IEEE T1)uaction4 on Nucecat Science, Vot.NS-24, No.l, Febuavy 1971

TAMPER AND RADIATION RESISTANT INSTRUMENTATIONFOR SAFEGUARDING SPECIAL NUCLEAR MATERIAL*

Barrett B. ParsonsJerry L. Wells

The BDM Corporation2600 Yale Blvd. S.E.

Albuquerque, New Mexico 87106

Summary

A tamper-resistant liquid level/accountabilityinstrumentation system for safeguards use has beendeveloped and tested. The tests demonstrate theaccuracy of liquid level measurement using TDR (TimeDomain Reflectometry) techniques and the accuracy ofdifferential pressure and temperature measurementsutilizing a custom designed liquid level sensorprobe. The calibrated liquid level, differentialpressure, and temperature data provide sufficientinformation to accurately determine volume, density,and specific gravity. Test solutions used includeordinary tap water, diluted nitric acid in varyingconcentrations, and diluted uranium trioxide also invarying concentrations. System operations and pre-liminary test results conducted at the General Elec-tric Midwest Fuel Recovery Plant and the NationalBureau of Standards, respectively, suggest that thesystem will provide the safeguards inspector with anadditional tool for real-time independent verifica-tion of normal operations and special nuclear materi-als accountancy data for chemical reprocessingplants. This paper discusses the system designconcepts, including a brief description of thetamper and radiation resistant features, the prelimi-nary test results, and the significance of the work.

Introduction

In recent years, there has been an increasinginterest in the development of more accurate methodsfor determining the contents of SNM (Special NuclearMaterials) in process and storage vessels.

Over the past few years, a radiation and tamper-resistant instrumentation system1,2,3 featuring TDRhas been developed and tested4. The system, asillustrated schematically in figure 1, measuresliquid level, temperature, and differential pressurehead using a custom built sensor probe. From thesedata, the volume, density, and specific gravity ofthe vessel contents can be obtained for remoteanalysis. Additionally, the TDR technique providescontinuous monitoring of liquid level, sensor probeoperation, and interconnections between the vesseland the TDR instrument to detect abnormal changeswhich may signify diversion or diversion attempts.

Sensor System

The sensor system consists of the sensor probe,the probe/vessel attachment, interface cabling, andan insertion manifold. The insertion manifold isused to facilitate insertion of system air and thetemperature sensing device, and to interface the TDRinstrument with the sensor probe. The sensor systemis a coaxial configuration constructed from materialsthat resist damage or corrosion in the applicationenvironment. For chemical reprocessing applications,

Figure 1. Safeguards Secure Instrumentation System

the materials used are 304L stainless steel conduc-tors and alumina ceramic conductor alignment insula-tors. The coaxial geometry of the sensor system isdesigned to match the 50-ohm output impedance of theTDR instrument. Figure 2 illustrates the relation-ship used to determine the conductor dimensions.The 50-ohm coaxial impedance is proportional to theratio of the conductor diameters.

Sensor Probe

The custom sensor probe is designed for measur-ing liquid level, differential pressure head, andtemperature using a single vessel penetration. Thisfeature is cost-effective, reduces the possibilityof accidental spills, reduces maintenance, andreduces the number of interconnections where diver-sion may take place. Figure 3 illustrates a cutawayview of a typical sensor probe4. The probe innerconductor is hollow to accommodate the temperaturesensor and to function as a purge tube (bubbler) fordifferential pressure head measurements. The innerconductor is electrically short circuited to theouter conductor at the probe bottom to provide a TDRcalibration reference when there is no liquid in theprobe. Holes in the outer conductor permit equali-zation of pressure between the probe conductors andthe volume above the vessel liquid. This is thereference pressure for differential pressure headmeasurement.

*This work was sponsored by the U.S. Arms Controland Disarmament Agency and the InternationalAtomic Energy Agency.

