Appendix

Radiation, Nuclear Technology, and Safety

The following is a summary of facts most relevant to this book. It draws from a variety of sources, including ever-expanding and changing material available in the Wikipedia encyclopaedia.

Radiation-Related Terms

• Radioactivity — the spontaneous transformation of an unstable atom, which often results in the emission of radiation. This process is referred to as the transmission, the decay, or the disintegration of an atom.

• Radioactive Contamination — radioactive material distributed over some area, object, or person. It tends to be unwanted in the location where it is, and has to be cleaned up, or decontaminated.

Common Types of Radiation

• Alphas — particles emitted from an atom that contain two protons and two neutrons.

• Betas — high-speed particles, identical to electrons, that are emitted from the nucleus of an atom.

• Neutrons — neutral particles normally contained in the nucleus of an atom that may be removed by various interactions or processes like collision or fission.

• X-rays — electromagnetic waves, or photons, normally emitted by energy changes in electrons. These energy changes are caused either by electrons moving between the orbital shells that surround an atom or by the process of slowing down electrons, as in an X-ray machine.

• Gamma Rays — electromagnetic waves, or photons, emitted from the nucleus (centre) of an atom.

Common Units — International Standard

• Becquerel (Bq) — measures radioactivity. One becquerel is the quantity of radioactive material that will have one transformation per second.

• Gray (Gy) — measures the amount of energy actually absorbed in some material. Used for any type of radiation and any material. Does not describe biological effect.

• Sievert (Sv) — measures the absorbed dose of radiation in human tissue and thus the effective biological damage. Bear in mind that not all radiation has the same biological effect, even for the same amount of absorbed dose. Dose is often expressed in terms of a thousandth of a sievert, or millisievert, or a millionth of a sievert, or microsievert.

Common Units — USA

• Curie (Ci) — measures radioactivity. One curie is the quantity of a radioactive material that will have 37 billion transformations in one second.

• Roentgen (R) — measures a quantity called exposure, but only of gamma rays and X-rays, and only in air.

• Rad — radiation-absorbed dose. Measures the amount of energy actually absorbed in some material. Used for any type of radiation and any material. Does not describe biological effect.

• Rem — roentgen equivalent in man. Measures the absorbed dose of radiation in human tissue and thus the effective biological damage. One hundred rems equals one sievert.

Nuclear Fission

• A chain reaction in fissile material occurs when subatomic particles called neutrons collide with nuclei, releasing further neutrons, which collide within milliseconds with other nuclei, creating heat and causing self-destruction. Without slowing the process down, a nuclear explosion will occur. With moderation, for example by the insertion of graphite rods among the fuel rods in a nuclear reactor, some of the neutrons are slowed down and absorbed, and the collisions are fewer. The reactions can thus be controlled and sustained, producing heat without kinetic energy.

• A nuclear reactor is a device to initiate and control a sustained nuclear chain reaction. In a power reactor, heat from the reaction raises steam from water surrounding the core. The steam drives a turbine, which can either propel a ship or power a generator to make electricity. Some smaller research reactors, like the one at Lucas Heights, Sydney, generate neutrons for medical and industrial use.

• Reactors are generally fuelled by uranium enriched to contain 3–5 per cent of the isotope uranium-235. Natural uranium can be used in heavy-water (deuterium oxide or D2O) reactors such as the Canadian CANDU. Nuclear weapons are fuelled either by uranium-235 or plutonium-239.

• Natural uranium contains over 99 per cent of the fertile isotope uranium-238 and less than 1 per cent of the fissile isotope uranium-235. To enrich it for use in a reactor, uranium is first converted into uranium oxide (U3O8), and then into a gas, uranium hexafluoride (UF6). The UF6 is passed through baffles (gaseous diffusion), spun in centrifuges, or treated by lasers to separate the two uranium isotopes. Separation of the chemically identical isotopes is possible because uranium-238 is slightly heavier than uranium-235, and tends to migrate to the outer area of a centrifuge. For civil use, the enrichment process stops when the uranium has been enriched to 3–5 per cent uranium-235, or in the case of some research reactors, up to 20 per cent uranium-235. For nuclear weapons, the enrichment process continues until the uranium is enriched to around 90 per cent uranium-235.

