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Nuclear Waste Management

Nuclear Waste Management

Claire Corkhill and Neil HyattNucleUS Immobilisation Science Laboratory, Department of Materials Science and

Engineering, University of Sheffield, Sheffield, UK

IOP Publishing, Bristol, UK

ª IOP Publishing Ltd 2018

All rights reserved. No part of this publication may be reproduced, stored in a retrieval systemor transmitted in any form or by any means, electronic, mechanical, photocopying, recordingor otherwise, without the prior permission of the publisher, or as expressly permitted by law orunder terms agreed with the appropriate rights organization. Multiple copying is permitted inaccordance with the terms of licences issued by the Copyright Licensing Agency, the CopyrightClearance Centre and other reproduction rights organisations.

Permission to make use of IOP Publishing content other than as set out above may be soughtat permissions@iop.org.

Claire Corkhill and Neil Hyatt have asserted their right to be identified as the authors of this workin accordance with sections 77 and 78 of the Copyright, Designs and Patents Act 1988.

ISBN 978-0-7503-1638-5 (ebook)

DOI 10.1088/978-0-7503-1638-5

Version: 20180601

Physics World DiscoveryISSN 2399-2891 (online)

British Library Cataloguing-in-Publication Data: A catalogue record for this book is availablefrom the British Library.

Published by IOP Publishing, wholly owned by The Institute of Physics, London

IOP Publishing, Temple Circus, Temple Way, Bristol, BS1 6HG, UK

US Office: IOP Publishing, Inc., 190 North Independence Mall West, Suite 601, Philadelphia,PA 19106, USA

To the materials scientists and engineers of the future—we need your new ideas andfresh perspectives to complete the task that the generation before ours started, and

that we do not have the lifespan to finish.

Contents

Abstract viii

Acknowledgements ix

Author biographies x

Nuclear Waste Management1 Introduction 1

2 Background 2

Waste immobilisation 4

3 Current directions 6

Geological disposal of high-level nuclear waste 6

Spent nuclear fuel 9

Plutonium: waste or resource? 13

4 Outlook 17

Additional resources 18

vii

Abstract

Nuclear waste—the radioactive by-product from nuclear power generation, nuclearweapons and medical isotope production—is one of the most challenging types ofwaste for our society to manage. Its high radioactivity requires that it be safelyisolated from humans and the environment until it no longer poses a hazard; of theorder of a million years. This review will show that nuclear waste management is aworld of materials science and engineering challenges that must stand the test oftime, from designing engineered facilities to isolate waste from future civilisations, toinventing new materials to immobilise weapons-grade and surplus civil plutonium.Due to the ever-changing nature of nuclear waste, which transforms its chemicalcomposition and physical properties through radioactive decay processes, nuclearwaste management is also a race against time that will continue to drive research anddevelopment for many years to come.

viii

Acknowledgements

C L C wishes to acknowledge the EPSRC for the award of an Early Career ResearchFellowship (EP/N017374/1) and for funding under the US Department of EnergyNEUP-EPSRC research programme (EP/R006075/1) and the UK–Japan CivilNuclear project, CHIMP (EP/R01924X/1). N C H would like to acknowledgefunding from EPSRC under the following grant awards: DISTINCTIVE (EP/L014041/1); EP/P013600/1; EP/M026566/1, EP/N017870/1 and EP/N017617/1.

ix

Author biographies

Claire Corkhill

Claire Corkhill is a Reader in mineralogy and materials science atthe University of Sheffield, UK, and currently holds an Engineeringand Physical Sciences Research Council (EPSRC) Early CareerResearch Fellowship. Her research focuses on understandingthe relationship between surface chemistry, microstructure anddissolution kinetics in nuclear waste materials. Her particular focusis the corrosion of oxide ceramics (e.g. uranium oxide), alumino-

borosilicate glass and cement materials. Recent work includes development ofsynchrotron-based diffraction and spectroscopy methodologies to observe slowmaterial degradation processes over long timescales, relevant to the geologicaldisposal of nuclear waste.

Neil Hyatt

Neil Hyatt holds a Chair in Radioactive Waste Management in theDepartment of Materials Science and Engineering at the Universityof Sheffield, UK, is a visiting Professor at Washington StateUniversity, and is a member of HM’s Government NuclearInnovation and Research Advisory Board (NIRAB). He is Directorof the NucleUS Immobilisation Science Laboratory, the UK’sleading academic centre for radioactive waste research. His research

focuses on developing strategy, materials, processes and policy to support the safe,timely and efficient clean-up of the UK radioactive waste legacy. A key aspect ofhis research is the design, manufacture and performance assessment of glass andceramic materials for the immobilisation of plutonium residues, legacy intermediatelevel wastes and high level wastes from reprocessing operations.

x

Physics World Discovery

Nuclear Waste Management

Claire Corkhill and Neil Hyatt

Nuclear Waste Management

1 IntroductionGlobally, nuclear power plants provide almost 11% of the world’s electricity. Asinterest in the generation of electricity by nuclear fission has been renewed by ademand for ‘low-carbon’ energy sources, this is set to increase. Nuclear power isgenerated in a process called fission, which involves the splitting of 235U atoms afterimpact by a slow neutron (n):

+ → + + +xU n A B n 200 MeV23592

10

10

Splitting of the 235U atom releases energy in the form of heat, which is used tocreate steam that drives electricity-generating turbines. The reaction is self-sustain-ing because splitting also releases more neutrons, which can go on to split morenuclei of 235U (called a chain reaction). When a 235U atom is split, it forms twosmaller atoms (labelled A + B for simplicity in the equation above), which areunstable. These smaller atoms include fission products and minor actinides, whichare highly radioactive.

