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 maximum interrogation depth in a conductive material is about two skin depths, or twice the value in equation 2.2, and will be referred to as the penetration depth for a given material being evaluated at a given frequency. Two “skin depths” is where the induced current density has diminished to about 13.5 percent of that found at the surface.
The coil excitation frequency range is chosen based on container construction, the metal type, and its thickness. For AL-R8 containers (DOT 17C) the coil is excited over the 40 to 1500 Hz range. At 1500 Hz, only the container surface is interrogated. As the frequency falls into the 700 to 1100 Hz range, the contents of the container may begin contributing to the coil impedance (current begins to penetrate items inside the container), depending upon container wall thickness. For the AT-400R container, frequencies in the 100 to 2500 Hz range are sufficient to accomplish the same objective.
Temperature Electrical resistivity is a temperature dependent property of most conductive materials, primarily metals. In most metals resistivity increases with temperature. This includes the coil itself, made from copper wire, a metal container inside the coil and any metal objects of interest stored inside. Therefore, any template or threshold determination devised for the EM coil application to low-intrusive verification must take the ambient temperature into account. This is not a limitation because the coil impedance measurement can be calibrated as an extremely accurate thermometer over the range of temperatures that we expect to encounter under most weapon component storage scenarios.
Signature of Interest As discussed above, the impedance analyser provides a measure of coil impedance when the coil is placed over a sealed container and its contents. The coil’s impedance changes significantly (relative to the coil in air) when a container is inside. The majority of this change results from the coil’s interaction with the container, not its contents. Only a small fraction of the induced magnetic field penetrates beyond the container walls and into the items of concern. This fraction is, however, frequency and container-type dependent. To separate coil impedance changes caused by the container from those caused by its contents, and obtain a signature of the items inside the container, the empty container impedance must be known or somehow characterized. In many situations where all containers have the same signature, this is as simple as measuring the coil impedance of an empty container placed inside the coil. However, if variances between containers occur this task becomes more complex unless only a finite number of
container signatures exist. In the case of the AT-400R storage containers, container signature variation has not been observed and this is not an issue once a standard empty container signature has been established.
The impedance of interest is typically normalized to the “background” using the formula: Z R R R X o o Zn i , Xo Xo Xo where R0 and X0 represent the background resistance and reactance respectively, and Zn represents the normalized impedance. When evaluating items stored in sealed containers the background values of R0 and X0 are most often obtained from an empty container measurement.
RESULTS
Initial laboratory tests were designed to evaluate the technical feasibility of confirming the presence of conductive objects stored inside metal storage containers using a non-intrusive, EM coil measurement. Results from these tests clearly demonstrated the EM coil’s capacity to obtain unique signatures of different metal objects placed inside a metal container. After establishing this baseline result, subsequent efforts focused on improving the coil design, choosing optimal coil excitation frequencies, determining signature sensitivity to variations in metal type and geometry, and developing test procedures for enhancing signature content.
One of the first enabling discoveries was that the EM field could not sufficiently interact with internal objects unless the coil excitation frequency is less than about 2000 Hz for the stainless steel AT-400R container and 1000 Hz for the carbon steel AL-R8 container. Subsequently, the operating frequency for optimal coil impedance response was found to be within the 100 Hz to 400 Hz range, depending on container, instrumentation, and coil design details.
Regarding container differences, the AT-400R is constructed from multiple layers of stainless steel, amounting to a greater total wall-thickness than found in the low carbon-steel AL-R8 container. However, the magnetic field penetration, and hence coil impedance response, was observed to be more than four times greater at a given frequency in the AT-400R. This result is due in part to the much higher magnetic permeability of the low-carbon steel. Estimates can be found using the skin depth formula derived from Maxwell’s equations as: r d 2 mn , d is the skin depth of induced magnetic field in inches, r is electrical resistivity in micro-ohm cm, m is the relative magnetic permeability, and n is frequency in Hz. The ratio of relative magnetic permeability of low-carbon steel to the 400 series stainless steel found in the AT-400R is at least 20, whereas the resistivity ratio between of the same materials is about three.
By normalizing the EM coil impedance measurements to the empty container, many of the coil and container effects are removed from the measurement, leaving changes to the overall coil impedance that result from container contents only.
Figures 3 and 4 show impedance measurements, normalized to the empty container, when various objects are placed inside a single AL-R8 container. Figure 3 shows the resulting normalized impedance measurements for coil excitation frequencies in the 100 to 1500 Hz range. Notice the difference between the hollow and solid 4.75-inch aluminium spheres. Each item placed in the empty container is of the same material but is a different size. In contrast, the items used in producing Figure 4 are of different materials and have different orientations inside the AL-R8 container.
