Provisions for the Permanent Disposal of Nuclear Fuel Waste.

 

 

Andrew L. Daley

Department of Mechanical Engineering

University of New Brunswick

Fredericton, N.B.

 

 

 

Abstract

 

Around the world the disposal of nuclear waste is an important issue.  Philosophies for disposal may vary in different regions and the response of Canada and the United States are presented.  Scientific, economic, and social issues are all important when regarding these philosophies.  The contributions of these issues to the prevalent attitudes on waste disposal are discussed.  Design considerations including site selection and waste containment are investigated for one philosophy of permanent nuclear waste disposal.  The International Atomic Energy Agency (IAEA) has stated that geological repositories are its preferred method of storage so the detail of such a repository is presented.

 

1. Introduction

 

The life cycle of the nuclear fuels used in power production is a long one.  Residual radioactivity from this fuel means that in order to protect humanity there must be provisions to isolate the fuel for time periods of the order of 10 000 years [1].  Currently, there is no operation in place that is capable of storing nuclear material for these significant time periods [2].  At this time the spent nuclear fuels are placed in containment devices located in on-site repositories.  For the first 3-10 years they are kept in pools of water which helps to dissipate the large amount of heat that the spent fuels will initially emit [3].  After this they are transferred to dry storage for the remaining life cycle of the plant.  This method of storage was implemented with the assumption that, at a later time, a permanent repository would be built and the waste from each individual plant would be transferred to this central repository [4].  Despite this assumption only two countries, the United States and Finland, have legislated plans to build a permanent repository [5].  Canada is expected to render a decision on the matter sometime in 2006 after the Nuclear Waste Management Organization (NWMO) submits its recommendations [6].

 

Because of the heel dragging by lawmakers in deciding what to do the problem has started to creep up on society.  Current projections indicate that the current availability of short term storage facilities (both wet and dry) located on-site at reactors will be exhausted in 2017 [4].  Either more short term storage or permanent repositories will

have to be built by then.  There are several philosophies regarding long term handling of waste.  The so called “wait and see” approach depends on dry storage exactly as is already involved in the life cycle of nuclear fuel.  By building more dry storage facilities the capacity will increase.  These facilities have proven to be safe and may be readily supervised.  Another approach is to deposit the waste in repositories deep underground with many barriers in place.  This would eliminate the need for human supervision.  The technical aspects are compelling themselves but there are many non technical issues involving the decision on how to store nuclear waste.  It is important to investigate what these are and how they can be resolved.

 

2. Non Technical Issues [7]

 

The non-technical side of nuclear waste disposal issues centres on ethical, social, and economic aspects of the problem.  Each of these aspects must be properly addressed before the development of permanent repositories can proceed.  There are, of course, people and organisations that would prefer nuclear power to be shut down altogether.  This paper assumes that nuclear power generation will, in the future, be an important, greenhouse gas free, source of power.  As a consequence of this assumption, and since there is already nuclear fuel waste waiting to be disposed of,  the following discussion will be a comparison of two forms of handling nuclear waste.  The first form is long-term storage.  Essentially, this means continuing to hold waste in dry storage containers for an indefinite amount of time.  The second form is disposal, specifically the emplacement of waste in a geological repository.  It must be mentioned that many of the non-technical issues share many of the same arguments.  A discussion that incorporates this fact would quickly degenerate into a circular one so each aspect will be presented separately.

 

2.1  Ethical Issues

 

Regarding disposal in geological repositories, the IAEA states that, “It is impossible to describe completely the evolution of an open system, such as a repository and its environment…”  This is undoubtedly true and represents the basis of the argument that building repositories that may in fact hinder future generations is unethical.  In essence, since humans cannot completely predict what will happen in a repository and geological system over the 10 000 years it is required to be stable, this approach is therefore unethical.  The counter argument starts with considering the alternative of leaving future generations the task of disposing of waste produced for the current generation.  This would be equally unethical, however many point out that long term storage is taking action and is therefore an ethical alternative to geological repositories since it would allow time to come up with conclusive evidence on disposal options.  Essentially not shoe horning future generations with our decision.  The IAEA states its position on the ethical issue:  “… pre-emption of future options is acceptable ethically provided that the current action is well motivated and reasonable in the light of current knowledge.”  In other words, since the intention is right i.e. protect humans from waste for a long time, and as long as all necessary studies and calculations are undertaken at the chosen sight then geological disposal is an ethical approach to disposal of nuclear waste.