Like other types of electricity generation, nuclear power produces waste.Managing this waste is similar to the management of toxic chemicals arising fromother industrial processes, but with one exception: due to the splitting of 235U intoelements with unstable nuclei, the waste generated is radioactive. This means thatthe processes used to handle, treat and store the waste must minimise the impacts ofradioactivity on human health and the environment. Uniquely, this hazard (theradioactivity) will reduce over time, owing to the radioactive decay of radionuclides(i.e. radioactive isotopes) within the waste to stable isotopes. This process can take ofthe order of at least one million years, making nuclear waste management one of themost significant, and long term challenges that our society faces.

Due to the very large amount of energy generated through nuclear fission from avery small amount of fuel, the quantity of nuclear waste from the last 60 years ofcivil nuclear power generation is actually relatively small. For example, the average

doi:10.1088/978-0-7503-1638-5ch1 1 ª IOP Publishing Ltd 2018

yearly waste output from nuclear power in the UK, including decommissioning, is~4000 m3 yr−1. Coal power stations, by comparison, generate ~130 000 m3 of ashand sludge per year, in addition to 1.45 million m3 of carbon dioxide (CO2).

Nuclear waste is not only produced from nuclear power, but is also generatedfrom the extensive use of isotopes in medicine (e.g. radioactive tracers ingested toidentify tumours) and research. Wastes are also created as by-products of mineralmining. Another source is ‘legacy wastes’, which are the remnants of historicmilitary nuclear operations; for example, by-products of plutonium weapongeneration at the end of the Second World War and during the Cold War era(particularly in the US, UK and Russia). These wastes were often generated withlittle regard to their future management, and thus are some of the most challengingand expensive to manage. For example, at the Hanford site in the US, operations areunderway to extract sludges and effluents from ‘tank farms’ where liquid waste fromfuel reprocessing was placed in inadequately designed underground storage silos thathave begun to leak into the surrounding environment. All types of nuclear wastemust be managed properly, now and in future generations, or there will be adverseeffects to human health and the environment.

The fundamental principles of nuclear waste management are: to ensure thegeneration of nuclear waste is kept to a minimum; to protect human health andthe environment; and to protect future generations (and perhaps, considering thetimescales involved, civilisations) while also ensuring they are not burdened withmanaging nuclear waste generated in our lifetime.

2 BackgroundThe nuclear fuel cycle describes the series of industrial processes that produceelectricity from uranium fuel—typically uranium oxide (UO2)—in nuclear powerreactors. Figure 1 demonstrates the different types of wastes that are producedthroughout the nuclear fuel cycle, from uranium mining, milling, conversion,enrichment and fuel fabrication (front-end processes) to the so-called back-endprocesses, performed once fuel has undergone fission and is removed from thereactor. At this time, the fuel is known as spent nuclear fuel (SNF).

Depending on whether a country operates an ‘open’ or ‘closed’ fuel cycle,different types of nuclear waste are produced. In an open fuel cycle, SNF is cooledfor several years under water in engineered cooling ponds, then placed withincontainers and sent for long-term storage (and ultimately disposal in a geologicaldisposal facility). The only additional wastes created in an open fuel cycle aretechnological wastes (for example, from handling SNF during cooling and placing incontainers) and effluents (for example, waste water from SNF cooling ponds).Countries including Sweden, Finland, Canada and the USA operate an open fuelcycle for civil nuclear power.

A closed fuel cycle takes advantage of the fact that SNF is composed of 96% UO2

and 1% plutonium oxide (PuO2), generated by neutron capture of UO2. Both arefissile materials and, if recycled, can be reused as fuel. To recover this usefulmaterial, it must be separated from the remaining 3%, which comprises fission,

Nuclear Waste Management

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minor actinide and activation products, representing the most radioactive portion ofSNF. Separation is typically achieved via a liquid separations process (such as thePlutonium and Uranium Refining by EXtraction process, PUREX), which involveschopping fuel and separating it from the metal alloy cladding, dissolving the SNF innitric acid, removing the UO2 and PuO2 using an organic solvent and, after drying,immobilising the remaining fission products by vitrification in a borosilicate glass.These reprocessing operations create a large number of different radioactive wastestreams. Countries that operate a closed fuel cycle include the UK, France, Japan,Russia and China.

Classification of nuclear waste differs according to the regulatory body of eachnation state, but generally depends on the radionuclide inventory and half-life.Wastes that comprise concentrations of radionuclides that give rise to radiogenicself-heating are known as high level waste (HLW). These wastes have substantiallevels of radioactivity and require shielding, personnel protection, remote handlingand consideration of the heat generated. They include SNF and some reprocessingwastes (e.g. vitrified fission, actinide and activation products). Other categories ofnuclear waste have lower activity levels than HLW, but may still require specialconsiderations with respect to the level of radioactivity; for example, shieldingduring processing and handling. Such wastes are known as intermediate level waste(ILW). Collectively, HLW and ILW are referred to as higher activity wastes. Wastes

Nuclear power plant

Fuel fabrication MOX fabrication

ConversionEnrichment Reprocessing

U

Immobilisation

Immobilisation

Interim storage

ConversionEnrichment

Milling

U + Pu

Mining

Disposal

SpoilRadon

Effluents

TailingsEffluents

LossesF2, HF, H2Effluent

LossesHex tails

Activation productsFuel corrosion

Effluents

Activation productsFuel corrosionEffluents

HLWILWLLWVolatilesEffluents

Technological wasteEffluents

Spoil

Figure 1. Wastes produced through the civil nuclear fuel cycle. Orange arrows indicate processes operating ina closed fuel cycle, while blue arrows indicate those that operate in an open fuel cycle.