616

20- 50s 1

SENSOR SYSTEM COAXIAL CABLE

0 CHARACTERISTIC IMPEDANCE

0 RELATIONSHIP: ZO =C Lnd2

0 v'7 di

dl = INNER CONDUCTOR OUTER DIAMETER

d2 = OUTER CONDUCTOR INNER DIAMETER

e = PERMITTIVITY OF DIELECTRIC (AIR = 1)

Z, - CHARACTERISTIC IMPEDANCE

FOR ZO = 50 OHMS d2 = Z3di

| ' | ~~~~~~~~~~~~~~~~LIQIUI1AHEEIGNTN POBE

CABLE IMPEDANCEy HISNATCH y Y

b. PRObE AND COONECTING CABLE TDR LENGTH - 11H LIQUID IN PROBE

SHORT CIRCUITTDR 0112P0T IN PROBE CAUSER

PULSE-AT INSTRUMENT BY LIQUIDINTERFACE PROBE/CONNECTING

IMPEDANCE MISIATCH CAOLE IMPEOCEAT AIR-TDR-T/C INSERTION KI SBOTCH

KAN FOLD

Figure 4. Idealized TDR Traces

COAXIAL CABLE

I- d1 -I

I- -aFe 2

Figure 2. Sensor System Coaxial Cable

1- VERELtt.OOCTOR

ALUMNA INSULATOR

BOTTO

Figure 3. TDR Sensor Probe

Liquid will flow between the probe conductorsresulting in a short circuit that is associatedwith the liquid height in the probe. Figure 4illustrates how the TDR measures the short circuitlocation.

Interface Cabling

The interface cabling connecting the sensor

probe to the instruments is designed to match thesensor probe. That is, the conductors are of thesame dimension and material, the center conductor ishollow and spaced insulators are used to maintainthe transmission line air dielectric characteristics.The hollow center conductor is required to permitpurging the sensor probe from a remote location.The insulators have holes (see figure 3) to equalizethe pressure between the conductors and permitdifferential pressure measurement at a remote location.

Insertion Manifold

The insertion manifold is custom designed tofacilitate interfacing the TDR instrument, tempera-ture probe, purge air, and differential pressuremeasurement instruments with the sensor system.This manifold is shown in figure 5. The TDR pulseis coupled to the system using an airtight connector.The low input of the differential pressure transduceris connected to the outer conductor air (referencepressure above the vessel liquid) and the high inputis connected to the inner conductor air which is thepurge air source supplied from a precision flow ratemeter. The manifold interfaces with the coaxialtransmission line used to link the remote instrumenta-tion to the probe. The insulator plug serves toelectrically isolate the inner conductor.

AIR

Figure 5. Insertion Manifold

617

Z20 50

Instrumentation

All instruments used in this program are commer-cially available. These instruments consist of theTDR, X-Y recorder, digital voltmeter, and standardchemical processing industry differential pressureand temperature transducers. The TDR instrument isthe significant contribution to measurement conceptdeveloped during this program. A brief descriptionof the TDR concepts is provided to illustrate howthe instrument is used to obtain liquid level dataand to maintain tamper surveillance for the sensorsystem. The X-Y recorder is used to obtain permanentTDR data records.

TDR Concepts

Briefly, the TDR Concept is best described as a"closed loop radar."6 A voltage step is propagateddown the transmission line being investigated andthe incident and reflected voltage waves are monitoredand related to one another. Comparison of thereflected and incident waves yields both time anddistance to the reflection source. Additionally,analysis of the TDR signature (a CRT display of theincident and reflected energy) reveals the trans-mission line characteristic impedance and the nature(resistive, inductive, or capacitive) of each reflec-tion source along the line.

The distance-time relationship is expressed by:

d =ct

where: d = Distance.

c = Speed of light.t = Total elapsed time

(two-way travel).C = Dielectric constant

(air = 1).

The reflection coefficient (p) is related tosystem impedance as follows:

Er z -zP = E. Z

i I 0

Due to the sensor probe construction andoperation the system always acts as a short circuittransmission line, that is, the reflection energy ofinterest (liquid/air interface) is related to theincident energy by

E = -E.r i

Variations in the horizontal displacement of thereflected energy signal provide the information fromwhich liquid level can be determined.

