SCOPE 50 - Radioecology after Chernobyl

8.1 INTRODUCTION TO RADIOACTIVE PROCESSES

Variants of every element exist which have the same number of protons in the nucleus but a different number of neutrons. These variants are known as isotopes. Many are unstable, transmuting into other types of nuclei, a process which is accompanied by the emission of radiation, in the form of particles or electromagnetic energy. Such unstable nuclei may occur naturally or form as a result of human activities. Instability of atomic nuclei may arise from excess energy within the nucleus, or imbalance between the number of protons and neutrons. Disintegration of an unstable nucleus may occur spontaneously or follow interaction with a nuclear particle. Forms of radiation which may be emitted from nuclei include alpha particles (identical with ⁴He nuclei), beta particles (electrons), gamma radiation (very short wavelength electromagnetic radiation) or neutrons. Alpha particles often carry the most energy but because of their double positive charge, interact strongly with matter and are readily stoppedfor example, by a sheet of paper. Beta particles, with a single negative charge, are usually more penetrating than alpha particles and can be stopped by a thin sheet of metal.

The neutron is uncharged and only interacts with matter when passing close to the atomic nuclei, so that neutron radiation is very penetrating. Gamma radiation is also chargeless and is the most penetrating of all, and when at high energy (short wavelength) can require many centimetres of heavy metal or metres of concrete to stop. Neutrinos, emitted in beta decay, with zero charge and mass, do not interact appreciably with matter and are ignored in this study. The emission of alpha particles is common only for nuclides of mass numbers above 209 and atomic numbers above 82.

The presence of natural radiation in the environment arises from cosmic and terrestrial sources. Earth is being constantly bombarded with cosmic rays (high-energy protons and charged nuclei) of solar and galactic origin. These cosmic rays cause direct exposure but also give rise to secondary radioactivity from their interaction with stable elements in the upper atmosphere forming radionuclides. Measurable doses to humans result from ³H, ⁷Be, ¹⁴C and ²²Na arising in this  way. Moreover, such secondary radioactivity may enter global biogeochemical cycles; for example by incorporation of ¹⁴C into plants during photosynthesis and into the oceans by absorption. Tritium enters into the global hydrological cycle.

Many artificially created transuranic elements, such as plutonium, neptunium and americium, occurred on Earth at one time, but decayed to zero long ago by virtue of their short half-lives. Some transuranium elements continue to be produced in very small amounts by the absorption in uranium isotopes of naturally occurring neutrons.

Artificial radionuclides may be released into the environment as a result of deliberate, accidental or inadvertent actions such as from nuclear fuel cycle operations, reactor accidents, spillages or detonation of nuclear weapons. Whilst there has been a decline of environmental radioactivity attributable to nuclear weapons since the ban on atmospheric testing in 1963, anthropogenic radioactivity remains of great importance. This results from the growth of the nuclear power industry and the increasing use of radionuclides in fields such as medicine and biology, research, space exploration and industry (e.g. radiographic examination of structures). Radionuclides are also used in consumer products such as smoke detectors and luminous watches. The importance of radionuclide discharge to the environment from the nuclear fuel cycle, and accidents such as those which occurred at Kyshtym, Chernobyl and Windscale, are considered in detail in Chapter 2.

Radionuclides would be injected into the environment in a nuclear war. A speculative war scenario involving the direct targeting of nuclear facilities (i.e reactors, spent fuel storage, fuel reprocessing plants, high-level waste facilities) reveals that this could release quantities of fission products comparable with weapon debris from a major nuclear exchange (SCOPE, 1989).

The movement of radioactivity among the terrestrial ecosystems, the oceans and the atmosphere by interactive physico-chemical and biological processes (known as biogeochemical cycling) involves many scientific disciplines, from biology and physical chemistry to hydrology, meteorology and oceanography.

The elucidation of pathways for man-made radionuclides contributes to the protection and enhancement of the quality of the environment. Knowledge of transport mechanisms in the environment is essential for the safe disposal of nuclear wastes from civil power generation.

Artificial radionuclides are also excellent environmental tracers, far easier to study than the corresponding stable nuclides. The knowledge gained from radionuclides gives understanding of the behaviour of the stable elements. For example, Chernobyl emissions have improved understanding of wet and dry deposition processes, and Sellafield effluents have proved a useful tracer for sediment behaviour in the Irish Sea. Hence radionuclides are an excellent tool in generally advancing knowledge of biogeochemical cycling.

