About the Reed Reactor
- Reactor Uses
- Reactor Irradiation Facilities
- Neutron Activation Analysis
- Neutron Activation Autoradiography
The Reed College Reactor Facility has been used for research and educational projects in the Portland area since its establishment in 1968. Cooperative programs between Reed and several public and private high schools, colleges, and universities in northwestern Oregon were established in 1970 through the Nuclear Science Consortium of the Willamette Valley. These programs have been an important part of the educational picture of the region.
The Reed College Reactor is a TRIGA Mark I water-cooled, "swimming pool" reactor at the bottom of a 25-foot-deep tank. It uses 58 zirconium hydride/uranium hydride fuel elements in a circular grid array. The uranium fuel is enriched to 20 percent in uranium-235. The reactor is surrounded by a graphite ring which minimizes neutron leakage by reflecting neutrons back into the core. Most reactor components are clad with aluminum.
The reactor can operate at any power up to the license ceiling of 250 kW. This makes it possible to provide a defined neutron flux as required for the experiment. The power level is usually maintained for periods ranging from a few minutes up to eight hours. Continued or repeated operation over several days is possible for longer irradiations.
The Reed Reactor Facility is primarily used for instruction, research, and analysis, especially trace-element analysis. Since the initial startup, the reactor, in addition to providing student research opportunities, has worked to educate the surrounding community on the principles of nuclear energy and fission-reactor operation.
The reactor and associated facilities are used to some extent in chemistry and physics courses, but they are mostly used for research projects. The reactor is operated almost entirely by undergraduate students who are licensed by the Nuclear Regulatory Commission. This allows them to conduct their own research and to be hired by the facility to conduct irradiations for educational organizations, private research organizations, and for industrial applications. A one year non-credit seminar is open to all interested students to prepare them for the licensing examination. Students and faculty from other institutions are also welcome to attend the seminar.
The facility provides tours for interested high school and college classes and other special groups in the Portland area. These tours include description and observation of the reactor and associated facilities, and most incorporate hands-on experiments demonstrating fundamental facts about the nature of radioactivity. The facility is also available to advanced classes and other special programs such as the Talented And Gifted (TAG) student program of the Portland Public Schools and the Mathematics, Engineering and Science Achievement (MESA) program which provides special experiences for minorities and disadvantaged middle and high school students.
Research at the Reed Reactor has centered around quantitative neutron activation analysis for trace-element concentrations. To facilitate this work a multi-channel analyzer and an intrinsic germanium gamma-ray spectrometer are used. A major research area for students using the reactor is the determination of trace-element concentrations in biological and environmental samples. More examples are included in the sections on neutron activation analysis and autoradiography.
The reactor facility is available as a neutron source for industrial applications. The most frequent use in the past has been for quality control and purity testing in manufacturing and electronics industries, and for environmental monitoring of industrial effluents.
PNEUMATIC TRANSFER SYSTEM
The pneumatic transfer system consists of an irradiation chamber in the outer ring of the core with its associated pump and piping. This allows samples to be transferred in and out of the reactor core very rapidly while the reactor is at power.
Routine use of the pneumatic transfer system involves placing samples into vials which in turn are placed into special capsules known as "rabbits." The capsule is loaded into the system in the radiochemistry laboratory next to the reactor and is then transferred pneumatically into the core-irradiation position for a predetermined time. At the end of this period, the sample is transferred back to the receiving terminal where it is removed for analysis. The transfer time from the core to the terminal is about seven seconds, making this method of irradiating samples particularly useful for experiments involving radioisotopes with short half lives. The flux in the core terminal is approximately 5 x 1012 n/cm3·s when the reactor is at full power.
ROTATING SPECIMEN RACK FACILITY
The rotating specimen rack ("lazy susan") is located in a well on top of the graphite reflector which surrounds the core. The rack consists of a circular array of 39 tubular receptacles. Each receptacle can accommodate two TRIGA-type irradiation tubes, so that up to 78 separate samples may be irradiated at any one time. Vials holding up to 17 ml (four drams) are routinely used in this system. Depending upon its geometry, a sample up to about 40 ml could be irradiated by joining two vials. Samples are loaded in the specimen rack prior to the startup of the reactor. The rack automatically rotates during irradiation to ensure each sample receives the same neutron flux. Typically the rotating rack is used by researchers when longer irradiation times (greater than five minutes) are required. The average thermal neutron flux in the rotating rack position is approximately 1.7 x 1012 n/cm3·s with a cadmium ratio of 6.0 at full power.