• SCRAM — Safety Control-Rod Axe Man: the name given to the man who sat on top of the first nuclear pile in Chicago in 1942. He held an axe to cut the rope suspending moderator rods above the nuclear pile, to shut it down if the reaction accelerated out of control. The acronym is used today to describe the emergency introduction of control rods into a reactor vessel to stop a nuclear reaction.

Reactor Types

• A pressurised-water reactor (PWR) uses a pressure vessel to contain nuclear fuel, control rods, moderator, and coolant. They are cooled and moderated by water under pressure. The hot, radioactive water leaves the pressure vessel and loops through a steam generator, which in turn flashes a secondary (non-radioactive) loop of water into steam to drive turbines.

• A boiling-water reactor (BWR) is like a PWR but without a secondary loop. The primary loop heats in the reactor, exits the reactor vessel to produce steam to run turbines and generators, and returns to the reactor vessel for reheating after going through a condenser.

• A pressurised heavy-water reactor (PHWR), known in Canada as a CANDU, is heavy-water cooled and moderated. Instead of using a single, large pressure vessel as in a PWR, the fuel is contained in hundreds of pressure tubes. Fuelled by natural (unenriched) uranium, PHWRs can be refuelled while at full power, making them very efficient in their use of uranium.

• A gas-cooled reactor is generally graphite-moderated and CO2-cooled. It can have a high thermal efficiency compared to PWRs, due to higher operating temperatures.

• A liquid-metal fast-breeder reactor (LMFBR) is cooled by liquid metal such as sodium or lead, and unmoderated, producing more fuel (plutonium) than it consumes. Fast breeders are said to ‘breed’ fuel as they produce fissionable fuel during operation because of neutron capture. They include BN-350 and BN-600 reactors in Russia, Phénix and Superphénix in France, and Fermi in the United States. In Japan, the Monju FBR suffered a sodium leak in 1995, was not restarted until 2010, and again went offline when a loading gantry got caught in the reactor vessel. No FBRs are at present commercially operational.

• Generation III reactors, or ‘advanced reactors’ are a heterogeneous collection of different reactor concepts, mainly evolving from existing Generation I and II reactors. One is the Advanced Pressurised-Water Reactor (APWR), developed by Westinghouse. Another is the Advanced Boiling-Water Reactor (ABWR) of General Electric. They are claimed to be safer than earlier generations, being less subject to human error and relying more on passive systems including gravity-fed pumps. Sceptics claim their designers are more interested in cutting ever-expanding capital costs than improving safety.

• The Pebble Bed Modular Reactor (PBMR) is a high-temperature reactor cooled by pressurised helium, supposedly safer than conventional systems. The fuel comprises billions of micro-spheres of enriched uranium coated with pyrolytic carbon and silicon, and consolidated in about 400,000 tennis-ball-sized graphite spheres, fed continuously from a silo into the reactor core. The designers claim the system will reduce reactivity and lower power density in the core to such an extent that dangers of a meltdown become extremely remote. Neither hands-on surveillance by operators nor a containment vessel will be necessary. Critics assert, however, that a core temperature increase above 1600 degrees Celsius could cause a graphite fire, and that if air intrudes into the primary helium circuit, the carbon coatings could ignite. It would also be difficult to prevent radioactive helium leaking from the reactor.

• Thorium reactors use a naturally occurring fertile element found in abundance around the world. Unlike uranium, thorium does not require enrichment, but does require irradiation and processing before use as a nuclear fuel, making it more expensive than uranium. It is believed to produce lower amounts of transuranic waste products than uranium, but its disadvantages include the fact that the isotope thorium-232 transmutes into fissile uranium-233, which can be used in nuclear weapons; that thorium is pyrophoric in powdered form, liable to spontaneous combustion; and that it is an alpha emitter which can cause cancers in the body if inhaled, as well as creating radon gas which can also cause cancers. Experimental thorium reactors have been built in the US, Russia, China, and Japan, but none are yet commercialised.

• Generation IV reactors are a set of theoretical nuclear-reactor designs not expected to be available for commercial construction until around 2030. Their primary goals are claimed to be to improve nuclear safety, resist proliferation, minimise waste, and decrease building costs. They include gas-cooled fast reactors, lead-cooled fast reactors, molten-salt reactors, sodium-cooled fast reactors, supercritical-water reactors, and very-high-temperature reactors.