8.2 UNITS

The intensity of a radioactive source is determined by the number of atoms present and their decay constant. The former is proportional to the mass of radionuclide divided by the atomic weight, the latter is the fraction of the total number of atoms disintegrating in unit time. Decay rate is more commonly expressed as half-life, the time taken for half the atoms of a given radioactive isotope to decay.

The unit of activity in the SI system is the becquerel (Bq), one disintegration per second. The earlier unit, the curie (Ci), was the activity of the radon in equilibrium with one gram of radium, later defined as 3.7 x 10¹⁰ disintegrations per second, but this became unwieldy to use when very low activities assumed importance.

Units of dose quantify the absorption of ionising radiation in material or tissue, usually in terms of energy implanted. The unit is the rad (radiation absorbed dose), defined as the liberation of 100 ergs per gram of absorber, or the gray, one joule per kilogram in the SI system. Since the forms and energies of the radiationalpha, beta, gamma, X-rays, etc.cause different degrees of biological damage for the same absorbed dose, a relative biological effectiveness (RBE) factor is applied in practical health physics, comparing the effect of the radiation with that of 200 keV X-rays. This gives the equivalent dose in units of rem (Roentgen equivalent man, rad x RBE), or the sievert (gray x RBE) in SI units.

Table 8.1 Radiation units

 8.3 METHODS OF ANALYSIS

The general principle underlying most methods of detection of nuclear radiation is that whatever the form of the radiation, it gives up part or all of its energy to the detecting medium either by ionizing it directly or by causing the emission of a particle which in its turn produces ionization in the medium. The ionization thus produced is then detected by one of an increasing variety of techniques, the most widely used of which are briefly reviewed in this section.

Although it is feasible to determine some radionuclides directly or by methods requiring the minimum sample preparation, e.g., gamma-spectrometry, where the radionuclide of interest is present at a very low concentration or emits no penetrating radiation, pre-concentration and radiochemical separation from a sample matrix containing interfering radioelements is usually necessary. In such cases the aim of the analyst is to extract from the sample sufficient of the radioelement of interest with a quantified chemical recovery and in a form suitable for nuclear counting.

Normally there are four steps in the measurement of environmental radioactivity: (a) selecting the measuring system; (b) collecting a suitable sample; (c) concentrating and/or separating the desired radioelement by physical and/or radiochemical means; and (d) assaying the radioactivity in the source with a detector that has been calibrated with an appropriate radioactivity standard (NCRP, 1978).

8.3.1 SELECTION OF A LOW-LEVEL MEASUREMENT SYSTEM

The choice of system for a particular application is usually based on a number of factors (NCRP, 1978) of which the most important are: (a) the quantity of sample available; (b) the nature and intensity of the radiation to be measured; (c) the time available for counting; (d) the efficiency; (e) the background; and (f) the cost. The sensitivity of a particular system is usually expressed as a lower limit of detection (LLD). A frequently used criterion is to express three standard deviations of the background in terms of activity as the LLD. However, the IAEA (1989) have recommended the use of the following expression:

         (LLD) = 4.66 S/eP

where S is the estimated standard error of the net count-rate, e is the counting efficiency for the specific nuclide energy and P is the absolute transition probability for emission of the above energy. Since P is physical quantity and cannot be adjusted to optimize the LLD, S must be minimized and e maximized. This leads to the following expression for the figure-of-merit (FOM) of the system (Mishra, 1991): 

         (FOM) = S_a ²/B

where S_a is the sample count-rate with background and B is the background countrate. To maximize the figure of merit, S_a should be made as large and B as small as possible. Since S_a occurs as the square, more benefit is gained by increasing S_a than by decreasing B. In other words, the main emphasis must be on selecting a system with the highest possible efficiency. Thus, the requirements of low-level counting systems are (a) high counting efficiencies, (b) low background count rates and (c) high stability, since long counting periods are often necessary.

Exact defining equations for the limits for qualitative detection and quantitative determination in radioactivity measurements, which are frequently quoted, have been given by Currie (1968). The statistical aspects of interpreting the results of counting experiments designed to detect low levels of radioactivity (when the detector efficiency is known), for both the case of the well-known and the poorly known background, have been discussed in detail by Sumerling and Darby (1981). 

