|Radiation Information Network's||Tritium Information Section|
Tritium is a radioactive form of hydrogen, used in research, fusion reactors and neutron generators. The radioactive properties of tritium are very useful. By mixing tritium with a chemical that emits light in the presence of radiation, a phosphor, a continuous light source is made. This can be applied to situations where a dim light is needed but where using batteries or electricity is not possible. Rifle sights and exit signs are two examples of where this phenomenon is commonly used. The phosphor sights help increase nighttime firing accuracy and the exit signs can be life saver if there is a loss of power. The radioactive decay product of tritium is a low energy beta that cannot penetrate the outer dead layer of human skin. Therefore, the main hazard associated with tritium is internal exposure. In addition, due to the relatively long half life and short biological half life, tritium must be ingested in large amounts to pose a significant health risk. In keeping with the philosophy of ALARA, internal exposure should be kept as low as practical.
TRITIUMTritium, as a form of Hydrogen, is found naturally in air and water. Most hydrogen is made up of one proton, and an orbital electron, but tritium has two extra neutrons in the nucleus. In nature, it is produced by cosmic rays in two source terms:
All atoms are composed of a center nucleus surrounded by shells of electrons. The tritium atom (3H) is unstable because it has two extra neutrons in its nucleus. These neutrons give tritium an excess amount of energy. Because of this, the atom will undergo a nuclear transformation or radioactive decay. In this, the atom emits two radiations: a beta particle (ß -), which is similar to an electron, and an anti-neutrino.
The beta is non-penetrating with a maximum energy of 18.6 keV and an average of 5.7 keV. This is a low energy beta compared to most radioactive beta emitters and it can be easily shielded. The outer layer of dead skin is enough to stop all of the beta external of the body. Only if tritium is taken into the body can it produce a significant dose.
Tritium has a single electron the same as the more abundant forms of hydrogen. This causes tritium to react chemically to form compounds in the same manner as hydrogen. The two primary forms that personnel will likely to be exposed to are HT (which is similar to hydrogen gas) or HTO (tritiated or heavy water). Of these two forms, the HTO is the only form that is a significant exposure hazard. HT gas is inhaled and exhaled with only of 0.005% of the activity being deposited in the lungs. The uptake of HTO vapor is near 100% for inhalation and ingestion. Tritium can also enter the body by absorption through the skin or open wounds. Skin contact should always be minimized to prevent absorption. Tritium will also be absorbed into materials such as gloves, clothing and metal. If not properly controlled, these contaminated materials can present an additional exposure source by releasing tritium when in contact with skin.
HTO is in the form of water, so one to two hours after an uptake, it will be evenly distributed through out the body's fluids. The amount of time it takes for half of the activity to be physically removed form the body is the biological half life. The biological half life of tritium varies significantly because of variations in bodily excretion rates, temperature dependence and fluid intake. Biological half-life of tritium is about 9.4 days, often rounded to 10 days. This can be shortened to 2-3 days (Fig 1) with ten fold increase of liquid intake (2 liters to 20 liters), or in serve cases to 4-8 hours by using dialysis machines.
Figure one. The percent of tritium left in a human based on removal half-life of 10 day (average for humans) and 3 days (based on increased water intake).
Methods of reduction of tritium in the body must be weighed against the potential harm that the tritium will cause. Any treatment should be based on known levels of uptake and made in consultation with medical personnel.
Dose from TritiumThe dose of tritium is dependent upon how much was initially ingested and the resident time in the body. Tritium will equilibrate through out the fluid compartments of the body and deliver the dose to the whole body. Taken form the National Council on Radiation Protection (NCRP) report 30, the Annual Limit for Intake (ALI) is 80 mCi and the Committed Effective Dose Equivalent (CEDE) in soft tissue is 64 mrem per millicurie (mCi) ingested. The ALI is the amount of activity required to receive a dose of 5 rem of equivalent whole body dose for the year. To use the given CEDE dose factor to calculate the dose, estimate the amount of tritium initially deposited in the body, and divide by 1 mCi/64 mrem. The ALI and the CEDE factor are based on the biological half life of 10 days. As an example of using the CEDE factor:
While there are several methods available to calculate the dose based on bioassay measurements, the following are from the DOE's Health Physics Manual of Good Practices for Tritium Facilities.
The dose rate, R(to), to an individual based on a bioassay concentration Co is:
An example of this would be if a worker had a urinalysis that indicated 1000 µCi/L, then they would be receiving an initial dose rate of 0.19 rem per day. So, if you would have a constant level of tritium in your system, you could simply calculate your dose (assuming your an average person) by:
Where lambda (up-side down "Y") is the elimination constant (ln(2)/biological
half-life). As shown in Figure 1 though, most of the dose is received in
the first month following exposure.
Gun SightsSome rifles use tritium for in their front sight. They will use about 12 mCi of tritium dissolved in a phosphor liquid contained in a small glass vial. Occasionally, a sight may develop a small leak or be completely broken and pose at least a potential for internal exposure. A small amount of the total activity could be transferred to the hands of the security personnel or armorer, then ingested orally or absorbed through the skin. The use of gloves can reduce the risk of exposure. The amount absorbed through the skin would probably be small compared to the amount ingested. Other likely pathways might be a localized cloud of HTO vapor if the sight were damaged and stowed in an air tight locker. The air space of the locker could reach equilibrium conditions with the tritium. If the locker where the rifles are stored has some ventilation, then that would be enough to dissipate the tritium.
Tritium in WatchesSee: The Use of Tritium in Plastic Watches
Tritium in Exit SignsTritium is used in exit signs to light the exit in the event of an electrical outage or a fire. Signs often have several curies of tritium in them. If the exit signs were severely damaged, HT gas might escape into the local area, but it should be dispersed by ventilation or wind quickly. The damaged sign would be expected to have relatively high levels of tritium on it, and should not be handled.
RisksThe risks from tritium are small, due mostly to:
To help evaluate the potential risks from tritium exposure, consider the following made-up scenarios:
TablesTable 1. Tritium characteristics
Other Sources of Information on Tritium:
Background and Suggested Further Reading
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