The TDR instrument used in this program is themodel HP 180a/1815A/1817A/1106A.12 The pulsesource (28 psec tunnel diode) represents the state-of-the-art fast risetime pulsers. The TDR distancesensitivity is determined by the length of thesystem being monitored. These sensitivities arelisted in table 1.

TABLE 1. TDR DISTANCE SENSITIVITY

SYSTEM DISTANCE SENSITIVITY SENSITIVITY WITH MAG-(FEET) (FEET/DIV) NIFIED (FEET/DIV)

0 TO 1010 TO 100

100 TO 1000

0.010.101.00

0.0010.010. 10

Tamper Protection

Tamper protection is provided by the inherentability of the TDR to detect changes in coaxialtransmission line impedance.8,9 Figure 6 shows TDRtraces from a transmission line prior to and followinga tamper (hole) in the transmission line outerconductor. The reflection shown in figure 6(b) ischaracteristic of an inductive impedance discontinuity.Attempts to enter the sensor system either todivert material or alter measurement data willfurther disturb the transmission line impedance andbe detected. The tamper location is easily determinedbecause the TDR automatically measures the distanceto impedance mismatch reflection sources. Inaddition, the TDR monitors the liquid/air interfaceto detect abnormal changes in level.

where: Er = Reflected energy.E; = Incident energy.

ZI = Load impedance.Z = Characteristics impedance.0

The incident and reflected energies for theopen and short circuit transmission lines6,7 areillustrated below.

PULSE @E; OPEN I~E ErETDR

. OPEN E ESOE.PULSE r

E.

p

E z -z0

E, Z] + z0ZLE. E

TDR (I)E. SHORTPULSE

E Z -zI E. E --E.0

0

r

Zc-'O E --E.L r

. 1

a. NO TAMPER -s i

b. TAMPER EVIDENCE* OWv

t t*TAMPER EVIDENCE - CABLE IMPEDANCE CHANGE CAUSED BY HOLE INCABLE OUTER CONDUCTOR

Figure 6. TDR Traces Illustrating Tamper Evidence

618

Data Acquisition and Reduction

There are three forms of data utilized toobtain accountancy information: calibration datafor each instrument, measurement data acquiredduring normal operations, and calculated data obtainedmathematically from the measured data. The calibra-tion data are unique for the sensor system andrespective instruments, thus, requirements forobtaining these data will vary. The measured andcalculated data requirements are discussed below.

Measured Data

The measured data (differential pressure,liquid level, and temperature) are obtained directlyfrom the calibrated instruments. The differentialpressure data are obtained from a standard differen-tial pressure transducer connected to the sensorsystem, as described in the previous paragraphs.Figure 7 illustrates the pressures involved. Measure-ments are obtained when the liquid is forced out ofthe sensor probe inner conductor and bubbling occurs.Liquid level is measured with the TDR instrument bycomparing two TDR signatures4,5 as shown in figure4. In this illustration, the short circuit on theprobe bottom is used for reference; however, anylocation of the sensor system that is the source ofa permanent TDR reflection that will not be submergedby liquid may also serve as a reference. An exampleis a reflection source caused by a vessel connectionimpedance mismatch. The temperature data are obtaineddirectly from the calibrated temperature transducer.These data are used to calculate the volume, density,and specific gravity using the expressions describedin the following paragraph.

- *0 20,l 0. l t'_.1\~~~~~~ 1P . PRESSURE PROBE BOTTERP . PRESSURE ADORE YESSEL LIODID

V,' UIHLE PRESSURE

- 2FtP, P BULI.G OCCURSAIO P, P8. P DIFFERENTIAL PRESSURE

00 - 0, ' 00 DIFFERENTIAL PRESSURE EAD (TDR LIQUID LEAEL)

Figure 7. Sensor Probe Pressure and Liquid Level Details

Calculated Data

Volume is obtained by calibrating the vesselvolume as a function of TDR liquid level. Densityand mass are computed using the following expression.The differential pressure (AP) and differentialpressure head (AY) are measured as described in thepreceding paragraph.

