Electromagnetic Coil (Em Coil) Measurement Technique to Verify Presence of Metal/Absence of Oxide Attribute

IAEA-SM-367/17/07 ELECTROMAGNETIC COIL (EM COIL) MEASUREMENT TECHNIQUE TO VERIFY PRESENCE OF METAL/ABSENCE OF OXIDE ATTRIBUTE R.L. Hockey and J.L. Fuller Pacific Northwest National Laboratory P.O. Box 999 Richland, WA 99352, USA ABSTRACT This paper summarizes how an Electromagnetic coil (EM coil) measurement technique can be used to discriminate between plutonium metal, plutonium oxide, and mixtures of these two materials inside sealed storage containers. Measurement results are from a variety of metals and Aluminium oxide in two different container types, the carbon steel AL-R8 and the stainless steel AT-400R. Within these container types two scenarios have been explored. 1.) The same configuration made from different metals for demonstrating material property effects. 2.) The same metal configured differently to demonstrate how mass distribution affects the EM signature. This non-radiation measurement method offers verification of the “presence of metal/absence of oxide” attribute in less than a minute. In January 2001, researches at Pacific Northwest Laboratory showed this method to discriminate between aluminium and aluminium oxide placed inside an AT-400R (a total wall thickness of over 2.5 cm) storage container. Subsequent experimental and theoretical investigations into adapting the EM coil technique for arms control applications, suggests a similar response for plutonium and plutonium oxide. This conclusion is consistent with the fact that all metals are electrically conductive while most oxides are electrical insulators (non-conductors). INTRODUCTION The Electromagnetic (EM) coil technique is suitable for rapidly determining whether stored fissile material is present in metallic or oxide form without also revealing additional sensitive information such as isotopic composition. This is possible using the EM coil technique because it can only provide a combined electronic structure signature of all the materials inside the coil. Properties associated with the nuclear structure of items inside the coil remain hidden from this technique. Since metallic plutonium is destined for storage after nuclear weapon dismantlement, there is an expressed interest in determining the chemical form of the material. A particular concern is the storage of weapons grade plutonium oxide, derived from ongoing reprocessing of spent nuclear fuel, rather than the metallic plutonium derived from weapons dismantlement. Determining the chemical form (oxide or metal) is a stated US Government goal. Applicability of the EM Coil Technique The EM coil technique can measure an oxide attribute in most storage scenarios. The greatest advantages of the EM coil technique are: 1. Extreme sensitivity to small electrical conductivity changes that result when oxide (or a different material) is substituted for a portion of the expected metal, thus altering the metal-to-oxide ratio. 2. Short measurement times of approximately one minute make it a viable tool for 100 percent screening, as well as periodic random sampling. 3. Inherent information barrier, since the measured signal cannot be analysed to yield sensitive information. 4. A simple, commercial-off-the-shelf instrument is attached to a coil of wire for straightforward implementation and operation. 5. The EM coil technology has been demonstrated. The short measurement time and inherent information security of the EM coil technique make it suitable for additional requirements at fissile material storage facilities. In particular, the EM coil could be used to rapidly verify presence of material in containers removed from storage. The EM coil can rapidly screen a large number of containers to show that they are all similar with regard to their contents. Further progression would be the addition of several smaller sets of coils, positioned near the container weld seams, including those on the lid. These coils would interrogate the welds with eddy currents and thereby furnish an intrinsic tag of the container and lid, as well as correlating container and contents. EM COIL FUNDAMENTALS Principle of Operation The Electromagnetic Coil (EM Coil) approach to obtaining a signature of select components in sealed containers, as well as verifying their absence from containers declared not to contain these items, utilizes a coil that encircles the container in question and connects to an impedance measuring device. The coil consists of copper wire wound around a hollow cylinder. The coil impedance depends primarily on the combination of electrical conductivity, magnetic permeability, mass and mass distribution of the materials placed inside the coil. An impedance measurement device displays electrical impedance change in the coil. Coil impedance can be monitored using either an eddyscope or impedance analyser because both instruments (of equal measurement accuracy) will respond similarly to a given change in coil impedance. A photograph of an EM coil system using an eddyscope and coils for two different applications is shown in Figure 1. There are advantages and disadvantages offered by each instrument type depending upon the application. Some of the largest differences between the eddyscope or impedance analyser are in how the measurement results are displayed, measurement speed, and differential versus absolute impedance measurement. An instrument choice is typically made after considering what data processing steps are required of the measurements. The impedance analyser is the obvious choice for evaluating the concept because absolute coil impedance measurements are available for a quantitative analysis and comparison with the many different experimental conditions to be evaluated. The electrical impedance of the coil is expressed as a complex number signifying its reaction (with respect to current flow) to an applied voltage. The two frequency dependent components of the coil impedance are resistance and reactance. Resistance is the real component, defined using Ohms Law, and reactance is the imaginary component, derived from electromagnetic theory to include capacitive and inductive properties. Coil impedance is typically written as Z R iX , Z being the complex impedance, R the resistance, and X the combined inductive and capacitive reactance, all having units in Ohms. FIG. 1 Photograph of Eddyscope (Left, atop AL-R8 FIG. 2. Equivalent Electrical Circuit. container) and EM Coils Held Above AT-400R Container with AL-R8 Containers in the background. Figure 2 shows the electrical impedance arrangement schematically. A small sinusoidal voltage applied to the primary circuit at the left of Figure 2, generates a magnetic field inside the coil which induces current into the secondary circuit (container and its contents) at the right of Figure 2, via mutual induction. Faraday’s law explains this behaviour by stating that a changing magnetic field generates an electric field. As the induced current flows inside (stored components) and through the container, it generates a secondary magnetic field according to Ampere’s Law. The total magnetic field influencing the coil is the combination of the primary and secondary magnetic fields. This combined, total field is what determines the measured coil impedance. Therefore, by connecting the impedance analyser to the coil via coax cable, a measurement of the coil’s complex impedance can be used as a signature of the electromagnetic properties of everything inside the coil. Parameters Affecting Impedance Measurement Although there are many physical parameters that may affect the coil impedance measurement, only two of these are material properties—the electrical conductivity and magnetic permeability of all items located inside the coil. Additional physical parameters affecting coil impedance are the distribution, total mass, and orientation of items inside the coil. There are many more physical parameters influencing coil impedance than are measured. Therefore, without additional measurement information, it is impossible to invert the EM coil measurements to derive the physical properties of individual items stored inside sealed containers. One of the most accurate and most versatile impedance analysers for evaluating the EM coil system is the Agilent 4294A with a basic impedance measurement accuracy of ±0.08 percent. However, with the exception of its high-accuracy and sweep frequency mode, very few of the 4294A’s many features and options are necessary for implementing this technology. For example, a convenient feature of the 4294A for evaluation purposes is the capability to sweep over a maximum of 800 frequencies, whereas in the final implementation fewer than 10 frequencies may be required. Coil impedance is monitored over a range of frequencies because electromagnetic theory tells us that the electromagnetic wave amplitude diminishes exponentially with depth in conductive materials. When plane electromagnetic waves arrive at a conductive surface some of their energy is dissipated upon interaction with the material, whereas the remainder is reflected. The induced current density diminishes exponentially with depth. The formula, from Maxwell’s equations, for estimating induced current penetration is: r d 2 mn , where d, the “skin depth,” is the depth at which the induced current has diminished to 36 percent of its value at the surface given in inches, r is electrical resistivity of the material in micro-ohm-cm, n is the frequency of the voltage applied to the coil in Hertz, and m is the (dimensionless) relative magnetic permeability of the material. Typically, the

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