After the Chernobyl nuclear power station exploded in 1986 a radioactive isotope, caesium-137, fell on northern England and Wales. At one place the activity of a soil sample was 64 Bq in 1986. The radioactivity due to caesium-137 was expected to fall to 32 Bq in 30 years. The level of radioactivity in the soil is higher than predicted. Discuss the factors that may have influenced the accuracy of this prediction.​