Tritium released to room air moves readily with normal air current. The room or building ventilation system should be designed to prevent the air from being carried to uncontaminated areas, such as offices or other laboratories where tritium is not allowed. For that reason, differential pressure zoning is commonly used, and released tritium is directed outside through the building stack. In some newer facilities where the large quantities of tritium are being handled, room air cleanup systems are available for emergency use. Following a significant release, the room ventilation system is effectively shut down, the room is isolated, and cleanup of room air is begun.
The most important control for preventing a release of tritium to the room atmosphere is the use of containment around the source of tritium. This containment usually takes the form of a glove box, which is then a secondary containment if the tritium is already contained within the process plumbing, which is the primary containment. Even if the tritium is on the outside surface of a piece of equipment and located inside the glove box, through popular usage, the box is still referred to as the secondary containment.
Glove boxes used for tritium work typically are made of stainless steel or aluminum and use gloves made of butyl, neoprene, or Hypalon. Windows are made of glass or Lexan. In order to reduce the amount of tritium released to the atmosphere, glove boxes where significant quantities of tritium are handled incorporate detritiation systems that process the glove box atmosphere and remove the tritium. These detritiation systems, including the room cleanup systems mentioned above, convert released HT to HTO and collect the HTO on a molecular sieve for later recovery or burial. Newer systems use metal getters that recover HT without resorting to oxidation. These getters, which can only be used in certain glove box atmospheres, can be heated to release and recover the HT easily.
The atmosphere in the glove box may be air, nitrogen, argon, or helium, depending on the type of activity in the box. Even in boxes with inert gas atmospheres, small amounts of moisture and oxygen exist. Any release of tritium gas in the box will eventually be converted to the oxide. As a result, the oxide will slowly diffuse through the gloves and contaminate their outside surfaces. For that reason, personnel using glove boxes that have had tritium releases are required to wear one or more additional pairs of disposable gloves when working in the glove box.
Glove box monitors are used to alert personnel of a release in the box and may be used to activate a cleanup system or to increase the rate of the cleanup process. With releases of tritium in the box, the monitor chamber will eventually develop a memory from contamination, mainly by HTO. Heated monitor chambers are useful in minimizing contamination by HTO.
The relative pressure of the glove box atmosphere is normally kept negative in order to prevent the gloves from hanging outside the box where passersby may brush against them and to prevent tritium from escaping into the room should a leak develop in the glove box. However, outward permeation of HTO through the gloves and inward permeation of room moisture are not affected by the pressure inside the glove box.
At times, maintenance or repair work is done on equipment that cannot be moved into a glove box or fume hood and that has a high potential to release tritium. For these activities a temporary box ("tent"), may be constructed over the equipment, and an existing cleanup system installed to process the air. Alternatively, if the tritium at risk is not significant, the enclosing atmosphere may be purged to the stack. If the enclosure is small, gloves and glove ports may be fitted to the side of the enclosure. For larger enclosures entry may be required. In such cases, personnel must work in air-supplied suits inside the enclosure.
In spite of the greater protection afforded by glove boxes, fume hoods are commonly used at tritium facilities for handling or storing material with low quantities of tritium or with low-level contamination. Limits are generally imposed on the quantities used or stored in these hoods.
Fume hoods are also used to protect personnel at the outside door of glove-box pass boxes where materials are passed into and out of the boxes. Ideally, any tritium released in a hood from outgassing or a leaky container, for instance, is routed to the hood's exhaust duct. However, turbulence may occur at the hood entrance, resulting in backwash and possible contamination of personnel if the face velocity is not adequate for the design of the hood, the activities in the hood, or the local conditions (such as traffic in front of the hood). No hood should be used that has not been thoroughly surveyed and judged acceptable for tritium use.
For small operations local ventilation is commonly provided at the work site through a flexible ventilation duct ("elephant trunk") directed to the room exhaust system. The exhaust of these ducts is generally directed to the building ventilation exhaust system, which of itself may be adequate to supply the needed air flow for the duct without help from an additional in-line blower.
Flexible ducts can provide adequate ventilation during maintenance in a glovebox with a panel removed. In this application, a flexible duct can be connected to a gloveport before the panel is removed, and then the work can proceed safely.
In general, only supplied-air respirators are effective in preventing inhalation of airborne tritium. Two types of air-supplied respirators are available: self-contained breathing apparatus (SCBA) and full-face supplied air masks.