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3

that do not generate heat, have a low radioactive inventory and do not require anyspecial considerations in their handling are known as low level waste (LLW).

While wastes arising from medical radionuclides can be acceptable for landfilldisposal since they are often extremely short-lived, disposal of HLW ismore problematic. These wastes contain radionuclides with long half-lives, forexample 99Tc and 239Pu have half-lives of 211 000 years and 24 100 years,respectively. Figure 2 demonstrates the time required for HLW to decay toeffectively safe levels. After several hundred thousand years, the radioactivity ofHLWwill have decayed to the same level of radioactivity as the UO2 ore from whichthe fuel was originally mined. After 1 million years, the HLW will still beradioactive, but at a much lower, safer level. Therefore, any disposal option forHLW must be viable for up to 1 million years. Disposal in a deep geological facilityseveral hundreds of metres below the ground is the favoured option for manycountries; this is discussed in detail later in this book.

The last estimate of the worldwide inventory of nuclear waste, taken in 2007 bythe International Atomic Energy Agency, gives the following values for wastegenerated from nuclear power production:

• 2.2 million m3 of ILW and LLW, with a total activity of 1.5 × 106 TBq.• 34 000 m3 vitrified HLW, with a total activity of 4.7 × 107 TBq.• 180 000 metric tonnes heavy metal (MTHM) of SNF, with a total activity of3.0 × 1010 TBq.

Waste immobilisation

The wastes generated at the back-end of the nuclear fuel cycle have great variety intheir physical (solid, liquid, gas), chemical (volatile, organic, non-organic, etc) andradiological (heat-generation, half-life) characteristics. To ensure that these diverse

0

1

10

100

1,000

10,000

100,000

1,000,000

0 10 1,000 100,000

Rad

ioac

tivity

(rel

ativ

e sc

ale)

Years after processing

Radioactivity of uranium ore

Total for spent fuel

Actinides & daughters

Fission & activation products

Figure 2. Graph showing the time required for SNF to radioactively decay to safe levels.

Nuclear Waste Management

4

wastes can be safely handled, stored and disposed of—in a way that minimises risksto human health and the environment for the long time periods they will remainradioactive—a process of immobilisation into a stable, passively safe wasteform isrequired. Any given wasteform material should be: a solid material to help withtransport and storage; stable under the required temperature range (i.e. will not meltor transform due to radioactive decay heat); stable under the required radiation field(i.e. will not be detrimentally affected by α, β or γ radiation); and durable, to ensurethat under conditions of long-term storage and disposal the wasteform is not easilydissolved and the immobilised radionuclides within the wasteform are not releasedto the environment.

A number of materials meet these requirements and are widely used in wasteimmobilisation, as described below. SNF is dealt with elsewhere in this book.

Borosilicate glass—glass is an attractive immobilisation matrix, particularly forHLW fission, actinide and activation products separated during reprocessing.Recently, vitrification has been trialled for immobilisation of ILW includingdecommissioning wastes—for example, contaminated masonry and metals fromdefunct nuclear fuel handling facilities—and it will be used to immobilise tank farmwastes from the Hanford site, USA. Vitrification involves melting glass-formingadditives (such as boron oxide and silica) with waste so that the final glass productincorporates the radionuclides into the atomic structure. It is possible to incorporatea large number of elements within a given glass matrix and to achieve a relativelyhigh waste loading (~35 wt% for HLW), resulting in a low-volume wasteform. Someelements are difficult to immobilise in glass, such as S, Cl and Mo; the flexibility ofborosilicate allows the chemistry of the glass to be fine-tuned to account for this. Theaddition of boron to silicate glass lowers the melt temperature to a range of 1100–1300 °C (depending on other additives), which is suitable for a number of differentvitrification technologies (such as Joule-heated ceramic melters and cold cruciblemelters) and also prevents the volatilisation of problematic radionuclides like 137Cs.Glass materials, being amorphous in nature, show good tolerance to radiationdamage and have a high chemical durability; Roman-era glass objects over 2000years old have been recovered intact from the seafloor, demonstrating the robustnature of this material. Borosilicate glass was first investigated as a wasteimmobilisation matrix in Canada in the 1950s. In the 1970s and 1980s, France,the US and the UK made the decision to begin vitrifying their HLW in borosilicateglasses, due to their increasingly large inventories of defence and civil nuclear wastes.Borosilicate is not the only glass material to be used in nuclear waste immobilisation;aluminophosphate glasses are also used to immobilise HLW from Russian fuelreprocessing activities.

Cement—cementation of ILW has been practised for many years in a number ofcountries. In contrast to vitrification, where radionuclides are chemically immobi-lised within the atomic structure of the glass, in cementation waste is simplysurrounded, or encapsulated, by a wet cement paste that hardens to form a solidblock. The advantages of using cement for nuclear waste immobilisation includeprovision of: a low-cost and simple processing route for a variety of effluents, sludgesand dry solids; a final product that is easily handled (with good thermal and physical

Nuclear Waste Management

5

stability); radiation shielding provided by the cement surrounding the waste;alkaline chemistry that lowers the solubility of cationic radionuclides; and sorptionof some radionuclides onto the main binder phase, calcium silicate hydrate. Themajor disadvantage is the large volume increase associated with cementitousencapsulation. This gives rise to a significantly larger volume of waste than priorto immobilisation, increasing the cost of waste management. Furthermore, theencapsulation of metallic wastes has proven problematic due to the ongoingcorrosion of metals within the cement matrix; the corrosion products occupy alarger volume than the original metal, and hydrogen is generated during corrosion,resulting in expansion of the waste package in the worst cases. CEM I cements (alsoknown as ordinary Portland cement) are the most common type of cement used.CEM I is often blended with cement additives including blast furnace slag (CEM III)or fly ash (CEM V) to improve properties such as compressive strength or fluidity.

