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Nondestructive assay and diagnostic techniques keep track of plutonium and uranium

Developments in NIS-5 are vital to Laboratory missions

Nondestructive assay (NDA) is a term applied to nuclear (mostly) measurement techniques for plutonium, uranium, and other actinides. Scientists in the Nuclear Materials Technology (NMT) Division rely on nondestructive assay in research areas as varied as plutonium disposition and heat source development for NASA missions. Many of the NDA technologies used at the Laboratory and around the world were developed by Los Alamos' Safeguards Science and Technology Group (NIS-5).

Collaborations between NIS-5 and NMT Division also have contributed to the success of stockpile stewardship, manufacturing, waste management, and nuclear material accounting programs. NIS-5, with support from the Department of Energy (DOE) Office of Policy Integration and Technical Support (SO-13), also works with other DOE facilities in addressing nuclear material accountability problems throughout the DOE complex.

NDA has two characteristics that make it attractive to researchers working with nuclear materials. First, it does not alter the physical or chemical state of the material. Second, the measurements can be made on bulk quantities of nuclear materials without breeching the container or containment of the material. These characteristics allow NDA measurements to be made outside of gloveboxes, on entire 55-gallon drums, on filters in air ducts, on solutions inside processing systems, and on bulk materials packaged for disposition.

How NDA works

NDA techniques measure either the naturally occurring radiation emitted from actinide isotopes or the radiation that is stimulated or induced by another radioactive source. Most of the measured radiations are characteristic of and can be used to quantify the mass of a single isotope inside a sealed container.

Three types of radiation provide the source for almost all NDA measurements: gamma rays or x-rays, neutrons, and heat.

Naturally occurring gamma-ray emission arises from the decay of most actinide isotopes. The energy and intensity of the gamma rays emitted from any isotope provide a unique signature that can be used to identify and quantify the isotope. X-rays are emitted in many decay processes or can arise from stimulation by other radioactive sources. X-ray emission is characteristic of the element while gamma-ray emission is characteristic of the isotope. Several processes lead to the emission of neutrons from actinide materials. The most common process is the spontaneous fission of the plutonium-240 isotope present at a level of about 6 percent in weapons plutonium. In this process, several neutrons are emitted simultaneously when the nuclide undergoes spontaneous fission. The simultaneous detection of two or more of these neutrons provides a signature and a means of quantifying the mass of the spontaneously fissioning isotope.

Neutron-based NDA measurements are typically made with a neutron coincidence counter, a device that measures the rate of simultaneous neutron emission. The technology behind thermal neutron multiplicity counting was developed at Los Alamos and won an R&D 100 Award in 1992.

Nuclear material accountability and cookie dough: What's the connection?

Accounting for all of the plutonium passing through a facility process can be likened to accounting for all of the flour used in baking a batch of cookies. Consider how difficult it would be to do this for your baking project.

The heat emitted from actinide materials arises primarily from alpha decay of the individual isotopes composing the sample. The amount of heat from a single gram of each isotope is a known physical quantity. The measurement of this heat in a calorimeter coupled with the measurement of the relative abundance of the individual isotopes in the sample (isotopic composition) provides a nondestructive measurement of the elemental mass in the sample. Detection and quantification of these types of radiation provide the basis for the NDA techniques that are used today for nuclear material control and accounting in all nuclear facilities (see sidebar on page 7).

These same techniques also form the basis for quantitative measurements on waste for disposal, qualitative detection of the presence of nuclear materials for physical security and homeland security applications, and the quantification of pure plutonium materials for disposition under international treaties.

Holdup measurements-portable instrumentation

The nuclear material residues that become trapped or held up in the piping and ductwork of a processing facility must be measured to provide assurances of criticality safety and to ensure complete accounting of all the material processed. These "holdup" materials provide a particularly difficult measurement challenge because of measurement geometry and access problems.

Los Alamos scientists recently tested two new portable gamma-ray technologies online.

Taking measurements of nuclear materials held up in piping and ductwork is easier and safer, thanks to analysis software developed by researchers in the Safeguards Science and Technology Group (NIS-5) in a cooperative effort with Oak Ridge Y-12 Plant staff. Several different types of small, portable detectors can be attached to a telescoping pole to quantify materials trapped in piping and ductwork, as shown in the photo at the right. The portable detectors make the job of the man in the cartoon on page 8 a lot easier and safer. In the photo below, Duc Vo (left) and James Pecos prepare to make a measurement of the isotopic composition of the plutonium in a container of plutonium oxide using a cadmium telluride (CdTe) detector at the TA-55 Plutonium Facility, The container is visible through the glovebox window. Vo holds the portable CdTe detector in his right hand and the battery-powered electronics in his left. The same portable electronics used with the CdTe detector is also used with other detector types for the holdup measurements pictured at the right.

The first is a compact, lightweight, battery-powered cadmium telluride (CdTe) detector that measures gamma-ray isotopic composition in-situ without liquid nitrogen. This commercially available, room-temperature semiconductor detector is easily positioned in online measurement locations without the difficulties associated with comparatively bulky, liquid-nitrogen-cooled, high-purity germanium detectors.

In a first-ever application, Los Alamos scientists have measured the complete plutonium isotopic composition for a wide range of materials using the room-temperature CdTe detector. The testing covered a range of three percent to twenty-six percent plutonium-240. Development for uranium analysis is under way. Applications currently include online inventory in process equipment or containers (before welding to verify loading limits), and verification of the inventory in storage vaults.

The second technology is a new automated system for making holdup measurements in process equipment. These measurements use gamma rays from individual isotopes to quantify plutonium-239 and uranium-235, as well as other isotopes. Software developed in a cooperative effort between Los Alamos and Oak Ridge Y-12 Plant staff automates data acquisition with the newest portable gamma-ray spectrometers.

