Guest essay by Roger Graves
Nuclear fuel typically consists of either uranium dioxide (UO2) or a mixture of UO2 and plutonium dioxide (PuO2), commonly known as MOX (mixed oxide). The uranium can be natural uranium, containing only 0.7% of the fissile uranium-235 isotope, or it can be enriched uranium containing up to 5% of uranium-235. Uranium oxide fuel is only mildly radioactive and can be handled without any special precautions, as the accompanying illustration shows. MOX fuel is somewhat more radioactive, but not dangerously so .
However, after both types have been used in a reactor they will contain fission products, many of which are highly radioactive. Spent fuel is known as high-level waste (HLW) and must be handled and disposed of with appropriate care and caution.
After a fuel bundle has been removed from a reactor, it will be both thermally and radioactively hot for a considerable period of time. Spent fuel bundles are normally kept submerged in water for several years after removal from the reactor until the radioactivity has died down and much of the short-lived radioactive fission products have decayed. However, there will still remain a number of longer-lived fission products which can remain dangerously radioactive for thousands of years, and for which a long-term solution must be found.
Radioactivity and Half-Life
The following section is a short primer for those not altogether familiar with the subject of radioactivity. You can skip this if you are familiar with it.
Elements, such as carbon or cobalt or uranium, each have a fixed number of protons in their nuclei, but can have different numbers of neutrons. Versions of the same element, differing only in the number of neutrons in their nuclei, are known as isotopes of that element. Some isotopes are unstable and disintegrate spontaneously, giving off alpha, beta and/or gamma rays in the process. (Alpha rays are high energy helium nuclei, beta rays are high energy electrons, and gamma rays are high energy photons.) For example, cobalt has a stable isotope, cobalt-59, with 27 protons and 32 neutrons in its nucleus, but it also has an unstable isotope, cobalt-60, with 33 neutrons.
Isotopes will be referred to in this article in both the long form, such as cobalt-60 or uranium-235, and in the short form, such as 60Co or 235U.
Unstable atomic nuclei, such as carbon-14 or cobalt-60 or uranium-235, will disintegrate spontaneously at a rate which is specific to that particular isotope. To put it in more mathematical terms, there is a fixed probability that a nucleus of any isotope will undergo radioactive decay in any given period of time. This probability is a constant for any particular isotope but varies from one isotope to another. The level of radioactivity associated with any particular isotope depends on how often decay occurs. The total radiation from a particular sample of that isotope depends on the energy of the particles released during decay, the rate at which decay occurs, and on the sample size, i.e. how many atoms there are available to undergo decay.
The half-life of a particular isotope is the time during which 50% of all the nuclei in a particular sample of that isotope will have undergone radioactive decay. Cobalt-60, for example, has a half-life of 5.3 years, so that if a sample of cobalt-60 is produced in a nuclear reactor, typically for medical purposes, then 5.3 years after it is taken out of the reactor one half of all the nuclei in that sample will have decayed. The total radiation from that sample will be correspondingly halved, since with only half as many cobalt-60 nuclei as there were originally, there will be only half as many nuclear disintegrations per second as there were initially. After another 5.3 years, or 10.6 years altogether, the radioactivity level will have dropped to on quarter of its initial value, and to one eighth after 15.9 years, and so on.
Isotope half-lives vary enormously from one isotope to another. Hydrogen-7 (one proton and six neutrons) has a half-life of 2.3×10-23 seconds, while tellurium-128 has a half-life of 2.2×1024 years, which is about 160 trillion times the age of the universe.
The longer the half-life, the slower the rate at which radioactive decay occurs, and hence the lower the level of radioactivity associated with that isotope. Cobalt-60 has a high radioactivity level because its short half-life means that nuclear disintegrations occur quite rapidly. Uranium-238, in contrast, has a half-life of 4.5 billion years, which is about the age of the Earth. A sample of uranium-238 present when the Earth was first formed would still have half its atoms yet to undergo nuclear disintegration. Cobalt-60 must be handled with extreme care. Uranium-238 can be handled without any radiation-related precautions at all.
Nuclear Fission Products
Almost all civilian nuclear power reactors, i.e. those used to generate electrical energy, use either 235U or 239Pu as a fuel. When either of these two nuclei undergo fission in a reactor, they split into two parts, together with the release of additional neutrons which keep the reaction going, plus energy in the form of heat. The latter, of course, is the whole point of a power reactor. Most fission products are unstable, i.e. they will undergo radioactive decay themselves, and it is these fission products which create the problem in nuclear waste, and for which a long-term disposal method must be sought.