M Pl PB APm V ~9g(YT YB) gA

where: Pm = Mass density.M = Mass.

V = Volume.

AP= Differential pressure.

AY = Probe liquid level (TDR).g = Acceleration due gravity.

Specific gravity is computed using the followingexpression:

Specific gravity = pM

where: Pm = Mass density.

PW = Mass density of water at T°C.

T = 4°C or some other temperature.

The density of water is known to vary withtemperature.10 The corrected density is computedusing the Taylor-Tilton formula shown below and themeasured temperature. The temperature correcteddensity of water is used to compute the specificgravity. __

gravity.~ ~ ~ 2 t + 288.9414 i

PW = 0.999973 [ [t 3.9863)2 t + 629 JJ

where: PW = Density of water in g/cc.

t = In degree centigrade (°C).

Data Reduction Techniques

The pressure and temperature instruments providemillivolt output signals directly proportional tothe pressure and temperature measurements respectively.The appropriate pressure and temperature units areobtained from the calibration data. The TDR instru-ment provides output signals (vertical and horizontal)that are proportional to the reflection coefficient(unitless) and distance (feet), respectively.These signals are connected to an X-Y recorder toobtain a permanent record.

TDR data may be recorded on separate graphs orsuperimposed on a single graph. For liquid leveldetermination, it is necessary to compare a measure-ment TDR signature with the calibrated TDR signature.This may be accomplished by overlay for separatelyrecorded signatures or by superimposing the calibra-ted and measured signals on a single graph andmeasuring the differences between them. The techni-ques used to evaluate the TDR test data during thisprogram are similar to the phase-slope methodsintroduced by M. DeCarolis.5

Test Description

The sensor and instrumentation systems weretested at the National Bureau of Standards using thesetup configuration shown in figure 1 and a specialclosed apparatus for testing the TDR using ordinarytap water, diluted nitric acid, and diluted uraniumtrioxide solutions.

The NBS sight tube reference and the TDR measure-ments were compared to determine the TDR liquidlevel measurement accuracy. For determination ofthe differential pressure measurement accuracy, thesensor system measurement data were compared withthe NBS furnished Ruska Model XR-38 measurementdata. The tests were conducted by adding and sub-tracting known quantities of liquid.

619

Test Results

The preliminary test results4 demonstrate thatthe TDR liquid level measurement resolution isdependent on the instrument horizontal scale settingwith the most sensitive scales yielding the bestaccuracy. Table 2 lists the average measurementresolution for 10 test runs with each test runconsisting of approximately 37 different liquidlevels. The solutions used during these tests arerelated to the test run as follows: test run 1-ordinary tap water, test run 2-diluted nitric acid,test runs 3 through 10-diluted uranium trioxide.The average TDR instrument liquid level resolution(neglecting the effects of the 1 foot per divisionsensitivity) is 1.83 millimeter. Normal resolutionbetween the minimum and maximum deviations is 1.95mm + 0.35 mm.

TABLE 2. AVERAGE TDR LIQUID LEVEL DEVIATIONSAND TDR HORIZONTAL SCALE SENSITIVITIES

TDR HORIZONTAL LIQUID LEVEL DEVIATIONS* (_) BY TEST RUN _ TOTALSCALE SENSITIVITY 2 3 4 5 6 7 8 9 iS AVERAGE

0.10 ft/div. 1.4 1.2 2.1 1.6 2.1 2.3 1.2 1.1 1.4 1.6 1.6

0.20 ft/div. 2.2 1.7 .9 2.0 1.7 1.6 1.3 2.2 1.0 1.9 1.6

5.50 ft/div. 2.8 1.3 1.8 2.7 2.8 3.7 1.8 3.0 - 1.0 2.3

1.00 ft/div. - - 2.3 3.4 - - - - - - 2.9

OVERALL AVERAGE 2.1_

Figure 8 is a plot of the differential pressuremeasurements versus the transfer number (liquidlevel change step) for both the sensor system trans-ducer and the NBS XR-38 reference. The averageresolution of the sensor system measurement is0.0015 psi. The normal resolution between minimumand maximum deviation is 0.0015 psi + 0.0015 psi.