 

2.2   Social Issues

 

One aspect of the social issue is the “not in my backyard” attitude that society has towards nuclear power in general.  It is hard enough to find a site for building a plant let alone a place to store the dangerous waste from that plant.  The argument that favours a long-term storage option is bolstered by this attitude.  Studies have shown that above ground facilities such as these are much more publicly favourable, especially in communities where a nuclear facility already exists.  While this is undoubtedly the result of both knowledge of the hazards by the residents, and the demonstrated safety of this storage method, it has also been shown to have a strong correlation to the fact that these structures are understood to be temporary.  In other words, public support for any kind of disposal still seems to elicit fear.  To counter this, the public needs to be made aware of the studies and tests conducted over the last twenty years at facilities such as Yucca Mountain and AECL’s underground research laboratory (URL), which have demonstrated successfully many concepts of underground geological repositories.  The next aspect of societal issues depends on the nature of society itself and how this affects the relative merits of either long-term disposal or disposal.  What about access to the fuel?  If in the future information becomes available about the nature of nuclear waste it may be prudent to keep the fuel accessible.  Also, if something should go wrong with waste containers or the waste, access would be a priority in order to perform maintenance and supervision operations.  Both of these points seem to favour a long-term storage approach.  A surface facility would be much easier to access.  However, a geological repository does have access to the fuel, at least at first.  This supervision period could be as little as 50 years or last for several hundred.  The geological option, for the same reasons above, is a good option in terms of security.  If the repository is sealed off then access is practically impossible to any terrorist or criminal who wants access to nuclear waste.  Even if the resources to obtain it are available the attempt to reach the waste would almost certainly be noticeable and thus stoppable.  The surface condition is much easier to break into and the release of nuclear waste by criminal elements is much more likely.  The argument of access and supervision is also dependent on society itself and the institutional controls provided by society.  The assumption being that there will always be a government and regulations to look after any surface waste.  This assumption may not be valid.  Throughout human history society has been particularly unstable, and should society descend into anarchy, then any surface level long-term storage facilities would be vulnerable to attack and exploitation.  Also, with no society the benefit of maintenance and supervision against degradation disappears.  Geological repositories are inherently protected against societal breakdown because they are inaccessible (if sealed) and because they are designed not to rely on direct supervision, in contrast to long-term storage.  Another related social issue, not only may future generations not have the institutions in place to supervise nuclear facilities but they may no longer have the knowledge to adequately do the supervising.  Paper records are vulnerable to decay and computer records to obsolescence.  If information cannot survive into the future then the ability to deal with nuclear matters will be lost.

 

 

 

2.3  Economic Issue

 

Contrasting the two methods, they can be summarized economically by saying that disposal has a large capital, or construction, cost and long-term storage has a significant operating cost.  In other words, disposal will cost much in the short term and storage will build up in the long term.  The problems economically seem to all lie in long-term storage.  To finance the long term operation either a trust fund must be set up to provide revenue, which adds considerable capital cost or future generations must in turn pay for the operation.  This is irresponsible and unethical and may not be feasible.  The ability of future generations to pay for storage depends just as much on society as does their ability to supervise, therefore this active approach may be insufficient.  This is also true for a trust fund approach, if society breaks down then the bank will no longer exist and the money wouldn’t be able to do anything anyway.

 

3.0 Geological Repositories

 

The message about dealing with nuclear waste is mixed at best.  Based on non technical issues there is no clear winner in disposal methods.  After consulting around the world the IAEA has determined that, “… disposal in deep underground engineered facilities – geological disposal – is the best option that is currently available or likely to be available in the foreseeable future.”  when dealing with nuclear waste.  This opinion is based on around 20 years of research around the globe at places like Yucca Mountain and URL.  The design of geological repositories is very site specific to account for groundwater flow, rock fractures, filler material and other factors.  In the next sections an investigation into the safety of repositories and the design of repositories in Canada and the United States will be undertaken.

 

3.1 Confidence in Safety [8]

 

As previously stated, the disposal of nuclear waste in underground geological repositories comprises an “open system”.  This renders the assessment of these repositories into an unusual realm of engineering design where a complete design is not possible until construction is underway.  This presents problems in convincing the public, especially opponents of nuclear energy, that the safety of the repositories can be assured.  Some of the relevant concerns include: model validation, characteristics of the site, and assumptions made.  All of the uncertainties result in the design approach employing a “flexible, step-wise” method.  Essentially, as the design progresses, new information about the site, the assumptions made, and the validity of the models used in the design will become available.  It is of utmost importance that all new information becomes incorporated into the design and that the design is able to accommodate any unforeseen problems.  If the problems become insurmountable, then the project should be terminated or changed to ensure safety.  In order to have this confidence in the design process a strong external and independent regulator must be visible to the public.  This regulator must, in essence, try to shut down the project unless the designers can resolve all questions through fact or by positively indicating where a factor is critical and solvable or is, with a high level of confidence, not critical to the safety of the facility.

