New technology strengthens the hand of IAEA safeguards1 February 1998
Among the most important roles of the International Atomic Energy Agency is as an independent inspector of all nuclear material and facilities subject to safeguards agreements. Backing up this vital work is a range of new techniques and equipment used for verification activities including non-destructive analysis, containment & surveillance, unattended and remote monitoring, and environmental sampling, the last an area now adding considerable capability to the Agency.
The IAEA has the task of providing continuing assurance to the international community that States that have entered into safeguards agreements with the IAEA are meeting their obligations. This requires, in particular, that any diversion of safeguarded nuclear material from civil use to a proscribed purpose would be detected. To this end, the IAEA must be able to verify the correctness and completeness of the statements it receives from States concerning the nuclear materials included in the State’s safeguards agreements with the IAEA. The basic verification measure used by the IAEA is nuclear material accountancy, with containment and surveillance as important complementary measures.
In applying nuclear material accountancy, inspectors make independent measurements to verify quantitatively the amounts of nuclear material presented in the State’s accounts. For this purpose, inspectors count items (eg fuel assemblies, bundles or rods; or containers of powdered compounds of uranium and/or plutonium) and measure attributes of these items during their inspections and compare their findings with the declared figures and operator’s records. This activity aims at detecting missing items (gross defects). The next level of verification, called partial defect detection, may involve the weighing of items and measurements with non-destructive assay (NDA) techniques such as neutron counting (active or passive) or gamma spectrometry. These NDA techniques are capable of measuring the amount of nuclear material with a relative uncertainty in the range of 1-10%. For detecting bias defects, it is necessary to sample some of the items and to apply physical and chemical analysis techniques having the lowest possible relative uncertainty, typically in the range of 0.1-1%. These techniques are termed destructive assay (DA) methods and their use requires that the IAEA has access to laboratories that apply such precise and accurate techniques on a routine basis.
Containment and surveillance (C/S) techniques are applied to reduce the safeguards inspection effort (eg by limiting the frequency of accountancy verification) and also to give assurance that nuclear material follows predetermined routes, that the integrity of its containment remains unimpaired, and that the material is accounted for at the correct measurement points. A variety of techniques are used, primarily optical surveillance and sealing. These measures serve to back up nuclear materials accountancy by providing means by which access to nuclear material can be controlled and any undeclared movement of nuclear material detected.
The IAEA obtains new equipment and techniques for safeguards purposes either by specific development activities or by testing and evaluation of off-the-shelf equipment. Since the IAEA has no research and development resources of its own, it depends mainly on dedicated national support programmes which assist the IAEA in this domain. The role of the IAEA lies in co-ordinating these programmes and in testing and evaluating the techniques developed and the equipment resulting from them. All aspects of equipment performance are evaluated, including compliance with specifications, reliability and suitability for transport and, most importantly, for use by IAEA inspectors in nuclear facilities. As a result of these assessments, the new equipment passes through different developmental stages until it reaches the status of a fully approved device for routine use in the field.
The widely used gamma-ray spectroscopy equipment now includes two new portable multi-channel analysers and new analysis software. Both are small and versatile inspection tools, supporting a broad variety of radiation detectors and verification methods. They can easily be transported in an aeroplane or a car and, hence, are particularly suited for unannounced or no-notice inspections. One instrument was completely developed through the German Support Programme, whereas the other instrument was an off-the-shelve equipment which has been tested and evaluated by the IAEA.
The Mini-Multi Channel Analyser (MMCA) and the Canberra Inspector Multi-Channel Analyser (IMCA) are the two recent successors to the Portable Multi-Channel Analyser (PMCA), used by the IAEA for about 15 years. These two instruments are at the beginning of their operational life at the IAEA. They support all the detectors that had been used with the PMCA – including NaI, CdTe and high purity Germanium detectors. The MMCA represents a truly significant reduction in size and weight and a three-fold increase in operating battery lifetime compared to the previous PMCA. The MMCA weighs just 680 grams including the Li-Ion battery. Combined with a HP Palmtop and a CdTe detector it makes a very powerful yet versatile system that fits in half a briefcase – ideal for many inspection uses. The operating battery lifetime is at least 12 hours for CdTe and NaI detectors. The IMCA is being used primarily at present in unattended monitoring applications, but will also serve all of the normal portable functions of the PMCA.
The verification of spent fuel has always been a challenge because of the difficult access to the assemblies in the pool and the need to reduce the impact on facility operation. Recent developments have concentrated upon facility specific variants of the Spent Fuel Attribute Tester (SFAT) and Cerenkov viewers which allows in-situ verification for long-time cooled and low burn-up fuel.