0.3000

0.2900

0.2800.

0.2700

O.Z600 -..

0.2500 4.

V.2400.

0.2300

0.2100

0.2000

PRESSURE CHANGES VERSUSTRANSFER NUER

X- MODEL 555 0IFF. PRESS. TRANSHITTER

MODEL XR-38 NBS REFERENCE - AVE PRESSURE- .2378 psiAVERAGE DEVIATION: .0015 psiMAXIMUM DEVIATiON: .0030 psiMINIMUH DEVIATION: .0000 psi

O 1 2 3 4 5 6 7 8 9 10 1 1 12 13 14 15 16 17

TRANSFER NUHBER

Figure 8. Sensor System-NBS PressureMeasurement Curves

For a vessel possessing a liquid height of 6feet, the TDR measurement error is approximately 0.1percent. The differential pressure measurement erroris approximately .06 percent. The overall measurementerror is less than 2 percent, a figure within theguidelines established for measuring SNM in thenuclear fuel cycle.

This technique provides the safeguards inspectorand the industry with a new approach for obtainingaccountability data without loss in accuracy. Thesystem advantages are that it possesses a tamper-resistant probe and data link, and provides all thenecessary accountancy measurements with a singlevessel penetration. The system can be automated forunattended operation by using the appropriate generalpurpose minicomputer and 1/0 interfaces. The TDR canservice more than one probe with the addition ofcoaxial switching.5

References

1. Parsons, B. B., D. L.Final Report - Tamper

Durgin, and H. E. Rexford.Resistant Monitoring of

Chemical Reprocessing Plants, Volume I - "Summaryand System Description," BDM/A-64-73-TR, for theInternational Atomic Energy Agency and U.S. ArmsControl and Disarmament Agency.

2. Parsons, B. B., D. L. Durgin, and H. E. Rexford.Final Report - Tamper Resistant Monitoring ofChemical Reprocessing Plants, Volume 11, "Develop-ment of the Leacher Reservoir Monitor System,"BDM/A-65-73-TR, for the International AtomicEnergy Agency and U.S. Arms Control and Disarma-ment Agency.

3. Parsons, B. B., D. L. Durgin, and H. E. Rexford.Final Report - Tamper Resistant Monitoring ofChemical Reprocessing Plants, Volume 111, "Deve-lopment of the Plutonium Loadout Vessel MonitorSystem," BDM/A-67-73-TR, for the InternationalAtomic Energy Agency and U.S. Arms Control andDisarmament Agency.

4. Parsons, B. B. Final Report - Tamper ResistantMonitoring of Chemical Processing Plants,Volume 1, "Field Test and Evaluation of TimeDomain Reflectometry (TDR) Liquid Level Monitorfor Safeguard's Use," BDM/A-204-75-TR, for theU.S. Arms Control and Disarmament Agency.

5. DeCarolis, M, Bardone G. TDR Method and Appara-tus of Levels and Physical Characteristics ofMoving on Static Liquids and Fluids in Pipelineson Tanks.

6. Time Domain Reflectometry, Application Note 62,Hewlett-Packard Company, Palo Alto, California.

7. Strickland, James A. Time Domain ReflectometryMeasurements, Tektronix Inc., Beaverton, Oregon,1970.

8. Braddock, Dunn and McDonald, Inc. The Developmentof the Design Philosophy for Tamper ResistantAids to Inspection for Arms Control, for theU.S. Arms Control and Disarmament Agency.

9. Braddock, Dunn and McDonald, Inc. Test of TamperResistant Data Transfer System - Final Report,for the U.S. Arms Control and Disarmament Agency.

10. Schoonover, R. M., and J. F. Houser. PressureType Liquid Level Gauges, National Bureau ofStandards, ID 396.

11. Suda, S. C. Instruments and Data AnalysisMethods for Volume Measurements, BrookhavenNational Laboratory BNL 50489, February 1976.

12. Manufactured by the Hewlett-Packard Company.

620