1.003 275 225 250 200 350 1.002 175 400
1.001 500
150 600 150 125 Hz 700 1 150 100 Hz125 Hz
0.999 Frequency (Hz) 100 Hz
0.998 125 Hz
8_inch_hollow_ball
Normalized Reactance 0.997 6_inch_hollow_ball 5.4_inch_hollow_ball 0.996 Constant Phase 4.75_inch_hollow_ball 100 Hz 0.995 3_inch_hollow_ball 4.75_inch_solid_ball
0.994 -0.008 -0.007 -0.006 -0.005 -0.004 -0.003 -0.002 -0.001 0 0.001 Normalized Resistance FIG. 3. Normalized impedance measurements of hollow/solid aluminium spheres inside an AL-R8.
A preliminary test using a plutonium pit placed in both an AL-R8 and a stainless steel container clearly showed that the resulting coil impedance measurement change is significantly larger than the ambient noise. To more thoroughly evaluate the EM coil measurement concept for verifying the presence or absence of plutonium pits, tests were conducted in the 1998—2001 timeframe.
The EM Coil responds to metal displaced by oxide in a way similar to simply removing metal. This makes sense considering the nonconductive (insulating) behaviour that most oxides exhibit. Figure 5 below uses aluminium and aluminium oxide to illustrate the extremely large resistivity difference between a metal and an oxide, on a logarithmic scale. Most other metals and their various oxide compounds follow this trend as well. As stated above, electrical conductivity is one of the fundamental material properties the EM coil is most sensitive to, making it very responsive to metals and unresponsive to oxides and electrical insulators.
1x1021 1x1020 1x1019 1x1018 1x1017 16 1x10 Insulators/Oxides 1x1015 1x1014 1x1013 1x1012 1x1011 1x1010 1x109 1x108 1x107 Semimetals & Semiconductors 1x106 5 Resistivity (micro-ohm-cm) 1x10 1x104 1x103 2 1x10 Metals 1x101
Al2O3 Pu U Al Element
FIG. 4. Electrical resistivity of metals compared to an oxide (insulator), Al2O3.
To further demonstrate this concept, equal volumes of aluminium metal and aluminium oxide (shown in Figure 6) were independently placed inside an AT-400R storage container to observe their affect on coil impedance. Introducing the metal had a significant affect on coil impedance (see Figures 3 and 4), however, coil impedance remained the same as for the empty container when only the oxide was introduced. This is the expected result for an insulator like aluminium oxide, having about the same conductivity as air when the comparison is relative to a metal.
FIG. 5. Aluminium metal and aluminium oxide in equal volumes.
Taking this demonstration to the next logical step, displacing small amounts of metal with air, coil impedance was monitored after successively machining thin layers off the aluminium disk. Coil impedance began showing noticeable and detectable changes at about one percent weight loss from the aluminium disk. It should therefore be possible to use an EM coil technique to detect cases where very small amounts of metal (sealed in storage containers) are replaced with oxide or completely replaced by oxide and can remain undetected by some of the higher-intrusion types of radiation detection methods.
CONCLUSIONS
In light of the simplicity and extensive development history behind the EM coil method at PNNL the following benefits and features have been demonstrated: · The EM coil method can determine if a container does or does not enclose metal in the expected configuration. Radiation detection methods on the other hand may only be able to establish the lack of oxide, which thereby implies the presence of metal, a weaker statement. · The EM coil data analysis is extremely simple, requiring the definition of a region of interest in a two-dimensional plot. · The EM coil is inherently less intrusive than a radiation measurement since this method reveals no classified information (does not reveal properties related to the nuclear structure of stored items),
and only is sensitive to a combination of parameters defining the electronic structure of stored components and their configuration. · Additionally, the EM coil’s rapid response time (less than a minute) qualifies it as a screening tool suitable for examining a large percentage of containers. Since a radiation measurement system requires measurement times on the order of one hour it could not evaluate nearly as many containers in a given inspection schedule. · With the addition of several smaller sets of coils, positioned near the container weld seams, including those on the lid, the EM coil method could interrogate the welds with eddy currents and thereby furnish an intrinsic tag of the container and lid, as well as correlating container and contents.
ACKNOWLEDGEMENTS
This work was performed by the Pacific Northwest National Laboratory; operated for the U.S. Department of Energy by Battelle Memorial Institute under Contract DE-AC06-76RLO-1830. The authors would like to acknowledge the past support of the U.S. Department of Energy and the current support of the Defense Threat Reduction Agency’s Arms Control Technology Division for this work as part of the Joint DOE/DoD Integrated Technology Implementation Plan.
REFERENCES
[1] R. L. Hockey, “EM Coil measurements” an unpublished report (1995)