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ESARDA BULLETIN, No. 43, December 2009

71

Abstract

Unknown nuclear material may originate rom sev-

eral sources. Nuclear orensics allows by using fn-

gerprinting and comparison with reerence data to

determine the origin, the intended use, the last legal

owner and the smuggling route. These inormation

are essential in the cause o thet or diversion as

measures o saeguards can be implemented to

prevent future thefts.

Certain measurable parameters can point to a spe-

cifc material and provide thereore a fngerprint o

the unknown material. Comparing the measured pa-

rameters with reerence material give clues to the

origin and the last legal owner.

Characteristic parameters and possible inormation

they contain are presented.

Keywords: nuclear orensics; uranium; plutonium.

1. Introduction: What is nuclear orensics?

As the ormer Soviet Union disintegrated in the ear-

ly 1990s a new phenomenon was discovered nu-

clear smuggling. The frst cases were reported in

1991 in Switzerland and in Italy. New questions had

to be answered: intended use o the unknown nu-

clear material, its origin, the last legal owner and the

smuggling route. Techniques to answer these ques-

tions were known rom nuclear saeguards and ma-

terial science. Combining these analytic methods

was the starting point o nuclear orensics.

Inormation that is obtained by nuclear orensic

analysis can be divided into two groups: endogenic

and exogenic inormation.

Endogenic inormation is meant as sel-explaining

inormation as age, intended use and mode o pro-

duction, or which only model-calculation might be

required or data interpretation.

Exogenic inormation is meant as inormation bycomparison. To interpretate exogenic inormation

which include geolocation and production data

comparison with reerence data and reerence in-

ormation is necessary. Great eorts are made to

set up databases containing such reerence data.

Case development in nuclear orensics ollows a

deductive way (Fig. 1). Taking the available results

into account a hypothesis or a set o hypotheses

are built. Databases and other experts serve or the

knowledge to build the hypothesis. Further analysis

has to be made to prove the presence o signaturesaccording to the hypothesis. I the signatures are

absent in the sample a new hypothesis has to be

developed.

Figure 1: The deductive way in the nuclear orensic

process [1].

2. The nuclear fngerprint method

To characterize an unknown nuclear material a set

o items are measured to establish the so called nu-

clear fngerprint. Inormation that orm the nuclear

fngerprint are:

Macroscopicparameters (e.g.pellet diameter,

height)

Isotopiccomposition

Fingerprinting of Nuclear Material for Nuclear

Forensics

A. Redermeier

Vienna University of Technology, ATOMINSTITUT, Bahnhofstrae 46, 2231 Strasshof, Austria

E-mail: [email protected]

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Elemental composition (suchaselementsand

impurities)

In cases o a mixture o components, a powder or i

it may be possible that substances o dierent

chemicals or isotopic composition have been add-

ed, it is necessary to investigate the microstructu-

rual fngerprint which consists o ollowing items:

Particlemorphology

Particlesizeandsizedistribution

Grainsizeandsizedistribution

Porositysizedistributionanddensity

Dislocationdensityandcharacter

Precipitationofotherphases.

2.1. Isotopic patterns o U and Pu

The ollowing inormation can be obtained by inves-

tigating the isotopic patterns:

ThepresenceofsmallamountsofU-236willin-

dicate a contamination with recycled uranium

and hence point at reprocessing activities.

Theisotopiccompositionofplutoniumisauseful

indicator o the reactor type in which the material

was produced.

Uraniumoxidecanbefoundindifferentforms,e.g. UO2 or U3O8, which give inormation on vari-

ous points o origin in the uranium uel cycle.

Plutonium is generated as by-product in nuclear re-

actors when 238U absorbs a neutron creating 239U,

which -decays into 239Np and fnally to 239Pu. Also

heavier isotopes o Plutonium are produced by ur-

ther neutron captures. Thereore the isotopic com-

position may give answers which reactor type was

used to produce the unknown nuclear material.

The isotopic composition o Plutonium provides in-ormation to indicate the type o the reactor:

Thehighertheinitial235U enrichment o the uel

is, the higher is the 238Pu abundance due to mul-

tiple neutron capture on 237Np.

TheneutronenergyspectruminuencesthePu

isotopic composition (the soter the spectrum,

the higher is the 242Pu/240Pu ratio).

The measured isotopic patterns can be compared

with model calculations using computer codes (e.g.ORIGEN or SCALE or the RBMK and the VVER). It

was demonstrated that the main reactor types can

be separated clearly rom each other (Fig. 2).