3.2 Canadian Disposal Concept [3, 9]

 

Canada has not yet legislated a long term nuclear waste disposal strategy.  Despite this fact Atomic Energy of Canada Limited (AECL) and the Canadian nuclear utilities have collaborated for two decades at the Underground Research Laboratory (URL) in Lac-du-Bonnet, Manitoba.  This facility has allowed researchers from Canada and around the world to study and experiment on the various concepts involved in underground geological repositories.  The concept to be discussed was developed by Ontario Power Generation (OPG), while differing slightly from the AECL studies; it employs all the same basic concepts.  The approach is to create several barriers to stop any radiation from escaping from the repository confines.  The first “layer” in this system is the fuel bundles themselves.  These are removed straight from the reactor core itself and are already processed into rods with cladding and all associated hardware.  The fuel bundles will be placed inside durable containers.  These will have an inner shell made of steel.  This will be the load bearing section of the container designed to withstand pressure and stresses placed on the container itself.  The outer shell would be made of copper and is designed primarily to prevent corrosion of the container.  There are several sizes being considered for the container.  These are summarized in Figure 1: Used Fuel Containers.  The preferred container of OPG is the IV-324-hex, which has a diameter of approximately 1 metre and a height of almost 4 metres.  This would contain 324 CANDU fuel bundles and weigh approximately 23.5 Mg.  The container design life is estimated at a million years  (under ideal conditions).  The next barrier is still under consideration with two competing concepts.  One approach is to drill bore holes in the floor of the emplacement room and place the containers in the boreholes.  The second approach is to leave the containers in the room itself.  These two approaches are shown in Figure 2: Geological Repository Concept.  Also shown in this figure is the proposed layout for the entire facility showing access tunnels and the network of emplacement rooms at the desired depth.  In whichever approach, the container is surrounded by a layer of 100%, highly compacted bentonite.  This is a clay material with a low porosity and is almost impermeable.  One of the major concerns with repository design is water contamination so this is very important.  After the bentonite seal, various backfill materials will take up all the space in the room.  These would be clay/sand buffers and slurries designed to fill all air gaps and voids around the container and prevent contamination.  Once the room itself has been filled with buffers and various layers of backfill the entrance would be sealed with a bulkhead.  This is to help ensure the integrity of the other systems by isolating any contamination mechanisms from the room itself.  This system is also shown in Figure 2 and consists of bentonite gaskets and a concrete main section with gaps filled by slurry or grout.  All materials involved in the sealing procedure and corresponding to labels in Figure 2 are shown in Table 1:  Sealing Material.  An important design feature is the retrieval aspect of the containers should something go wrong.  The Canadian concept allows for rooms to be sealed separately from sealing of the repository.  This allows for monitoring and institutional controls to be in place indefinitely to ensure safe operation.  The design does not however rely on this human presence to be there and contingency to close the facility is available.

 

 

Figure 1: Used Fuel Containers [3]

 

Figure 2: Geological Repository Concept [3]

 

 

 

Table 1: Sealing Material [3]

 

 

Perhaps the most difficult portion of designing underground repositories is determining the behaviour of the regions surrounding the emplacement rooms.  Since the surrounding rock is also an essential component of the repository barrier, the behaviour of this rock must be characterized and understood.  Unfortunately the determination of the relevant properties is very site specific.  Canada has not yet chosen a repository site so this component of the design is necessarily in the preliminary stages.  What engineers and scientist have been able to do is develop models based on research at the URL.  If the models are accurate for the geology at URL then it may be reasonable to assume that inputting site specific geology at the eventual repository location into the same model will produce valid results.  The location of URL and the envisioned location of the repository will be in the stable granite rock of the Canadian Shield.  There are several aspects of this rock that need to be investigated.  One such aspect is the shield hydrogeology of the region.  In other words, this topic involves study of water flow in the area of the repository.   This is important in several ways.  If water breaches the emplacement room it would lead to a significant increase in the corrosion rate of the waste container.  Another important factor is water as a means of transportation for radioactive materials.  If the containers are breached, the water could act as a driving mechanism for radioactivity to contaminate human accessible regions.  To prevent this from happening the sealing system must be as water tight as possible and, perhaps most importantly, the emplacement rooms themselves must be constructed in regions of the shield that are separate from fractures or pathways that water is most likely to travel in.  This requires years of on-site study, which is why it is imperative for a policy decision to be made on Underground repositories as soon as possible.

