Nuclear Options : Beyond Uranium

Thorium and the nuclear fuel cycle

There are five basic fission fuels. Three are what is called fissile materials : they can start and sustain a chain reaction all by themselves. Two are fertile materials, which can be converted into fissile materials, but cannot sustain a chain reaction on their own. The fissile materials are uranium-233, uranium-235, and plutonium-239, and the fertile materials are thorium-232 and uranium-238.

Of the fissile materials, only ²³⁵U occurs in nature, to the extent of 0.7% in natural uranium. The remainder is ²³⁸U, which can capture a neutron and be converted (through two successive β decays) to ²³⁹Pu. If a fission chain reaction can be started in natural uranium (it can, with graphite or heavy water as moderator), and if the fission of ²³⁵U yields more than two neutrons (it does, 2.44 on average), it would appear that the available supply of fissile material can be continuously increased (what is known as ‘fuel breeding’), since the neutrons beyond the one per fission required to keep the chain going can be applied to the transmutation of uranium. Alas, things are not so simple.

To the mechanics of the chain reaction, neutrons emitted per fission is a less important number than neutrons emitted per neutron absorbed, and for natural uranium that value is about 1.6 for slow neutrons (and even less for fast neutrons, so that a chain reaction cannot be supported at all). Therefore, the ‘conversion ratio’ or number of new fuel atoms produced per fuel atom consumed is always less than unity, and the fissile material will ultimately be exhausted while there is still fertile material present. This could be overcome by the costly and laborious process of fractionating the uranium into an ‘enriched’ moiety containing a higher proportion of ²³⁵U, and a ‘depleted’ moiety containing less, to improve the chance that a given neutron will cause fission ; but another obstacle appears.

Plutonium-239 releases 2.88 neutrons per fission on average, but it also has a nasty tendency to absorb a neutron and not disintegrate. This produces ²⁴⁰Pu, which is not fissile, but is converted to fissile (albeit short-lived) ²⁴¹Pu by absorbing a second neutron. This process continues, so that more neutrons are wasted in creating ever-heavier isotopes of transuranic elements such as americium and berkelium than are put to use converting ²³⁸U. As a result, the conversion ratio (the number of new fissile nuclei produced per fission) in a thermal reactor using uranium-plutonium fuel never reaches unity ; the overall quantity of fissile material constantly decreases, and fresh fuel is required long before the theoretical maximum of energy is produced.

Breeder cycles

There is a known solution to this problem. The ‘parasitic absorption’ occurs much more strongly with slow than with fast neutrons. The plutonium can be chemically separated and put to use in an unmoderated reactor, where its large excess neutron yield will serve to produce additional ²³⁹Pu from ²³⁸U quite efficaciously. It is not even necessary to perform the separation before the plutonium has a chance to absorb too many excess neutrons — the higher transuranics also make adequate fuel for a ‘fast reactor’, so that the uranium can be left in the thermal reactor until it has achieved a high fuel value.

Unfortunately, such reactors have proven difficult to make work. Furthermore, early techniques for separating plutonium from irradiated uranium were intended for producing bomb material. Not only was little attention given to cost-effectiveness for power-plant use, or even to recycling the uranium, but the whole subject of ‘reprocessing’ became tainted with the idea of atomic weapons — even though the accumulation of higher isotopes tends to make plutonium from thermal power reactors less than suitable for such uses. Consequently, much of that plutonium simply sits, along with the used uranium, in spent fuel casks, awaiting a political (rather than an engineering) solution. Where plutonium is used for power production, it mostly goes into thermal reactors, mixed with uranium, giving mediocre results. Even a mediocre nuclear fuel is still very good, but the result is that new uranium is constantly required, and unusable spent fuel continues to accumulate, although more slowly.

Thorium offers another path to sustainable atomic energy. Thorium in nature is composed almost entirely of the isotope ²³²Th, which has a half-life of fourteen billion years. Since this is close to the present age of the universe, and heavy elements are being formed constantly by stellar nucleosynthesis, we can say that more thorium exists now than has ever existed in the past, and the quantity is increasing. More to the point, it is considerably more plentiful in the crust of the Earth than uranium, and quite readily obtained from sources such as monazite sand.