8.3.2 SAMPLING

Before reviewing the radiochemical and radiometric techniques currently in use, it is appropriate to emphasize the fundamental importance of the protocols used to obtain environmental samples. Sampling is often considered to be the weakest link in the chain of planningsamplinganalysisreporting activities, and with some justification. Accurate and precise environmental sampling is an essential prerequisite to analysis and the development of meaningful sampling protocols necessitates careful planning of the practical procedures used in sample collection, handling and transfer. To this end, a preliminary sampling experiment can provide the experience and validation necessary to design larger-scale sampling protocols which will permit meaningful evaluations of radionuclide concentrations, their distributions in time and space, and their impact on the environment and humans. The many variables and techniques required to design and conduct reliable sampling programmes have been thoroughly reviewed in a recent publication by the American Chemical Society, entitled Principles of Environmental Sampling (Keith, 1988). Although aimed at sampling for the purpose of chemical rather than radionuclide analysis, many of the protocols and techniques discussed are equally applicable to the latter. Particular attention is given to sampling equipment and techniques, and sample preservation involving water, air and stacks, biota, and solids, sludges and liquid wastes, as well as quality assurance and quality control considerations, and the reader is directed to this and other sources (Kratochvil and Taylor, 1981; Green, 1979; Gy, 1983) for further reading.

The importance of assessing and controlling sample contamination has long been recognized and is yet another aspect which is worthy of the maximum consideration. Some of the problems arising in the whole process from planning of experiments and collection of samples through handling and radiochemical manipulations to the ultimate interpretation of results have been considered by Harvey and Lovett (1991). 

8.3.3 DETECTION AND MEASUREMENT OF ALPHA-EMITTING RADIONUCLIDES

The extremely short range of alpha particles in matter dictates that the amount of material between the sample and the detector be minimized as much as possible. Various methods are used to achieve this condition including evacuating the air between the sample and the detector, optimizing the counting geometry, removing as much of the inactive part of the sample as possible and mounting the source inside the detector. Alpha particle measurements are normally classified into two categories, namely, (a) gross alpha counting and (b) alpha-particle spectrometry.

8.3.3.1 Gross alpha counting

Polycrystalline ZnS(Ag), the first solid material to be used as a particle detector, is still the most widely used scintillator for gross alpha counting. For high sensitivity the source thickness should never be permitted to exceed the range of the alpha particles. Discs of transparent plastic coated with silver-activated ZnS and photomultiplier tubes coupled to appropriate electronic circuitry remain in regular use. The background count-rates over the entire alpha-energy range are relatively low and of the order of one event per hour for ZnS(Ag) on a thin plastic disc of 24 mm diameter (NCRP, 1978).

Gas-filled ionization chambers and proportional counters have been used to measure alpha activity in thick or thin solid sources mounted inside the detector or outside next to a thin entrance window. The most modern systems have the capability of recording alpha and beta particles simultaneously using the technique of pulse shape discrimination.

Conventional liquid-scintillation counting has been used to assay gross alpha activities in certain environmental samples. However, due to the high background countrates, the technique is seldom indicated for the measurement of low-level sources.

Etchable plastic nuclear track detectors such as lexan and CR-39 have been used for many years to detect very low alpha activities in environmental samples, provided the exposure periods are sufficiently long, typically weeks or longer. The method is a form of autoradiography. The tracks produced in these dielectric materials are usually less than 0.1 µm in length and special techniques must be employed to treat the exposed material. Chemical etching, electrochemical etching and grafting are the three methods most widely used for track enlargement (Mishra, 1991). Various automated techniques for counting track etch detectors have been reported (Mishra, 1991) and include optical scanning, light scattering, electrical conductivity measurement, spark counting and etched-pit counting using an alpha particle flux.

8.3.3.2 Alpha-particle spectrometry

In the case of alpha-particle spectrometry, maximum energy resolution is achieved with a source which approximates as closely as possible to that of an infinitely thin, weightless source on a perfectly flat substrate. If absorption corrections are to be less than 1 per cent, the source thickness should be less than 1 per cent of the average range of the alpha particles. Methods for the preparation of alpha-particle-emitting sources are summarized below.

