The tritium monitoring system at a tritium handling facility is critically important to its safe operation. Operators and others at the facility need to be informed of the status of the processes, the development of any leaks in the primary or secondary containments, or of any releases to the room or environment so that protective measures and corrective action may be taken quickly. The location and degree of surface contamination are equally important to prevent accidental uptakes of tritium by personnel.
In this section, the various techniques used to monitor for tritium in gases (including air), in liquids, and on surfaces will be discussed.
Fixed ionization chamber instruments are the most widely used instruments for measuring gaseous forms of tritium in laboratory and process monitoring applications. Portable ionization chamber instruments are also used to control contamination and to supplement fixed instrument measurements. Such simple devices require only an electrically polarized ionization chamber, suitable electronics, and a method for moving the gas sample through the chamber usually a pump. Chamber volumes typically range from a tenth to a few tens of liters, depending on the required sensitivity. The output is usually given in units of concentration (typically {mu}Ci/m{sup 3}) or, if a commercial electrometer or picoammeter is used, in current units that must be converted to those of tritium concentration. The following rule-of-thumb can be used to convert current to concentration: 10{sup 15} x current (amps)/chamber volume (liters) = concentration ({mu}Ci/m{sup 3}). For real-time tritium monitoring, the practical lower limits of sensitivity range from 0.1 to 10 {mu}Ci/m{sup 3}. External background radiation or the presence of radon can lower the sensitivity of the instrument.
For measurements of low concentrations, sensitive electrometers are needed. For higher concentrations (>1 mCi/m{sup 3} for example), the requirements on the electronics can be relaxed, and smaller ion chambers may be used. Smaller chambers also need less applied voltage. Because of a greater ratio of surface area to volume, residual contamination in the chamber is more likely and is called "memory." This residual contamination elevates the background chamber current. Response times for higher level measurements can be made correspondingly shorter. However, because small chambers and chambers operated at low pressures may have significant wall effects, the above rule-of-thumb may not apply. Such instruments would have to be calibrated to determine their response.
Although most ionization chambers are the flow-through type that require a pump to provide the flow, a number of facilities use "open window" or "perforated wall" chambers. These chambers, which employ a dust cover to protect the chamber from particulates, allow the air or gas to penetrate through the wall to the inside chamber without the need for a pump. These instruments are used as single point monitors to monitor rooms, hoods, gloveboxes, and ducts.
Because HTO is more toxic than HT (10,000 to 25,000 times greater), it may be desirable to know the relative amounts of each species following a significant release into a room or to the environment. In the case of stack monitoring, discrete samples of the stack effluent should be taken using bubblers or desiccants with a catalyst for oxidizing the HT. Another technique for differential monitoring uses a desiccant cartridge in the sampling line of an ionization chamber monitor. The result is a measurement of the HT concentration. Without the cartridge, the total tritium concentration is measured. Subtraction of HT from the total produces the HTO concentration. The technique may be used with two instruments or one instrument in which the desiccant cartridge is automatically switched in and out of the sampling line.
Another technique uses a semipermeable membrane tube bundle in the sampling line to remove the HTO (preferentially over the HT), which is directed to an HTO monitor. After removing the remaining HTO with another membrane dryer, the sampled air is directed to the HT monitor. Although this technique is slower than the one requiring a desiccant cartridge, it does not require a periodic cartridge replacement. Furthermore, it can be adapted to measure tritium in both species in the presence of noble gases or other radioactive gases by adding a catalyst after the HTO dryers, followed by additional membrane dryers for the HTO. However, because of its slow response, it is more suitable for effluent or stack monitoring than for room monitoring. Because significant releases into a room are quite rare, it is easier to treat any such release as one of HTO than use complicated techniques for continuous differential monitoring.
Discrete sampling differs from real-time monitoring in that the sampled gas (usually air) must be analyzed for tritium content (usually by liquid scintillation counting). The usual technique is to flow the sampled air through either a solid desiccant (molecular sieve, silica gel, or Drierite) or water or glycol bubblers. For low-flow rates (about 0.1 to 1 L/min), bubblers may be used. Bubblers are more convenient for sampling, but are less sensitive than the solid desiccant cartridges if the water in the desiccant is recovered by heating. Glycol or water may be used, but glycol is preferred for long-term sampling. In any case, the collected water is then analyzed for HTO. For differential monitoring of HTO and HT, a heated catalyst (usually a palladium sponge) is used between the HTO desiccant cartridge or bubblers and the HT cartridge or bubblers. This is currently the preferred method for monitoring stacks for reporting purposes. In a different arrangement, palladium is coated on the molecular sieve in the HT cartridge to oxidize the HT into HTO, which is then absorbed by the molecular sieve. However, this technique is used primarily for environmental monitoring.
Another technique for sampling HTO in room air is to use a "cold finger" to freeze HTO out of the air. An alcohol and dry ice mixture in a stainless steel beaker works well. To determine the concentration, the relative humidity must be known. Another sampling technique is to squeeze a soft plastic bottle several times to introduce the air (containing the HTO) into the bottle. A measured quantity of water is then introduced, and the bottle is capped and shaken. In a minute or less, essentially all the HTO is taken up by the water, which is then analyzed.
Other techniques involve placing a number of vials or other small specially designed containers of water, cocktail, or other liquid in selected locations in the area being monitored. After a period of time (usually a number of days), the liquid in the containers is analyzed. The result is qualitative (for open containers) to semiquantitative (for specially designed containers).
