The release and migration of gaseous carbon-14 has been identified as a key issue for geological disposal of intermediate-level radioactive wastes in the UK. A significant fraction of carbon-14 in the UK inventory is in irradiated graphite. This paper describes measurements of gaseous carbon-14 releases from irradiated graphite on immersion in alkaline solution. Apparatus has been developed to discriminate organic and inorganic ((CO)-C-14/(CO2)-C-14) species in the gas phase by means of selective oxidation and capture. In the initial experiment, small amounts of gaseous carbon-14 (similar to 4 Bq) were released from 9 g of crushed graphite within a two-week period. In a long-term experiment, cumulative releases were measured periodically from an intact specimen of graphite over a 14 month period. A small fraction of the graphite carbon-14 inventory was released to the gas phase (similar to 0.004% as CO/CO2 and similar to 0.001% associated with organic compounds). A larger quantity of carbon-14, about 0.1%, was released to the solution phase and was thought to be mainly (CO2)-C-14, with some possible organic component. In general, the rate of gaseous carbon-14 release decreased with time. The results suggest a small initial release of relatively labile, accessible carbon-14, with longer term release occurring at a much slower rate. Tritium (T) releases were also measured.

Passive diffusion samplers have now been in use in the UK for the measurement of tritium at environmental levels in the form of HTO and HTO + HT for more than ten years. At the outset their main purpose was to identify the direction of a possible inadvertent release of tritium into the environment. More recently, however, there has been growing interest in their use as stand-alone devices for tritium determination at environmental levels. This has necessitated a more rigorous design to reduce the "draughts effect" in exposed locations and has also required modifications to enable the measurement of HTO + HT.The paper describes the recent design improvements and the results of validation tests carried out against dynamic, discriminating (dry-bed) samplers.

Direct reading tritium-in-air-monitors are widely used for immediate observation of discharges or working area levels where significant sources of tritium are held. These monitors require initial and periodic calibration certification at the levels likely to be encountered We describe a purpose built gas handling apparatus which enables various activity levels of tritiated gas (H-3 + N-2) to be precisely metered and circulated in a closed loop through the monitor under calibration at a wide range of activity levels.

Dynamic air samplers, designed as part of a broader project which included sulfur-35 (presented in Chapter 20), were installed at Magnox (Bradwell and Oldbury) and AGR (Heysham) power stations and BNFL Sellafield during 1997 to study the speciation of tritium and carbon-14 in environmental air close to the discharge points. Species sampled were HTO, HT and CH3T (+ other organics) for tritium and CO2, CO and CH4 (+ other organics) for carbon-14. Samples were taken both during normal running and reactor shutdown and start-up periods. Results are presented of measurements obtained during the first twelve month study period and it is seen that speciation was observed at all sites. The distribution of the species and the implication of the results for dose assessment calculations are discussed.

Knowledge of the current 'natural' background level is important to 14C-related studies, especially dose assessments in the vicinity of and distant from nuclear establishments since its value must be subtracted from measured levels to determine local enhancement. Although measurements have been made world-wide to monitor the decline of the background level, from its peak in the early 1960s to the present, there exists a paucity of precise data for UK natural materials. Accordingly this study was carried out to establish an average figure for the 'natural' 14C level in foodstuffs in 1992 from a survey of materials collected from a wide range of sites over England and Wales, using this single year value, to infill missing years prior to 1992 and to extrapolate for some years afterwards. Results are presented from the survey, and from the derived extrapolation with comparison made with measurements from other laboratories.

The Archaeological Site Index to Radiocarbon Dates for Great Britain and Ireland, pioneered by the Council for British Archaeology in 1971, is now being prepared as a fully computerized database. This note describes the genesis and format of this invaluable new tool.

Reliable measurement of Tritium for regulatory and radiological protection purposes requires the design of samplers which are simple to operate and easy to maintain and check during operation. Where measurements of Tritium in only the oxide form suffice, passive (diffusion tube type) samplers are very effective, requiring no moving parts or power supplies for their operation. Where separate measurement of Tritium in the oxide (HTO) and elemental form (HT) is required dynamic samplers, involving pumped gas systems, are generally necessary. Designs of both type of sampler, commissioned by JET, are described and the results of one year's operation, for environmental applications, are presented and discussed.

The Nuclear Energy Cycle comprises all the stages involved in the generation of power from fission (and possibly fusion in the future), and presently includes mining and milling, fuel fabrication, its utilization during reactor running, post-reactor handling (including reprocessing) and the subsequent disposal or storage of waste. During reactor running, 14C is produced in the fuel, moderator, coolant and structural materials. A small amount (estimated at only a few percent of the total) may be produced during the fission process itself by ternary fission, but the major fraction is produced by the neutron activation reactions given in Table 32.1. The target nuclei exist in the basic components of the reactor, such as the coolant and moderator, or in impurities in the fuel and reactor materials generally. The most important contribution to the total 14C produced comes from nitrogen impurities in these materials via the familiar 14N(n,p)14C reaction. Consequently, although the activation cross-sections are well known for the neutron spectra of the different reactor types, estimation of the quantities of 14C actually produced is often complicated by difficulty in assessing precise levels of nitrogen impurity present in the various target materials (Bush, Smith & White 1984).

We have been measuring 14C in natural, biological materials growing in the vicinity of Sellafield, Cumbria as a continuing project with BNFL to understand the dispersion of releases from the site over several seasons. We have measured locally grown foodstuffs for monitoring purposes, individual tree rings to establish a chronology of releases, hawthorn berries for spatial investigations and are now carrying out controlled plot uptake experiments. We have been attempting to validate a current dispersion model (Clarke 1979) over a six-year period, and although we observe general agreement in most years, certain anomalies, which probably relate to topographical features, are leading to more detailed correlation with local meteorological data.

The use of computer data bases for storage and retrieval of 14C data is a logical application for the rapidly expanding numbers of 14C determinations. Harwell has established a data base for all samples originating from sites in the United Kingdom and Eire. The core of the data is the Council for British Archaeology's published Index of Radiocarbon Dates which we are expanding to include all Harwell UK dates released for publication by the submitters plus dates from other laboratories both within and outside the UK. As a demonstration of the feasibility of direct database-to-database communication, cooperation has been sought from Groningen and Oxford to transfer computer files containing 14C results for UK sites. Neither of these laboratories use the same system as Harwell for their in-house data base and this exercise highlights the importance of a transfer language for both the national and international schemes as it is no longer practical to carry out such procedures through keyboard typing.

Current calibration methods for single and replicate 14C dates are compared. Various forms of tabular and graphic output are discussed. Results from all the methods show reasonable agreement but further methodological development and improvements in computer output are required. Comparison of existing techniques for a series of non-contemporaneous dates showed less agreement amongst participants on this issue. We recommend that calibrated dates should be presented as a combination of graphs and ranges, in preference to mean and standard deviation.

A segment of earlier Bronze Age arable landscape incorporating isolated round barrows on the high chalk spur of Hog Cliff Hill became the chosen location for a later Bronze Age earthwork of considerable dimensions. The area excavated within the bank and ditch was densely occupied by two major phases of buildings of timber construction, lasting into the earliest Iron Age. Sometime during the early Iron Age the oval enclosure was replaced by a more substantial one which partly followed its line and contained a series of unusual structures comprising dry-stone flint banks or wall-footings. The site was subsequently abandoned, the land probably being returned to agricultural use, until the Roman period when the agger of the Roman road from Dorchester to Ilchester was constructed across the earthwork.