A wide range of different detector types are employed in alpha-particle spectrometry today including semiconductor detectors, liquid scintillation counters, ionization chambers, proportional counters and magnetic spectrometers. The most commonly used are silicon surface barrier (SSB) detectors, though passivated ion-implanted silicon (PIPS) detectors are beginning to supersede them, having the advantages of higher resolution and more rugged construction and, hence, the exposed surface can be wiped to decontaminate it. In a typical system, the source (e.g. electroplated stainless steel disc or polypropylene membrane filter) is counted, under conditions of `close geometry', with a PIPS detector (active area 300600 mm²) coupled in series to a pre-amplifier, spectroscopy amplifier and multi-channel analyser (MCA), either free-standing or PC-based. Although typical counting times are long (days to weeks), spectrum stability is seldom a problem and, thus, the use of digital spectrum stabilization techniques is unnecessary.

Low-level liquid scintillation counting, using pulse-shape discrimination to separate alpha- from beta-particle-induced events and anti-coincidence shielding to reduce background, is also employed (Yu et al., 1991). The disadvantages are much inferior energy resolution and sensitivity compared to PIPS- or SSB-based systems and problems associated with chemiluminescence in the scintillator cocktail.

8.3.3.3 Mass spectrometry

Conventional thermal ionization mass spectrometry (TIMS) or inductively coupled plasma (ion-source) spectrometry (ICP-MS) is sometimes the method of choice for very low-level measurements of long-lived radionuclides; as the sensitivity of these techniques for nuclides such as ²³⁹Pu, ²⁴⁰Pu, ²³⁷Np and ⁹⁹Tc is some orders of magnitude greater than that of the standard radiometric techniques usually used (Halverson, 1984; Buesseler and Halverson, 1987). Because of isobar and chemical interferences it is necessary, particularly when using TIMS, to first carry out a high-quality chemical separation of the element of interest. The instrumentation itself is also very expensive. ICP-MS, on the other hand, offers an attractive alternative, being considerably less expensive and requiring less complex sample preparationsolution sources can be used. Its sensitivity, however, is inferior to that of TIMS and its use more effective at A > 80 due to the presence of background ions below this mass range. Sensitivities of < 1 pg/g are readily achievable for heavy elements. The technique has been used to determine with accuracy the ²³⁹Pu/²⁴⁰Pu ratio in environmental samples (Kim et al., 1989) (²³⁹Pu and ²⁴⁰Pu cannot be resolved by conventional high-resolution alpha spectrometry).

Resonance ionization mass spectrometry (RIMS) has recently been shown to be an ultra-sensitive technique for the detection of trace elements. The method is based on the stepwise excitation of atoms from a defined state to highly excited states by resonant absorption of photons followed by ionization with subsequent mass analysis of the ions. A detection limit for plutonium of 10⁷ atoms has been reported. 

8.3.3.4 Source preparation techniques in alpha spectrometry

Techniques for the preparation of sources suitable for alpha spectrometric measurements have been reviewed by Lally and Glover (1984) and include vacuum sublimation, direct evaporation using a spreading agent such as tetraethylene glycol, electrodeposition, electrospraying and electrostatic precipitation. For metrology measurements with pure solutions either vacuum sublimation, direct evaporation or electrodeposition is usually used. However, for low-level radiochemical analysis the favoured techniques are either electrodeposition of actinides onto a polished stainless steel or platinum disc from an aqueous solution containing one or more ammonium salts to which a complexing agent such as EDTA is sometimes added, or coprecipitation with rare earth metals in various media.

8.3.4 RADIOCHEMICAL SEPARATION TECHNIQUES

Radiochemical analysis differs from classical analytical chemistry in that the mass of the substance being determined is extremely small. It is usually in the sub-microgram or, indeed, the sub-femtogram range when dealing with environmental samples contaminated with artificial radioactivity. The selection of chemical techniques for the analysis of environmental matrices should be based on their ability to eliminate potential gravimetric and radiometric interferences. Other factors, such as their flexibility and suitability within a given laboratory, should also be taken into account. Two main approaches exist in the analysis of actinides in the environment, namely liquid/liquid solvent extraction and ion exchange separation. A third possibility, that of column reversed-phase partition chromatography, also known as extraction chromatography, is less widely used.

