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Studying possibilities for nuclear waste disposal

Prototype Repository inSwedish BedrockWith nuclear power providing about 16% ofglobal electricity, safe solutions for the storageand disposal of spent fuel are being sought inexperiments all over the world. Geologicaldisposal is internationally considered to be themost appropriate method for dealing withlong–lasting radioactive waste. In Sweden, thepossibilities of disposing of spent fuel in thebedrock are being studied in a prototyperepository at the Äspö Hard Rock Laboratory(Äspö HRL). Vaisala equipment is being usedfor humidity measurements.

Ritva Siikamäki, M.A.Acting Editor-in-Chief VaisalaHelsinkiFinland

bout 50 % of theelectricity in Swedenis currently pro-duced by nuclear

power plants. The SwedishNuclear Fuel and WasteManagement Company (SvenskKärnbränslehantering AB, orSKB), jointly owned by theSwedish nuclear power plants,is responsible for the manage-ment and disposal of radioac-tive waste from these plants.SKB has been carrying out re-search into the final disposal ofradioactive waste since the mid-1970s. The Swedish governmenthas accepted the geological dis-posal of spent fuel and othernuclear wastes as a legitimatebasis for SKB’s research and de-velopment work.

As in many other countries,it is planned to use geologicaldisposal as a part of the wastemanagement systems in Sweden.The final disposal of nuclearwaste is to take place in a deeprepository approximately 500meters down in the bedrock.Until now, feasibility studieshave been carried out in eightmunicipalities around Swedento find a suitable site for a deeprepository, and three of thesehave been proposed for moredetailed studies by SKB. TheSwedish bedrock provides a sta-ble mechanical and chemicalenvironment, in which changesoccur only extremely slowly.Approximately two meters ofrock are quite sufficient to stop

At Äspö HRL, the research areas are located in several locations. Thetunnel is connected to the surface via shafts for the hoist and ventilation.

Currently, spentfuel in Sweden issent to an interimstorage facility atthe CentralInterim StorageFacility for SpentNuclear Fuel(CLAB) near theOskarshamnnuclear powerstation.

A

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the direct radiation emitted byradioactive waste.

Construction of the ÄspöHard Rock Laboratory (ÄspöHRL) in southeast Sweden inthe vicinity of the OskarshamnNuclear Power Plant was startedin 1990 and the facility wascompleted in 1995. The ÄspöHRL consists of a research cen-ter and a large descent tunneland system of disposal tunnels.In this underground labyrinth,the descent is 3600 meters longand extends to a depth of 460meters. The last 65 meters ofthis is the prototype repository.Electric heaters are used to si-mulate the heat energy flow thatwould be created by spent fuelin an actual deep repository.

Multidisciplinaryresearch at Äspö

The prototype repository hasbeen constructed in order tostudy the thermal, hydraulic,mechanical, chemical and bio-logical processes that would re-late to any future deep reposito-ry of spent fuel. Participants inthe research include a number

of universities, research insti-tutes and other experts in vari-ous fields such as geology, rockmechanics, investigation andmeasurement methods, ground-water chemistry, the chemistryof radioactive materials, andsafety analyses.

The prototype repository isan EU-funded project withbroad international participa-tion, involving eight countriesbesides Sweden. A lot of re-search has focused on issues re-lating to the bedrock’s functionas a barrier or filter for radioac-tive substances. The move-ments and chemical composi-tion of the groundwater havealso been central subjects.Additionally, the experimentshave produced in-depth knowl-edge of the interaction of ben-tonite clay, copper canisters andthe bedrock under realistic con-ditions.

At Äspö HRL, the main re-search and design consultant re-garding clay-based engineeringbarriers is Clay Technology AB,a soil and rock engineeringcompany specializing in clay-based barriers for waste dispos-

al. Clay Technology has beeninvolved in the Äspö HRL ex-periment for more than 10years, acting as the project man-ager in the Backfill and Plugproject as well as in the LongTerm Test of Buffer Material(LOT) project. Due to its broadexpertise, the company also actsas a consultant in similar inter-national projects.

For Vaisala, the prototyperepository project involved R &D work to find optimal solu-tions for special needs. Vaisalahas been involved in the projectfor two years, with Mr. DraganMorovic of Vaisala Malmö re-sponsible for Vaisala’s contribu-tion.

Realistic environment forexperiments

The Äspö HRL allows the ex-periments to be performed in arealistic environment, with con-ditions similar to those in an ac-tual repository. The key para-meters and phenomena thathave been studied at Äspö in-clude microorganism build-upand the oxygen content in thegroundwater - an issue affectingthe corrosion rate of the canis-ters in a deep repository.

Large-scale testing anddemonstration is still needed be-fore a deep repository for spentnuclear fuel can be built, inorder to make sure that the safe-ty requirements are met. One ofthe key aspects will be to ana-lyze the long-term effects on the

Copper canisters act as a barrier for radiation. Each canister is about 5 meterslong, has a diameter of one meter and weighs 25 - 27 tons when filled with fuel.

At the Äspö HRL, accuratehumidity measurements are obtainedwith Vaisala HMP237 HumidityTransmitters, which are ideal fordemanding applications requiringan extremely rugged design.

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bedrock of various futureevents. Spent nuclear fuel mustbe isolated for a very long time,which sets very strict require-ments on encapsulation tech-niques and materials. Moreover,the long-term stability of thematerials is an essential issueand, in this respect, copper andclay have performed very well.

