This section reviews the nuclear properties of tritium and discusses of some of the physical and chemical properties that are important in understanding tritium handling, containment, and contamination control.
Being an isotope of hydrogen, tritium has many of the properties of ordinary hydrogen (such as chemical reactions, permeability, and absorption). Differences may occur because the decaying tritium atoms can speed up (catalyze) reactions of undecayed tritium, or because atoms that have undergone decay have changed into helium atoms ({sup 3}He). Additionally, small differences in chemical reaction rates may result from the relative masses of the isotopes.
Some of the useful properties of tritium are listed in Table 2. Note that the properties listed are those of T{sub 2}. The specific activity and power density of HT and DT are approximately one-half those for T{sub 2}. The activity density of HT and DT is exactly one-half that of T{sub 2}.
The penetration and absorption of beta particles in a material are important factors for detecting tritium and understanding the mechanisms by which tritium can degrade materials. A beta particle interacts with matter by colliding with electrons in the surrounding material. In each collision, the beta particle may lose several electron volts (keV) [Footnote b] of energy, and the electron is stripped from its atom (ionization) or promoted to an excited state. The beta particle has a finite penetration depth that depends on its energy.
Recall that tritium undergoes beta decay according to the following equation:
The helium daughter ({sub 2}{sup 3}He) is stable, but lighter than common helium ({sub 2}{sup 4}He). The decay energy is constant (18.6 keV), but is shared between the beta particle and an antineutrino (a tiny particle). The result is that not all beta particles have the same energy. The average energy is 5.7 keV. Consequently, not all tritium betas have the same penetration depth in a given material. Where beta ranges are given, it is customary to list both the highest energy and the average, most representative energy, as listed in Table 3.
With one unimportant exception, tritium is the weakest beta emitter known. The range of the most energetic tritium beta particles is only about 5 mm in air or 0.005 mm in water or soft tissue. This range makes it a nonhazard outside the body, but presents a detection problem. Where other radioisotopes can be detected by virtue of their penetrating radiation, tritium has to be introduced directly inside the detector or counter to be measured.
Laboratories that have large quantities of tritium usually handle it in the form of HT. However, at any time the tritium may be stored on metal getter beds (such as titanium, zirconium, or uranium). These beds form weak chemical compounds with hydrogen. Some of the beds are stable in air; and others are not and can only be used in certain atmospheres. The tritium is released (or delivered) by heating the beds to the required temperature.
Laboratories may also handle tritiated gases (such as ammonia and methane) and other compounds. By far the most common of these is HTO, which is formed from HT whenever it is exposed to oxygen or water vapor. The conversion reactions are oxidation and exchange:
These reaction rates are increased by radiation (from nearby tritium at high concentrations), heat, or the presence of metal catalysts (especially palladium or platinum). All chemical reactions involving hydrogen can also be performed with tritium, sometimes at a higher rate if the tritium concentration is high enough to catalyze the reaction. One of the most important reactions occurs when a tritium atom exchanges with a loosely bonded hydrogen atom of an organic molecule. However, where HT is dissolved in water (H{sub 2}O), the exchange process is fairly slow because the hydrogen in is tightly bonded and the reaction is not catalyzed.
Tritium as HT or HTO will readily adsorb onto the surface of most metals (such as stainless steel, copper, or aluminum), plastics, and rubbers. The tritium will remain fairly close to the surface unless the metal is heated to a high temperature. At room temperature, permeation into these metals is usually extremely slow.
In the case of metal contamination, the tritium remains on or very close to the surface. The contamination can be removed with water or water vapor if the surface is contaminated with HTO or with hydrogen (H{sub 2} or D{sub 2} ) if the contamination is HT. Heating also speeds up the decontamination process. The initial application of heat to surfaces can also be used to prevent or lessen the contamination by HT or HTO. Metal surfaces exposed to high pressures of HT or HTO for extended periods, especially at high temperatures, may allow enough penetration to cause structural damage to the metal. This is especially true if the decaying tritium causes a buildup of helium within the structure of the metal.
If adsorbed onto hydrogenous material, the tritium will easily permeate into the material. The HTO will move much more rapidly into the bulk material than will HT. The permeation rate varies with the type of material and is accelerated by increasing the temperature. As a result of this movement, plastics and rubbers exposed to tritium (especially as HTO) are readily contaminated deep into the bulk material and are impossible to decontaminate completely. After a period of time, the tritium exchanges with bulk hydrogen and presents little biological risk.
