Post-Chernobyl monitoring and controls survey reports

In 1986, an accident at the Chernobyl nuclear power plant in the former USSR (now Ukraine) released large quantities of radioactivity into the atmosphere. Some of this radioactivity, predominantly radiocaesium-137, was deposited on certain upland areas of the UK, where sheep farming is the primary land-use.

Due to the particular chemical and physical properties of the peaty soil types present in these upland areas, the radiocaesium is still able to pass easily from soil to grass and hence accumulate in sheep.

A maximum limit of 1,000 becquerels per kilogram (bq/kg) of radiocaesium is applied to sheep meat affected by the accident to protect consumers. This limit was introduced in the UK in 1986, based on advice from the European Commission's Article 31 group of experts.

Under powers provided in the Food and Environment Protection Act 1985 (FEPA), Emergency Orders have been used since 1986 to impose restrictions on the movement and sale of sheep exceeding the limit in certain parts of Cumbria, North Wales, Scotland and Northern Ireland. The Emergency Orders define geographical areas, often termed 'Restricted Areas', within which the controls must be followed. Under the FEPA Orders, sheep with levels of contamination above the limit are not allowed to enter the food chain. Initially these restricted areas were large, but have reduced substantially as levels of radioactivity have fallen, with all restrictions lifted in Northern Ireland in 2000.

A management system known as the 'Mark and Release' scheme operates in Restricted Areas. Under this scheme, a farmer wishing to move sheep out of a restricted area can have them monitored to determine the level of radiocaesium.

A live monitoring technique is used, where an external monitor is held against the sheep, giving a count rate (becquerels per second), which is converted to a concentration (bq/kg) using a derived conversion factor. To allow for inherent variability in live monitoring results, a Working Action Level of 645 bq/kg is applied (rather than 1000 bq/kg), which has been set so there is only a 1 in 40 chance of a sheep above the limit giving a monitor reading below.

Any sheep that exceed the working action level are marked with a dye and are not released from restrictions. Those that pass are allowed to enter the foodchain.

A continuous programme of assessing the need for restrictions has been undertaken. Special surveys, termed 'de-restriction surveys', or sometimes 'summer surveys', are performed at farms where there is confidence that restrictions are no longer needed.

During these surveys, the whole flock is monitored at the time of year (mainly July and early August) that gives maximum radiocaesium concentrations. Monitoring takes place within 24 hours of the sheep being brought down from the hills, to ensure levels of radiocaesium have not started to decline. If the whole flock is below the radiocaesium limit, the farm is considered for removal of restrictions.