A Girl’s Guide to Nuclear Nasties

It usually takes a nuclear disaster before people take an interest in nuclear energy and in the subsequent panic, a lot of misinformation starts to fly about. Here is a simple guide which will come in several parts which will hopefully help you make sense of what you hear in the press.

Radiation Measurement

Everyone knows nuclear reactors unless contained properly gives off dangerous stuff called radiation. It’s made all the more scary because you can’t see or smell it. It tends to be dangerous because it can pass through flesh (like X-rays) and do considerable damage in the process.

People will talk about what is an acceptable amount of radiation to be exposed to and what is dangerous, although all that can be said is if you look at the history of ‘acceptable’ radiation limits since World War II it tends to be revised continuously downwards.

Measuring nuclear radiation to decide whether it’s safe or not is a complex business because there are three quite different types of radiation, added to which depending on the source they can have quite different energies, which governs how easily they can penetrate stuff like you and me. The picture is made more complex by the fact that the three different types of radiation have quite different ionising capabilities ie the amount of damage that they do when the pass through flesh.

So what scientists do when they measure radiation is they use different measures depending on whether they are measuring:

radiation coming from a radioactive source, like a leaking reactor
the radiation dose absorbed by a person,
or the risk that a person will suffer health effects (biological risk) from exposure to radiation.

Because of what we discussed above about the different types of radiation which can have widely differing energies, it basically means that while there may be a lot be a lot of radiation about, but depending on it’s type and energy you may not absorb all of it and again depending on the type will determine how serious it is (biological risk)

Units of Measure

If that weren’t complicated enough scientists in the international community measure radiation using the System Internationale (SI), a uniform system of units evolved from the metric system. However in the United States, they still use the old (conventional) system of measurement the rest of the word abandoned thirty or so years ago.

Scientists tend to name physical measurement units after famous scientists although the names are not capitalized in the SI system.

For example, the amount of radiation being given off by a radioactive material is measured using the conventional unit curie (Ci), named for the famed scientist Marie Curie, or the SI unit becquerel (Bq).

The radiation dose absorbed by a person (that is, the amount of energy deposited in human tissue by radiation) is measured using the conventional unit rad or the SI unit gray (Gy).

The biological risk of exposure to radiation is measured using the conventional unit rem or the SI unit sievert (Sv).

The measurement that is of most interest to us is the Biological Risk (that is, the risk that you will suffer health effects from an exposure to radiation) measured in rem or the SI unit Sv.

To make sense of this let’s look at some common radiation Exposures. People are exposed to radiation daily from naturally occurring radioactive materials in the soil and cosmic rays from outer space (of which we receive more when we fly in an airplane). Some common ways that people are exposed to radiation and the associated doses are shown below.

Exposure to cosmic rays during a roundtrip airplane flight from New York to Los Angeles: 3 mrem or 0.03 mSv

One dental x-ray 4–15 mrem or 0.04–0.15 mSv

One chest x-ray 10 mrem or 0.1 mSv

One mammogram: 70 mrem or 0.7 mSv

One year of exposure to natural radiation (from cosmic rays, etc.): 300 mrem or 3 mSv

The prefix 'm' means milli or one thousandth of a whole unit. So you can see that the siervert and Rem are huge units, when you consider that each year you on average are exposed to 3/1000th of a sievert. Hopefully this will help you make sense of the figures being bandied about on the news. Sadly the likelihood of the Japanese workers heroically working on the damaged reactors surviving is not good.