An SCBA, consisting of a full-face mask fed by a bottle of compressed air carried on the worker's back, provides excellent protection against HTO inhalation. Because the mask provides no protection against absorption by most of the skin, the SCBA is normally reserved for emergency use only. The protection factor of 3 or more afforded by the SCBA may be adequate for some applications. An SCBA can be used as an added precaution during certain maintenance or operations that experience has shown should not result in the release of significant amounts of HTO. Nevertheless, the potential for exposure is real, and the SCBA gives the worker time to leave the area if necessary before a skin exposure occurs.
Full-face supplied-air masks are also available. Because the air is normally supplied by a fixed-breathing-air system, they are not practical for many emergency situations and, consequently, are not as popular as SCBAs.
Because of the inherent disadvantages associated with respirators and other breathing apparatus, supplied-air plastic suits that completely enclose the body are often used by facilities that handle large quantities of tritium. Although they afford reasonably complete body protection, they are slow to don and cumbersome to wear. For these reasons, they are not favored for rescue work where time and mobility are important considerations. For certain maintenance operations outside of glove boxes with a high degree of risk, supplied-air suits may be quite useful.
For tritium work, supplied-air suits are constructed of materials that have acceptable permeation protection against HTO and provide good tear and abrasion resistance. Because of the closed environment, and the additional background noise caused by the flow of air into the suits, communication between personnel may require special equipment or methods.
Experience at tritium laboratories has shown that many tritium exposures to personnel occur as a result of contact with highly contaminated surfaces. Sudden and significant releases of airborne tritium occur mostly as the less toxic form HT and are quickly detected by portable or strategically placed, fixed tritium monitors. The result is that the exposure and uptake of airborne tritium are minimized. (Heavy-water reactors, of course, present a more significant risk of exposure to tritiated water vapor than to tritium gas.) The presence and degree of contamination may be unknown until measurements are made. Consequently, the importance of routine and special monitoring surveys for surfaces that personnel might contact cannot be overestimated.
Protective clothing worn by workers is one of the most important aspects of an effective health physics program. Because tritium can be absorbed easily through the skin or by inhalation, personnel protective equipment must protect against both exposure routes. The following paragraphs describe protective measures and equipment.
Lab coats and coveralls (fabric barriers) are worn in most tritium facilities. Lab coats are routinely worn to protect personal clothing. Coveralls are sometimes worn for added protection instead of a lab coat when the work is unusually dusty, dirty, or greasy. The protection afforded by lab coats and coveralls is minimal (except for short exposures) when tritium is airborne, but they are more effective in preventing skin contact with contaminated surfaces.
Disposable water-proof and water-resistant lab coats and coveralls have been tested at various laboratories. They are not popular for everyday use because of the cost and excessive discomfort inflicted on the worker. Most facilities prefer using ordinary open-weave fabrics for lab coats and coveralls and using an approved laundry for contaminated clothing. Some facilities have chosen to use disposable paper lab coats and coveralls, exchanging the costs associated with a laundry for the costs associated with replacement and waste disposal.
Although shoe covers provide protection against the spread of contamination and exposure, the routine use of shoe covers in a tritium facility is usually weighed against actual need. Shoe covers can offer both a degree of personnel protection and control over the spread of contamination on floors. However, in modern facilities where tritium is largely controlled by the use of secondary containment, shoe covers may not be required. Such facilities can easily maintain a clean laboratory environment by the use of regular smear surveys and good housekeeping. Using liquid-proof shoe covers until spills are cleaned up should be considered following spills of tritium-contaminated liquids and solids to prevent the spread of local contamination.
In most operations, the hands and forearms of workers are vulnerable to contact with tritium surface contamination. The proper use and selection of gloves are essential.
Many factors should be considered in selecting the proper type of glove. These include chemical compatibility, permeation resistance, abrasion resistance, solvent resistance, glove thickness, glove toughness, glove color, shelf life, and unit cost. Gloves are commercially available in butyl rubber, neoprene, polyvinyl chloride (PVC) plastics, latex, etc.
The most common gloves found in tritium laboratories are the light-weight, disposable short glove (usually PVC or latex) used for handling lightly contaminated equipment. Depending on the level of contamination, such gloves may be changed frequently (every 10-20 minutes), a second pair may be worn, or heavier gloves may be used instead. When using gloves for this purpose, the work should be planned so that contaminated gloves doe not spread contamination to surfaces that are being kept free of contamination.
