- s ~ n s i ~ i ~ e  ort

MICHAEL HOTCHIUS, DAVID CHILD, CLAUD10 TUNIZ and MEGAN WILLIAMS  Australian Nuclear Science and Technology Organisation  PMB 1 Menai 2234 NSW Australia

Summary

The International Atomic Energy Agency (IAEA) applies a range of ultra-sensitive detection techniques to  provide assurance that Member States are in compliance with their safeguards agreements. Environmental  samples are collected which can contain minute traces of nuclear material or other evidence. Careful analysis of  these samples reveals the nature of the activities undertaken in the vicinity of the sampling point. This paper  reviews the analytical techniques that are being applied.

To ensure that the IAEA has access to the best available methods, samples are distributed to a group of qualified  laboratories around the world for analysis. The Accelerator Mass Spectrometry facility at the Australian Nuclear  Science and Technology Organisation (ANSTO) is part of this select group of laboratories, and is the only AMs  facility currently accredited with the IAEA. AMs provides the highest sensitivity available for detection of  particularly useful signature radioisotopes, including I, U and plutonium isotopes. 129 236

Introduction

Traditional safeguards methods have been applied by  the IAEA using verification measures based on  nuclear material accountancy and containment and  surveillance. These methods verify the correctness

of the declaration of nuclear material inventories  made by Member States. The challenge for the  safeguards system is to be able to verify the

completeness of declarations made by States. In  particular, the system must be able to provide  assurance of the absence of undeclared nuclear  material and activities, and be able to provide that  assurance with a high level of confidence. Also, it  must be able to provide assurance of the absence of  undeclared facilities. The new strengthened  safeguards system, developed by the IAEA over the  past twelve years, shifts focus from safeguards  implementation at the facility level to the State as a  whole. Information is obtained from a variety of  sources: from States themselves, inspection  activities, open sources, satellite imagery and  environmental sampling. In the following we will  focus on the latter source of information.

Samples taken from within and around declared  facilities are analysed. Present and past activities  undertaken at a facility disperse and leave behind  minute traces of nuclear material and other evidence.  The elemental and isotopic composition of these  traces, or the presence of certain radioisotopes, act as  signatures of the particular activities undertaken. The  results are compared to the signatures that would be

expected to result from the declared activities of the  facility. Any anomalies found are subject to further  investigation and inquiries. To provide assurance of  the absence of undeclared facilities, the concept of

wide area environmental sampling (WAES) has been  studied [I] and is being further evaluated.

Environmental Sampling

Since 1996, environmental samples in the form of  cotton surface wipes (swipes) have been taken from  within a large number of enrichment plants and  facilities with hot cells [2]. A range of analytical  methods is engaged to study the material collected.  The isotopic ratios of uranium and plutonium are of  most interest, as this gives information on degree of  enrichment and/or fuel burn-up. The first step  involves screening by high resolution gamma  spectroscopy, performed at the IAEA’s Safeguards  Analytical Laboratory at Seibersdorf near Vienna.  Detailed further analyses are then undertaken either  at Seibersdorf or through distribution of samples  among a group of expert laboratories around the  world. These laboratories must undergo a rigorous  accreditation procedure to qualify for membership of  the Safeguards Network of Analytical Laboratories

( W A C ) .

Bulk analysis of swipes is undertaken using X-ray  fluorescence, isotope dilution thermal ionisation  mass spectrometry (TIMS) and other techniques.  Very high sensitivity can be achieved - with TIMS,

isotopic analysis of samples containing a total of less

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than lpg of nuclear material can be performed.  However, the nuclear material often exists on the  swipes as an array of minute particles, and bulk  analysis provides only the average isotopic  composition. Information about the extremes of  isotopic composition, which is of particular interest  for nuclear safeguards, may be lost in the bulk  analysis.

The most powerful technique available to elucidate  the present and past activities of a facility is particle  analysis: the isotopic analysis of individual particles  picked up on a swipe. The particles are typically a

few microns diameter. First it is necessary to identify  the ‘hot’ particles -those containing a predominance  of nuclear material - then each hot particle is  analysed individually, to determine the U and Pu

isotopic ratios.

Figure 1: SIMS image of uranium-238  obtained by rastering a <10pm beam spot  across a 150pm field of view. Particle sizes  ranee from 6-15um.

Two methods are used for particle analysis. In the  first, particles are removed from the swipe and  deposited on a track-etch polymer (Lexan), which is  then irradiated in a reactor. Particles containing a  high proportion of fissionable material are identified  visually by their high track density; these particles  are then transferred individually on to rhenium  filaments for isotopic analysis by TIMS. This  method has high sensitivity, high precision, and high  cost.

In the second, less costly method, secondary ion  mass spectrometry (SIMS) is used both to find the  particles, in scanning mode, and then to perform the  isotopic analysis on particles identified as containing  uranium or plutonium. ANSTO participated in an

inter-laboratory comparison of SIMS analysis  capabilities for uranium particles; Figure 1 shows a  scan of one of the test samples. A technique has also  been developed whereby scanning electron  microscopy (SEM) is used to locate the hot particles  prior to transfer to and re-location in SIMS, where  the isotopic analysis is performed [3].

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Wide Area Environmental Sampling (WAES)

It is a considerable challenge for the IAEA to be able  to provide assurance of the absence of undeclared  facilities. Such facilities may be located anywhere  where the necessary infrastructure exists. The first

obstacle is a political one: under standard safeguards  agreements, the IAEA is only permitted to inspect  and take samples at or around declared facilities.  Under the Additional Protocol to Safeguards

agreements, much more extensive inspections are  permitted. Parties to the non-proliferation treaty are  being encouraged to sign up to the Additional  Protocol; Australia was the first signatory back in

1997.

