Tritium - The back story

On 26th February, 2017, The Age published a story with the original headline '$1bn clean up bill for Werribee's radioactive 'zombie; cows', but now modified to ' ... Now meet Werribee's radioactive cows.' at The story about Tritium as a radioactive tracer contained some correct factual content, but there were factual errors and omissions of relevant content.

A recent article in The Age, titled' ' referred to the use of radioactive Tritium in animal research. The article did have some correct information, but there were factual errors and it omitted other relevant information. The language used was also quite exaggerated and sensationalist, suggesting that the article was either a beat up or an example of fake news.

Outlined below is information teachers might like to refer to when mentioning Tritium or the article in question.

Decay Details

Tritium is an isotope of Hydrogen. The nucleus has two neutrons and one proton and Tritium is radioactive. It is commonly written as 1T3.

Its half life is 12.3 years. It decays by a neutron decaying into a proton, an electron and an anti neutrino, producing Helium 3.

1T3 → 2He3 + -1e0 + υ

The decay releases 19 keV of energy, with the emitted electron, or beta particle, having an average of 5.7 keV. At this energy, the electron does not penetrate the outer layer of human skin, but it is a risk if inhaled or ingested. This low energy also makes it difficult to detect the beta particles, the particle cannot penetrate the thinnest mica window of ordinary counting equipment. Liquid scintillation and gas counting methods are needed.

Production

Tritium doe not exist naturally on Earth. It is produced when cosmic rays collide with atoms in the upper atmosphere. The principal reaction is

7N14 + 0n1 → 6C12 + 1T3

It is also produced in nuclear reactors when Lithium is bombarded with neutrons.

3Li6 + 0n1 → 2He4 + 1T3

A large amount of Tritium was released into the atmosphere as a result of the atmospheric testing of nuclear weapons. For example, the ratio of Tritium to Hydrogen in rain increased from 8 x 10-18 in 1953 to a peak of 1,240 x 10-18 in 1954. It is projected that it will not be until 2030 that the Tritium produced by this means will decay to a level comparable with the Tritium produced by cosmic rays.

Exposure:

The US EPA has a Maximum Contaminant Level (MCL) of 4 millirems per year (40 microsieverts per year) for beta particle and photon radioactivity from artificial radionuclides in drinking water. To produce this level of exposure the concentration of Tritium would need to be 20,000 picoCuries per litre, which is 740 Becquerels per litre.

Other countries have different levels:

Country / Tritium limit (Bq/l)
Australia / 76000
Finland / 30000
WHO / 10000
Russia / 7700
Canada / 7000
EU / 100

The value for Australia is calculated from the accepted standard of 1 mSv per year. The dose in milliSieverts per Becquerel for Tritium is 1.8 x 10-8 . In comparison, data for some other radioactive elements is in the table below.

Source / mSv/Bq
Carbon 14 / 5.8 x 10-7
Potassium 40 / 6.2 x 10-6
Americium 241 / 2.0 x 10-4
Strontium 90 / 2.8 x 10-5

Source: page 121

The annual dose from an individual radionuclide consumed in water is calculated using the following equation:

Annual dose (mSv/year) = dose per unit intake (mSv/Bq) x annual water consumption (litre/year) x radionuclide concentration (Bq/L)

Assuming the average Australian consumes 730 l of water each year the tritium exposure is about 1mSv per year.

Uses:

Tritium is used as a radioactive tracer. The reasons for this are:

  • The energy of the radiation is quite low, so the risk is less,
  • The half life is long enough for manageable monitoring, but not so long as to be a long term storage problem or environmental risk,
  • As an isotope of hydrogen, it can be introduced into a plant or animal as part of a water molecule, called tritiated water or as part of an organic molecule, although tritiated water is the common means.
  • It does not accumulate in living things. In humans the biological half life of Tritium in tritiated water is 7 to 14 days.

Tritiated water (HTO) is used to study the absorption of water by plants and its distribution within the plant. In animals, Tritium is used with peptides to study the binding of peptides to receptors. If one Tritium atom is added to each molecule, then the activity of the samples are of the order of about 50 Curies per millimole. In a sugar study, Tritium labelled sugars had readings ranging from 1 to 30 microcuries per milligram depending on the sugar.

Tritium is also very useful in hydrology when investigating water circulation patterns. The Tritium content of rain, snow, rivers, lakes, springs, wells, artesian basins and oceans enables the investigation of the age of different water resources, how water circulates within a continent and also the ocean mixing rates.

Tritiated water can be purchased from Perkin Elmer. The cost is AUD$140 for10 ml. The water is rated at 2.5 x 106 DPM/g, which stands for disintegrations per minute per gram.

Fusion

Because it has 2 neutrons, Tritium is considered as a possible fuel for a fusion reaction.

1T3 + 1D2 → 2He4 + 0n1

Radioluminescence

The beta particle can excite phosphors to glow. It is used in emergency exit signs, watches and military instrumentation.

Inconsistencies in the article:

  • The by-product of Tritium decay is not, as described, radioactive
  • Given the short biological half life of Tritium, was there any Tritium left in the bodies of the cows when they died or were killed?

Information not provided:

  • What was the initial dose of Tritium administered to the cattle?

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