Radioanalytical Methods in Interdisciplinary ... - ACS Publications

Chapter 7

Downloaded by UNIV OF GUELPH LIBRARY on July 18, 2012 | http://pubs.acs.org Publication Date: November 4, 2003 | doi: 10.1021/bk-2004-0868.ch007

Applications of Californium-252 Neutron Irradiations and Other Nondestructive Examination Methods at Oak Ridge National Laboratory 1

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R. C. Martin , D. C. Glasgow, and Μ. Z. Martin 1

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Nuclear Science and Technology Division, Chemical Sciences Division, and Environmental Sciences Division, Oak Ridge National Laboratory, Oak Ridge, T N 37831 3

The U.S. Department of Energy (DOE) program for the production and distribution of heavy (transplutonium) elements is described, along with the variety of applications for californium-252 ( Cf) neutron sources and neutron irradiation experiments at die Californium User Facility for Neutron Science (CUF) at Oak Ridge National Laboratory (ORNL). ORNL's Radiochemical Engineering Development Center (REDC) is the center for the Transuranium Element Processing Program (TEPP), which produces curium, berkelium, californium, einsteinium, and fermium for basic research, and the distribution of Cf neutron sources for commercial and research applications. Capabilities for neutron and laser-based in situ nondestructive examination techniques for elemental analysis in the R E D C and collaborative O R N L programs are also described. 252

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© 2004 American Chemical Society In Radioanalytical Methods in Interdisciplinary Research; Laue, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2003.

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Transuranium Element Processing Program The TEPP was established in the late 1950s to produce transplutonium heavy elements and distribute them to interested researchers ( i ) . The production is based at ORNL's R E D C . The R E D C has two large hot cell buildings. Building 7920 has nine heavily shielded hot cells for fabrication of target rods, dissolution of the irradiated rods, and subsequent radiochemical processing and purifications. Building 7930 has three hot cells for the preparation, handling, and distribution of C f neutron sources and recovery of the C m daughter. Building 7930 also has two unused hot cells, which are reserved for fiiture radiochemical recovery of P u from irradiated N p target rods for use as radioisotopic power sources in space and elsewhere (2). Heavy element (HE) production begins with long, thin target rods containing 35 pellets made from a blend of curium oxide and aluminum. Each target contains up to 10 g of actinide. After a year-long irradiation within the High Flux Isotope Reactor (HFIR) at O R N L , the targets are returned to the R E D C and dissolved. Radiochemical processing removes impurities and fission products, HEs are separated and purified using ion-exchange chromatography, and unreacted curium is recycled into the next batch of target rods (J). Existing actinide oxide inventory can maintain H E production at current rates for two decades. The remaining HFIR lifetime is projected at >30 years. The H E production path of neutron capture and (J decay is shown in Figure 1. H E Campaign 73 will begin in early 2003 with the dissolution of nine curium target rods and the subsequent recovery of the following estimated masses: £275 mg of C f , 241 * m 242

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ORNL DWG 93A-5511


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Californium-252 Properties and Source Forms Californium metal melts at 900°C and boils at 1472°C, with a roomtemperature density of 15.1 g/cm (4,5). The pure metal is very reactive and apparently bright silver in color as prepared in an inert atmosphere. The C f radioisotope is an intense neutron emitter that is readily encapsulated in compact sealed sources. For ease of encapsulation, C f is converted into either the oxide or oxysulfate form (6). Commercial and medical sources typically incorporate a cermet wire or pellet containing C f 0 in a palladium matrix. Industrial and research sources contain either a C f 0 powder (obtained by filtering and heating a C f oxalate slurry inside a source capsule) or a pressed pellet containing aluminum powder and C f oxysulfate microspheres (obtained by firing an ionexchange resin containing Cf). The inherent safety of C f source encapsulations is well demonstrated by 30 years of experience, even under explosive impact (6). Electroplates of C f can be encapsulated inside small ionization chambers (7) for use as timed neutron sources (i.e., the fission pulse acts as a trigger to time the release of the neutrons). Electrodepositions of trivalent actinides (e.g., C f and C m ) at the R E D C range from nanograms of actinide to surface loadings as high as 300 ng/cm . With 3.092% of C f decay occurring via spontaneous fission, 1 \xg of C f emits 2.31434 x 10 fast neutrons/s with a 2.645-year half-life (0.5362 mCi/|ig). The average of the Maxwellian neutron energy distribution is 2.14 M e V , with the most probable energy at «-0.7 M e V . A cylindrical source capsule the size of a person's little finger can contain >50mg of C f , emitting up to ~ 1 0 neutrons/s. Radiological exposure in air, at 1 m from a 1-^g C f source, totals 2.21 mrem/h (22.1 jiSv/h) from fast neutrons and 0.19 mR/h (1.9 ^iGy/h) from gamma rays. Californium-252 sources contain several C f isotopes. The isotopic distributions from a recent campaign (early 1999) are typical: 4.3 atom % C f ; 10.8% C f ; 3.3% C f ; 81.5% C f ; 0.04% *Cf; and 0.01% C f . Neutron emission from C f decay (13-year half-life) as well as from C f must be considered for sources >15 years old. Neutron emission from C f must be considered in assays of recently produced C f . With 99.7% of decay by spontaneous fission and a 60-day half-life, neutron emission from2ngof Cf equals that from 1 mg o f C f . Pure C f is obtained from decay of T3k. The R E D C recently supplied fairly pure C f (with acceptably low neutron emission for routine handling) from the reprocessing of 30-year-old C f sources. 3