Uranium-233 is produced by neutron capture in ²³²Th, in a manner exactly analogous to the production of ²³⁹Pu from ²³⁸U (although thorium is the stronger absorber), and releases 2.50 neutrons per fission. Better yet, its tendency to absorb a thermal neutron is less, giving a net 2.31 neutrons per neutron, as against 2.08 for either ²³⁵U or ²³⁹Pu. The picture is complicated by parasitic absorptions in the intermediate nuclide ²³³Pa (half-life 27 days), but this poses no insuperable problems. Hence, thorium irradiated with neutrons from uranium or plutonium fission will tend to accumulate ²³³U, and will ultimately become enriched enough in fissile material to support a chain reaction in a thermal reactor. When that fuel is used, its proportion of fissile material will continue to increase, and the excess can be separated, mixed with fresh thorium, and used as fuel for additional reactors.

Thorium can be used in existing power reactors. Thorium is often discussed as a prospect for use with technologies such as the molten-salt reactor, which are not in commercial use, and would generally require some degree of research and development, followed by the construction of entirely new power plants. This makes the widespread use of thorium appear as a remote prospect.

In fact, however, thorium-uranium fuels are nothing new. From an early period, at plants such as Indian Point, Peach Bottom, and Fort St Vrain, thorium was mixed with enriched uranium as a sort of extender, to provide a longer fuel life from a given amount of ²³⁵U than would ²³⁸U. From 1977 until its decommissioning in 1982, the Shippingport plant in Pennsylvania (America’s first full-scale utility atomic power station) operated as a Light Water Breeder Reactor, using a thorium-uranium core, with a small but appreciable production of excess fissile material.

The experience of Shippingport shows that existing light-water reactors, which comprise most of the atomic power capacity in America, France, Japan, and much of the rest of the world, might reasonably be converted to thorium-based fuel now. Work on Th-U and Th-Pu fuels for heavy water reactors is also well in hand. The breeding performance will not be as good as with purpose-designed plants, and periodic reprocessing will still be required, but the buildup of unusable fuel (including depleted uranium from enrichment plants) will stop. If desired, depleted uranium could even be re-enriched with ²³³U. This is especially attractive in the period after proper thermal breeders have been deployed, but before existing reactors have reached the end of their useful lives or fast breeders have become common.

“All that waste”, and other misconceptions

Radioactive waste is an ambiguous term. After all, coal-fired power plants customarily dump a great part of their waste right into the air we all breathe, and the remainder (everything from cinders to residue from acid ‘scrubbers’) may end up in waterways or old mine workings. And that waste is all radioactive. Coal is the residue of dead vegetation from past ages ; plants take up what the soil yields, and virtually all soils contain radioactive minerals. So coal typically contains at least a trace of uranium and thorium, with their daughter products, and some lignites are considered economic uranium ores. The mineral content of the coal concentrates in the ash, whether that goes up the stack (along with radon) or into some kind of dump where extracting its uranium content may be worthwhile.

Another form of radioactive waste consists of material which has been contaminated from external sources. For example, pipe and other items used in oil and gas drilling accumulate radioactive encrustations, principally from waters containing dissolved radium salts. In atomic power plants and radiation laboratories, neutron bombardment can leave fixtures and tools bearing activation products. This kind of ‘low-level waste’ constitutes by far the bulk of the material to be dealt with, although the radiation associated with it is small. Unfortunately, in the public mind it is often lumped together indiscriminately with something quite different.

What people who complain about ‘the waste problem’ are usually referring to, whether they entirely understand it or not, is spent nuclear fuel. Typically, when a charge of fuel is removed from a civilian power reactor, more than 95% of the mass is uranium and plutonium. We already know that these fuels can be recycled. The true waste products are the fission residues, and some of these have appreciable commercial value. Even the ‘hot’ isotopes may find medical and scientific markets, although the extent of such applications is limited.