New Los Alamos-developed algorithms accurately determine holdup quantities using analysis algorithms that correct for departures of real deposits from ideal geometries and for gamma-ray self-attenuation. Automation allows rapid execution of large numbers of measurements in very short count times (five to fifteen seconds) and rapid quantification of measurement results. These new measurement methods reduce radiation exposure as well as minimize overall measurement costs.

The instrument operator places a plastic vial containing a plutonium-bearing solution into the measurement head of the Solution Assay Instrument (SAI) inside a glovebox. A high-purity germanium detector, located outside the glovebox underneath the SAI head, measures the gamma rays from the samples after they penetrate the stainless steel floor of the glovebox.

Solution assay

Researchers have been processing and purifying uranium and plutonium in solution since the dawn of the atomic age, and in the 1970s they began using nondestructive assay because it is relatively straightforward to apply to solutions because of their uniform composition.

The gamma-ray emission from plutonium-239 is most often used to quantify that isotope, while gamma rays from uranium-235 serve the purpose well for uranium. The measurement techniques incorporate methods for correcting for the self-absorption of the gamma rays produced in the solution; a correction required to measure solutions with varying concentrations of plutonium or uranium.

NDA methods for solutions can be applied to many different configurations, including ion-exchange columns for plutonium, solvent-extraction systems for uranium, solutions flowing in pipes, and small samples of approximately 50 milliliters in plastic vials.

At Los Alamos, plutonium solution accountability measurements are usually performed with the solution in a plastic vial inside a glovebox and the detector outside "looking" through the glovebox floor.

By carefully applying these techniques, researchers can get results that are equivalent to conventional analytical chemistry

These techniques were applied to enriched uranium in a technology called the Nuclear Materials Solution Assay System, which also earned an R&D 100 Award for the Laboratory in 1988.

Calorimetry

NDA measurements of plutonium in bulk samples in sealed containers are primarily done through a technique called calorimetry, which measures the total power (watts) produced by a plutonium sample.

Calorimeters traditionally have been fabricated using a sensor of nickel wire wound around the measurement chamber. The nickel wire provides a temperature-sensitive resistance leading to highly accurate and precise electrical measurements of the power produced by the sample.

This fabrication method, however, is somewhat of an art and the few experienced practitioners are retiring. A team of researchers is developing calorimeters with thermopile-based solid-state sensors. These new sensors reduce fabrication costs and can be configured to make the calorimeters more sensitive than traditional wire-wound calorimeters‹measuring samples as small as 1 gram of plutonium.

This new calorimetry technology is being applied to the measurement of kilogram quantities of enriched uranium, an application that will improve controlling and accounting for this difficult-to-measure material.

Researchers at Los Alamos also are building a mobile calorimetry laboratory that will take this new technology to sites around the country to assist in measurements of materials slated for disposal. This mobile user facility will make measurements using four different calorimeters. The calorimeters range in size from two inches in diameter to more than two feet in diameter, which is large enough to do the first-ever calorimetry measurements on fifty-five-gallon waste drums

Los Alamos isotopic analysis software can determine the isotopic composition of plutonium components inside a weapons storage container (above) as well as plutonium in lead-shielded short-term storage containers (below).

Gamma-ray isotopic analysis

Because most NDA measurements only quantify a single isotope, these measurements require knowledge of the isotopic composition of the measured material to convert the measurement results for the single isotope to elemental mass. Gamma-ray spectrometry is used to nondestructively determine the isotopic composition of essentially all materials present in the nuclear fuel cycle, both plutonium and uranium.

Gamma-ray spectrometry measurements are performed on samples of arbitrary size, geometry, and physical and chemical composition, and do not require calibration, or even knowledge, of the containment of the sample. This technique, now in worldwide use, was developed at Los Alamos in the mid-1970s.

Los Alamos researchers also developed software called PC-FRAM for this type of analysis. PC-FRAM is the most advanced software of its type in the world and affords an analysis of plutonium isotopic composition of samples contained in as much as 2.5 centimeters of lead.

The commercially available software analyzes all the isotopic measurements at the TA-55 Plutonium Facility as well as all Los Alamos waste sent to the Waste Isolation Pilot Plant (WIPP). It can verify the contents of weapons storage containers and is used in inspections by the International Atomic Energy Agency.

The nondestructive assay module of the Advanced Recovery and Integrated Extraction System (ARIES) at TA-55 is used to quantify the plutonium oxide converted from the plutonium metal in dismantled weapons and encapsulated in containers like the one shown in the inset. The triple-ply containers are approved by the Department of Energy for long-term storage of up to 50 years.

Integrated NDA systems

Los Alamos researchers have combined several NDA instruments into a robot-controlled system under a central host-computer control. The Advanced Recovery and Integrated Extraction System (ARIES) is a set of six modules that extracts the plutonium metal from surplus weapons components, converts the metal to oxide, and packages the oxide in containers suitable for long-term storage or disposition.

The ARIES NDA system quantifies the plutonium oxide from dismantled weapons components

The ARIES NDA module performs unattended, around-the-clock measurements with calorimetry, neutron coincidence multiplicity counting, and gamma-ray isotopic analysis. This part of the ARIES system is the model for a functionally similar system being built in Russia to assist the Russian plutonium disposition program.

This system was developed as the prototype for the NDA system to be installed in the Pit Disassembly and Conversion Facility (PDCF) to be constructed at the Savannah River Site.

This article was contributed by Thomas E. Sampson of Safeguards Science and Technology (NIS-5)


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