The SFAT consists of a multi-channel analyser and a NaI or CdTe detector mounted and enclosed to be used under water in spent fuel ponds. SFAT provides an attribute verification of the presence of spent fuel through detection of particular fission product gamma rays – either Cs-137 (662 keV) for fuel which has cooled longer than four years or Pr-144 (2182 keV) for fuel with shorter cooling time. The SFAT can be employed in situations where Cerenkov viewing cannot provide verification, eg in the case of low burn-up or long-cooled spent fuel where Cerenkov radiation is too weak, or where the water in the storage pool is too murky. The detector and its lead shielding are housed in a stainless steel watertight container, which can be submerged in a spent fuel storage pool and positioned over the items to be examined. The increase in intensity of the selected gamma-rays from the item is compared with the intensity when positioned over the gap separating the fuel assembly from its neighbour.
The Cerenkov Viewing Devices (ICVD and ACVD) are image intensifier viewing devices that are sensitive to the ultraviolet radiation in the water surrounding the spent fuel. These viewing devices operate with facility lights turned on in the spent fuel pond area. They are optimised for ultraviolet radiation by filtering away most of the visible light and by making the image intensifier tube to be sensitive primarily to ultraviolet radiation. Spent fuel assemblies are characterised by Cerenkov glow patterns that are bright in the regions immediately adjacent to the fuel rods. This variation in light intensity is very apparent when viewed from an aligned position directly above the fuel rods. This can be used to differentiate an irradiated fuel assembly from a non-fuel item.
CONTAINMENT & SURVEILLANCE (C/S) TECHNIQUES
IAEA optical surveillance equipment is presently in a transition period from analog video systems to digital surveillance systems. Digital surveillance systems are essentially mandated by the strong commercial industrial trend to low cost digital components and by the hoped-for improvements in system reliability. There are other inherent benefits for IAEA safeguards, such as the enhancement of data evaluation and automated review, the prospect for remote surveillance, as well as improved authentication and encryption, to name only a few. Three new digital surveillance systems (EMOS from France, GDTV from the USA and VDIS from Germany) are being extensively field tested for potential future routine use. The ideal system should incorporate authentication, encryption, image compression, scene change detection, connection to triggering devices (door opening, radiation or temperatures changes), connection for remote communication through wire or satellite, battery backup and local storage using an international standard. Such a system should also be suitable for a stand-alone, single camera set up, and for integration in a local network of adjacent equipment.
Containing the nuclear material and sealing it off until the next inspection provides a high confidence that no tampering has taken place. A sealing system comprises the containment enclosing the nuclear material to be safeguarded, the means of applying the seal (eg a metal wire) and the seal itself. All three of these components must be examined in order to verify that the sealing system has fulfilled its function of ensuring continuity of knowledge of the identity and integrity of the nuclear material concerned. Seals can be verified in-situ with an appropriate automated seal verifier or shipped back to IAEA Headquarters for subsequent verification. Recent developments are focused on improved tamper indication and authentication capabilities for seals and on automated seal verifiers.
The In-Situ Verifiable Seals fall into three main categories: fibre optic loop; ultrasonic; and electronic seals. One of the most widely used in-situ verifiable seals is VCOS, an electronic seal that provides a high level of tamper indication using electronic encoding methods in conjunction with fibre-optic loops. The seal is intended for high reliability, long duration surveillance in those applications that require periodic access. Time, date, and duration of openings and closings of the loop are recorded internally for later retrieval. For installations with multiple seals in proximity, the seals may be daisy-chained in a “Party-Line” mode permitting a sequential read out of seal data without changing connection. The seal housing is opened only to replace the internal batteries. All openings are recorded as tamper events. The electronics are embedded in X-ray resistant compound of epoxy and ceramic particles to frustrate attempts at reverse engineering. The use of the seal has been integrated with existing analogue video surveillance systems as well as with new digital systems to allow a facility operator – in case there is a need for operating reasons – to attach or detach seals under surveillance and thereby eliminate the need for inspector presence during this operation. The software guides the operator correctly through the procedure. In this type of application the seal data are also stored on video tape for ease of review.
UNATTENDED AND REMOTE MONITORING
Unattended systems have come very early to IAEA safeguards. Optical surveillance systems, whether photo or video cameras, are inherently unattended systems since their prime function is to survey an area for safeguards relevant activities over extended periods of time when inspectors are not around. Unattended monitoring systems employing radiation detection sensors are increasingly used to detect the flow of nuclear material past key points in the facility process area. For complex nuclear facilities where the plant is automated (remotely operated), unattended assay and monitoring techniques are an integral part of safeguards verification. The unattended use requires attention already at the design stage if the system is to be a reliable, cost-effective tool that provides credible, independent data. This means that the system must operate without failure over extended time period, even in case the facility power supply is interrupted. The unit should also automatically record its state of health and, if data has to be transmitted over unsecured data pathways, data must be authenticated.
