiation Center

Radiation Hardness Testing of Materials at the UC Davis/ McClellan Nuclear Radiation Center


The UCD/ MNRC research reactor of the TRIGA type is designed to be operated at a nominal 2 .0 MW steady state power as well as pulse and square wave operation. It is cooled and moderated by light water and reflected by graphite. The reactor core is located near the bottom of a water-filled aluminum vessel 7.0 ft in diameter and 24.5 ft in height. It went first critical in 1990 and has since become the highest power TRIGA reactor in the U.S. Radiation hardness testing of materials is made possible through the so-called “neutron irradiator” which provides fast neutron exposure to samples with minimal contamination from thermal neutrons and gamma rays. This neutron irradiator has three primary components; conditioning well, exposure vessel, and detachable upper shield for the exposure vessel. The conditioning well is installed adjacent to the annular graphite reflector inside the reactor tank. It is held vertically in place and rests at the bottom of the tank. The well-structure is shielded with sufficient boron nitride and lead encased in aluminum to remove thermal neutrons and gamma rays, respectively. The removable and water-tight exposure vessel has a usable inner space of approximately 7” in diameter and 9” in height. There are six removable titanium plates with holes arranged in a hexagonal shape which can hold the components to be irradiated. It also contains a valve at the bottom to purge and pressurize an assembled unit with helium in order to reduce Argon-41 production during irradiation. The exposure vessel is lined with boral and gadolinium paint to insure minimal leakage of thermal neutrons. The detachable upper shield contains boron nitride and lead encased in aluminum to complete the upper shield for the exposure vessel before it is lowered into the conditioning well for irradiation. Monte Carlo code simulation is benchmarked with multiple threshold neutron flux measurements. The converted 1 MeV equivalent silicon neutron flux at 1.5 MW operating power is 2.3 x 10^10 n/cm2.sec. Among others, materials such as silicon based devices, coatings for metals, superconducting magnets which are susceptible to fast neutron exposure and damage in their working environments are examined. This unique irradiation facility enables us to provide credible information regarding fast neutron radiation tolerance of materials used in crucial applications.