The explosion of reactor number four of the Chernobyl nuclear power plant in 1986 is widely regarded as the worst radiation disaster in human history. The radioactive fallout spread from Northern Ukraine throughout Northern Europe, dispersing large quantities of radioactive elements, including two caesium isotopes, Cs-134 and Cs-137. In the United Kingdom, this radiocaesium-laden cloud mingled with heavy rain falling in mountainous areas of North Wales and Cumbria, depositing substantial quantities of radioisotopes in uplands areas and introducing radioactivity into the food chain: plants took up the radiocaesium deposited in topsoil, livestock ate the plants and humans ate the livestock. In response, the Food Standards Agency (FSA), the independent authority that makes sure all food in the UK is safe and hygienic, implemented a series of laws restricting the movement, sale and slaughter of livestock grazed on contaminated uplands pastures. For sheep, a Summer monitoring program was designed to remove highly radioactive animals from the food chain.
More than 25 years later, a government report released in November 2011 showed that although environmental radiocaesium still persists in Welsh and Cumbrian farms, levels found in sheep have gradually declined over time, with recorded measurements (on average 0.09 mSv per year) now well below the safe limit established by the ICRP (1 mSv per year). No radioactively unsafe animals have been identified for several years in Cumbria, and less than 0.5% (out of ~75,000 sheep) in North Wales. This is partly due to the common farming practice of fattening up sheep for several months on tasty lowland protein-rich, clover-heavy pastures before taking them to market. Lowland grasses contain much less radiocaesium, and since its biological half-life (the time required for the amount of active radionuclide to be reduced by 50%) is only 10-20 days, lengthy periods of grazing in these areas allows the natural process of radioactive decay to lower whole body levels.
Mathematical modelling estimates that even extremely hungry people who eat more than 25 kg of delicious sheep products per year couldn’t begin to approach unsafe levels of radiation exposure, thus all Chernobyl livestock legislation was recently fully revoked in the UK. While other areas of Europe, such as Scandinavia, continue to record substantial levels of radioactivity in stock animal populations, the UK has provided happy evidence that the impact of Chernobyl radiation is diminishing.
You can read the original FSA report in full here.
Field, A. (2011). An Assessment of Radiocaesium Activity Concentrations in Sheep in Restricted Areas of England and Wales and Potential Consumer Doses Food Standards Agency