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In Radioanalytical Methods in Interdisciplinary Research; Laue, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2003.

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Applications of C f Neutron Sources Californium-252 neutron sources are used whenever compact, portable, and reliable neutron sources are required. Recent American Nuclear Society conference sessions surveyed the range of applications (2,5). A recent estimate indicates that - 7 0 publications per year involve either use o f C f sources throughout the world or calculation of C f properties. Common industrial and research applications include prompt gamma neutron activation analysis ( P G N A A ) of coal, cement, and minerals and PGNAA-based detection and identification of explosives, land mines, and unexploded military ordnance. Other uses include neutron radiography, materials characterization and nuclear assay, fuel rod scanning for uniformity of fissile material, reactor start-up sources, cancer therapy, calibration standards, and standard neutron fields. Some applications use the high-energy (60- to 80-MeV) fission-fragment (FF) emission from unsealed C f (typically an electroplate). Etching of thin plastic samples exposed to FFs produces tiny holes along the F F tracks, for use in specialized applications. A benchtop system uses a low-activity Cf electroplate for FF simulation of cosmic ion damage to electronic devices. Subcritical multiplier assemblies driven by C f were used in the past for enhanced flux neutron irradiations. When a C f source is placed within a carefully designed array of fissile fuel rods or plates, the total neutron emission can be 1 to 2 orders of magnitude greater than that of the C f source but remains safely below criticality. This approach has been suggested for generation of non-reactor-based external beams for cancer therapy. 2 5 2


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Cancer Therapy Brachytherapy is a method of cancer treatment that places a small radioactive source inside the patient's body, at the location of the tumor, to kill the cancerous cells. Intense sources are typically attached to a cable and inserted through a catheter to the tumor site. Californium-252 neutron brachytherapy (NBT) is more effective than photon and gamma brachytherapy in treating radioresistant tumors such as bulky and late-stage tumors, melanomas, and glioblastomas (brain tumors) and causes rapid regression of bulky, localized tumors (2). Over 6000 patients have been treated using N B T , with the number of treatments accelerating. The Linden Neutron Knife Company installed the first N B T treatment unit in China in 1999 and now has 10 treatment centers in operation (>350 patients treated to date) and up to 30 centers planned. Iridium-192 photon brachytherapy is the current standard for high-dose-rate (HDR) brachytherapy. Small (~l-mm outer diameter) Ir H D R sources can provide treatments in £10 minutes. Russian C f H D R sources with an outer diameter of - 3 mm are appropriate for intracavitary brachytherapy (e.g., gynecological, rectal, head, neck, and oral cavity treatments) but are too large for 192

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93 interstitial brachytherapy (inside organs, such as the brain and prostate). To date, C f interstitial brachytherapy has been limited to low-dose-rate treatments with lengthy treatment times because of the difficulty in miniaturizing highactivity C f sources. In 1999, O R N L and Isotron, Inc., entered into a Cooperative Research and Development Agreement to design a new family of medical sources suitable for interstitial and intracavitary H D R N B T . The first prototype sources have been fabricated for use with existing gamma-source applicators, achieving the desired miniaturization. Isotron's N B T system will provide optimized treatments of adult and pediatric cancer in 18 sites throughout the body. 2 5 2


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Neutron Radiography Unlike X-radiography, neutron radiography provides nondestructive examination and visual contrast for low-Z elements (such as hydrogen) and moisture. A Cf-based facility for neutron radiography was used at McClelland A i r Force Base in California in the 1990s to detect debonding of composites and moisture, fuel leakage, and corrosion within the aluminum honeycomb structure of F - l l l and F-15 aircraft, without requiring disassembly (8). A major Cf radiography facility (up to 150 mg of C f ) is used for the nondestructive examination of components at the Pantex Plant in Amarillo, Texas. With collimated neutron fluxes of ~2 x 10 cm" s" , up to nine radiographs per day can be obtained with image quality comparable to that of reactor-based radiography (P). 252

