The Primer on Tritium Safe Handling Practices is approved for use by all DOE Components. It was developed as an educational supplement and reference for operations and maintenance personnel involved in tritium handling. This Primer is intended to help operations and maintenance personnel perform their duties safely by increasing their knowledge of tritium handling and control, thus reducing the likelihood of tritium releases and exposures, and by providing basic emergency responses in the case of a tritium release.
The Department of Energy (DOE) Primers are a set of fundamental handbooks on safety-related topics of interest in the DOE Complex. The Primers are written as an educational aid for operations and maintenance personnel. The Primers attempt to supply information in an easily understandable form which will help them perform their duties in a safe and reliable manner. Persons trained in other technical areas may also find the Primers useful as a guide or as a reference source for further investigation.
The DOE Primer series draws heavily upon the subject-specific Primers and training materials previously developed by DOE sites (Savannah River, Rocky Flats, and Mound) and is intended for distribution to all DOE contractors. Information is also drawn from the applicable volumes of the DOE Fundamentals Handbook series developed by the DOE Office of Nuclear Safety Policy and Standards. References to other material sources are indicated in the text where applicable and a bibliography is included.
Beneficial comments in the form of recommendations and any pertinent data that may be of use in improving this document should be addressed to
by using the U.S. Department of Energy Standardization Document Improvement Proposal Form (DOE F 1300.x) appearing at the end of this document or by letter.
Hydrogen, Tritium, Half-life, Radiation, Beta particles, Contamination, Health Physics, Monitoring, Bioassay, Metabolism, Emergency Response
Tritium Safe Handling Practices was prepared as an information resource for personnel who perform tritium handling functions at DOE facilities. A basic understanding of the properties and hazards associated with tritium will help personnel understand the impact of their actions on the safe and reliable operation of facility systems.
Tritium Safe Handling Practicescontains an introduction and sections on the following topics:
This Primer is provided as an information resource only, and is not intended to replace any radiation worker or hazardous materials training. The Primer presents the theoretical concepts and good practices that form the basis of safe tritium handling.
Specific references have been cited as footnotes, and a bibliography is available at the end of the Primer. These sources provide further reading and specific guidance. This document contains selected information from the Health Physics Manual of Good Practices for Tritium Facilities, which was prepared by EG&G Mound Applied Technologies at the Mound tritium facility. The authors acknowledge their expertise and experience.
This Primer is designed for use by operations and maintenance personnel to improve their knowledge of tritium safe handling practices. It is applicable to many job classifications and can be used as a reference for classroom work or for self-study. It is presented in general terms so that it can be used throughout the DOE Complex.
The information in this Primer should enable the reader to do the following:
The organization of the Primer is as indicated in the Overview. The following section contains background information on "Radiological Fundamentals." Those familiar with these topics may elect to skip this section and begin reading at the section entitled "Physical and Chemical Properties of Tritium." Additional information about tritium is available from the sources listed in the "Bibliography" section.
This section provides a review of radiological fundamentals. The reader is assumed to be familiar with this information from radiological worker training. The section discusses hydrogen and its isotopes and describes basic radiological concepts.
Atomic nuclei of a particular element (such as hydrogen or oxygen) have the same number of protons (positively charged), but may have a different number of neutrons (no net charge). Those that have a different number of neutrons are isotopes of that element. Most elements exist in nature in several isotopic forms. For example, hydrogen has one proton. The isotopes of hydrogen either have no neutrons (normal hydrogen, called protium), one neutron (deuterium), or two neutrons (tritium) (Figure 1). Although isotopes of an element have almost the same chemical properties, the nuclear properties can be quite different.
Nuclear notation uses the chemical symbol (H for hydrogen) and an arrangement of subscripts and superscripts. The total number of protons and neutrons is shown as a superscript: {sup 1}H for protium, {sup 2}H for deuterium, and {sup 3}H for tritium. The number of protons (which identifies the element) is shown as a subscript. However, the common practice of using H, D and T for these isotopes, respectively, will be followed in this document, except where nuclear reactions are illustrated.
The atomic masses, symbols, and natural abundances of the three isotopes of hydrogen are given in Table 1.
Tritium occurs naturally in the environment. Reactions between cosmic radiation and gases in the upper atmosphere produce most of the world's natural tritium. For example,
Tritium converts into water and reaches the earth's surface as rain. An estimated production rate of 4 x 10{sup 6} Ci/yr results in a world steady-state natural inventory of approximately 70 x 10{sup 6} Ci. [Footnote a]
In addition, commercial producers of radioluminescent and neutron generator devices release about 1 x 10{sup 6} Ci/yr. Atmospheric nuclear test explosions from 1945 to 1975 added about 8 x 10{sup 9} Ci of tritium to the environment, much of which has since decayed. However, about 5 x 10{sup 8} Ci remain in the environment, mostly diluted in the oceans. Underground nuclear tests appear to add little tritium to the atmosphere. The nuclear power and defense industries now release 1-2 x 10{sup 6} Ci/yr, a small fraction of which comes from light-water reactors. Tritium is also a by-product of light-water and heavy-water nuclear reactor operation. In their coolants, these reactors produce about 500 to 1,000 and 2 x 10{sup 6} Ci/yr, respectively, for every 1,000 MW(e) of power. Tritium is a fission product within nuclear fuel, generated at a rate of 1-2 x 10{sup 4} Ci per year/1000 MW(e). U.S. DOE reactors have produced tritium by the neutron bombardment of lithium ({sup 6}Li).