Highly contaminated metal or plastic surfaces may release some of the loosely-bound tritium immediately after exposure to the contaminating tritiated atmosphere or liquid. This is referred to as outgassing. The personnel risk from outgassing tritium is generally much less than that from making unprotected skin contact with the outgassing surface.
At most tritium facilities, the most commonly encountered forms of tritium are tritium gas (HT) and tritium oxide (HTO). Other forms of tritium may be present, such as metal tritides, tritiated pump oil, and tritiated gases such as methane and ammonia. As noted earlier, deuterated and tritiated compounds generally have the same chemical properties as their protium counterparts, although some minor isotopic differences in reaction rates exist. These various tritiated compounds have a wide range of metabolic properties in humans under similar exposure conditions. For example, inhaled tritium gas is only slightly incorporated into the body during exposure, and the remainder is rapidly removed (by exhalation) following the exposure. On the other hand, tritiated water vapor is readily taken up and retained in the body water. In this Primer, we will address only those compounds likely to be found at DOE laboratories: gaseous tritium, tritiated water, other tritiated species, metallic getters, and other tritiated liquids and gases.
During a brief exposure to tritium gas, the gas is inhaled and a small amount is dissolved in the bloodstream. The dissolved gas circulates in the bloodstream before being exhaled along with the gaseous waste products (carbon dioxide) and normal water vapor. If the exposure persists, the gas will reach other body fluids. A small percentage of the gaseous tritium is converted to the oxide (HTO), most likely by oxidation in the gastrointestinal tract. Early experiments involving human exposure to a concentration of 9 {mu}Ci/mL resulted in an increase in the HTO concentration in urine of 7.7 x 10{sup -3} {mu}Ci/mL per hour of exposure. Although independent of the breathing rate, this conversion can be expressed as the ratio of the HTO buildup to the tritium inhaled as HT at a nominal breathing rate (20 L/min). In this context, the conversion is 0.003% of the total gaseous tritium inhaled. More recent experiments with six volunteers resulted in a conversion of 0.005%. For gaseous tritium exposures, there are two doses: (a) a lung dose from the tritium in the air inside the lung and (b) a whole body dose from the tritium gas that has been converted to HTO. The tritiated water converted from the gas in the body behaves as an exposure to tritiated water.
Intake of gaseous tritium through the skin has been found to be negligible compared with that from inhalation. Small amounts of tritium can enter the skin through unprotected contact with contaminated metal surfaces, which results in organically bound tritium in skin and in urine. Ordinarily this is not a serious problem because surfaces highly contaminated with tritium gas are inaccessible to skin contact. Also, most tritium exposed to air will be converted to the oxide form (water vapor) before the internal surfaces of equipment are handled during maintenance or repair operations.
The biological incorporation (uptake) of airborne HTO can be extremely efficient: up to 99% of inhaled HTO is taken into the body by the circulating blood. Ingested liquid HTO is also almost completely absorbed by the gastrointestinal tract and quickly appears in the blood stream. Within minutes, it can be found in varying concentrations in the organs, fluids, and tissues of the body. Skin absorption of airborne HTO is also important, especially during hot weather, because of the normal movement of water through the skin. For skin temperatures between 30 and 40 degrees C, the absorption of HTO is about 50% of that for HTO by inhalation (assuming an average breathing rate associated with light work, 20 L/min). No matter how it is absorbed, the HTO will be uniformly distributed in all biological fluids within one to two hours. This tritium has a retention that is characteristic of water. In addition, a small fraction of the tritium is incorporated into easily exchanged hydrogen sites in organic molecules. Hence, retention of tritiated water can be described as the sum of several terms: one characteristic of body water, and one or more longer-term components that represent tritium incorporated into organic hydrogen sites.
Most tritium handled in laboratories is in the form of tritiated gas or tritium oxide. However, tritium handling operations may form other compounds, such as tritiated hydrocarbons and metal tritides. Tritium may also contaminate surfaces and liquids such as pump oil. These materials may present special safe handling problems.