The most important fact about spent nuclear fuel is that there isn’t very much of it. This is a natural consequence of the fact that nuclear reactions yield millions of times the energy of chemical reactions. The total quantity of spent fuel discharged each year by all the atomic power plants in the world is about twelve thousand tonnes, which is the mass of fuel consumed by one large coal power plant in sixteen hours.

If the proportion remaining to be disposed of, after reprocessing, is five per cent (certainly an overestimate), this is all of six hundred tonnes. The freight cars on American railways carry a hundred tonnes each, so if we imagine the waste to be blended, to the extent of one per cent by mass, with some inert material to absorb the radiation it emits, six trainloads of a hundred cars each would suffice. Piled up into a heap, that would approach the size of the fuel pile which might be found at a coal-fired power plant.

Suppose the inert material to be a good grade of fireclay, with a specific gravity of 2.25 (which will not be appreciably altered by the addition of 1% of fission products), made up into bricks. The radioactive material might be very effectively kept from escaping into the environment by concentrating it at the center, with an impermeable coat of ‘clean’ clay over it. Further suppose that those bricks are made up into cubical stacks, ten meters on a side, with fully fifty per cent of gap spaces for air to circulate and carry away the decay heat. Allowing an aisle ten meters wide around each stack, it would take the best part of fifty years to cover a square kilometer of ground. The thirty-five hundred arid, inhospitable square kilometers of the Nevada Test Site, where atomic bombs were formerly exploded at ground level (a place where hardly anyone goes, and erosion would act but slowly on the bricks), would suffice for over one hundred fifty thousand years at this rate — a span of time much longer than we have any reason to consider.

Ignore it long enough, and it goes away. The other distinctive feature of radioactive wastes is that the inexorable process of decay will get rid of them in time. Furthermore, that is not a long time. The process of fission itself uses up most of the energy available to drive radioactive decay, and the activity of fission products is principally in ephemeral isotopes. The nuclides of greatest concern, strontium-90 and cesium-137, have half-lives of about 30 years. Longer-lived products are not only less active, but also much scarcer.

After a few centuries of standing, the bricks described above would be less radioactive than many common building materials, such as granite, and would pose little real hazard even if crushed for rubble. As a result, even if all the world’s people consumed energy at American rates, and all that energy was provided by fission, all the waste (about 3.5 grams of fission products per person per year) could be accommodated at the Nevada Test Site until it had aged to complete safety, with no danger of ever running out of room. The extravagant claims that nuclear wastes must be isolated for thousands of centuries are based on the idea that spent fuel is to be discarded wholesale, plutonium and all. This obviously makes no sense unless atomic power is abandoned entirely, and using it as an argument for abandonment is circular reasoning to say the least.

Fusion will save us! Right?

Some say that further investment in present-day, practical atomic power is irresponsible because new forms of nuclear energy are coming soon. But it’s hard to see why this should be true. Even if useful energy were to be produced from fusion tomorrow, the money invested in fission infrastructure would not be wasted. After all, a fission plant may have a lifetime of sixty years or more, and construction of new fusion generating capacity would have to be rapid enough just to keep up with demand growth that withdrawing cheap, clean, safe fission capacity would be uneconomic.

In fact, it would make sense to continue building fission power plants for some time. Why? Simply because it takes time to go from proof of concept to full commercialization. The slow-down in new fission construction since the 1980s has been accompanied by a large growth in the use of combustion, and that is more than coincidence. It is a trend which we can expect to continue.

“Fusion is the energy source of the future, and always will be.” If that is a trifle harsh, it is nevertheless true that practical power from fusion has been expected within ten or twenty years for about sixty years now. Fusion research has yielded a wealth of discoveries in plasma physics, almost all of them in the form of reasons why the latest machine won’t work. Good for basic science, not so good for practical applications.