Since most chemical separations are not quantitative, a known quantity of an isotopic, non-interfering, radiometric or stable yield monitor must be added to the sample at the outset. The least desirable technique is to determine the recovery in a series of spiked samples and to use the resultant, averaged value to assess the activities of future samples. Where the recovery is to be measured gravimetrically, such factors as constant composition and hygroscopicity must be taken into account. Moreover, chemical equilibrium must be established with the determinant at the earliest possible stage and certainly before any loss of either determinant or tracer occurs (Harvey and Lovett, 1984). Details of the radiochemical separation techniques currently employed are beyond the scope of this limited review and may be found elsewhere (De Regge and Boden, 1984; Singh, 1988).

8.3.5 DETECTION AND MEASUREMENT OF BETA-EMITTING NUCLIDES

8.3.5.1 Actively shielded gas flow counting

For the low-level measurement of solid (planchet) sources anti-coincidence shielded gas flow proportional counters (or GeigerMueller counters) have been in use for many years. Typically, a large-area gas flow counter acts as a common guard detector located above a set of ultra-flat gas flow proportional counter tubes. The whole assembly is encased in a 10-cm thick shielding of interlocking lead bricks. A number of planchets can be measured simultaneously and detection limits for beta radiation (3 sigma accuracy) of 15 mBq for a 100-minute counting time using 60 mm diameter planchets are quite usual. These systems are also suitable for gross alpha measurement, in which case detection limits are significantly lower. 

8.3.5.2 Liquid scintillation counting

Liquid scintillation counting systems equipped with active shielding are now available, whose performance characteristics greatly outstrip those of conventional liquid scintillation counters. Not only does the anti-coincidence guard counter suppress cosmic radiation and much of the residual environmental gamma radiation, but it also rejects the cross-talk from Cerenkov light-induced pulses which are caused primarily by cosmic radiation in the photomultiplier tubes themselves. Careful design and the selection of low-activity components ensure that the intrinsic level of radioactivity is minimized. In one system the use of specially designed highpurity Teflon/copper counting vials is recommended as a further aid in reducing background. For example, in the case of tritium counting using a window which allows 25 per cent efficiency, backgrounds as low as 0.4 cpm (figure-of-merit, E²/B = 1850) have been reported (Schonhofer and Henrich, 1987). These systems are now used to measure a wide variety of beta- (and alpha-)emitting nuclides, including ¹⁴C, ⁹⁰Sr/⁹⁰Y, ¹³⁷Cs, ²²⁶Ra, ²⁴¹Pu and ²⁴¹Am, and are also ideally suited to Cerenkov counting.

8.3.5.3 Cerenkov counting 

Cerenkov radiation may be observed when a charged particle, such as an electron, passes through a dielectric medium at a velocity, v, greater than the velocity of light in the medium. The resulting pulse of light is called `Cerenkov emission'. Cerenkov counting has a number of advantages in the assay of radionuclides, not least in the ease of sample preparation. In the case of water, it is found that the threshold energy for Cerenkov production is 263 keV for electrons.

The intensity of Cerenkov emission, and hence the detectability, is governed by the margin by which the electron's energy exceeds the threshold value. A starting point for the evaluation of the intensity may be found in the work of Frank and Tamm (1937). Ross (1969) calculated a series of Cerenkov photon yields for electrons up to 4 MeV. He found, for example, that 89 per cent of beta emissions from ⁹⁰Y are available for Cerenkov production.

8.3.6 DETECTION AND MEASUREMENT OF GAMMA-EMITTING NUCLIDES

Traditionally, scintillation detectors, in particular thallium-activated sodium iodide (NaI(Tl)) crystals, have been used in gamma-ray spectrometry. The latter's high light yield, excellent linearity and high atomic number of its iodine constituent makes sodium iodide a very efficient and versatile tool in many applications. However, due to its limited energy resolution (~78 per cent FWHM at 662 keV), the deconvolution of complex spectra is by no means a simple matter. In the past two decades, semiconductor detectors, based initially on lithium-drifted germanium but now almost exclusively on intrinsic germanium, have taken the place of the scintillation detector in high-resolution analysis applications. Greatly improved energy resolution (1.72.3 keV FWHM at 1.332 MeV) is one of the principal characteristics of these detectors. Moreover, intrinsic (p-type) germanium detectors with efficiencies (relative to a 3 x 3 inch NaI detector) in excess of 100 per cent have now become available commercially.