Although many metals are commonly used for gettering (chemically combining with) tritium, little information on their metabolic properties is available. Some of these compounds (such as uranium tritide and lithium tritide) are unstable in air. For these, exposure to air produces different results. Uranium tritide, being pyrophoric, releases large quantities of tritiated water; lithium tritide, a hydroxyl scavenger, releases mostly tritium gas.
Tritides of metals (such as titanium, niobium, and zirconium) are stable in air. For particles of these tritides, the primary organ of concern is the lungs. Some of the tritium may leach out in the lung fluids and then be incorporated into the body water. These particles may also produce organically bound tritium from contact with lung tissue, which would further complicate the metabolic process. However, in laboratories where such tritiated metals are handled, the possibility for exposure to airborne particulates of these metals is extremely remote except in accident situations.
Next to HTO, the most common tritiated liquid is tritiated vacuum pump oil. Experience at DOE facilities has shown that the specific activities of pump oils can easily range from a few mCi/L to a few tens of Ci/L. The wide range in specific activities may result from variations in the tritium concentration and total throughput of tritium. Depending on the history of these pumps, the tritium may be found as HT, HTO, or tritiated hydrocarbons.
Next to pump oils, the next most common group is tritiated solvents. All solvents can be absorbed through the skin and are relatively volatile and toxic. The overall toxicity of tritiated solvents is usually dominated by the chemical nature of the solvent.
If tritium is released in a nitrogen- or air-filled glovebox, other tritiated gases may be formed, such as ammonia and methane. The conversion of tritium to tritiated ammonia is small unless the tritium concentration is very high. The toxicity of these gases is not believed to be greater than that of tritium oxide.
Studies of biological elimination rates of body water in humans date back to 1934 when the body water turnover rate was measured using HDO. Since that time, several additional studies have been conducted with HDO and HTO. A simple average of the data suggests a value of 9.5 days for the measured biological half-life of water in the body with a deviation of {+-}50%. Calculations based on total fluid intake indicate a similar value. This is reasonable because the turnover rate of HTO should be identical to that of body water. In other words, the biological half-life of tritium is a function of the average daily throughput of water.
The biological half-life of HTO has been studied when outdoor temperatures varied at the time of tritium uptake. The data suggest that biological half-lives are shorter in warmer months. For example, the 7.5-day half-life measured in southern Nigeria is not surprising because the mean outdoor temperature there averages 27 degrees C. In contrast, an average 9.5-day half-life was measured in North America, where the mean outdoor temperature averages 17 degrees C. Such findings are consistent with metabolic pathways involving sensible and insensible perspiration. As such, the skin absorption and perspiration pathways can become an important part of body water exchange routes. It is important to note that personnel who are perspiring will have a greater absorption of tritium from contact with tritiated surfaces. For planning purposes, it is customary to use an average half-life of 10 days. However, it is not used to calculate doses from actual exposures.
Prolonged exposures can be expected to affect the biological half-life. Tritium's interaction with organic hydrogen can result in additional half-life components ranging from 21 to 30 days and 250 to 550 days. The shorter duration indicates that organic molecules in the body retain tritium relatively briefly. The longer duration indicates long-term retention by other compounds in the body that do not readily exchange hydrogen or that metabolize more slowly. However, the overall contribution from organically bound tritium is relatively small, that is, less than about 5% for acute exposures and about 10% for chronic exposures. Methods used to compute the annual limits on intake of air and water specify only the body water component and include the assumption of a 10-day biological half-life, as mentioned above.
Exposure to tritium oxide (HTO) is by far the most important type of tritium exposure. The HTO enters the body by inhalation or skin absorption. When immersed in tritiated water vapor, the body takes in approximately twice as much tritium through the lungs as through the skin. Once in the body, it is circulated by the blood stream and finds its way into fluids both inside and outside the cells.
According to International Commission on Radiological Protection (ICRP) Publication 30, the derived air concentration (DAC) [Footnote c] for tritium gas (HT) and HTO are 540,000 {mu}Ci/m{sup 3} and 21.6 {mu}Ci/m{sup 3}, respectively. The ratio of these DACs (25,000) is based on a lung exposure from the gas and a whole body exposure from the oxide. However, as was noted earlier, when a person is exposed to HT in the air, an additional dose actually results: one to the whole body. During exposure to HT, a small fraction of the tritium exchanges in the lung and is transferred by the blood to the gastrointestinal tract where it is oxidized by enzymes. This process results in a buildup of HTO until the HT is removed by exhalation at the end of the exposure. The resultant dose from exposure to this HTO is roughly comparable to the lung dose from exposure to HT. Thus, the total effective dose from an HT exposure is about 10,000 times less than the total effective dose from an equal exposure to airborne HTO. For both HTO and HT exposures, a bioassay program that samples body water for HTO is essential for personnel monitoring at tritium facilities.