• Generation V+ reactors are designs which are theoretically possible, but which are not being actively researched at present. Although such reactors could be built with current or near-term technology, they trigger little interest for reasons of economics, practicality, or safety. They include liquid-core reactors — which have a closed-loop liquid core, where the fissile material is molten uranium, cooled by a working gas pumped in through holes in the base of the containment vessel — and gas core reactors.

Radioactive Isotopes

In a 2006 report entitled The Biological Effects of Ionising Radiation VII, the United States National Academy of Sciences determined that there is no safe minimum dose of ionising radiation, that all radiation doses in the human body accumulate, allowing the stronger possibility of cancers to develop, and that children, especially girls, are more susceptible to contracting cancers from radiation than adults. The academy’s findings have been endorsed by the United States Nuclear Regulatory Commission.

Most radio-toxic isotopes are the product of nuclear fission. Some have very short lives; others, extremely long ones. The most dangerous need to be isolated from the biosphere, often for a very long time. Plutonium-239, for example, has a half-life of 24,400 years. The following are among the worst from a human-health point of view:

• Caesium-137, an isotope with a half-life of 30 years, is radioactive for 600 years. A potassium analogue, caesium-137 tends to concentrate in animal muscle and fish, and it deposits in human muscles where it irradiates muscle cells and nearby organs. It is a dangerous beta and high-energy gamma emitter, and is very carcinogenic.

• Iodine-131, with a half-life of eight days, is a volatile isotope, usually released from a reactor as a gas, either routinely or accidentally. A beta and gamma emitter, it can enter the bloodstream after being inhaled into the lungs or ingested in contaminated food. It is absorbed by the thyroid gland. Children are at special risk because their tiny thyroids avidly absorb iodine from the blood like a sponge.

• Plutonium-239 is an artificial isotope, with a half-life of 24,400 years, and which in a nuclear reactor transmutes from uranium-239. It is used in nuclear weapons and nuclear reactors. Because it emits alpha particles, plutonium is most dangerous when it is inhaled and lodges in the lungs. The alpha particles can kill lung cells, which causes lung scarring, leading to cancer. Plutonium can also enter the bloodstream and circulate around the body. Once plutonium circulates, it concentrates in the bones, liver, and spleen, and can cause cancers. Mutations in reproductive genes increase the incidence of genetic disease.

• Strontium-90 is produced commercially through nuclear fission for use in medicine and industry. It is found in the environment from nuclear testing that occurred in the 1950s and 1960s, and in nuclear-reactor waste. A beta and gamma emitter with a half-life of 28 years, it is therefore radioactively dangerous for 600 years. A calcium analogue, it can be inhaled, or ingested in food and water. Once in the body, it acts like calcium and is readily incorporated into bones and teeth, where it can cause cancers of the bone and bone marrow.

Problems of Disposal

The long life of many radioactive isotopes produced as part of nuclear-reactor waste has created physical and political problems in developing international permanent disposal regimes. Even the procedures before permanent disposal are complex and expensive. The intensely hot and highly radioactive spent fuel is unloaded from the power reactors into a water-filled pool, usually immediately adjacent to the reactor, where its heat and radiation are allowed to decrease. The rods remain in the pool for periods ranging from a few years to decades. After cooling, they may be transferred to massive air-cooled dry casks on site, or to a centralised facility. Spent fuel rods may then be permanently stored in a geological repository, either in an unprocessed state, or after having been reprocessed to extract remaining uranium-235 and plutonium-239, both of which are fissile, and may be used in nuclear weapons or converted back into fuel rods for re-use in reactors.

Whether processed or not, the spent fuel or its waste remain highly radioactive for centuries, and the challenge, still unsolved, is to find suitable locations for permanent storage. At the time of writing, there are 437 nuclear-power reactors in use around the world in 31 countries, yet no repository as yet exists for permanent disposal of high-level waste. Finding suitable sites is fraught with political problems, as hardly anyone wants such waste in their neighbourhood. Only Sweden, Finland, Canada, Germany, and France have come close to solving the problem. But even where sites acceptable to local populations may be found, these have to be able to be of such a geological nature as to resist the heat and corrosion of the nuclear waste for thousands of years without leaching into the geological substrata or a water table. They must also be rendered irrecoverable by some future regime that wants to mine the waste for fissile material for nuclear weapons. The problem continues to grow as spent-fuel storage pools fill up and as temporary storage facilities become overburdened and prone to theft or terrorism.