Other wasteform materials include geopolymers, bitumen and crystallineceramics (discussed later in this book).

3 Current directionsGeological disposal of high-level nuclear waste

With significant inventories of HLW that will be radioactive for timescales out-lasting our civilisation, long-term above-ground storage is considered only aninterim management measure. A final solution for the long-term disposal of thesematerials is required to reduce the risk to the environment and future populations. Anumber of potential solutions have been considered but ruled out for technicalreasons. For example, sending the waste into outer space with a target destination ofthe Sun entails enormous risks (e.g. launch failure) and costs. There is internationalconsensus that the safest option is to remove nuclear waste from the dynamic surfaceof the Earth, where human intrusion, climate change and tectonic processes maydisturb it, and to place it within an underground storage facility several hundreds ofmetres or more below ground. In a stable rock formation, the environment willremain largely unchanged over the 100 000 to 1 million years required to allow thewaste to safely radioactively decay, isolated from the biosphere.

This concept is known as the geological disposal of nuclear waste and is proposedfor wastes that are unsuitable for long-term storage in near-surface facilities,including SNF, HLW from reprocessing and other radioactive wastes that generatea significant amount of heat or contain particularly long-lived radionuclides.

Multi-barrier conceptThe general principle of maintaining safety after the facility has been closed relies onthe ‘multi-barrier’ concept. This concept employs engineered and natural barriersthat act together to contain the waste and to prevent, or lower, the release ofradionuclides to the biosphere, as follows:

• Several layers of engineered barriers will contain the nuclear waste until mostof the radioactivity has decayed;

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6

• The host geology, also known as the natural barrier, will isolate the wastefrom the biosphere and reduce the likelihood of human intrusion into thefacility;

• The location of the facility, several hundreds of metres below the surface, willensure long transport pathways to delay any significant migration of radio-nuclides from the waste to the biosphere until far into the future when muchof the radioactivity will have decayed.

When considering the great uncertainties associated with the spatial (hundreds orthousands of metres) and temporal (up to a million years) scales of geologicaldisposal, this approach provides confidence that the facility will effectively containthe nuclear waste and retard radionuclide migration to the biosphere.

The engineered barrier system, which can be likened to a set of Russian dolls fromthe inside to the outside, comprises the wasteform, the waste package and thebackfill that surrounds them, as shown in figure 3. Arguably, the wasteform is themost important component of the engineered barrier system; it is the dissolution ofthe wasteform—the process by which solid materials dissolve into a solution—ingroundwater that controls the release of radionuclides to the environment. Forexample, under ideal conditions of geological disposal, SNF would take significantlymore than 1 million years to dissolve completely, by which time most of theradioactivity of the SNF will have decayed.

The wasteform is placed within a waste package—a metal container composed ofa corrosion-resistant metal such as stainless steel or copper—which helps to mitigateexposure of the wasteform to groundwater. The final component in the engineeredbarrier is the backfill, which fills the gap between the waste package and the hostgeology. The functions of this barrier, typically composed of clay or cement, are tocontrol the movement of groundwater to the waste package—for example, by havinglow porosity and ensuring groundwater movement is only by slow diffusion—and tosorb dissolved radionuclides.

While the engineered barriers can be considered to be extremely important atmitigating the release of radionuclides from the waste in the relative short-term of

Waste PackagePrevents access of groundwater until

significant radioactive decay has occurred

BackfillSlows down water ingress

through low porosityProvides sorption capacity

for radionuclides

Host geology Disposal at depth in a

suitable and stable environment provides

isolation

WasteformMatrix dissolves slowly,

limits radionuclide release

Figure 3. The multi-barrier concept.

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geological disposal (1000–10 000 years), the geology and hydrogeology of the hostrock controls the long-term containment (>10 000 years) of nuclear wastes. It is thispart of the multi-barrier system that provides a long travel path for radionuclides toreach the biosphere if they breach the engineered barriers. A host rock of ageological facility for nuclear waste should have minimal fracture systems, lowporosity and permeability to reduce the mobility of groundwater and radionuclides,and should have minerals that readily sorb released radionuclides. Several rock typesare under consideration for hosting a geological disposal facility for nuclear waste.These fall into three categories: high-strength crystalline rock (e.g. granite),sedimentary rock (e.g. clay) and salt. Each of these rock types has advantages;high-strength rocks tend to have low porosity and are favourable for construction,while clay and salt formations have extremely low permeability (in the case of saltthere is no groundwater), are plastic and can form a tight seal around the engineeredbarrier.

The exact design of a geological disposal facility, and the engineered barriersystem employed, depends on the type of host rock available in a suitable location(e.g. tectonically stable, away from large population centres) and also on the typeand volume of nuclear waste to be emplaced. For example, in one of the proposeddesigns for a UK facility shown in figure 4, two types of vault and engineered barriersystem are envisaged for disposal of cementitious ILW and for HLW, includingvitrified HLW and SNF. The former will use a cement backfill and the latter will useclay. In contrast, other countries including Sweden and Finland will dispose only ofSNF, necessitating only one type of engineered barrier system (known as the KBS-3concept, discussed later).

Figure 4. UK generic geological disposal concept for nuclear waste in a high-strength crystalline rock,including separate but co-located disposal concepts for intermediate level waste (ILW) and high level waste(HLW)/spent nuclear fuel (SNF).