Figure 2: Calculated isotopic composition o Pluto-

nium or various reactor types (continuous line) and

measured isotopic composition (points) [2].

2.2. Age determination

The age is defned as the time elapsed since the

last chemical processing took place (e.g. produc-

tion, reprocessing or purifcation). The age o the

material may serve as exclusion parameter in the

search or the production or reprocessing plant.

Since radioactive isotopes decay at a rate deter-

mined by the initial amount and the hal-lie o the

isotope, the relative amounts o decay products

(daughters) in comparison to the parent isotope can

be used as chronometer.

The age o the material is short in comparison to

the hal-lie o the observed nuclides. Using the ta-

ble o nuclides many parent/daughter pairs can be

ound. The optimal nuclide ratios or Uranium are:

234U/230Th

235U/231Pa

and or Plutonium:

238Pu/234U

239Pu/235U

240Pu/236U

241Pu/241Am.

The radioactive decay o each o these nuclides is

unique, thereore measuring the parent/daughter

ratio allows to calculate the time elapsed since the

last chemical separation. I the material has not

been ully separated, the chronometer hasnt been

set properly to zero and so misleading inormationabout the age is gained. To avoid this systematic

error it is recommended to measure various parent/

daughter ratios.

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Figure 3: The parent/daughter mass ratios o aged

plutonium material [3].

2.3. Metallic impurities

Nuclear material contains metallic impuritites at var-

ying concentrations. In the starting materials metal-

lic impurities are accompanying elements, during

processing to the intermediate and fnal product the

impurities are drastically reduced. A reduction ac-

tor o 103 in the impuritity level is possible ater

processing. Impurity patterns within the processing

remain or the most mines the same.

Otherwise metallic impurities may enter in the nu-

clear material at the dierent processing stages. The

systematic behind this is not well understood up tonow, but a ull theory must contain that the concen-

tration o the impurities are a unction o exposure

time to the container material or storage tank as

they are leached rom the surace o the walls.

Nowadays in sample analysis the ratio o elements

o similar chemical behaviour are examined be-

cause the ratio will vary only within narrow limits.

2.4. Stable Isotopes

Measuring the stable isotopes is a established tech-nique in geolocation. Two substances which are

chemically identical have dierent stable isotope

compositions i either their origin and/or their history

are not the same. In nuclear orensics the oxygen and

the lead isotope ratio measurements are applied.

2.4.1. Oxygen

Natural oxygen exists in three stable isotopes: 16O

(99.762%), 17O (0.038%) and 18O (0.200%). Tem-

perature, latitude and distance to the sea cause

variations up to 5% in the 18O/16O ratio.

Water is used as a common solvent in uranium

processing. During the processing isotopic ex-

change takes place and the fnal U-oxide product

carries the signature o the 18O/16O ratio o the used

water. Measuring the oxygen isotope ratio provides

inormation about the geographical region where

the material was processed. Figure 4 shows the

correlation between the geographic location o the

production site o uranium oxid examples and thevariation in the (18O)/(16O) ratio.

Figure 4: Comparison o the 18O/16O ratio o U3O8

or dierent uranium mines. Olympic and Ranger

are Australian mines [4].

2.4.2. Lead

From the our stable lead isotopes one is primordial

(natural) 204Pb and the other three are endproducts

o radioactive decay series: 238U 206Pb, 235U 207Pb, 232Th 208Pb. Thereore the stable lead iso-

tope composition gives inormation on the initial

U/Th ratio in the mine and on the age o the ore.

Due to the act that the variations in the composi-

tion or dierent mines are signifcant, investigating

the stable lead isotopes can locate the origin mine.

2.5. Anionic impurities

Uranium crude ore undergoes dierent chemical

processes beore it becomes uranium ore concen-

trate (yellow cake). Since uranium is mined rom

ores with dierent mineralogical natures (acidic, al-

kaline and phosphatic ores) dierent chemical leach-

ing processes are used. In the subsequent process-

ing also dierent chemical processes are used or

the precipitating and dissolving the uranium.

Dierent acids are used or the dierent processesand may leave anionic impurities (Cl-, F-, Br-, NO2-,

NO3-, SO4

2- and PO43-). These anions may be an in-

dicator or the process used.

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In practice anion ratios are used because the leach-

ing rate in the mines can change and absolute anion

concentrations dier more than the ratios.