Spent nuclear fuel

Nuclear fuel consists of urani-um dioxide, UO2, that is heatedand compressed into pellets.The structure of the sinteredfuel pellets resembles that of ce-ramic materials, which makesthem sparingly soluble in water.In fact, the fuel acts as a barrierin itself. The fuel is enclosed infuel rods, consisting of a zirco-nium alloy.

Spent fuel is irradiated fuelthat is discharged from a reactorat the end of a fuel cycle, sinceit is no longer considered effi-cient. In the process of generat-ing energy with nuclear power,two kinds of radioactive wasteare produced: low and interme-diate level (LLW and ILW) op-erating waste and high-levelwaste (HLW), i.e. spent fuel.The spent fuel is initially placedin water pools in the reactorbuilding and allowed to coolfor some years. The fuel rod as-semblies are moved to an inter-im storage facility where theyare submerged in water for sev-eral decades before being placedin a final repository.

Multiple-barrierapproach

The Äspö HRL has experiment-ed with a method in which thespent nuclear fuel - simulated inthis case by heaters - is protect-ed by a number of barriers formaximum safety. Should one ofthe barriers fail, other barriersstill remain in place to protectthe environment and people.The greatest threats to safety arecorrosion or movement in thebedrock. The copper canisterthat isolates the fuel in a com-pletely tight housing providesthe primary barrier for radioac-tive substances. Copper is high-ly resistant to corrosion, andbentonite clay acts as a bufferaround the canister, absorbingmovements in the rock andkeeping it securely in place. In

addition, the clay has the abili-ty to bind radioactive sub-stances, which makes it an ef-fective barrier.

In the multiple-barriermethod, the fuel rods are firstencapsulated tightly in a coppercanister that is transported byspecial vehicles to the reposito-ry and placed in the holes. Thecanisters are embedded in ben-tonite clay that reacts with waterby swelling and seals the holes.In addition to this barrier, thetunnels are then filled up with amixture of clay and crushedrock. The descent tunnels willalso be filled up at closure ofthe repository. When the repos-itory is sealed, no further moni-toring, maintenance or supervi-sion will be required in order toensure that it remains secure. Inthe Äspö HRL experiment, thecanisters will, however, be re-trieved. The LOT project in-volves a test series comprisingseven test parcels, which will beexposed to repository condi-tions for 1, 5 and 20 years.

Underground humiditymeasurements withHMP237 Transmitters

In the test areas at the ÄspöHRL, several environmental pa-rameters such as temperature,pressure and humidity are mea-sured to study the conditionsand their effects. In particular,humidity measurement is im-portant in studying the bufferand backfill materials in thetunnels. The relative humidityreadings of the tunnels andholes are constantly monitored

Spent fuel – a matter of national and international concern

According to the statistics of the International Atomic EnergyAgency (IAEA), there were 438 operational nuclear powerplants in the world at the end of 2000. Nuclear power pro-vides 16% of global electricity, with 83% of nuclear capacityconcentrated in the industrialized countries.

Although decisions concerning the management of nuclearwaste are a matter for each national government, there areseveral international dimensions, especially with respect to themovement of spent nuclear fuel across borders and disusedsealed sources. Recently, 25 countries signed and ratified theJoint Convention on the Safety of Spent Fuel Management andon the Safety of Radioactive Waste Management. This was thefirst international protocol on the safe management and stor-age of spent fuel and radioactive waste, and entered into forceon 18 June 2001. While each country is still responsible for themanagement of its radioactive waste, the Joint Conventionclearly sets the obligations that regulatory regimes must meet.

Storage vs. disposal

The term “storage” is defined by the IAEA as the placement ofwaste in a nuclear facility where isolation, environmental pro-tection and human control are provided with the intent that thewaste will be retrieved at a later time. The primary distinctionbetween “storage” and “disposal” relates to the intention of re-trieving the waste at a later time. Disposal refers to placing thewaste in a facility finally, with no intention to retrieve it.

by 140 Vaisala HMP237Humidity Transmitters inte-grated into a measurement anddata logging system.

According to Mr. OlaKarnland of Clay Technology,who works as a project managerfor one of the experiments, theVaisala equipment has per-formed well at the site. Accuracyand reliability were essentialconcerns when selecting themeasuring devices. The extreme-

ly rugged design and compactsize of the Vaisala transmitterswere also important factors.

Mr. Dragan Morovic ofVaisala highlights the impor-tance of Vaisala’s R & D in thisproject which was significant indeveloping a new probe design.Another advantage of theVaisala instruments is their sta-bility. “Long calibration inter-vals are definitely a benefitwhen the devices are buried un-derground,” notes Mr. Morovic.“The location of the Malmö of-fice made things a lot easier,too. In a very complex projectsuch as this, it is essential to beable to arrange meetings at shortnotice when needed,” he says.

Thorough assessment of the method

SKB will carry out an assess-ment after an introductory op-erational phase of the reposito-ry. Should the method thenprove to have shortcomings, orbetter methods are found, thecanisters will be retrieved. Ifthis method proves to be se-cure and tenable, SKB willapply for a license to progressinto regular operation. �

Vaisala has been involved in the project for two years, with Mr. DraganMorovic of Vaisala Malmö responsible for Vaisala’s contribution.

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