Although, in general, the analysis of a sample by gamma spectrometry is considered to be non-destructive, certain sample/source preparation steps are essential for precise measurements. For example, it is necessary that the sample be completely homogenized and the measurement carried out in the same geometry as used in the efficiency calibration. The full energy peak efficiency as a function of energy must be determined experimentally using sources whose activities are traceable to national or international activity standards. Ideally, the calibration source and the samples to be measured should also have the same chemical composition and density. If this is not the case, corrections must be made for differences in the degree of self-attenuation. Corrections may also have to be made for coincidence summing, which occurs with radionuclides which emit gamma rays in cascade and which is particularly important for low source-detector distances. Chance coincidence summing (pulse pile-up) corrections may also be necessary at high count rates.

To suppress background radiation and thus improve sensitivity, all gamma detectors must be surrounded by a passive shield made from `aged' lead or pre-World War II battleship steel. Actively shielded gamma-ray spectrometers, in which the intrinsic germanium detector is surrounded by a large NaI(Tl) coincidence shield, have found application in ultra-low-level environmental studies (Hotzl and Winkler, 1981; Wogman, 1981). Greatly enhanced background reduction and Compton continua suppression are achieved when operating such systems in anti-coincidence mode. For example, Hotzl and Winkler (1981) have demonstrated that the identification and measurement of ⁸⁵Sr (514 keV) in soil samples is greatly helped by operating in AC mode as the strongly interfering annihilation peak at 511 keV is drastically reduced.

8.3.7 CONCLUSIONS

The relatively brief discussion given above is not intended to be a comprehensive review of all of the methods of analysis presently in use. It will, however, have succeeded in its purpose if it has provided an insight into the diversity and complexity of the methods employed to assay trace concentrations of radioactivity in the environment. New and improved techniques continue to be developed and the reader's attention is directed to those periodicals and proceedings in which the latest scientific and technical advances are traditionally published. Of particular interest are the Journal of Radioanalytical and Nuclear Chemistry (articles), the Journal of Nuclear Instruments and Methods in Physics Research, Health Physics and proceedings of conferences organized by international (and national) bodies such as the IAEA, ICRM, IRPA, NCRP (US), etc., in the general fields of low-level measurements and their application to environmental radioactivity studies.

8.4 GLOSSARY OF TERMS

Absorbed dose: Quantity of energy deposited from incident radiation per unit mass of absorber. SI unit: gray, symbol Gy = 1 joule per kg.

Actinaria: An order of coelenterates, the sea anemones.

Actinides: Series of 15 elements with atomic numbers 89 (actinium) to 103, and including uranium and the transuranics.

Activation: Induction of radioactivity in a stable element by irradiation, usually with neutrons.

Activation product: Radionuclide generated by irradiation of a stable nuclide. 

Activity: The intensity or strength of a radioactive source; the number of atoms disintegrating per unit time. SI unit is the becquerel, symbol Bq, = 1 disintegration per second. The earlier unit is the curie, symbol Ci, = 3.7 x 10¹⁰ Bq.

Adsorption: Uptake of a substance by physical or chemical reaction on the accessible surface of a solid, or at a liquid interface.

Aerosol: Solid or liquid particles suspended in a gas.

Alpha particle: Doubly positively charged particle, an ⁴He nucleus, comprising two protons plus two neutrons, emitted during the decay of some radionuclides.

Anion: Ion with a negative charge.

Atom: The smallest unit of an element, comprising a positive nucleus with orbiting electron(s).

Atomic number: Number of protons in the nucleus of an atom. Symbol Z. 

Becquerel: SI unit of radioactivity. Symbol Bq = 1 disintegration per second. 

Benthic: Pertaining to, or with the characteristics of, the benthos.

Benthos: The animal and plant life on the sea bottom.

Beta particle: An elementary particle emitted during the decay of some radionuclides. Usually it is the electron, the fundamental unit of negative electric charge. More rarely its charge is positive, when it is known as the positron.

Biological cycling: General term meaning biologically mediated transformations and pathways.

Biota: The flora and fauna of a given region.

Bioturbation: The mechanical disturbance of sediment by animal movement. 

Boundary layer: The layer of a moving fluid with modified dynamics immediately adjacent to a solid surface.