In addition to the fission process, both uranium and plutonium atoms can capture neutrons and be converted into transuranic elements, otherwise known as actinides. These too are radioactive and require long-term disposal.
The chemistry of fission products and actinides is too complex to be discussed at length here. Interested readers are recommended to consult The Chemistry of Nuclear Fuel Waste Disposal by Donald R. Wiles .
High-Level Waste Disposal
It is generally agreed that the best way to deal with high-level waste on a long-term basis is to bury it deep underground. The International Panel on Fissile Materials (IPFM), a group of independent nuclear experts from sixteen countries, has stated that “There is general agreement that placing spent nuclear fuel in repositories hundreds of meters below the surface would be safer than indefinite storage of spent fuel on the surface” . However, numerous objections have been raised to this, which can be summarized as the possibilities of accidental dispersal into the biosphere, and accidental unearthing by our distant descendants. The best way to meet these objections is to look at the currently proposed methods for HLW disposal and see how they meet these objections. Before we do this, some basic parameters need to be established.
1. Minimum Isolation Time
First, let us be clear about the purpose of long-term burial. No burial method can be guaranteed to keep the buried material isolated for ever, because ever is a very long time indeed. The purpose instead is to keep the waste isolated for long enough that the radiation levels associated with it will have decreased to a level where any harm associated with it is minimal.
The decay characteristics for HLW from a natural uranium fuelled CANDU reactor  are show below . Characteristics for other spent fuel types are broadly similar. Although it will take about a million years until the radioactivity level reaches that of natural uranium, it is not necessary to wait this long for the risk to future generations to become negligible. The human race has coexisted with uranium and other radioactive ores for its entire history, and we do not seem to be any the worse for it. A radiation level ten times that of uranium ore is probably acceptable, considering the fact that by the time the buried material will have resurfaced again it will be very much diluted. The time for this to occur is about 43,000 years, and this can be used as a figure of merit when designing a disposal method.
2. Isolation Process
Second, no long-term disposal method can rely on administrative measures to keep the material safe, such as a fenced-in area patrolled by guards, because while we can perhaps guarantee the maintenance of such measures for a generation or two, beyond that we cannot possibly foresee whether social conditions will permit them to be continued. Consequently, any long-term burial must be done ultimately on a seal-and-forget basis.
3. Water and Oxygen
Third, the two enemies of long-term burial are water and oxygen. Water is important because, without water, there will be no corrosion and any containers used will not corrode. Oxygen is important because, while UO2 is not water-soluble, the UO4— ion is soluble, thereby providing an easy path for water dispersal. The problem of safe, long-term burial therefore devolves largely to one of keeping out both oxygen and water.
There are, broadly speaking, three methods proposed or in use for permanent disposal:
- -Deep burial
- Vitrification followed by deep burial
- Ultra-deep boreholes
Deep burial involves the creation of a geological depository below the level at which atmospheric oxygen can penetrate through solid rock, which is typically 500 metres depth. It is assumed that water can penetrate to this depth, so the burial method must take this into consideration. Burial vaults are designed to have a series of engineered barriers so that if and when one barrier fails, another will come into play.
Deep burial methods have been selected for use in a number of countries, including Finland, Sweden, Britain and Canada. A typical multiple-barrier system which is described here has been adopted, but not yet implemented, by Canada .
Canadian reactors use natural uranium in the form of uranium dioxide (UO2) pellets, which are assembled into fuel bundles, 0.5m x 0.1m diameter. Fuel bundles spend about 18 months in a working reactor before they are replaced. There are about 2.6 million used fuel bundles in Canada today, and about 90,000 are added each year.
The Canadian specifications state that the area chosen for the depository must be geologically stable and must provide a minimum of 50 metres of unfractured rock enclosing the depository. The area chosen must have little likelihood that the surrounding rock would ever be exploited as a mineral resource – no recoverable oil, gas, metals or other useful minerals nearby. (This is one reason why old salt mines are not considered suitable for permanent burial because, although they can reasonably be presumed to be water-free for any foreseeable future, they might be re-opened at some future time for salt mining.) Burial would be 500-1000 metres below ground.
Used fuel bundles will be placed into cylindrical stainless steel containers coated with a 3 mm layer of copper for corrosion resistance. The containers are embedded in bentonite, which is a clay that swells on contact with water, thereby providing a self-sealing, low-permeability barrier. (It is commonly used to line the base of landfills to prevent migration of leachate.) The vault is then sealed with a clay- or cement-based backfill. When all vaults are full, the access tunnels and other holes will be sealed, and the land above returned to a pristine condition .