There is also a considerable technical challenge  posed by the problem of undeclared facilities. A  combination of theoretical studies and field trials has  been undertaken to evaluate the possibilities. A  report prepared for the IAEA in 1999 [ 11 concluded

that the most promising technical approach would  involve the use of networks of high volume air  samplers. Air filters can reveal signatures of nuclear  activities through high sensitivity bulk analysis. As  with swipe analysis, isotopic ratios of U and Pu are  of particular interest. Other signatures have also been  considered. A small reprocessing operation, for  example, would release certain long-lived fission and

activation products including *%, 90Sr, 134,’37Cs, lZ9I  and Pu isotopes [ 13.

At ANSTO we have pursued the use of the 12’1 and  2 3 6 ~  as particularly promising signatures for use in

WAES. lZ9I can be detected at extremely low levels  by Accelerator Mass Spectrometry (AMs - see  below), although this was not appreciated by the  authors of the IAEA report [ 11. Field trials and other  studies [4] have demonstrated the potential of lZ9I. In

addition, we have developed a unique capability for  detection of 236U in environmental media [5], and  demonstrated its usefulness as a signature of  reprocessing. Using AMs allows the detection of  minute traces of irradiated uranium, which contains  236U, against a background dominated by natural  uranium, which is virtually free of 236U [6].

Accelerator Mass Spectrometry (AMS)

The AMs laboratory at ANSTO is currently the only  AMs facility which is accredited as a member of  NWAL. It qualified on the basis of its quality-  assured measurement capability for lZ9I and 23&u.

AMs (see ref. [7]) differs from other kinds of mass  spectrometry in accelerating ions to energies in the  range 10-100 MeV. AMs measures isotopic ratios,  by counting the ions of the rare radioisotope while  measuring the beam current of the corresponding  stable isotope. With the use of a Tandem accelerator,

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the method has three key properties:

(i) negative ions are injected; this eliminates stable  isobars of elements which do not f o m  negative ions;  for example, lZ91 is free from ‘ 2 9 ~ e  interference;

(ii) interfering molecular ions are destroyed by  stripping ions to high charge states in the high  voltage terminal; for example z36U detection is free  of 235UH;

(iii) at high energies, ions can be positively identified  through precise measurement of their energy, rate of  energy loss and time-of-flight; for example 1291 can  be distinguished from multiply-scattered lZ7I ions.

As a result, AMs has a very high abundance  sensitivity: in some cases, isotopic ratios can be  measured with a detection limit as low as loi5 (for  example, radiocarbon, where the “C: 12C ratio is  measured in samples containing 1mg carbon or less).  In the case of uranium, where typical environmental

samples yield about 1pg of uranium, the current  detection limit is for the z36U:238U ratio. This  corresponds to a detection limit of around  1 femtogram for 2 3 6 ~ .

To illustrate the high sensitivity of AMs, Figure 2  shows the results for a sample from a recent inter-  laboratory comparison exercise, NUSIMEP-2  (Nuclear Signatures Interlaboratory Measurement  Evaluation Programme), run by the Institute for  Reference Materials and Measurement [ 81. The

sample contained 0.1pg of uranium and had a  236:238 ratio of los7. This ratio proved to be below  the detection limit of all other participating  laboratories, who were using a variety of mass  spectrometry (ICP-MS, TIMS) and radiometric  methods. While AMs can eliminate interference  from molecular species, ICP-MS and TIMS must  rely on background subtractions to correct for  molecules. The number of measurements with  inaccurate results for this sample indicates problems

NUSIMEP- 2 : 2ssU/238U isotopic ratio

.

3 4  I

Results from all participants

Figure 2: All results for Sample 111 in the  NUSIMEP intercomparison including our AMs  result.

with such corrections, or possibly problems with  cross-contamination between samples. AMs has also been shown to have femtogram-level  sensitivity for Plutonium isotopes and 237Np [9]. We  are now extending our capabilities at ANSTO to  include Pu isotopes, with a view to providing the  IAEA network with higher sensitivity bulk analysis  of swipe samples. At present, there is sometimes  insufficient material picked up on swipes for analysis  by methods such as TIMS, leaving the IAEA unable

to draw conclusions from those samples.

Discussion

In recent times, the role of the IAEA in nuclear  safeguards has become a subject of high importance  in international affairs and a subject of intense public  interest. ANSTO’s technical and scientific expertise  with a cutting-edge technique - AMs - has enabled  us to assist the IAEA in its most vital mission. We  have fulfilled this role by originating the ideas, such

as the use of 236U in WAES, by doing the necessary  research and development, and now by providing a  unique analytical service as part of the IAEA’s  network of analytical labs.

The effectiveness of environmental swipe sampling  is now well established. However, further  improvements are possible and we are developing  higher sensitivity Pu detection for this purpose. Wide  area environmental sampling continues to be  evaluated through field trials and other sampling. Air  sampling systems were deployed in Iraq [lo] by the  IAEA Action Team.

The techniques discussed in this paper can also be  applied to other situations where the presence and  nature of minute traces of nuclear or radioactive  material may be evidence of unauthorised activities.  This area of “nuclear forensics” was the topic of a  recent IAEA conference [ 111 which brought together  scientists and other experts in these areas. Particle  analysis and isotopic ratio measurements have been  used in combination with a range of other techniques  in a number of cases of theft and illicit trafficking of  nuclear material.

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