The most recent developments in the unattended monitoring field are the new Bundle Counter (VIFB) and Core Discharge Monitor (VIFC) for use in Candu plants, for counting the irradiated fuel bundles transferred to the spent fuel bay and those discharged from the core, respectively. The central key component of both VIFB and VIFC is a generic autonomous data acquisition module (ADAM) which is essentially a counter timer with 8 channels input, designed for low power operation so that it can operate solely on battery power for at least three months. The ADAM is a generic module capable of operating also in a stand alone mode, independent of the VXI bus, and of supporting 8 nuclear pulse detectors.
Remote monitoring brings unattended monitoring a further step forward, with the transmission of the collected data to an IAEA regional office or to the IAEA headquarters in Vienna, through telephone lines, network cables or satellites Better safeguards effectiveness and better cost efficiency are the prime justification for remote monitoring implementation. In fact, the possibility of data transmission to a remote location, in essentially real time, can reduce the frequency of inspector visits to the facility and increase the capability for data review and evaluation. In addition, the supplementary transmission of State-of-Health information, can also result in a significantly more reliable operation than an unattended system that is serviced by inspectors. Routine use of remote monitoring by the IAEA is in the preliminary phase. Limited IAEA human resources, an ever-growing stockpile of nuclear material and economics are likely to accelerate implementation of remote monitoring in the near future. A specific additional requirement of remote monitoring is data encryption to maintain the facility and State’s need for confidentiality of information.
Among other locations, remote monitoring is now demonstrated in Switzerland with regular images and data directly transmitted back to IAEA headquarters. The system includes two self-contained digital video cameras providing authenticated and encrypted surveillance images and data, and two remotely verifiable electronic seals. Power monitoring and control, back-up battery power and scene change detection are all integral system features which contribute to the overall reliability. The images and data are transmitted to data storage computers at IAEA headquarters via communication satellite and ultra-small aperture terminal (USAT) satellite transceivers. Subsequently, the images and data are periodically transferred to the Safeguards Department’s local area network for review, upon demand, by authorised inspectors.
Environmental dust samples at or near a nuclear site, combined with ultra-sensitive analytical techniques, such as mass-spectrometry methods, particle analysis, and low-level radiometric techniques, can reveal signatures of past and present activities in locations where nuclear material is handled. The initial implementation of environmental sampling for safeguards has since 1966 been focused on the collection of swipe samples inside enrichment plants and hot cells. Other types of nuclear facilities will come later as well as other types of environmental samples (eg vegetation, soil, water) outside facilities and sites.
Samples are analysed in either bulk or particle mode depending on the sampling objectives and the activity levels of the swipes. Bulk analysis involves the analysis of an entire sample; the analytical measurements represent average results of the material contained. Particle analysis relies on the detection and analysis of individual particles in the micrometer size range and provides as results isotope ratios of uranium and/or plutonium in particles.
An IAEA Clean Laboratory for Safeguards was inaugurated in December 1995 in Seibersdorf near Vienna with a Class-100 clean-room for the provision and certification of sampling kits and for the receipt, screening and distribution of environmental samples coming from safeguards inspections. This facility for clean handling of sampling materials and samples significantly reduces the risk of cross contamination that might lead to false alarms. The Clean Laboratory consists of over 200 m2 of laboratory space, with approximately 100 m2 at Class-100 cleanliness level. The laboratory is equipped with a suite of analytical techniques, including alpha, beta, gamma and X-ray fluorescence spectrometry, scanning electron microscopy with electron probe attachment as well as high-sensitivity thermal ionisation mass spectrometry.
Environmental swipe samples are received at the Clean Laboratory and are given a code number to maintain confidentiality about their origin. The samples are then measured by low-background gamma spectrometry to detect the presence of actinide elements (primarily U and Pu) and fission or activation products (such as Co-60, Cs-137, Ru-106, etc); the samples are then measured by radioisotope-excited X-ray fluorescence spectrometry to detect the presence of U, Pu or other important elements. Alpha/beta counting is then applied to the samples to detect actinides or beta-emitting isotopes such as H-3, Sr-90 or Tc-99m. Scanning electron microscopy is used to examine small particles removed from environmental samples. The size and morphology of these particles can be examined at high magnification and their elemental and isotopic composition can be measured with X-ray fluorescence spectrometry using the electron probe attachment.