BWR: Boiling water reactor. Nuclear reactor in which water is boiled within the core to raise steam.

Cation: Ion with a positive charge.

Cephalopoda: A class of marine molluscs with the foot modified into tentacles, with or without shell, e.g. squid, octopod, nautilus.

Cladding: The material, usually a metal, forming the fuel can in a nuclear fission reactor, resistant to chemical and physical attack and preventing fuel corrosion and fission-product release.

Collective effective dose: Product of the mean effective dose to a group or population from a given source and the number of individuals in the group. Sl unit; person-sievert.

Colloid: A state of subdivision of matter, with particle sizes ranging from molecular dimensions up to one micron (10^-6 m).

Committed equivalent dose: Integral of equivalent dose-rate in a given tissue or organ over time following an intake of radioactive material.

Control rods: Rods of neutron-absorber for controlling the power of a nuclear fission reactor.

Core: That part of a nuclear reactor in which the fission process takes place. 

Coriolis: An effect or `force' deflecting the path of a moving body as a result of the Earth's rotation.

Critical group: Sub-group of the public most affected by a given release of radioactivity.

Daughter: Stable or radioactive nuclide resulting from the decay of a parent radionuclide, sometimes down a chain of radioactive daughters.

Decay: The transformation of a radionuclide, usually by the emission of radiation, into a more-stable nuclide (although this may be via less-stable intermediate daughters).

Decay-corrected fallout: Measured contamination corrected for decay to give activities at time of initial fallout.

Decontamination: The removal or 'clean-up' of radioactivity. 

Deposition velocity: The ratio of the activity deposited per unit surface area per second to the activity per unit volume of air in contact.

Diagenesis: Chemical modifications taking place over time in buried sediments by interaction with pore waters.

Discharges: Releases of effluents (gaseous, aerosol or liquid) from industrial operations.

Dispersion coefficient: A measure (m² s^-1) of hydrodynamic dispersion resulting from molecular diffusion and processes associated with the flow of water.

Distribution coefficient: The ratio of the concentrations of a given solute dissolved in two immiscible liquids at equilibrium.

Dose rate: Absorbed dose per unit time.

Eddy diffusion: Rapid diffusion in turbulent flow by the mixing of eddies. 

Effective dose: Sum of weighted equivalent doses in all tissues and organs in the body.

Electron: A stable elementary particle, the basic unit of negative electric charge. 

Element: Substance comprising only atoms of the same atomic number.

Enrichment: Process of increasing the proportion of a chosen isotope of an element. 

Equivalent dose: Product of the absorbed dose averaged over a tissue or organ and the quality factor of the radiation. SI unit is the sievert, symbol Sv.

Fast reactor: Type of nuclear reactor utilizing high-energy (i.e. fast-moving) neutrons to cause fission.

Fermentation: The decomposition of organic substances by micro-organisms or enzymes, usually under anaerobic conditions and evolving heat and gas.

Fission: The exoenergetic break-up of the nucleus of an atom into two or more smaller nuclei. It may be spontaneous or follow the absorption of an energetic particle, usually a neutron in a fission reactor, e.g. ²³⁵U +¹n = ⁹⁵Mo +¹³⁹La +2¹n + 200 MeV.

Free energy: Thermodynamic potential; the capacity of a system to perform work. 

Fucus: A genus of branched flat-fronded seaweeds.

Fuel element: Rod or pin of clad fuel, or an integral assembly of such rods. 

Fuel, nuclear: Fissionable material, such as ²³⁵U, processed into suitable form for reactor use.

Gamma ray: Photon of high-energy (= short-wavelength, < 0.01 nm). electromagnetic radiation emitted in some radioactive decay processes.

Half-life: Time in which half the nuclei of a given radioisotope disintegrate.

Halocline: Zone where salinity changes markedly with ocean depth. 

High-level waste: Highly radioactive heat-generating fission and activation products from the reprocessing of spent nuclear fuel.

Holothurioidea: A class of the phylum Echinodermata, the sea cucumbers.

Humic: Pertaining to the humus, the biologically generated organic soil fraction. 

Hypogene: Geological processes within or below Earth's crust.

Intermediate-level waste: Radioactive waste from nuclear plant, etc., having an activity too high for direct discharge or simple burial, i.e. greater than 4 x 10⁹Bq of alpha per tonne, or 1.2 x 10¹⁰Bq betagamma, but not generating sufficient heat to be classified as high-level waste.