It has been shown that anionic impurities can distin-

guish between dierent mines (Fig. 5), but there are

also dierences between dierent sampling cam-

paigns in the same mine (Ranger-old vs. Ranger-resh and Beverly-old vs. Beverly-resh in Fig. 5).

Figure 5: Anion ratios in dierent uranium ore con-

centrate samples [5].

2.6. Limitations o these techniques

2.6.1. Cross contamination

There are two problems which may cause crosscontamination by investigating the stable lead iso-

topes. First, natural lead (204Pb) is omnipresent so

special care has to be taken perorming the chemi-

cal separation second, lead is oten used as

shielding material.

Presently the methods o the analysis are continu-

ously improved. I the methods are sensitive enough

to detect even metallic impurities rom the spatula

used to collect the evidence, new problems or the

analysts are created.

2.6.2. Reprocessing and enrichment

Presence o 236U can point to reprocessing activi-

ties. It has also been shown that in natural uranium

variations in 236U and in 234U abundances occur.

Better measurement methods have to be developed

or 236U abundance levels close to natural abun-

dance.

I nuclear material was reprocessed or non-peace-

ul purposes the identifcation o this material is verychallenging. Additional inormation is necessary to

calculate predictive signatures or enriched uranium

produced rom reprocessed uranium [8]:

The 232U content does not only depend on the

operation mode o the production reactor but

also on non-reactor-related history (i.e. length o

storage periods beore and ater irradiation).

HEUmaybeenrichedinasinglecascade,butit

is also possible to use several interconnected

smaller cascades.

The identication of the enrichment process

(gaseous diusion vs. gas centriuge) due to

small dierences in the concentrations o the

trace uranium isotopes (232U, 234U and 236U) is

very challenging.

2.6.3. Blending

A special challenge to determine origin o nuclear

material arises i it is blended. In ormer times Rus-

sia blended the spent VVER uel with spent uel opropulsion reactors, ater reprocessing this nuclear

material is suitable or RBMK reactors [9].

Thereore the nuclear material measured value o

the 238Pu/tot.Pu vs. 242Pu/240Pu value will not ft the

simulated values (Specimen R1 in Fig.2).

Sometimes however cross-contamination has oc-

curred beore collection to disguise the origin.

Transmission electron microscopy (TEM) is used to

determine the grain-size distribution and can there-

ore indicate that a powder consists o dierent par-

ticle types.

2.6.4. Computer codes

Estimating the uncertainty o all quantities used in

the calculations (i.e. cross sections) and determin-

ing how these uncertainties propagate through the

entire simulation is called uncertainty analysis.

Uncertainty analysis is not implemented in the ORI-

GEN2 and SCALE code.

An interesting overview how to determine the un-

certainty in such computer codes is given in [10].

3. Overview o analytical techniques used in

nuclear orensics

Due to the tremendous fngerprint diversity and the

requirement o high accuracies o measurement

many dierent analytic techniques have to be ap-

plied in nuclear orensics. Figure 6 gives a short

overview o inormation gained rom Uranium andPlutonium samples and which analytic techniques

are required to obtain this inormation. A short over-

view o most widely used techniques is given in [2].

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3.1. Including traditional orensic methods

Wherever he steps, whatever he touches, whatever

he leaves, even unconsciously, will serve as a silent

witness against him. Not only his fngerprints or his

ootprints, but his hair, the fbers rom his clothes,

the glass he breaks, the tool mark he leaves, the

paint he scratches, the blood or semen he deposits

or collects. All o these and more, bear mute wit-

ness against him. This is evidence that does not for-

get. It is not confused by the excitement of the mo-

ment. It is not absent because human witnesses

are. It is factual evidence. Physical evidence cannot

be wrong, it cannot perjure itself, it cannot be whol-

ly absent. Only human ailure to fnd it, study and

understand it, can diminish its value.

Dr. Edmond Locard

This is the amous Locards Exchange Principle that

states that every contact leaves a trace. Traditional

orensic methods establish relations between loca-

tions, events and individuals - entirely other inorma-

tion than the nuclear orensic methods. In summary

orensic methods provide inormation adherent to

the material while nuclear orensic methods provide

inormation inherent to special nuclear material [7].

A detailed description o collecting both nuclear

and traditional orensic evidence is given in the Nu-

clear Forensics Support [1]. During the collection o

evidence and analysing the evidence in a laboratory

special attention has to be taken on sufcient pro-

tection against the radiation.

To ulfl good radiological saety practice the Insti-

tute or Transuranium Elements developed jointly

with the German Federal Criminal Police a so called

glove-box, where contaminated evidence can be

visually inspected, photographed and fngerprints

and DNA samples can be taken (Fig. 7).