3.3 United States Disposal Concept [10]

 

The United States is ahead of the Canadian program in disposal.  The U.S. has also been studying repository design for two decades at their facility located at Yucca Mountain in Nevada.  The difference between the programs in terms of development is that in 1987 Congress authorized the investigation of this site in particular for the repository itself.  That means that all the geological data and modelling done on the site over the program’s two decades can be applied directly to the design of the repository itself.  Canada has not yet chosen its site so the URL data is not specifically relevant.  The geology of the site is also different from the Canadian Shield.  Yucca Mountain is located in a material called Tuff.  This is volcanic residue from eruptions millions of years ago.  An advantage of Yucca Mountain is that it has a very big “unsaturated zone” where virtually no water passes through.  The water that does pass through the proposed repository has been well studied over the facilities 20 year life.  As with the Canadian design, Yucca Mountain has a contingency to provide institutional controls for as long as deemed necessary but it too can be sealed permanently with no active human controls to secure the waste.  The U.S. approach to waste containers also has a two-metal approach.  The inner shell containing the fuel bundles would be 316 Stainless Steel and the outer corrosion barrier would be Alloy 22, a nickel based alloy.  Tests on these packages have shown that the expected life is in excess of the 10 000 years that the waste should ideally be isolated.  Interestingly the U.S. approach is to pre treat the fuel by converting the waste into a form of glass.  The containers in the rooms would be protected by drip shields.  In the event of water breaching the room or dripping as moisture from the walls the container itself would not contact water.  The drip shield would direct any water safely away from the container.  The containers and drip shield would be supported by drift inverts.  They are composed of a steel structure and sealed with backfill against contamination.  In 2002 the U.S. Senate gave the go-ahead and it was confirmed that the United States will develop Yucca Mountain as its disposal site for nuclear and radioactive waste.

 

4.0 Conclusions

 

Nuclear waste is here to stay.  Even if all the nuclear facilities in the world shut down today, there would still be a 10 000 year waiting period before the waste that has already been produced is no longer completely dangerous to humans.  It is only ethical that the generation that produces the waste must also dispose of it and that means action has to be taken now on how to permanently dispose of it.  Weighing all the factors is a difficult task but the consensus of the international teams of experts consulted by the IAEA is that underground geological repositories are the best option to dispose of nuclear waste.  These facilities are advantageous in numerous areas, in particular they may remain open and under human supervision for as many years as deemed necessary but they also do not depend on human intervention to remain safe.  These repositories will be constructed 500-1000 metres underground.  They will employ multiple barriers to prevent contamination getting in or waste getting out.  Research has been done over approximately two decades.  While these systems are inherently uncertain, it has been shown that they are indeed feasible.  Humans must take steps to dispose of nuclear waste and the clear step to take is underground geological disposal.

5.0 References

 

[1] International Atomic Energy Agency (IAEA), “Nuclear Waste Bulletin #13”, December 1998.

 

[2] IAEA, “USA to Select Deep Disposal Site for Spent Nuclear Fuel”, http://www.iaea.org/NewsCenter/News/2002/13022002_news01.shtml

 

[3] Russell, S. & Simmons, G., “Engineered Barrier System for a Deep Geological Repository in Canada”, Las Vegas High Level Waste Conference, 2003.

 

[4] Fakuda, K. et al., “IAEA Overview of Global Spent Fuel Storage”, IAEA-CN-102/60

 

[5] IAEA, “High Science Inside the Belly of the Alps”, http://www.iaea.org/newscenter/features/grimsellab/grimselsite20040123.html

 

[6] Russell, Sean., “Canadian Developments in Nuclear Fuel Waste Technology and Regulatory Activities”, The 5^th Plenary Meeting of the OECD/NEA Integration Group for the Safety Case, October 15-17, 2003, Paris.

 

[7] IAEA, “The Long Term Storage of Radioactive Waste: Safety and Sustainability”, IAEA-LTS/RW

 

[8] Nuclear Energy Agency, “Confidence in the Long-term Safety of Deep Geological Repositories”, 1999.

 

[9] Russell, S. et al., “Deep Geological Repository Technology Program Annual Report 2002”, Report #: 06819-rep-01200-10100-r00, Ontario Power Generation, 2003.

 

[10] Office of Civilian Radioactive Waste Management, “Executive Summary”, http://ocrwm.doe.gov/documents/ser_b/execsum.htm