Special reports: Safeguards & non-proliferation

Internationalsafeguards

aspectsof

spent-fuelstorage

Techniques and approachesare evolving

to meet future challenges

by V. Pushkarjov and E.Tkharev

Spent-fuel storage is an important independentelement within the nuclear fuel cycle. Because of thedelay in taking decisions regarding the ultimate dispositionof spent-fuel assemblies in some countries, an everincreasing amount of spent fuel is accumulating instorage. Storage must be provided either at-the-reactor(AR) or away-from-the-reactor (AFR), regardless of themanagement option chosen.

The first power plants were constructed with normalreactor spent-fuel pools capable of storing fromtwo-thirds to one-and-two-thirds of a core. It wasexpected that after an initial cooling time of 2 to 3 years,the spent fuel would be moved to a reprocessing plant.When the original programmes for reprocessing weredelayed, plans on spent-fuel storage were modified toincrease existing storage capacity at the same location.

On the other hand, it appears there may bepossibilities for alternative storage of spent fuel. Thesetechnologies involve the use of long-term, separatespent-fuel storages of two types, wet and dry. These areforeseen as large, centralized facilities capable ofaccepting spent fuel from a number of reactors. Atpresent, separate spent-fuel storages are becomingincreasingly important facilities in the nuclear fuel cyclebecause most spent fuel will be stored there for an

Mr Pushkarjov is Head, Section for System Studies in the Agency'sDepartment of Safeguards, and Mr Tkharev is a member of thatsection.Photo: Inside Sweden's CLAB facility for AFR spent-fuel storage,a transport flask is being lowered into the pool. (Credit: SKB)

extended period of time before reprocessing or finaldisposal.

Wet storage of spent fuel is still the predominantstorage method, with long and positive experienceavailable. In some European countries, different designsof wet storages have been developed and these facilitieseither have already been constructed or will beconstructed (in France, the German Democratic Republic,Sweden, the United Kingdom, and the USSR, amongothers).

Dry storage of spent fuel currently is being evaluatedas an alternative option to water-cooled pools, and as amethod of final spent-fuel disposal. Dry storageprogrammes are under way and are being investigatedin countries including Canada, the Federal Republicof Germany, Spain, Switzerland, the United Kingdom,and the United States.

Safeguarding stored spent fuel

Safeguarding spent fuel is one essential element of theinternational safeguards system implemented atnuclear fuel cycle facilities. The IAEA has always paiddue attention to the development of safeguards pro-cedures for irradiated-fuel storage but, at present, thisproblem is becoming one of particular importancedue to the situation described earlier.

A few years ago, the IAEA Secretariat began a studyon the development of safeguards approaches for separatespent-fuel storages. Related to this, tasks are beingdeveloped under support programmes for IAEA safe-

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guards of the FRG, USA, and USSR. In 1983, a meetingtook place in the USSR on safeguards problems for aparticular design of wet storage planned for constructionin a number of countries of the Council for MutualEconomic Assistance (CMEA). In 1984, the IAEA •organized a consultants' meeting on problems related tosafeguarding AFR storages and future trends in this area.

All this strongly emphasizes the importance ofproblems concerning safeguards of spent fuel and itspractical implementation.

Safeguards procedures for stored spent fuel

In classifying nuclear fuel cycle facilities with respectto material accountancy, spent-fuel storage is consideredas an item facility. This means that all nuclear materialis contained in identifiable items (e.g., fuel assemblies,sealed canisters, or casks of spent-fuel assemblies)during its residence at the facility. The quantity of

nuclear material contained in each item should be knownfrom measurement or estimate. Furthermore, thenuclear material content is assumed to be constant (or tochange only in known ways, e.g., due to radioactivedecay) so long as the integrity of the items is preserved.(See the accompanying box for a review of basicsafeguards concepts.)

In such cases, IAEA safeguards are based on itemaccounting procedures, which include item counting andidentification, examination of the continued integrityof the item, and non-destructive measurements.Containment and surveillance (C/S) measures supple-menting nuclear material accountancy have particularsignificance in safeguarding spent fuel. In internationalcontrol practice, the Agency seeks to use the nuclearfacility's containment or physical barriers (e.g., walls,transport containers, storage casks, etc.) to restrictor control the movement of, or access to, nuclear material.Through surveillance, such as TV or film cameras, the

Basic safeguards concepts

Safeguards for spent-fuel storage are being implementedin accordance with the IAEA Statute and twofundamental agreements (INFCIRC/66/Rev. 2 andINFCIRC/153 [corrected]).

