How much Iodine-131 remains?

The television drama Chernobyl inspired a series of articles in the August 2019 issue of Physics World. One of the articles was an interview with Richard Wakeford, editor-in-chief of Journal of Radiological Protection, discussing why the assessment of radiation risk is so difficult.

Responding to a question asking whether access to some areas around Chernobyl will remain restricted indefinitely, Wakeford’s answer included the following statement: “At Chernobyl, the principal contaminant of concern was iodine-131. That has a half-life of eight days, so it is gone in three months.”

In this context, “gone” can be interpreted as meaning that the level of remaining decay is below the natural background radiation level rather than there being not a single atom of iodine-131 left in existence.

Use your knowledge of half-life to calculate how much iodine-131 actually remains after 88 days (a calculation-friendly estimate for three months).

HINT: Assume that there is 100% of unstable iodine-131 at the start of day one and find the percentage that has still not decayed after three months (88 days).

Email me your answer, with your working out, when you have completed this task.

Extension task… Wakeford went on to add: “Then there are the radioisotopes of caesium; caesium-134 has a half-life of around two years so that is essentially all gone now…” Given that the Chernobyl accident occurred in April 1986, estimate the percentage of caesium-134 remaining today. Using this information, determine whether the percentage threshold used for “gone” is the same when talking about iodine-131 as it is for caesium-134 and suggest what other factor may be important if you find that the percentage figures are not the same. Email me your findings and explanations.