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Fissile Material and Nuclear Waste Characterization 2 5 2

Passive-active neutron shufflers employing C f have been used to monitor vehicles' contents and to determine the fissile content of waste drums, spent fuel assemblies, and process materials. The source is placed near the sample, and the delayed neutrons from induced fissions are counted after source removal. Detection of milligram masses of fissile isotopes has been demonstrated. O R N L researchers have developed a cart-portable Nuclear Materials Identification System (NMIS) containing a low-intensity C f ionization chamber (7) to probe and characterize fissile material (10). Neutrons from the 2 5 2

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source induce fission of fissile atoms present within the sample, releasing additional neutrons. These neutrons are detected by the detector array, providing a sample response to the incident neutrons that can be analyzed using standard time-correlation and/or frequency-analysis techniques. The N M I S uses the C f ionization chamber to time the C f neutrons so as to exclude their signal from the response of the detector array. Spontaneous fission of a C f atom is accompanied by production of energetic fission fragments as well as several neutrons. The fission fragments create an electronic spike within the ionization 2 5 2

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94 chamber that is used to time the detector array to reject the prompt C f neutron signal, but accept the subsequent fission-induced neutron signal from the sample. This technique has been used to determine the spatial distribution, mass, and hydration of deposits of U 0 F « H 0 in process piping in order to estimate subcriticality and plan for deposit removal (8). Other applications include identification and inventory of items for nuclear materials control and accountability, measurement of mass flow rate in a U F gas stream for downblending Russian weapons-grade uranium, and the determination of the effective neutron multiplication factor (k ff) for optimized storage and shipping of fuel elements. A value of k ^ less man one represents a safe, subcritical system while a value greater than one represents a potential criticality accident. 2

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Neutron Backscattering for Land Mine and Contraband Detection Californium-252 ionization chambers have been incorporated into a handheld land mine detector. Modern land mines use a nonmetallic casing such as plastic to elude detection, but this detector measures the time delay between neutron emission and backscattering from the plastic (correlated detection) to differentiate readings from background (11). Land mine detection using uncorrected neutron backscattering has also been demonstrated using -3.5 jig of C f in a prototype handheld detector weighing - 8 kg. With this device, longterm radiological exposure to the operator is at an acceptable level (12). Nova R & D , Inc., of Riverside, California, has developed a handheld instrument for the U . S . Coast Guard to detect contraband hidden in compartments behind metal and other structures (8). Fast neutrons from this Compact Integrated Narcotics Detection Instrument penetrate the barrier material, and unexpected backscatter of neutrons by any hydrogen-rich materials present suggests the potential presence of contraband. Californium-252 neutron backscattering has also been used to measure the void fraction of boiling water in a nuclear fuel rod bundle testing facility (2). 2 5 2

Prompt Gamma Neutron Activation Analysis

On-line Process Monitoring and Borehole Logging P G N A A involves capture of a neutron by an atom's nucleus. De-excitation of the nucleus occurs with instantaneous emission of a high-energy (multi-MeV) "prompt" gamma ray. The gamma spectrum peak energies identify the elements present. N o significant radioactivity is induced by rapid sampling using lower-


95 intensity neutron sources. P G N A A provides information concerning the principal components of a sample. Subpercent sensitivities are common. P G N A A analysis of raw materials for process control or material management (blending, etc.) in the coal and cement industries is common. Conveyor belt and slurry analyzers quantify over 20 elements. Analysis of coal provides sulfur content (to meet smokestack release criteria), ash and moisture content, and information on its calorific content (Btu/lb). Over 200 analyzers are in service worldwide (8). Subsurface P G N A A measurements of elemental chlorine in an old radiochemical processing waste pit were made using a high-purity germanium gamma detector and - 5 ng of C f in a cylindrical borehole probe (73). Although P G N A A measurements of H , Si, Ca, Fe, A l , and CI were obtained within the boreholes, only chlorine was quantified, with concentrations ranging from 1,000 to 30,000 ppm and a minimum detection limit of 300 ppm.


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Ancore Corporation uses C f sources in several explosives detection systems (e.g., Small Parcel Explosive Detection System, Vehicular Explosive and Drug Sensor, etc.) based on P G N A A detection of the nitrogen present in explosive compounds (14). A prototype system was demonstrated at airports for detection of explosives in luggage in the early 1990s. The Canadian Department of National Defence has developed a mobile P G N A A system to detect buried land mines; detection of nitrogen using

Radioanalytical Methods in Interdisciplinary ... - ACS Publications

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