Nuclear Waste Management

8

International status in 2018A number of countries have active geological disposal programmes for LLW andILW. One example is the Waste Isolation Pilot Plant (WIPP) in New Mexico, US,where materials contaminated with by-products of plutonium processing, fromhistoric US military nuclear operations, are being disposed of 660 m below thesurface in a salt formation. However, there is currently no operating facility for thedisposal of civil HLW. Finland is the most advanced nation with respect to theirplans for geological disposal; a licence has been granted to construct an under-ground facility in a granite host rock at Onkalo and it is envisaged that the firstemplacement of SNF will be in 2023. In 2011, the Swedish Nuclear Fuel and WasteManagement Company (SKB) applied for a licence to build a repository atForsmark for 12 000 MTHM of SNF. The review process for this licence is stillin progress (as of June 2018). In the US, disposal of SNF and HLW from SNFreprocessing has been proposed in a facility embedded within Yucca Mountain,Nevada, above the water table. This facility has continued to encounter technicaland political challenges since 1987 and is yet to be constructed or licensed to acceptwaste. A number of other countries have either selected sites for disposal but are yetto begin construction (e.g. France, Switzerland), have identified geological disposalas the preferred option for long-term nuclear waste management but are yet to locatea site (e.g. UK, Canada, Japan), or have not yet made a decision on the long-termdisposal solution for their waste (e.g. Belgium, Netherlands).

Spent nuclear fuel

SNF is the most problematic type of nuclear waste; it continuously transforms interms of its physical and chemical characteristics and it has a susceptibility tocorrode in the presence of oxygen, making storage and disposal challenging. Afterbeing removed from the reactor, the activity of SNF is six orders of magnitudegreater than the original UO2 fuel, ~1017 Bq MTHM−1 fuel. A person exposed tothis level of radioactivity would absorb a lethal dose in less than a minute if theywere standing just a metre away.

Until several hundreds of thousands of years have passed (when the radioactivitywill be the same as the original UO2 ore; see figure 2), the SNF is highly hazardousto living organisms and must be safely stored and isolated. Several storage practicesare currently utilised. SNF is first stored in cooling ponds for 2–5 years to removethe initial intense heat generated by radioactive decay of fission products. After thistime, depending on the type of fuel cycle operated, the SNF is either sent forreprocessing (closed fuel cycle) or stored in dry casks (open fuel cycle). In dry caskstorage, SNF bundles are placed within a sealed steel cylinder and surrounded byconcrete, providing radiation shielding to the environment around the cask. Theymay be stored vertically or horizontally, either within a secure warehouse or on aconcrete pad at nuclear power stations or other nuclear licensed sites. Convectivecooling is still required for dissipation of decay heat.

For many countries, there is presently no final disposal solution for the 180 000MTHM (metric tonne of heavy metal) worldwide inventory of SNF. This is

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problematic since many new nuclear reactors are under construction (50 reactors in2018) and commercial SNF reprocessing operations are earmarked for closure (foreconomic reasons). Hence, the inventory of SNF will continue to grow. There isinternational consensus that disposal in a geological facility is the most suitablesolution. However, as discussed above, only Sweden and Finland have madesignificant progress towards this goal. In the absence of a final destination, long-term storage of SNF in dry casks is the plan for 100 years or more. However, it is notwell understood how the SNF will behave in these conditions over such timescales. Itis likely that this option will begin to face significant public and political challengesin the near future as stockpiles of SNF begin to grow.

Evolution of spent nuclear fuel with timeWithin the reactor, UO2 fuel undergoes a number of transformations driven bynuclear fission that affect the physical and chemical state of the fuel. These processesinclude: generation of fission, actinide and activation products through radioactivedecay; increased temperature; defects caused by radiation; swelling; andrestructuring.

The distribution of fission products and actinides within SNF after it has beenremoved from a nuclear reactor is shown in figure 5. The majority of the fissionproducts (e.g. Sr, Zr, Nb and lanthanide elements), and also the higher actinides (e.g.Pu, Np, Cm and Am), are incorporated into the UO2 structure. Metallic fissionproducts (e.g. Mo, Tc, Ru, Rh and Pd), known as epsilon particles, are found asimmiscible precipitates in the grain boundaries. Other fission products precipitate asoxides (e.g. Rb, Cs, Ba and Zr). The final composition of the fuel is typically 96%UO2, 3% fission products and 1% PuO2, but this can vary depending on the ‘burn-up’—the amount of fission experienced—of the fuel. Owing to a steep thermalgradient between the centre (~1700 °C) and the outer rim (~400 °C) of the fuel pellet,volatile fission products, including Cs and I, tend to migrate towards the gapbetween the pellet and the cladding. Due to the cooler temperature in the rim, atomsdisplaced from their lattice positions as a result of radiation damage cannot returnthrough thermal annealing, so this region undergoes a significant restructuringprocess, forming a structure known as the ‘high burn-up’ or ‘rim’ structure. In thisregion, the grains are significantly smaller than in the centre of the SNF pellet.

Fission product gases (e.g. Xe and Kr) form finely dispersed bubbles within theUO2 matrix, and can coalesce at grain boundaries. The presence of helium bubbles,generated by α-decay can also accumulate in the SNF over time, leading to volumeswelling. Swelling may result in a breach in the fuel cladding that surrounds theSNF, which is highly detrimental to the long-term behaviour of SNF in storage anddisposal. This may enhance corrosion and release of radionuclides to the surround-ing environment.