Ion: Electrically charged atom or group of atoms.

Ionization: The adding or removing of electrons(s) from a neutral atom or molecule to make an ion.

Isohalines: Profiles joining points of equal salinity. 

Isotopes: Nuclides of a given chemical element which, while having the same number of protons in their nuclei, have different numbers of neutrons and hence different atomic masses.

Langmuir circulation: A type of wind-driven cellular circulation pattern, observed in the surface layers of lakes and oceans. Cell convergence often marked by visible lines of buoyant particulate matter.

Leaching: Removal of the soluble component of a porous solid by the permeation of water.

Limnetic: Pertaining to the pelagic zone (upper and middle depths) of a freshwater body.

Lithostatic: Pertaining to ground pressure, arising from superimposed strata or rock movement.

LWR: Light water reactor. Nuclear reactor using ordinary (`light') water as moderator and core coolant.

Marl lakes: Lakes in which calcium carbonate is precipitated, generally through biological mediation.

Mass number: Total number of protons plus neutrons in the nucleus of a given atom. Symbol A.

Meromictic: Pertaining to permanently stratified lakes.

Moderator: Material used in the core of a fission reactor to reduce the energy (velocity) of the neutrons generated by fission.

Molecule: The smallest particle of a compound capable of independent existence while still retaining its chemical properties.

Natural radiation: Radiation in the environment from natural radioactive elements and cosmic rays.

Neutron: Uncharged elementary particle of approximately unit atomic mass. 

Nucleus: The positively charged core of an atom, composed of protons and neutrons.

Nuclide: An isotope of an element, characterized by its unique total and ratio of protons and neutrons. (Sometimes, in addition, by the energy state of the nucleus.)

Parent radionuclide: A radionuclide which yields a daughter nuclide on disintegration.

Photic: Responding to light.

Phytoplankton: Passive or weakly motile floating plant life.

Pleistocene: A recent geological epoch extending over the last 23 million years up to the Holocene, 10 000 years BP.

Primary production: Biomass per unit area produced by photosynthesis (or less commonly chemosynthesis).

Purex: Chemical process for the separation of uranium and plutonium from spent nuclear reactor fuel.

Quality factor: Factor to take account of the different degrees of damage to tissue by different types and energies of radiation.

Radioactivity: Emission of particles or electromagnetic radiation from the nuclei of unstable atoms.

Radiobiology: The study of the use, involvement or consequences of radioactivity in biological systems.

RBMK: Russian design of nuclear reactor with graphite moderator and ordinary (`light') water boiling directly in the core to raise steam.

Reprocessing: Separation of spent reactor fuel into unused uranium, plutonium, and waste fission products, usually by chemical methods.

Salps: Small pelagic filter-feeding gelatinous invertebrates of the families Salpidae and Doliolidae.

Shear stress: (1) The force deforming a solid body by displacing a plane relative to parallel planes; (2) the drag exerted on a boundary by contiguous fluid flow.

Shear velocity: A parameter (`fictitious' velocity) whose square equals the quotient of shear stress and fluid density.

SI units: Système Internationale, the agreed system of scientific units based primarily on the kilogram, metre, second, ampere, kelvin, mole and candela. 

Smectite: A series of clay minerals, such as montmorillonite and saponite, with high absorptive and ion-exchange properties.

Sorption: A general term for the processes of adsorption, absorption, and persorption.

Spent fuel: Nuclear fuel in which neutron-absorbing fission products have accumulated sufficiently to inhibit the chain reaction.

Stratigraphic: Of the form and relationships of sedimentary rock strata. 

Telluric: (1) Pertaining to Earth; (2) derived from or containing the chemical element tellurium.

TR: (Tritium ratio) represents one ³H atom in 10¹⁸ atoms of ¹H or an activity of 0.118 Bq kg^-1 in water.

Tritium: Hydrogen isotope, ³H, with nucleus of one proton and two neutrons. It is radioactive with a half-life of 12.4 years.

Void coefficient: Quantifies the effect of voids (vapour bubbles in liquid moderator or coolant) on the power level of a nuclear reactor.

Zooplankton: Microscopic animals moving in aqueous ecosystems.