About the Reed Reactor
- Reactor Uses
- Reactor Irradiation Facilities
- Neutron Activation Analysis
The Reed College Reactor Facility has been used for research and educational projects in the Portland area since its establishment in 1968. These programs have been an important part of the educational picture of the region.
The Reed College Reactor is a TRIGA Mark I reactor at the bottom of a 25-foot-deep tank. It uses zirconium hydride/uranium hydride fuel elements in a circular grid array. The uranium fuel is enriched to 20 percent uranium-235. The reactor is surrounded by a graphite ring which minimizes neutron leakage by reflecting neutrons back into the core.
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 some experiments. 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 engineering and energy.
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 and labs for middle school, high school and college classes as well as other special groups in the Portland area. 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 using a multi-channel analyzer and an intrinsic germanium gamma-ray spectrometer. 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 section on neutron activation analysis.
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. Samples are loaded into a terminus in the radiochemistry lab, and then transferred to the in-core irradiation position using differential air pressure. After a predetermined amount of time, the sample is transferred back to the receiving terminus, where it can be retrieved 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/cm2·s when the reactor is at full power.
ROTATING SPECIMEN RACK FACILITY
The rotating specimen rack ("lazy susan") is located in between the reactor core and the graphite reflector which surrounds the core. The rack consists of a circular array of 40 tubular receptacles. Each receptacle can accommodate two TRIGA-type irradiation tubes, so that up to 40 separate samples may be irradiated at any one time. Vials holding 1.5 ml to 8 ml are routinely used in this system. Depending upon its geometry, larger samples could be irradiated by joining two vials. Samples are loaded in the specimen rack prior to the startup of the reactor and removed after shut down. The rack automatically rotates during irradiation to ensure each sample receives the same neutron flux. The average thermal neutron flux in the rotating rack position is approximately 1.7 x 1012 n/cm2·s with a cadmium ratio of 6.0 at full power.
The specimen rack can also be used for shut down irradiation in order to minimize neutron dose. The shutdown gamma flux in the specimen rack is approximately 3 R/min.
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/cm2·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.
Flux Foil Insertion Holes
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 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).