The specimen rack can also be used for gamma irradiations when the reactor is shutdown. The shutdown gamma flux in the specimen rack is approximately 3 R/min.
IN-CORE AND IN-POOL FACILITIES
The central thimble, which is a water-filled irradiation chamber about 3 cm in diameter, provides the highest available neutron flux, about 1 x 1013 n/cm3·s. However, it holds only one specially positioned irradiation container containing a cavity 7.5 cm in length and 2.5 cm in diameter. The chamber fits into a fuel-element position within the core itself. Use of the chamber as an irradiation facility necessitates special arrangements.
Foil-insertion holes, 0.798 cm in diameter, are drilled at various positions through the grid plates. These holes allow inserting special holders containing flux wires into the core to obtain neutron flux maps of the core.
Near core in-pool irradiation facilities can be arranged for larger samples if required by the experimenter. Neutron fluxes will be lower than in the lazy susan and will depend on the sample location.
|Rotary specimen rack||2 x 1012|
|Pneumatic transfer system||5 x 1012|
|Central thimble||1 x 1013|
|In-core||2 x 1013|
|In-pool||<1 x 1012|
NUCLEAR SCIENCE LABORATORY
In addition to the reactor itself, the radiochemistry laboratory is available for experiments involving radioactive materials. Equipment can measure simple and complex half-lives by both periodic counting and multi-scaling, verify the inverse square law, and measure the attenuation of radiation in shielding materials. Equipment for more complex measurements using sodium iodide crystals, intrinsic germanium gamma spectrometry, beta spectrometry, and alpha spectrometry can be performed. Sample preparation facilities are also available.
Neutron Activation Analysis (NAA) is a very important tool for trace element detection. NAA yields abundance data for many elements by measuring the characteristic gamma-ray energies of radioactive nuclides. It can be performed as a non-destructive examination. NAA can achieve sensitivity several orders of magnitude greater than other commonly used analytical methods, down to tenths of a part per billion for many elements. The sample may be in any form: solid, liquid, or gas.
The principle of NAA consists of irradiating a sample with neutrons in a nuclear reactor thereby producing specific radionuclides. After the irradiation, the characteristic gamma rays emitted by the radionuclides are quantitatively measured by suitable radiation detectors yielding a gamma-ray spectrum. The activity or number of detected gamma rays of a particular energy is directly proportional to the disintegration rate of the specific radionuclide, which in turn is directly proportional to the amount of its parent isotope in the sample. Data reduction of gamma-ray spectra by means of a computer yields the concentrations of the parent elements in the samples. Accuracies for quantitative measurements of trace and rare elements are generally in the range of ±1 to 10 percent depending on the concentration of the particular trace element and the background from other radioactivities induced in the sample.
NAA has been used at the Reed Reactor Facility in the fields of geology (rock sample analysis), anthropology (tracing trade routes by elemental fingerprinting of sources), medicine (detection of selenium concentrations in the internal organs of rats), archeology (age dating), chemistry (identification of contaminants), biology (trace element analysis), forensics (matching powder to guns), computers (silicon wafer analysis), and environmental science (testing for elements in factory air filters).
Autoradiography is a photographic method for recording the distribution of radioactive materials within a specimen. With this method large areas can be examined quickly for elemental content.
Autoradiography involves irradiation in the core followed by exposure of film by radiation from the irradiated sample. In practice, a flat radioactive sample is placed in contact with a photographic emulsion which, after exposure and development, yields an autoradiograph. Both single and many element autoradiographs can be obtained.
Autoradiography has been used in many fields including geology and biology. Projects at Reed in recent years have included localization of elements in rock samples and monitoring of the uptake of iridium in plants.