After HTO enters the body, it is quickly distributed throughout the blood system and, within 1 to 2 hours, throughout all water in the body. Once equilibrium is established, the tritium concentration is found to be the same in samples of blood, sputum, and urine. For bioassay purposes, urine is normally used for determining tritium concentrations in body water.
Workers who may be or who have been exposed to tritium are normally required to submit urine samples for bioassay periodically. The sampling period may be daily, biweekly, or longer, depending on the potential for significant exposure.
Special urine samples are normally required after an incident or a work assignment with a high potential for exposure. After a possible exposure, the worker should empty the bladder 1 to 2 hours later. A sample taken after the bladder is emptied should be reasonably representative of the body water concentration. A sample collected before equilibrium is established will not be representative because of dilution in the bladder, or because of initial high concentration in the blood. However, any early sample may still be useful as a sign of the potential seriousness of the exposure.
A pure HT exposure is considered as a combination of a lung exposure from the HT and a whole body exposure from HTO. The HTO comes from the conversion of HT dissolved in the blood. The whole body dose can be determined as outlined above by analysis for HTO in the urine. Because the effective dose equivalents from the lung and whole body exposures are about equal, the total effective dose can be obtained conservatively by multiplying the HTO whole body dose by 2. However, in general, this is too conservative because a release of pure tritium gas with less than 0.01% HTO is highly unlikely. With only a slight fraction (approximately 0.1%) of HTO in the air, the total effective dose is essentially the HTO whole body dose determined by bioassay.
As noted above, tritium-labeled molecules in the skin result from contact with metal surfaces contaminated with HT. This form is associated with a longer half- life. Lung exposure to airborne metal tritides may also cause unusual patterns of tritium concentrations in body water because of the slow release of tritium to the blood stream. If such exposures are possible at the facility, it is good practice to follow the elimination data carefully and to look for organically bound tritium in the urine.
The results of the bioassay measurements and their contribution to the worker's dose and general health must be shared with the worker in a timely fashion.
The committed dose following an HTO exposure is directly proportional to the biological half-life, which in turn is inversely proportional to the turnover rate of body water. This rate varies from individual to individual. Such things as temperature, humidity, work, and drinking habits may cause rate variations. Although the average biological half-life is 10 days, it can be decreased by simply increasing fluid intake, especially diuretic liquids such as coffee, tea, beer, and wine. Even though the half-life may be easily reduced to 4 to 5 days in this way, a physician must be consulted before persons are placed on a regimen that might affect their health. Chemical diuretics require medical supervision because the resultant loss of potassium and other electrolytes can be very serious if they are not replaced. Such drastic measures can result in a decrease in half-life to 1 to 2 days. Even more drastic is the use of peritoneal dialysis or a kidney dialysis machine, which may reduce the half-life to 13 and 4 hours, respectively. Such extreme techniques should be used only in life-threatening situations involving potential committed dose equivalents that would exceed about 100 rem without any treatment. Based on a 10-day half-life, the committed dose for an intake of 1 mCi of HTO is approximately 63 mrem.
Individuals whose urine concentrations exceed established limits should stop work that involves possible exposure to radiation, whether from tritium or other sources. Work restrictions are suggested or imposed to make certain that the annual dose limits for workers are not exceeded. The operating group may impose stricter limits on their staff than those imposed by the health physics group. Depending on the number of workers available and the importance of the work to be done, doses can be managed to safe levels (from 5 to 100 {mu}Ci/L in urine).
Results of bioassay sampling should be given to workers who have submitted samples as soon as they are available. The results may be posted, or the workers may be notified personally. Moreover, the results must be kept in the workers' radiation exposure records or medical files. Like any other radiation exposure, any dose in excess of the limits specified by applicable regulations must be reported to DOE.
Table of Contents
Next Section
