Radioactivity and the Meaning of (Half) Life


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Following the incident at the Fukushima Daichi plant, anyone living in the areawhere the4 received dose was estimated to be in excess of 20mSv in the first year was advised to leave the area. Image: US Department of Energy

Following the incident at the Fukushima Daichi plant, anyone living in the area where the 4 received dose was estimated to be in excess of 20mSv in the first year was advised to leave the area. Image courtesy of the US Department of Energy.

The incident at Fukushima has raised concerns about the long-term environmental impact of radioactive contamination.

There is often the perception that such contamination could last for thousands of years.

So leaving aside the hype and hysteria, what is the real risk from radioactive contamination – and what does ‘half life’ mean when it comes to earth science?

What is Radiation?

Radiation is the name given to the energy arising from electromagnetic waves or high-speed particles.

Ionizing radiation is capable of breaking the bonds between a nucleus and its electrons (creating ions – hence the name). There are several types of ionizing radiation, including:

  • Alpha particles – two protons and two neutrons (in other words a helium nucleus). Outside of the body, they cannot do much harm –  these particles are unable even to break through the skin. However, if this radiation is emitted within the body (as a result of inhaling or ingesting an alpha-emitter), it can cause serious damage to internal organs.
  • Beta particles – these are fast-moving electrons. They can penetrate human skin (causing a an effect similar to sunburn, though longer-lasting), but simple protection (even from normal clothing) will stop them.
  • Gamma radiation – this is high-energy electromagnetic radiation. It can penetrate deep into the body (and even pass the whole way through), and can only be stopped by using extensive shielding.

There are other types of ionizing radiation (such as neutron emission, X-rays or cosmic rays), but these are less likely to occur in a contamination incident. We can measure ionizing radiation in a number of different ways, two of the most important being:

Activity: is a measure of the amount of radiation that is released (or the amount of isotope decayed) from a sample in a given time period. Activity can be measured in Bequerels (Bq), or kilobequerels (kBq).

Effective Dose: measures the effect of radiation on an organism. It is measured in Sieverts (Sv), or more often millisieverts (mSv).

Conversion from activity into effective dose is far from straightforward, as it needs to take into account a number of factors, including:

The source of the radiation – different elements and isotopes emit radiation at different energies.

The type of radiation – within the body, alpha particles are much more damaging than beta/gamma.

The method of exposure – particles that have been inhaled or ingested will deliver a more concentrated dose.

Radiation’s Half-Life Conundrum

The half-life of an isotope represents the time taken for half of a quantity of an isotope to decay. The remainder will decay further over future half-life periods.

The half-life is therefore an inverted exponential measure of activity, meaning that the most active isotopes have the shortest half-lives.

We can look at an example from a grassland meadow (previously used for animal forage) near Fukushima.

Cesium-137 activity (half-life approx. 30 years) in the plants was measured just after the incident at 15,000 kBq per kg. After the first half-life period (30 years), we would expect the activity to reduce to 7,500 kBq per kg, and 3,750 kBq per kg after a further 30 years, It will take 5 half-lives (150 years) before the activity level fell below the legal limit in Japan for animal forage (500 Bq per kg).

Examples of half-lives include:

Radon is a gas produced from the decay of Radium (itself a product of Uranium decay). It’s produced naturally from Uranium-bearing rocks, and the longest half-life is for Rn-222 at just under 3 days.

Cesium is a common component of radioactive fallout. Cesium-134 has a half-life of  2 years plus, while cesium-134 has a half-life of just over 30 years.

Carbon occurs in living (and dead) organisms, as well as inanimate materials. C-14 has a half-life of just under 6,000 years, and its relative abundance is responsible for its role as a commonly-used dating method.

Uranium in nuclear fuel rods contains two isotopes: U-235 (half-life 700 million years), and U-238 (4.5 billion years).

Hold on, I hear you say. If a long half-life indicates a less active isotope, what’s happening with Uranium?  It’s true – Uranium decays very slowly, but the radiation in a nuclear reactor is produced via a different process: fission.

Where Does Radiation Come From?

Some elements can occur in different isotopes, where the number of protons in the nucleus are the same, but with different numbers of neutrons. Some of these isotopes (particularly those of heavier elements) are unstable, and will decay to form a different isotope. In doing so they will emit radiation.

An example of radioactive decay, by the Cesium isotope Cs-137. The result is stable Barium (Ba-137), with the emission of beta and gamma radiation. Image: Tubas-en

In a nuclear power plant, these unstable isotopes are generated as a by-product of the power generation process.

If a leak occurs (or a catastrophic explosion), it is these isotopes that are released, where they can be distributed by wind, sea and river currents. They can also be accumulated into plants and animals, and therefore find themselves into the human food chain.

Are There Safe Limits to Radiation?

Ionizing radiation also occurs naturally. In the UK, the average human dose is 2.7 mSv (6.2 mSv in the US).

This can be influenced by food (0.005mSv from a 135g bag of Brazil nuts), transport (0.07 mSv for a transatlantic flight), and geology (the annual dose rises to 7.8 mSv for people living in Cornwall, where the rocks contain uranium, and release radon gas).

UK legislation sets a maximum limit of 20 mSv above background per year (equivalent to the dose from 2 whole-body CT scans), for the amount of radioactivity that a nuclear industry worker is allowed to receive. The same legislation states that members of the public should be exposed to no more than an additional 1 mSv per year.

Following the Chernobyl incident, the radioactive contamination of upland grazing land in Northern England prompted the UK government to ban the consumption of sheep meat that contained radiation above 1000 Bq per kilogram. This ban remained in place for 26 years. Japan has set a tighter limit of 500 Bq per kg on plants grown in the Fukushima area for animal forage.

As an illustration, consider a 250g meat portion that is contaminated with Cesium-137 (a beta/gamma emitter) at 15,000 Bq per kilogram (the initial level measured in Fukushima grassland). If such meat is eaten in two meals per week, this could increase the effective dose by up to 5.25 mSv during the first year. This would therefore double or treble the background dose, depending on the area.

It should be stressed that this is just an illustration: Contamination of this level would certainly render the meat as unfit for human consumption.

What are the Effects of Radiation?

While it’s beyond the scope of this article to look in detail at the medical effects, they will generally depend upon the amount of radiation, and the time span of exposure.

Sudden exposure to a large radiation dose will lead to radiation sickness. The symptoms begin at around 100 mSv, when changes can be detected in blood chemistry. Higher doses will lead to nausea (at 1000 mSv), and eventually death (50% probability at 5000 mSv). Exposure to these levels may occur within a power plant during a catastrophic accident, but are unlikely to be experienced in the wider environment.

At doses lower than these (particularly over a prolonged time), exposure could increase the  risk of cancers, resulting from DNA damage. However this increase needs to be quantified in the context of the overall risk of cancers.

UK figures indicate that between 20 and 25% of the population will be diagnosed with cancer at some point in their lives. This includes around 1% that are linked to radiation – most of which will be from natural sources. The contaminated food example used above could therefore increase the risks by a further 1 or 2%. This is far outstripped by other risk factors, including smoking, poor diet and over-exposure to sunlight.

What Does All This Mean?

A serious incident (such as Fukushima) could release sufficient contamination to require close monitoring for 150 years or more. Of course, this assumes that no other remedial action is taken to try to remove radioactive material from the site – but that is the subject of another article.

The ionizing radiation from contamination incidents can therefore result in in an increase in the cancer risk factors, particularly when the contamination is inhaled or ingested. This risk increase has to be considered alongside the much greater risks that already occur through environmental factors and lifestyle choices.

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