Radioactive Sheep

Justin Buck
March 23, 2017

Submitted as coursework for PH241, Stanford University, Winter 2017


Fig. 1: Sheep and lambs graze upland pastures in Wales. (Source: Wikimedia Commons)

On April 26, 1986, a nuclear reactor in Chernobyl, USSR exploded, releasing a cloud of radioactive material. A week after the explosion, a series of storms deposited significant levels of radioactive material from Chernobyl to certain areas in Wales, Scotland, and Cumbria. Two isotopes of Caesium that were deposited from the Chernobyl reactor fallout, Cs-134 and Cs-137, were of particular concern to public health officials. [1] Inhalation or ingestion of Cs-137 increases the risk of various types of cancers, and has an alarming long half-life of roughly thirty years. [2] In the UK, officials use the becquerel (Bq) as the base unit of radioactivity in an area. Prior to the accident in Chernobyl, levels of Cs-137 in the UK hovered around 5 Bq/m2. Immediately after the accident, officials measured levels of Cs-137 as high as 1785 Bq/m2 in some areas of Wales. [1] However, months after the Chernobyl fallout, officials measured Caesium deposition from rainfall, rather than vegetation, and discovered that areas in Scotland received an astounding 20,000 Bq/m2. [3]

Following the heavy rainfall and deposition, environmental officials became increasingly concerned with the ability of livestock in the affected areas to ingest the radioactive material while grazing, and passing the radioactive material to humans. As the grazing patterns of sheep expose them to the highest risk of radioactive exposure of all animals used by citizens, officials began testing flocks of sheep for radioactive contamination in high risk areas, such as the Welsh sheep in Fig. 1. [4]

The UK Ministry of Agriculture, Fisheries, and Food Reacts

Following the high becquerel levels in North Wales, Cumbria, and Scotland, officials from the UK Ministry of Agriculture, Fisheries, and Food (MAFF) began testing sheep in contaminated areas. Initial testing was performed by removing roughly 1 kg of muscle from a slaughtered sheep and subjecting it to gamma-spectrometry analysis. Testing showed that sheep from many of the areas initially suspected of being severely contaminated exceeded MAFF's reaction threshold of 1000 Bq/kg. [1] This threshold was originally established under Article 31 of the Euratom Treaty by experts in Luxembourg on May 23, 1986, and was subsequently adopted as the standard by MAFF. [5]

The initial testing methods posed problems for MAFF. Transporting, arranging, slaughtering, and disposing of the sheep at the Ministry's laboratory proved too rigorous to perform on every sheep that required testing. Furthermore, the widespread slaughtering of affected sheep was extremely wasteful. In response, MAFF was able to develop NaI detectors that were portable and able to accurately measure the caesium levels in live sheep. The devices allowed the ministry to test 28,000 sheep in the first two months of using the NaI detector, a significant improvement from the 2800 sheep they were able to test over three months using gamma-spectrometry methods. [4]

MAFF issued a 3-week ban on slaughtering and the movement of sheep in Cumbria and Wales starting on June 20, 1986. The ban was issued across 5000 farms and restricted 2.5 million sheep from being slaughtered or moved. [1] MAFF claimed that the half-life of caesium in sheep is 25 days, so roughly half of the caesium currently in sheep would be gone in 21 days. They also claimed that within the three week window, new grass would replace the contaminated grass. However, four days after the ban was announced, testing showed that the caesium in certain sheep was not decreasing, and was in fact increasing in some sheep. [3] The ban was extended indefinitely until proper testing could be performed on all affected farms. During the 21-day window, approximately 1750 farms showed becquerel levels less than the 1000 Bq/kg threshold and were taken off the restricted list. By October of 1986, 315 farms remained on the restricted list. [1]

Upland vs. Lowland Sheep

After the ban on sheep movement and slaughter, caesium levels remained higher than many experts were predicting in upland areas of the United Kingdom. Sheep that were grazing in exclusively upland areas persistently showed Caesium activity levels above the 1000 Bq/kg threshold for safe consumption. In 1987, researchers funded by MAFF tested the effects of moving upland sheep to lower lands. The researchers tested Cs-137 and Cs-134 levels before and after restricting the sheep to lowland pastures. Results showed that more than 80% of the original caesium was no longer in the sheep after 40 days. Furthermore, the transfer coefficient of caesium to lamb muscle was 0.79 day/kg for upland sheep, roughly six times the transfer coefficient measured in previous MAFF studies in lowland farms. Further studies showed that the upland pastures had significantly higher Cs-137 levels, and areas prone to high water accumulation were also associated with higher Cs-137 levels. [5]


While the widespread effects of the Chernobyl fallout were unforeseeable for the period, government entities should be better prepared to serve the public when there is a possible health risk stemming from nuclear energy issues. Further research is required to understand the long-term effects of the Chernobyl fallout on non-participating countries and how to best prepare for similar accidents in the future.

© Justin Buck. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.


[1] W. A. Kerr and S. Mooney, "A System Disrupted - The Grazing Economy of North Wales in the Wake of Chernobyl," Agr. Syst., 28, 13-27 (1988).

[2] J. E. Hoeve and M. Z. Jacobson, "Worldwide Health Effects of the Fukushima Daiichi Nuclear Accident," Energy Environ. Sci. 5, 8743 (2012).

[3] B. Wynne, "Sheepfarming After Chernobyl: A Case Study in Communicating Scientific Information," Environment 31, No. 2, 10 (1989).

[4] B. Walters, "Chernobyl Derived Activity in Sheep: Variation within a Single Flock and With Time," J. Environ. Radioact. 7, 99 (1988).

[5] B. J. Howard et al., "A Comparison of Caesium 137 and 134 Activity in Sheep Remaining on Upland Areas Contaminated by Chernobyl Fallout With Those Removed to Less Active Lowland Pasture," J. Soc. Radiol. Prot., 7, 71 (1987).