In the lighter elements, the ratio of neutrons to protons in their stable nuclei is usually 1 (one neutron for every proton). For the heavier elements, this ratio gradually increases to about 1.5 (1.5 neutrons for every proton). Although one cannot always predict from its ratio whether an isotope is stable or unstable, the relationship between the number of protons and neutrons is extremely important.
When an isotope is unstable, its nucleus will emit rays or particles or it may split into two different nuclei. Some combinations of neutrons and protons lead to stable nuclei. If there are too many or too few neutrons, the resulting nucleus is not stable. This unstable nucleus tries to become more stable by releasing excess energy. Atoms with unstable nuclei are radioactive. The process of nuclei releasing this energy is referred to as radioactive decay or disintegration (Figure 2). If a nucleus is still unstable after radioactive decay, further decay will occur.
Atoms can combine chemically to form molecules. Atoms and molecules are surrounded by orbiting electrons (negatively charged). If the number of electrons equals the total number of protons (positively charged) in the nucleus, the atom or molecule is neutral (uncharged). Electrically charged atoms or molecules are called ions. Ions are either positively or negatively charged, depending on the number of orbiting electrons relative to the number of protons in the nucleus. As shown in Figure 3, ions with more electrons than protons are negatively charged, while ions with more protons than electrons are positively charged. The process of breaking a neutral atom or molecule into electrically charged parts is called ionization. This process requires energy. Ionization removes electrons from the atom, or molecule, leaving an ion with a positive charge. The negatively charged electron (which can attach itself to a neutral atom or molecule) and the positively charged ion, are called an ion pair. Radiation that causes ionization is called ionizing radiation.
There are four basic types of ionizing radiation emitted from nuclei: alpha particles, beta particles, gamma rays, and neutrons.
) -- consists
of two protons and two neutrons
and is the same as the nucleus of a helium atom ({sup 4}He)
(Figure 4).
Generally, only the heavy nuclides can emit alpha particles.
A typical example of an alpha-emitting nuclide is uranium-238:
The mass of an alpha particle is about four times the mass of a single neutron or proton, and has a positive charge of +2 (it has no electrons). This positive charge causes the alpha particle to ionize nearby atoms as it passes through body tissue. The strong positive charge and its relatively slow speed (resulting from its large mass) causes the alpha particle to interact strongly with orbiting electrons of atoms and molecules and to lose large amounts of energy in a short distance. This limits the penetrating ability of the alpha particle, making it easy to stop. A few centimeters of air, a sheet of paper, or the outer layer of skin stops alpha particles (Figure 5).
Alpha particles are not an external radiation hazard because they are easily stopped by protective clothing or the outer layer of skin. However, if an alpha emitter is inhaled or ingested, it becomes an internal radiation hazard. Because the source is in close contact with body tissue, the alpha particle will dissipate its energy in a short distance of the tissue.
) -- is equivalent
to an electron
except for its source. Beta-emitting nuclides have too many neutrons.
A neutron emits a beta particle, and the neutron is then converted to a
proton. Tritium decay provides a good example of this process:
A beta particle is identical to an electron, and its mass and charge are the same as those of an electron. As in the case of alpha particles, beta particles ionize atoms by removing electrons from their orbits. This reaction occurs from charged particle interactions or "collisions" with orbiting electrons.
Beta particles penetrate further than alpha particles of the same energy. A high-energy beta particle can penetrate a few centimeters of organic tissue. The higher the energy, the greater the penetrating ability. However, low-energy beta particles of tritium can be shielded by skin, paper, or only about 6 mm of air.
) -- is
emitted when the nucleus of a nuclide
releases stored energy without releasing a particle. Many gamma-emitters
are found among the products of nuclear fission.
During pure gamma emission the nucleus does not emit particles or change its nuclear structure or chemical characteristics. For instance,
Gamma radiation is in the form of electromagnetic waves (or photons). Gamma rays are similar to x-rays, but they differ in their origin and energy. Gamma rays originate within the nucleus, and x-rays originate outside the nucleus.
Gamma rays have a very high penetrating power because they have no charge or mass. Depending on their energy, a stream of gamma rays may penetrate with gradually diminishing intensity through several inches of concrete or similar material. They can be shielded effectively by very dense materials, such as lead and uranium. Gamma rays are a whole-body hazard. That is, because of their penetrating ability, the damage caused by gamma rays is not restricted to any particular body organ.
A neutron has about 2,000 times the mass of an electron, but only one-fourth the mass of an alpha particle. Neutrons are difficult to stop because they lack a charge. Neutrons mainly interact with matter by striking hydrogen nuclei or interacting with the nucleus of atoms. These collisions generally cause charged particles or other radiation to be emitted. These particles may then ionize other atoms. Collisions between neutrons and hydrogen nuclei (protons) are effective in stopping or slowing down high-energy neutrons. Neutrons are best shielded by materials with a high hydrogen content, such as water or plastic (see Figure 6).
As radioactive isotopes decay, the number of radioactive nuclei decreases. The time required for half of the nuclei in a sample of a specific radioactive isotope to undergo decay is called its (physical) half-life (Figure 7). Each radioactive isotope has its own characteristic half-life. Radioactive isotopes decay to less than 1% of their original quantity after about seven half-lives.
Half-lives vary widely with different radionuclides, as shown by the following examples:
The activity of a radioactive isotope sample is defined as the number of nuclei that decay per unit of time.
It has been shown that for a pure radioactive isotope the number of nuclei decaying per unit time (rate of decay) is proportional to the number of nuclei available to decay. If the substance is not being replenished, its activity will decrease accordingly. Therefore, in terms of half-life, the remaining activity after a period of time can be expressed as follows:
For example, assume we have stored 10,000 Ci of tritium. How many curies will be left after 50 years of storage?
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