When working in a glove box using the box gloves, disposable gloves are worn to prevent uptake of HTO contaminating the outside of the box gloves. Again, depending on the level of contamination, more than one additional pair may be required, one of which may be a longer, surgeon's length, glove.
In spite of all the precautions normally taken, workers may occasionally be contaminated with tritium. The skin should be decontaminated as soon as possible after any potential skin exposure to minimize absorption into the body. Effective personal decontamination methods include rinsing the affected part of the body with cool water and soap. If the entire body is affected, the worker should shower with soap and water that is as cool as can be tolerated. Cool water keeps the pores of the skin closed and reduces the transfer of HTO across the skin. The importance of washing the affected skin as soon as possible after contamination cannot be over-emphasized. Figure 8 [Footnote d] illustrates the effect of speed on reducing the uptake and the resultant dose. Even if gloves are worn when handling contaminated equipment or when working in contaminated glove box gloves, it is good practice to wash the hands after removing the gloves.
It is important to examine the history of accidents that have occurred in tritium facilities and to consider foreseeable unplanned events in order to minimize or mitigate their effects or to prevent their taking place at all. When an accident does occur, requirements for reporting accidents must be followed.
Facilities that handle significant quantities of radioactive material must have a site-specific emergency plan. All radiological workers at the site must be familiar with certain aspects of this plan. In addition, job assignments involving radiological hazards are typically covered by procedures and work permits that include steps for emergency situations that may arise during the course of the work. Radiological workers must be familiar with these procedures or be accompanied by a radiological control technician (RCT) to provide guidance in case of an emergency.
The initial steps to be undertaken following a serious accident must always include the following:
Personnel should be decontaminated by the following procedure:
Following a tritium spill involving a liquid with high specific activity, the area may have to be isolated and other protective measures taken before cleaning up the liquid. Monitoring for possible airborne tritium must be started to determine the need for respiratory protection or skin protection. After the spill has been cleaned up, residual contamination will remain. Depending on the level of contamination, any further steps needed to prevent the spread of contamination and reduce the level to an acceptable value should be determined.
Following a release of tritium gas, surfaces would not be expected to be heavily contaminated. If tritiated water vapor is released, the contamination may be greater, depending on the amount and activity of the released vapor. In any case, smear and air surveys will be used to determine the course of action needed to control and reduce the contamination safely.
EG&G Mound Applied Technologies, Health Physics Manual of Good Practices for Tritium Facilities, MLM-3719, Draft, Miamisburg, Ohio, December 1991.
International Atomic Emergency Agency, Safe Handling of Tritium, IAEA- 324, Vienna, Austria, 1991.
U.S. Department of Energy, Radiological Control Manual, DOE/EH-0256T, June 1992.
"Occupational Radiation Protection," 10 CFR 835, Federal Register, 58, No. 238, December 1993.
T.B. Rhinehammer and P.H. Lamberger (eds.), Tritium Control Technology, WASH-1269, Monsanto Research Corporation, Miamisburg Ohio, 1973
U.S. Department of Energy, Radiation Protection for Occupational Workers, DOE Order 5480.11, Change 3, June 1992.
National Council on Radiation Protection and Measurements, Tritium Measurement Techniques, NCRP 47, 1976.
U.S. Department of Energy, Occurrence Reporting and Processing of Operations Information, DOE Order 5000.3B, January 19, 1993.
U.S. Department of Energy, Radiation Protection of the Public and the Environment, DOE Order 5400.5, Change 2, January 7, 1993.
a. The curie (Ci) is a unit of activity defined as 3.7 x 10{sup 10} disintegrations per second (dps). A more basic unit is 1 dps, which is the definition of the becquerel (Bq). Throughout this Primer, the curie will be used instead of the becquerel.
b. An electron volt is a small unit of energy used in descriptions of nuclear and chemical reactions. It equals the energy gained by an electron when it moves across a potential of 1 volt.
c. The DAC is defined as that concentration of a gas, which, if a worker were exposed to it for one working year (2,000 hours), would result in an annual dose of 5 rem.
d. W. R. Bush, Assessing and Controlling the Hazard from Tritiated Water, AECL-4150, Atomic Energy of Canada LTD., Chalk River, Ontario, 1972.
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