The only stable, energy-positive fusion process we know of, so far, occurs in stars. Attempts to harness unstable fusion, as a gasoline engine harnesses unstable combustion, have so far not been successful. Self-sustaining artificial fusion will almost certainly be achieved, but nobody can say when.

Beyond the problem of producing the fusion reaction itself is the problem of converting the energy it releases into a useful form. The reactions which have a low enough energy barrier and a high enough reaction rate to be pursued in the current state of the art are of deuterium with itself, and with tritium, helium-3, and possibly protium : ²H + ²H → ³H + ¹H (3.27 MeV)
²H + ²H → ³He + n (4.03 MeV)
²H + ¹H → ³He + γ (5.49 MeV)
²H + ³H → ⁴He + n (17.6 MeV)
²H + ³He → ⁴H + ¹H (18.4 MeV) Deuterium is present to the extent of one atom in six thousand in essentially all the waters of the world. Practical deuterium fusion would thus be an abundant energy source which no country could monopolize, and this was the primary motive for its development in the 1950s, when the extent and distribution of uranium reserves was unknown. For some years, however, research has concentrated on the deuterium-tritium reaction, which is more easily achieved. Although tritium is present in surface and ocean water, due to the action of cosmic rays, it is not sufficiently abundant to be extracted, and the available supply is produced in fission reactors, for industrial and military purposes, by the neutron bombardment of lithium. Through β-decay, this is also a source of ³He, which also exists as a primodial constituent of helium obtained from natural gas, but again, only in trace quantities (diluted by ⁴He from α-decay of radioactive minerals).

Of the energy produced in the reactions based on deuterium, much is carried off by the fast-moving neutrons ; the remainder is deposited in the plasma, which does not readily give it up. It has been suggested that the neutrons can be used to produce transmutations in a blanket of lithium, yielding tritium, and heat which can be tapped off to a steam system. Fast fission in a blanket of uranium is also possible, but (given that the energy per fission is 200 MeV) it is hard in that case to justify the additional trouble and expense compared to a fission reactor. Other than such roundabout processes, it is not easy to see how power is to be obtained. There has been some work on the use of magnetohydrodynamic techniques to tap electric current directly from the moving charged particles, which would be the ideal arrangement, but so far this only seems to confound the problems involved with confining and heating the plasma.

Light-element fission is another possibility. Decades ago, television pioneer Philo Farnsworth invented an electrostatic-confinement apparatus for nuclear fusion, based on his work with vacuum tubes. This ‘fusor’ is a simple mechanism which has largely escaped the interest of the government-funded fusion researchers, perhaps because it offers no insights into plasma physics. Its cheapness and ability to reliably produce deuterium fusion have made it a favourite with hobbyist experimenters, although it presents no better prospect of nett power from fusion than any other approach.

Recently, fission in a Farnsworth reactor has been proposed. The reaction of interest is ¹H + ¹¹B → 3×⁴He. Although it releases only a moderate amount of energy, its activation energy is moderate, and it yields only charged particles. This offers the possibility of taking the output energy as high-voltage current directly from the electrified grids, without interposing a steam loop or other elaborate conversion apparatus.

If such a technique can be reduced to practice, the result would be simple, low-cost plants which could be erected anywhere, and would produce pure electricity with minimal waste heat, with no radioactive substances involved, from a fuel which can be bought for a pittance at any dry-goods store. The very high expected power per unit plant mass makes it especially attractive for electric propulsion of spacecraft. But it is not known with certainty whether it can be achieved, and current fusion researchers are unlikely to support funding for a development program which, if successful, would completely kill interest in their billion-dollar assemblies of magnets and lasers.

‘Cold fusion’ is a complete question mark. Since the 1980s, there have been repeated claims of anomalous energy release in solid or liquid materials at ordinary temperatures, which have been variously attributed to fusion or some kind of ‘low-energy nuclear reaction’. It is certainly not out of the bounds of possibility that such reactions could occur, although the specifics are not obvious from present physics. Unfortunately, the field is so completely populated with obvious frauds and charlatans that serious research is almost impossible.