The objective of safeguards as stated in I NFCIRC/153is "the timely detection of diversion of significantquantities of nuclear material from peaceful nuclearactivities to the manufacture of nuclear weapons or ofother nuclear explosive devices or for purposes unknownand the deterrence of such diversion by the risk ofearly detection". In safeguards agreements concludedunder the non-NPT system (INFCIRC/66 Rev. 2),there is no specific definition of objective, but in currentpractice essentially the same concepts apply.

In the practical implementation of internationalsafeguards, the Agency is guided by the technicalobjectives based on three fundamental parameters("detection goals"):• The significant quantity of nuclear material• The detection time (the maximum time that mayelapse between diversion and its detection by the IAEA)• The detection probability, which determines therequired reliability of IAEA safeguards measures.

In designing a safeguards approach for spent-fuelstorage facilities and evaluating results obtained, the IAEAassigns the following quantitative values to the detectiongoals relating to spent fuel:• Significant quantity: eight kilograms of plutonium(all isotopes); 25 kilograms of uranium-235 contained inhighly-enriched (greater than or equal to 20%) uranium;75 kilograms of uranium-235 contained in low-enriched(less than 20%) or natural uranium• Detection time: 1 to 3 months for plutonium andhighly-enriched uranium in spent fuel; order of 1 yearfor uranium-235 in low-enriched uranium;• Detection probability: 90 to 95%.

Using the detection goals as guidelines, the Agencydraws up for every facility the inspection goals, takinginto account actual operational conditions, the provisionsof the safeguards agreement, and the current technicalpossibilities for applying safeguards measures.

The fundamental concept of IAEA safeguards isverification that nuclear material subject to safeguardsunder the agreement is not diverted from peaceful

- nuclear activities. In practice, this verification isperformed by means of regular inspections with afrequency defined by the detection time. IAEA verifi-cation is based on nuclear material accountancy as asafeguards measure of fundamental importance, withcontainment and surveillance (C/S) as importantcomplementary measures. It includes examination ofthe operator's records and reports regarding the amountsof nuclear material present and its use, as well asindependent measurements and observations conductedby the Agency.

As a first step for developing a safeguards scheme,a safeguards approach is designed for a specific facilitytype. The safeguards approach is a system of nuclearmaterial accountancy and C/S measures to attain theverification goals, taking into consideration thecapabilities of these measurement systems, facility designfeatures, and facility practice.

To simplify verification of spent fuel, an lon-1 detector(shown here during safeguards training) involving grossgamma-ray and neutron measurements was developed.

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Agency collects information using devices and/orinspector observation to detect any undeclared movementof nuclear material, tampering with containment,falsification of information related to locations andquantities of nuclear materials, or tampering with IAEAsafeguards devices.

This combination of C/S as complementary measuresto nuclear material accountancy can provide forcontinuous knowledge of the verified nuclear material,on the integrity of items, and on movements of nuclearmaterial. The most desirable combination of thesemeasures is the one that permits the safeguards objectivesto be achieved at acceptable costs and with minimumintrusion into routine plant operations.

IAEA experience safeguarding spent fuel

It must be understood that basic safeguards conceptswith regard to spent fuel may serve only as guidelineswithin the scheme of practical implementation ofinternational safeguards at a specific facility. At first,spent fuel appears at the reactor after irradiation offresh fuel, then it is placed in reactor storage for sometime. Then, following a chosen policy, it is transferredto a reprocessing plant, or to either interim or long-termstorage for future reprocessing or final disposal. It isobvious that at each stage practical safeguards measuresmay differ significantly depending on facility type,its design features and regime, and length of spent-fuelstorage. If spent fuel is transferred from one nuclearfacility to another, this interrelationship betweencorresponding facilities should be taken into account inthe design of a safeguards application scheme at eachof the facilities involved.

For a long time in the practice of IAEA safeguards,the only locations at which irradiated reactor fuelwas kept under international control were in storagesites at the reactor. In the late 1970s when reprocessingfacilities came under Agency safeguards, the IAEAstarted to gain experience in the control of irradiatedreactor fuel in the buffer storages of such facilities.Since then the Agency has accumulated experience anddeveloped safeguards approaches and procedures forsuch types of storages. Recently, the first long-term, AFRstorages have been constructed in a number of countriesand now are subject to safeguards.

In the following sections, possible ways of safeguardsapplication, and IAEA experience regarding spent fuelat different nuclear facilities, are briefly described.

Storage at power reactors

In the majority of reactors, irradiated reactor fuel isstored in pools filled with water to provide coolingand proper shielding. At present there are about 100 suchstorages at reactor sites under IAEA safeguards..

