In order to understand nuclear technology and its impact on human health, three atomic-level events must be understood: fissioning, activation and ionisation. Fissioning, i.e. the splitting of the uranium or plutonium atom, is responsible for producing radioactive fission fragments and activation products. These in turn cause the ionisation of normal atoms, leading to a chain of microscopic events we may eventually observe as a cancer death or a deformed child.
        Radioactive fission products are produced in nuclear reactors. They are variant forms of the ordinary chemicals which are the building blocks of all material and living things. The radioactive forms of these chemicals were, prior to 1943, present in only trace quantities in isolated places in the environment as, for example, in South Africa where it appears that a small nuclear fission reaction occurred spontaneously about 1700 million years ago.
        When a uranium atom is split or fissioned, it does not always split in the same place. The two pieces, called fragments, are chemicals of lower atomic weight than uranium. Each fragment receives part of the nucleus and part of the electrons of the original large uranium atom. The uranium atoms, of course, cease to exist after they are split. Instead, more than 80 different possible fission products are formed, each having the chemical properties usually associated with their structure, but having the added capability of releasing ionising radiation. X-rays, alpha particles, beta particles, gamma rays (like X-rays) or neutrons can be released by these `created' chemicals. All these can cause `ionisation', i.e. by knocking an electron out of its normal orbit around the nucleus of an atom they produce two `ions', the negatively charged electron and the rest of the atom which now has a net positive electrical charge.
        The atomic structure of fission fragments is unstable. The atom will at some time release the destabilizing particle and return to a natural, low-energy, more stable form. Every such release of energy is an explosion on the microscopic level. With each fissioning, 2 or 3 neutrons are released which can strike a nearby U235 atom causing more fissioning in what is usually called a chain reaction.
        The violence of the chain reaction is such that it can also yield what are called activation products, i.e. it can cause already existing chemicals in air, water or other nearby materials to absorb energy, change their structure slightly and become radioactive. As these high-energy forms of natural materials eventually return to their normal stable state, they can also release ionising radiation. About 300 different radioactive chemicals are created with each chain reaction.[1] It takes hundreds of thousands of years for all the newly formed radioactive chemicals to return to a stable state.
        In a nuclear power plant the fissioning takes place inside the zirconium or magnesium alloy cladding which encloses the fuel rods. Most of the fission fragments are trapped within the rods. However, the activation products can be formed in the surrounding air, water, pipes and containment building. The nuclear plant itself becomes unusable with time and must eventually be dismantled and isolated as radioactive waste.
        After fissioning, the fuel rods are said to be `spent'. They contain the greatest concentration of radioactivity of any material on the planet earth -- many hundreds of thousands of times the concentration in granite or even in uranium mill tailings (waste). The spent fuel rods contain gamma radiation emitters (which are similar to X-ray emitters) so they must not only be isolated from the biosphere, but they must also be shielded with water and thick lead walls. Direct human exposure to spent fuel rods means certain death.
        In reprocessing, spent fuel rods are broken open and the outer cladding is dissolved in nitric acid. The plutonium is separated out for use in nuclear weapons or for fuel in a breeder or mixed oxide nuclear reactor. The remaining highly radioactive debris is stored as liquid in large carbon or stainless steel drums, awaiting some kind of solidification and burial in a permanent repository. Waste of lower radioactivity is buried in dirt trenches or -- as in Windscale (Sellafield) in England -- piped out to sea. The spent nuclear fuel rods and liquid reprocessing waste are called `high level radioactive waste'. It must be kept secure for hundreds of thousands of years -- essentially forever. Lower level waste may be equally long-lived, but it is less concentrated.
        In above-ground nuclear weapon testing, there is no attempt to contain any of the fission or activation products. Everything is released into the air and on to the land. Some underground tests are also designed to release most of the radioactive particles; these are called crater shots or shots with unstemmed holes. Even when below-ground shots are designed to be contained, they normally lose the radioactive gases and some particulates. The radionuclides trapped in the ground can also migrate downwards in the earth to water reservoirs which provide irrigation and drinking water for human purposes, although this process is slow. Radioactive debris piped out to sea can be washed back on shore or can contaminate fish.
        In all nuclear reactions, some radioactive material -- namely the chemically inert or so-called `noble' gases, other gases, radioactive carbon, water, iodine, and small particulates of plutonium and other transuranics (i.e. chemicals of higher atomic number than uranium) -- is immediately added to the air, water and land of the biosphere. In the far-distant future, all the long-lived radioactive material, even that now stored and trapped, will mix with the biosphere unless each generation repackages it. Our planet earth is designed to recycle everything.
        The radioactive chemicals which escape to the biosphere can combine with one another or with stable chemicals to form molecules which may be soluble or insoluble in water; which may be solids, liquids or gases at ordinary temperature and pressure; which may be able to enter into biochemical reactions or be biologically inert. The radioactive materials may be external to the body and still give off destructive penetrating radiation. They may also be taken into the body with air, food and water or through an open wound, becoming even more dangerous as they release their energy in close proximity to living cells and delicate body organs. They may remain near the place of entry into the body or travel in the bloodstream or lymph fluid. They can be incorporated into the tissue or bone. They may remain in the body for minutes or hours or a lifetime. In nuclear medicine, for example, radioactive tracer chemicals are deliberately chosen among those quickly excreted by the body. Most of the radioactive particles decay into other radioactive `daughter' products which may have very different physical, chemical and radiological properties from the parent radioactive chemical. The average number of such radioactive daughters of fission products produced before a stable chemical form is reached, is four.
        Besides their ability to give off ionising radiation, many of the radioactive particles are biologically toxic for other reasons. Radioactive lead, a daughter product of the radon gas released by uranium mining retains the ability to cause brain damage exercised by non-radioactive lead. Plutonium is biologically and chemically attracted to bone as is the naturally occurring radioactive chemical radium. However, plutonium clumps on the surface of bone, delivering a concentrated dose of alpha radiation to surrounding cells, whereas radium diffuses homogeneously in bone and thus has a lesser localized cell damage effect. This makes plutonium, because of its concentration, much more biologically toxic than a comparable amount of radium. Some allowance for this physiological difference has been made in setting plutonium standards, but there is evidence that there is more than twenty times more damage caused than was suspected at the time of standard setting.[2]
        The cellular damage caused by internally deposited radioactive particles becomes manifest as a health effect related to the particular organ damaged. For example, radionuclides lodged in the bones can damage bone marrow and cause bone cancers or leukaemia, while radionuclides lodged in the lungs can cause respiratory diseases. Generalised whole body exposure to radiation can be expressed as a stress related to a person's hereditary medical weakness. Individual breakdown usually occurs at our weakest point. In this way, man-made radiation mimics natural radiation and causes the ageing or breakdown process to be accelerated.