In order for the buried waste to become a problem for future generations, the containers must corrode and their contents must be transported back to the surface where they can enter the biosphere. Calculations on the rate at which this could occur indicate that this would be 50,000 years at a conservative minimum. The maximum level of radioactivity at this point would be less than ten times that of the original uranium ore.
Vitrification Followed by Deep Burial
Vitrification can usefully be used in cases where the fuel is reprocessed after use, so that the radioactive fission products can be separated out into a much more compact form than the original fuel bundles.
Reprocessing is only useful where enriched fuel (uranium or MOX) is used, in which the spent fuel still has a significant amount fissile material, and so is worth reprocessing to recover it for further use. In the process of recovery the fission products can be separated out and then mixed with molten glass. The resulting glass blocks are very much more corrosion resistant than the original fuel bundles. They can then be permanently disposed of by deep burial in a manner similar to that described above. Although long-term calculations are subject to many variables, the indications are that vitrified HLW would take hundreds of thousands of years before its radioactive burden was released to the biosphere .
The US is actively examining a method of burial in which vertical shafts up to 5 km deep and a metre in diameter are bored into the Earth’s surface . Nuclear waste in strong steel containers would be lowered into crystalline rock in the lower 1 to 2 km of the hole, and the remaining 3-4 km would then be steel-lined and filled with layers of sealing materials such as bentonite, asphalt, concrete and crushed rock.
Under some variants of this plan, the waste would still be radioactively hot when inserted into the borehole, and the heat produced would melt the surrounding rock. When it cooled after a period of years, the waste would be completely entombed in the rock. Lest anyone should think in terms of an underground nuclear explosion as a result, the geometries involved make this quite impossible.
One problem with this scheme is that current technology only permits boreholes of less than 50 cm diameter to be bored to this depth, so nuclear waste would have to be repacked before it could be inserted. However, it is a reasonable expectation that, were this method to be fully implemented, the required one-metre boring technology could be developed.
An advantage of this method of disposal is that it would be very sparing in land use. It is estimated that the entire US nuclear waste stockpile would require no more than 800 boreholes, which could be situated on an area of no more than a few square kilometres. Filled and capped boreholes could be covered over and the land returned to a pristine condition.
Problems with Finding a Depository Site
The physical properties required of a permanent disposal site are well-defined, and such sites can reasonably easily be located. The social properties can be more problematic. Any site selected must be acceptable to the local inhabitants, acceptable to nearby communities, and acceptable to the wider public.
It is an undeniable fact that there is a deep-seated fear of nuclear energy in our society. Part of this presumably stems from the use of nuclear weapons in World War II and their continuing deployment to this day. However, part of it must be ascribed to what can only be described as hysterical overreaction by journalists and public intellectuals to any nuclear-related accident. Isaac Asimov, a normally level-headed science fiction writer, based some of his books around a future uninhabitable, intensely radioactive Earth arising from the Three Mile Island accident (which actually hurt no-one and released only miniscule amounts of radiation ). The claim by some public intellectuals after Fukushima that the whole western seaboard of North America would have to be evacuated arose from the same fount of baseless hysteria.
Nonetheless, any attempt to impose a nuclear waste disposal facility on a particular location by government fiat is going to meet with resistance from local inhabitants unless the concept is explained very carefully in advance. The first attempt by the US to drill an experimental deep borehole, in Pierce County, North Dakota, failed when local officials first heard of the project through the media , which may well be described as a textbook example of how not to get the local populace on your side.
Disposal sites are most likely to be located in remote, sparsely populated areas where employment opportunities are few and far between. Any disposal site, once opened, is likely to stay in operation for many years before it is finally sealed off, thereby providing well-paid employment for local people. This, together with a completely honest and open description of what the disposal facility is and is designed to do, has a much better chance of getting approval from the local population.
High-level nuclear waste is a problem but not an insoluble one. Our ancestors have had other problems disposing of toxic waste in the past, and have come up with solutions for it. One advantage of HLW is that it will gradually become less dangerous as time goes by, unlike, say, arsenic waste from mining which will remain toxic no matter how old it is. A properly designed nuclear disposal facility will be capable of isolating high-level waste until it is no longer a threat.
My thanks go to Don Wiles, emeritus professor of radiochemistry at Carleton University, Ottawa, Canada for some of the material used here and for his much appreciated advice and comments.
Roger Graves is a physicist and risk management specialist who, much to his chagrin, is not associated with big nuclear, big oil, or big anything else.