For the first 10 000 years after removal from the reactor, the dominant source ofradioactivity in SNF arises from β, γ-radioactive decay of fission products incorpo-rated within the UO2 matrix of SNF (figures 2 and 5). After 10 000 years, α-radiationfrom the actinide and daughter products is the dominant form of radiation. In theα-decay of SNF, the α-particle dissipates its energy along a 15–22 μm pathway in

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the UO2 crystal lattice. Elastic collisions with atoms along this pathway produceseveral hundreds of displacements per atom. In a collision, the α-particle transferssome of its energy to the collided atom, which then collides with other atoms. Thisis known as α-recoil. Together, a single α-particle and its α-recoil nuclei can cause~1200 atomic displacements, which generate a significant effect on the structure ofUO2 in SNF. The displacement of atoms (U and O) in SNF by α-radiation results inthe formation of a type of lattice defect known as a Frenkel defect pair, whichoccurs when a uranium atom leaves its place in the lattice, creating a vacancy, andbecomes lodged elsewhere in the lattice as a so-called ‘interstitial’. This can occur inthe reactor during fission, but here the high temperatures are sufficient to allowdefect annhiliation and recovery of the UO2 lattice (i.e. the interstital returns to thevacancy, promoted by the thermal energy generated through nuclear fission), asshown in figure 6. However, the temperatures experienced by the SNF understorage conditions are much lower and do not allow this process, so that defectstend to accumulate, leading to an increase in the UO2 lattice parameter andswelling of the SNF.

Figure 5. Schematic of spent nuclear fuel (SNF) microstructure and the distribution of fission products andminor actinides after fission. Reproduced with permission from Bruno J and Ewing R C 2006 Spent nuclearfuel Elements 6 343. Copyright 2006 Mineralogical Society of America.

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Disposal of spent nuclear fuelIn a geological disposal facility, dissolution of SNF by groundwater will solubiliseradionuclides, making it easier for them to be transported to biosphere, should theeningeered barriers be breached. The dissolution of SNF under geological disposalconditions is summarised in figure 7, and occurs in two main stages. The first is aninitial fast dissolution step, where volatile fission products (I, Cs, Cl) and fission gasbubbles (Xe, Kr) are instantaneously released from the grain boundaries and fromthe gap between the cladding and the fuel. This occurs when the metal wastecontainer and fuel cladding are first breached and is known as the instant releasefraction. The second stage of dissolution involves the slow corrosion of the UO2

matrix via the oxidation of U(IV) to U(VI) and the formation of higher oxide defectstructures, such as U4O9 clusters, at the surface and the grain boundaries. Secondaryalteration products, such as coffinite (USiO4) or uranyl (UO2

2+) can be formed uponSNF matrix dissolution, under reducing or oxidising conditions, respectively. Sinceaqueous U(VI) species such as uranyl are highly soluble, and U(IV) compoundsincluding UO2 and coffinite are insoluble, it is preferable that reducing conditionsare maintained in the engineered barrier during SNF disposal.

The maintenance of reducing conditions is complicated by the production ofradiolytic species in groundwater through radioactive decay of actinide daughterproducts in the SNF. Alpha particles emitted from the surface of fuel can travel adistance of ~40 μm in water, so only a thin film of water needs to be present on thesurface of the SNF to be affected by α-radiolysis. Both oxidising (e.g. HO•, HO2

•,H2O2) and reducing (e−(aq), H

•, H2) radiolytic species are produced; the presence of

Figure 6. The UO2 lattice (space group Fm3m) with a lattice parameter of a = 5.468(1) Å. The relationshipbetween dose (displacements per atom) and change in the lattice parameter of UO2 (Δa/a0) as a function of α-damage, α-recoil damage and fission damage. Fission damage has a negligible effect on UO2 volume swellingdue to thermal annealing, while α-damage and α-recoil damage during storage and disposal result in significantlattice parameter increase and volume swelling.

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oxidising species could lead to the oxidation of SNF and the subsequent release ofradionuclides to the engineered barrier and biosphere. However, the presence ofreducing species, especially H2, can counteract the oxidising species and keep theconditions favourably reducing. A further complication is the radiolysis of speciescommonly found in groundwater (e.g. carbonate), which may alter the balance ofradiolytic species towards more oxidising; this is not yet well understood.

The Swedish KBS-3 engineered barrier concept has been designed to minimise, asfar as possible, the dissolution of SNF by groundwater. The SNF waste package willbe surrounded by a bentonite (clay) backfill that will slow down the transport ofgroundwater towards the waste package. The SNF will be placed in a gas-tight,corrosion-resistant copper container, with a cast iron overpack. If the coppercontainer is breached by groundwater, the overpack will provide reducing con-ditions through the production of hydrogen upon its corrosion. This will also help tocounter the production of oxidising radiolytic species. It is expected that this designwill be highly effective. As such, it has also been chosen for the Finnish geologicaldisposal facility.

Plutonium: waste or resource?

A significant stockpile of plutonium has been created in the UK from reprocessing ofcivil SNF, projected to reach 140 metric tonnes by the end of reprocessingoperations. Substantial stockpiles of separated civil plutonium are also held inRussia and France, and surplus military plutonium in Russia and the US. Suchstockpiles constitute a proliferation and security concern due to risk of diversion ormisappropriation for use in nuclear weapons.

The motivation for separating plutonium from civil nuclear fuel, by reprocessing,was to use this fissile material to fuel fast-breeder reactors, with the production of

Figure 7. Schematic of the key processes involved in the dissolution of SNF.

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additional plutonium from a 238U blanket. The economic rationale was that thescarcity and demand of natural uranium resources would outstrip supply and,against the backdrop of rising fossil-fuel prices, the conservation and reuse of fissilematerial was deemed essential. However, fossil fuel prices remained largely stable,known reserves of uranium increased and the anticipated expansion of global civilnuclear-energy programmes was not realised. Since economic deployment of fastreactors could not be demonstrated, and reliable operation proved challenging, fastreactor programmes were largely abandoned. Nevertheless, reprocessing of civilSNF continued, giving rise to the present substantial stockpiles.