Figure 7: Glove-box or handling o special nuclear

material [7].

4. Data interpretation

Measured exogenic inormation (such as 18O/16O ra-

tio, Pb isotopic composition, impurities and micro-

structure) needs comparison with reerence data

rom known samples.

4.1. Supporting inormation

It is important to have access to reerence data and

to keep inormation, that may vary with time (e.g. as

seen in 2.5. level o impurities), up to date. The re-erence inormation can be gained rom Nuclear Ma-

terials Database, Open Literature, ITDB, IAEA Re-

search Reactor Database, other databases and

comparison samples.

The Nuclear Materials Database was established in

collaboration between the ITU in Karlsruhe with the

A. A. Bochvar Institute in Moscow. The database

contains inormation on uels or commercial reac-

tors as collected rom open literature and rom bi-

lateral agreements with uel suppliers.

Figure 6: Inormation that can be obtained rom nuclear (U, Pu) material and used analytic techniques [6].

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Certain data have limited accessibility (commercial-

ly sensitive data such as chemical impurities) or are

not shared (detailed inormation on weapons grade

material).

4.2. Exclusion Principle

In order to avoid a ull characterization each timewhen unknown nuclear material is ound, the exclu-

sion principle is applied. The exclusion principle

works as ollows:

1. The frst measured inormation (e.g. pellet dimen-

sions and isotopic composition) are compared

with the database entries.

2. The non-matching records are rejected as they

could not be manuacturer o the unknown nu-

clear material.

Exit condition: A single record is let. This is thebest case.

3. The remaining records, which match the meas-

urements, are compared to each other in order

to identiy parameters which could distinguish

between these records.

Exit condition: No urther parameters can be in-

vestigated.

4. These parameters are measured next.

5. The measured data are compared with the re-

duced database entries.Go back to 2).

5. Conclusion

Combining various analytic techniques can give in-

ormation about the origin, the intended use, the last

legal owner and the smuggling-route o unknown

nuclear material. But this is only possible i enough

reerence datas are available and accessible.

Thereore it is necessary to strengthen the interna-

tional cooperation and the cooperation with the uel

manuactors.

Since the beginning o nuclear orensics in the early

1990s more and more parameters proved to be use-

ul and could be applied or the nuclear fngerprint.

Hence it is necessary to do urthermore research

and development and to keep close cooperation

with other sciences whether new characteristic pa-

rameters can be investigated.

6. Acknowledgement

The author would like to thank the European Sae-

guards Research and Development Association,

Working Group on Training & Knowledge Manage-

ment or organising the 5th ESARDA course.

Reerences

[1] IAEA; Nuclear Security Series No 2, Nuclear Forensics Sup-

port, Vienna, Austria, 2006, http://www-pub.iaea.org/MTCD/

publications/PDF/Pub1241_web.pd[2] Mayer K. et al; Nuclear orensicsa methodology providing

clues on the origin o illicitly trafcked nuclear materials, Ana-

lyst, 2005, 130, 433441

[3] Wallenius M., Mayer K: Age determination o plutonium mate-

rial in nuclear orensics by thermal ionisation mass spectrom-

etry, Fresenius J Anal Chem (2000) 366 :234238

[4] Advances in destructive and non-destructive analysis for envi-

ronmental monitoring an nuclear orensics, Karlsruhe, Germa-

ny, 2002, http://www-pub.iaea.org/MTCD/publications/PDF/

pub1169_web.pd

[5] Badaut V. et al;Anion analysis in uranium ore concentrates by

ion chromatography, Journal o Radioanalytical and Nuclear

Chemistry, Vol. 280, No.1 (2009) 5761

[6] Wallenius M. et al; Nuclear orensic investigations: Two case

studies, Forensic Science International 156 (2000) 55-62

[7] Mayer K. et al; Nuclear orensic science From cradle to ma-

turity, Journal o Alloys and Compunds 444-445 (2007) 50-56

[8] Joint Working Group o the American Physical Society and the

American Association or the Advancement o Science; Nu-

clear Forensics Role, State of the Art, Program Needs

[9] Wallenius M. et al; Origin determination o Plutonium material

in nuclear orensics, Journal o Radioanalytical and Nuclear

Chemistry, Vol. 246, No. 2 (2000) 317-321

[10] NUREG/CR-5625, ORNL-6698; Technical support or a pro-posed decay heat guide using SASH2/ORIGEN-S data

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