Accountancy measures such as item counting,identification, and non-destructive assay (NDA) measure-ments can, in principle, be performed at all reactor pool

storages if they are provided with appropriate instrumentsand equipment. Different C/S measures, using TV andfilm cameras and seals, may be applied in the storage areaas complementary measures. NDA measurements providethe ability to re-verify the inventory of spent fuel in theevent of failure of C/S measures.

A few specific safeguards features relevant to thedifferent design concepts of power reactors are presentedbelow:

• Light-water reactors (LWRsj. Spent-fuel storagesat LWRs are foreseen as short-term ones. Spent fueldischarged from a reactor is stored underwater, usuallyin "baskets" that hold the assemblies in a vertical positionproviding favourable conditions for counting andidentification. The minimum storage time before ship-ment is variously quoted as 90 to 180 days, but morefuel currently is stored for longer times (years) due tolack of reprocessing capabilities. Most reactor storagefacilities were designed with a capacity of perhaps two tofour complete reactor cores.

All LWR fuel assemblies are manufactured with aserial identification number on the top.- In theory, thisserial number is readable after irradiation undertypical spent-fuel storage conditions. In actual practice,the storage conditions are such that readability issometimes questionable. The core reloading at LWRsnormally occurs at 12 to 18 month intervals, duringwhich spent fuel is transferred from the core to thestorage ponds, where it is verified by the inspector. Attimes other than reactor reloading, the flow of spentfuel occurs only during shipment, either to reprocessingor to long-term storage.

• Heavy-water reactors (HWRs) - CANDU type.The CANDU reactor was not designed on the assumptionthat reprocessing will occur, and due to this fact long-term storage of irradiated fuel is foreseen. Capacities atspent-fuel storage bays vary with reactor type, but inall cases capacity is expandable by adding secondarystorage facilities. Storage capacity of approximately1 year is usually provided in the primary storage bay atmulti-unit CANDU power stations. Bundles arestored horizontally in containers (trays or baskets ormodules) suitable for stacking. After this period,the stored fuel will be transferred via an underwater canalto the secondary storage bay having a storage capacityof 30 operating years.

The safeguards approach developed for CANDU usesunique instruments and techniques because of particulardesign features (namely, continuous on-load refuelling,large inventory, and difficulties of access for routineinventory measurements). Item accountancy forinventory and flow verification forms the foundation ofthe safeguards approach, although for routine purposesthe CANDU approach uses flow-verification methodsin combination with C/S to verify irradiated-fuelinventory. Use of seals in the secondary bay will providea significant decrease of inspection effort required forinventory verification.

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A spent fuel bundle counter was'developed for Candu reactorsthat are refuelled on line. (Credit: AECL)

• Gas-cooled reactors (GCRs). The IAEA has onlylimited experience in safeguarding nuclear reactors withgas-cooled storages of irradiated reactor fuel. Thereare only three such storages at GCRs, and one demon-stration facility for experimental storage of irradiatedreactor fuel in concrete canisters. Due to the delayin reprocessing irradiated fuel, there has been considerableeffort recently to increase the capacities of existingstorages and those under construction at reactor sites.Among proposed technical solutions, which may createadditional difficulties for verification and influencepractical inspection activities, are storage compaction,addition of a second level of storage racks, andconsolidation of spent-fuel rods by placing them intospecial containers. These modifications have to betaken into account in the design scheme of safeguardsimplementation and, in some cases, the developmentof new safeguards methods and techniques is required.

Storage at reprocessing plants

Storage pools at spent-fuel reprocessing plants weregenerally designed on the assumption that receivedspent fuel would soon be reprocessed. Sufficient spacewas provided for fuel storage covering 3 to 4 monthsof operating capacity for a reprocessing plant. Somelater designs of large-scale reprocessing plants exist whereit is foreseen to increase the storage time of spentfuel in buffer storage up to several years (4 to 5)before reprocessing.

Fuel assemblies are received at reprocessing facilitiesin shielded casks typically weighing 20 to 100 metrictonnes and holding in the range of 10 to 20 assemblies.In the storage ponds, spent-fuel assemblies may bestored either in simple racks designed to support the

assemblies in a vertical position and to maintain thespacing required for nuclear safety, or the assemblies maybe kept in various types of baskets or containers forease of handling or for other reasons. These specialcontainers often are of safeguards interest because theyresult in covering the top of the assemblies, preventingvisual observation of serial numbers, observation ofCerenkov radiation, or even simple physical presence ofassemblies.

If spent-fuel assemblies are accessible for directverification in storage ponds, the inspection proceduresare, in principle, similar to those performed at spent-fuelstorages at the reactor site, though they may requireconsiderable efforts. In other cases, when fuel assembliesstored in special closed containers are inaccessible forexamination, these storage containers are defined as basicitems of material accountancy. It is assumed that theircontent is verified by inspectors either at the reactorimmediately before shipment, and then sealed, or at thereprocessing plant upon receipt, and then sealed.