One option for managing current civil plutonium stockpiles is to reuse thismaterial as mixed oxide (MOX) fuel, composed of (U,Pu)O2, in light water reactors.MOX fabrication involves a number of processes that include blending of PuO2,mixing with UO2, milling, pelletising and sintering, followed by loading of MOXfuel pellets into zircaloy fuel cladding to make fuel assemblies for light waterreactors. All of these complex processes must be performed remotely, withautomated operation due to the high radioactivity associated with PuO2.Successful deployment of MOX fuel has been achieved in several countries, mostnotably France, Japan, the US and Russia. The prospect for MOX uptake in theUK remains weak, however: no reactor operator has expressed an interest inutilising MOX fuel, and neither the Sizewell B nor future Hinkley Point Cpressurised water reactors are yet licensed to do so. The commercial and technicalviability of industrial scale MOX production is also unclear. The MOX FuelFabrication Facility (MFFF) under construction in the US is far behind schedule,over budget and is now scheduled for cancellation.

A further challenge in MOX fuel fabrication is the ingrowth of 241Am from 241Pu(with a half-life of 14 years). Since 241Am is a strong γ-emitter, blending of theplutonium feedstock and adequate shielding of production facilities are required toreduce worker dose uptake. Alternatively, chemical extraction of 241Am may beconsidered. However, this has not been demonstrated at industrial scale, and wouldadd both cost and hazard to the overall fuel production process.

Immobilisation options for plutoniumThe economic and technical challenges of commercial MOX fuel manufacture haveled the UK to consider the immobilisation of its plutonium stockpile for ultimatedisposal, since long-term managed storage is not a sustainable or cost-effectivestrategy. In any case, a fraction of the UK plutonium stockpile will not be suitablefor reuse as MOX fuel and will require suitable treatment and disposal. Technologydeveloped for this purpose could also be utilised to treat plutonium declared surplusto military requirements in the US and elsewhere.

Several immobilisation options for plutonium have been evaluated.Encapsulation in cement is not considered a viable option; the achievable incorpo-ration rate, based on safety and criticality concerns, would afford a prohibitivelylarge volume of waste and life time management cost. Moreover, illicit recovery ofplutonium from a cement waste package would not be as technically challenging asfor glass and ceramic wasteforms. Immobilisation of plutonium by vitrification is

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considered a viable option, using special glass formulations. However, plutoniumaccountancy (i.e. verification of material amounts) in vitrification processes, withreuse of a melter crucible, is a concern. The most promising materials forimmobilisation of plutonium are tailored ceramic materials, in which plutonium isaccommodated by solid solution in the crystal structure, with appropriate chargecompensation. The batch-wise nature of ceramic production technology offers aninherent advantage in terms of plutonium accountancy, criticality and safeguardingconsiderations. The technology necessary for both ceramic and vitrification optionsrequires considerable development and maturation.

Research into crystalline ceramic wasteforms for actinides has been underwaysince the 1970s, when a multi-mineral phase titanate ceramic material called Synrocwas conceived to immobilise radionuclides from reprocessed SNF. Synroc is anassemblage of hollandite, perovskite, zirconolite and rutile minerals, where actinidestypically partition into the crystal structure of the zirconolite phase. Thus,zirconolite—prototypically CaZrTi2O7—could also be utilised as a dedicatedceramic wasteform for plutonium. Indeed, naturally occurring zirconolites incorpo-rate a considerable inventory of the actinides uranium and thorium, which have beenretained, under sustained self-radiation damage, for millions of years. Such naturalanalogues (which are lacking for glass materials), provide strong evidence of thelong-term efficacy of ceramic materials for plutonium retention and can be used toparameterise and test predictive models of long-term performance in geologicaldisposal systems. The chemical composition of synthetic zirconolite analogues canbe engineered to accommodate the desired plutonium content, with the incorpo-ration of neutron absorbers such as gadolinium and hafnium as an additionalsafeguard against criticality.

Table 1 shows the range of mineral-ceramic phases that may fulfil the keyrequirements of a wasteform for plutonium. Each host phase has advantages ordisadvantages with respect to durability, waste loading, chemical flexibility (i.e.ability to incorporate impurities and contaminants), processing compatibility (i.e.sintering temperature, pressure, compatibility with the radioactive environment),volume swelling (arising from α-decay that may also impact the aqueous durability)and natural analogues (to build confidence with respect to wasteform longevity).The choice of wasteform for immobilisation depends on the relative importance ofeach of these factors. In the US, significant research efforts have identifiedpyrochlore and zirconolite as the most suitable immobilisation hosts for plutoniumunsuitable for MOX, while recent UK efforts (for example, at the University ofSheffield) have focused on the development of zirconolite glass-ceramics—a mixtureof a ceramic host phase to partition plutonium, distributed within a sodiumaluminoborosilicate glass matrix to immobilise contaminants such as chloride—for plutonium residues.

One technology under development for ceramic immobilisation of plutonium atindustrial scale is hot isostatic pressing (HIPing). Although there are currently nofull-scale radioactive HIP facilities for nuclear waste immobilisation, HIP technol-ogy has been successfully demonstrated at full scale with inactive simulants, and iswidely used in the fabrication of metals and ceramics on an industrial scale. The HIP

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process simultaneously applies heat and pressure to consolidate and sinter materials.The potential benefits of using HIP technology for nuclear waste immobilisationinclude:

• The use of isostatic pressure, applied using an inert gas, promotes densifica-tion and eliminates internal porosity. This is advantageous for promotinggood durability (lower porosity results in lower surface area for dissolution)and achieves volume reduction (up to 60%), leading to significant lifetimewaste management cost savings.