Taking into consideration the operation regime of thereprocessing plant, item accountancy and bulk materialaccountancy are combined for verification of spent-fuelflow from storage to the process area.

Long-term spent-fuel storage

The Agency has gained some experience in safeguardsimplementation of wet storages at reactors andreprocessing plants, and it seems that safeguardsprocedures at long-term, separate wet storages will be .similar in many respects. Meanwhile, the extended timeperiod of storing spent fuel, the capacity of long-term,separate storages, and their design features raise newquestions on safeguards application at such facilities.

Verification procedures based on accountancymeasures are limited at a number of wet-type storagefacilities because of the design features and the limitedtechnical possibilities of performance of NDA measure-ments. In addition, it appears that these measurements(where they are feasible) are likely to be very time-consuming because of the large number of spent-fuelassemblies (up to several thousand) stored in thesefacilities. In some cases, when spent-fuel assemblies arestored in special storage baskets or multi-element"bottles", even item counting and identification ofassemblies may cause serious difficulties.

This situation is even more complicated for drystorages where spent fuel cannot be verified in practiceby accountancy measures because it is physicallyinaccessible. Based on these facts, the role of C/Smeasures is becoming extremely important for safeguardsimplementation at long-term, separate spent-fuelstorages for both wet and dry types.

Fortunately, favourable conditions for applicationof C/S measures at these facilities are provided in so faras spent-fuel assemblies remain in a static positionfor long periods of time. Diversity of C/S measuresapplied at storage should be such that corresponding

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C/S devices do not possess a common mode of failureand provide a high level of technical performanceand redundancy.

For the same reasons, the establishment of an initialinventory of irradiated fuel loaded at the reactor is atask of high priority. It is particularly important for drystorages where transport/storage containers cannotbe re-opened for verification upon their arrival at thestorage facility. Reliable sealing systems are to be appliedto transport containers to preserve integrity andcontinuity of knowledge on nuclear material transferredfrom reactor to storage.

It is clear that operating procedures related to transferof spent fuel from the reactor to long-term storagewill considerably influence the safeguards inspectionscheme. For designing a safeguards approach, this inter-relationship between shipping (reactor) and receiving(storage) facilities should be taken into consideration forthe optimization of inspection efforts and effectivesafeguards implementation.

It appears that the regular presence of an inspectorduring shipment of spent fuel from the reactor and itsreceipt at storage will demand considerable inspectorresources. The development of advanced C/S techniquesmay solve this problem and provide potential benefitsto reduce the demand for inspection resources. As anexample of these new techniques, two particular devicesare currently being developed under support programmesfor Agency safeguards. These are the spent-fuel assemblycounter and the electronic sealing system, which arecapable of storing all opening and closing times of sealsattached to transport casks. Information collectedthrough these instruments and supported by surveillancecameras at both facilities may allow continuity ofknowledge on spent-fuel shipments from the reactor tostorage without an inspector's presence at either facility.

The described devices have conceptual safeguardsmeanings and form the basis of new ideas for safeguardsimplementation relevant to the storage of spent fuel.

The practical use of these devices should, under realisticconditions, be properly analysed on the basis of futuretests of this instrumentation, relevant to technicalperformance, reliability, and tamper resistance. Thefeasibility of applying this technique to particularfacilities also will depend on the specific design featuresof these facilities.

Design guidelines

All nuclear facilities are subject to many economic,technical, legal, security, safety, environmental, andother design constraints, and it is the function of thefacility design team to find solutions that are in somesense optimal. For nuclear facilities, the ability tofacilitate safeguards implementation is an additionalconcern that should be taken into account to the greatestextent possible at the design stage. This is particularlytrue for spent-fuel storage, where C/S measures play avery important role. This problem has been thoroughlydiscussed at a number of meetings held by the Agency.Member States have expressed their understandingof this problem's significance and are taking into accountcorresponding guidelines and recommendations in theearly design stages of spent-fuel storages.

Satisfactory application

In this article, some general aspects of safeguarding.spent fuel have been described. Of course, they do notreflect all details and peculiarities relevant to theinternational safeguards system regarding spent fuel.With respect to storage at the reactor and buffer storageat reprocessing plants, the IAEA has a proven schemeof safeguards implementation. Current safeguards 'techniques may be satisfactorily applied at long-termspent-fuel storages to achieve the safeguards objectives,in spite of a number of unique features at these nuclearfacilities.

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