• The waste is processed in hermetically sealed canisters so that no radioactiveoff-gas is produced, precluding the use of expensive off-gas treatment systemsand generation of secondary wastes. The production of a sealed canister isalso beneficial for disposal purposes; this can form part of the engineeredbarrier in a geological disposal facility.

• There is no requirement to pour the discharge product, as in a melter system,improving material accountability, important for fissile materials likeplutonium.

The combination of tailored ceramic wasteforms and HIP technology for theirprocessing is also under consideration for 99Tc-bearing wastes arising from medicalisotope production (Australia). In future fuel cycles, where advanced separationstechnologies could be used to separate radionuclides according to their chemicalproperties and half-life, each radionuclide could be immobilised in an individual,

Table 1. Properties of mineral-ceramic phases identified as potential plutonium immobilisation hosts. Ln =lanthanide element (for fission products or Gd), An = actinide (U or Pu); green = high, yellow = intermediate,red = low.

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tailored crystalline ceramic phase. Until political decisions regarding the fate of theUK and US plutonium stockpiles are made, this immobilisation route will continueto be researched. Since such decisions may be far in the future, immobilisation maybecome an essential alternative to MOX fabrication.

4 OutlookAlthough nuclear waste management has been practised and researched for morethan 40 years, we are still further than desired from achieving the goal of safedisposal of nuclear waste. This should be a necessary ethical prerequisite for theconstruction of new nuclear power stations and is required to fulfil the principle ofintergenerational equity. The reasons for this lie behind the long timescales involved;political decisions are typically made with an outlook of only four to eight years, nothundreds of thousands.

However, some countries are setting an example to the rest of the world; Swedenand Finland will have operational geological disposal facilities for SNF within thenext few decades and other European nations will follow within this century. Forthose countries that have struggled to gain public (or political) acceptance fornuclear waste disposal (notably the UK and US), an alternative disposal option maybe the solution. Deep borehole disposal—where HLW and excess plutonium couldbe disposed of, permanently, in a small number of boreholes drilled severalkilometres deep into the Earth’s crust—may be viewed more favourably than theshallower facilities currently planned. No such facility yet exists, but technologicaldevelopments in oil-well drilling may facilitate future progress.

As old nuclear power stations are retired from service, new materials andtechnologies will be required to immobilise decommissioning wastes, incorporatingmaterials ranging from contaminated soil to pipework and masonry used in nuclearfacilities. Recent advances have been made to use an approach of size reduction andvitrification for such wastes, but the melter technologies are yet to be used onradioactive materials at full scale. Much recent focus has been on developingdecommissioning and immobilisation strategies for the Fukushima Daiichi andChernobyl nuclear power plants. In these accident scenarios, SNF was melted withfuel cladding and fuel reactor components, generating a highly radioactive andheterogeneous material known as corium, which remains poorly understood.Advances in radiation-tolerant robotics are required to retrieve these materials,which must be characterised to understand how best they should be immobilised.

In the future, it may be possible to significantly reduce the volume of HLW by aprocess known as transmutation. This would not eradicate HLW completely and thereduced volumes would still require disposal, but if it were possible to separate thelong-lived actinides and fission products from SNF, they could be transmuted toshorter-lived radionuclides by irradiation in a reactor or an accelerator.

The development of nuclear fusion as an alternative to nuclear fission wouldinevitably cause the demise of electricity generation by fission. The nuclear wastegenerated through fusion will arise from neutron activation of reactor materials(which are yet to be selected). However, since this will produce very short-lived

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radionuclides, these wastes may only need to be managed for 100 years before theyno longer pose a risk to human health.

Additional resourcesBodansky D (ed) 2004 Nuclear Energy: Principles Practices and Prospects (New York: Springer).

An introduction to the nuclear fuel cycle.Choppin G, Liljenzin J O, Rydberg J and Ekberg C 2013 Radiochemistry and Nuclear Chemistry

(Oxford: Elsevier). An introduction to basic radiation physics and radiochemistry.Chapman N and McCombie C (ed) 2003 Principles and Standards for the Disposal of Long-lived

Radioactive Wastes (Oxford: Elsevier). An introduction to geological disposal of nuclearwaste.

Donald I W 2010 Waste Immobilisation in Glass and Ceramic Based Hosts (Oxford: Wiley).Further details on glass and ceramic materials for nuclear waste immobilisation.

Konings R (ed) 2012 Comprehensive Nuclear Materials (Oxford: Elsevier). An introduction toradiation damage in nuclear materials, including nuclear waste.

Lee W E, Ojovan M I and Jantzen C (ed) 2013 Radioactive Waste Management and ContaminatedSite Clean-Up (Cambridge: Woodhead Publishing). An overview of radioactive wastemanagement.

Ojovan M I and Lee W E 2005 An Introduction to Nuclear Waste Immobilisation (Oxford:Elsevier). An overview of radioactive waste management, focusing largely on UK practices.

Ojovan M I (ed) 2011 Handbook of Advanced Radioactive Waste Conditioning Technologies(Cambridge: Woodhead Publishing). An overview of the technologies used to immobilisenuclear waste.

Yardley B W D, Ewing R C and Whittleston R A 2016 Deep-mined geological disposal ofradioactive waste Elements 12 233. A collection of articles on different geological disposalconcepts and site selection.

Hyatt N C 2017 Plutonium management policy in the United Kingdom: The need for a dual trackstrategy Energy Policy 101 303. Further details on the Pu stockpile crisis.

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