Ionization chambers are typically used for monitoring stacks, rooms, hoods, glove boxes, and processes. The outputs can be used to sound alarms, activate ventilation valves, activate detritiation systems, and perform other functions. In general, it can be expected that stack, room, and hood monitors will require little nonelectronic maintenance (i.e., chamber replacement because of contamination). Under normal circumstances, the chambers are constantly flushed with clean air and are not exposed to high tritium concentrations. However, glove box monitors can be expected to eventually become contaminated, especially if exposed to high concentrations of HTO. Process HT monitor backgrounds can also be expected to present problems if a wide range of concentrations (4 to 5 orders of magnitude) are to be measured.
Mass spectrometers, gas chromatographs, and calorimeters are the main instruments used for process monitoring. Because of their relative insensitivities, these instruments cannot detect tritium much below a few parts per million (Ci/m{sup 3}). For this reason, the analytical results and the related health physics concerns must be interpreted carefully. It is not uncommon to find that samples showing no trace of tritium when analyzed on a mass spectrometer may actually have a concentration of several curies of tritium per cubic meter. In spite of their contamination problems, ionization chamber instruments are useful for measuring these lower concentrations and for providing instant indications of changing concentrations that are not possible with the more sophisticated instruments.
Any material exposed to tritium or a tritiated compound has the potential of being contaminated. Although it is difficult to quantify tritium contamination levels, several methods are available to evaluate the extent of contamination, including smear surveys and off-gassing measurements. Good housekeeping and work practices are essential in maintaining contamination at acceptable levels.
For health or safety implications, an indication of loose, removable tritium contamination is more valuable than a measurement of the total surface contamination. Loose tritium can be transferred to the body by skin contact or inhalation if it becomes airborne. As a result, loose contamination is routinely monitored by smears, which are wiped over a surface and then analyzed by liquid scintillation or proportional counting.
The smears are typically small round filter papers used dry or wet (with water, glycol, or glycerol). Wet smears are more efficient in removing tritium, and the results are more reproducible, although the papers are usually more fragile when wet. However, results are only semiquantitative, and reproducibility within a factor of 2 agreement (for wet or dry smears) is considered satisfactory. Ordinarily, an area of 100 cm{sup 2} of the surface is wiped with the smear paper and quickly placed in a vial with about 10 mL of liquid scintillation cocktail, or 1 or 2 mL of water with the cocktail added later. The paper must be placed in liquid immediately after wiping because losses from evaporation can be considerable, especially if the paper is dry. The efficiency of the liquid scintillation cocktail is only slightly affected by the size of the swipe. Foam smears are also available commercially. These smears dissolve in most cocktails and do not interfere significantly with the normal counting efficiency.
Smears may be counted by gas-flow proportional counting. However, because of the inherent counting delays, tritium losses before counting can be significant. Moreover, counting efficiencies may be difficult to determine and may vary greatly from one sample to the next. Another drawback is potential contamination of the counting chamber when counting very "hot" smears. For all of these reasons, a liquid scintillation spectrometer is the preferred system.
An effective tritium health physics program must specify the frequency of routine smear surveys. Each facility should develop a routine surveillance program that may include daily smear surveys in laboratories, process areas, step-off pads, change rooms, and lunchrooms. In many locations within a facility, weekly or monthly routine smear surveys may be sufficient. The frequency should be dictated by operational experience and the potential for contamination. In addition to the routine survey program, special surveys should be made following spills or on potentially contaminated material being transferred to a less controlled area to prevent the spread of contamination from controlled areas.
The surface contamination levels acceptable for the release of materials from radiological areas may be found in the DOE Radiological Control Manual and DOE Order 5400.5.
In general, the total tritium contamination on a surface can be measured only by destructive techniques. When tritium penetrates a surface even slightly, it becomes undetectable because of the weak energy of its beta particles. With open-window probes operated in the Geiger Mueller (GM) or proportional regions, it is possible to measure many of the betas emitted from the surface. Quantifying that measurement in terms of the total tritium present is difficult because the history of every exposure is different. Consequently, the relative amounts of measurable and unmeasurable tritium are different. Such monitoring probes are used to survey areas quickly before more careful monitoring by smears, or to monitor the smears themselves while in the field.
The probe must be protected carefully from contamination. When monitoring a slightly contaminated surface after monitoring a highly contaminated one, contamination of the probe can be an immediate problem. Placing a disposable mask over the front face of the probe can reduce, but never eliminate this contamination, particularly if the tritium is rapidly outgassing from the surface. Sensitivity of the instrument depends on many factors, but should be about 10{sup 3} to 10{sup 4} dpm/cm{sup 2}.
For highly contaminated surfaces (>1 mCi/100 cm{sup 2}), a thin sodium iodide crystal or a thin-window GM tube can be used to measure the characteristic and continuous x-rays (Bremsstrahlung) emitted from the surface as a result of the interaction of the beta particle with the surface material.
Off-gassing can be measured using one of two methods. The simplest method is to "sniff" the surface for airborne tritium using a portable or fixed tritium monitor. The most reliable method, however, uses a closed-loop system of known volume and a flow-through ionization chamber monitor. By placing the sample inside the volume and measuring the change in concentration over time, tritium off-gassing rates can be determined accurately on virtually any material. The initial off-gassing rate is the required value because the equilibrium concentration may be reached quickly in a closed volume, especially if the volume is small because of recontamination by the airborne tritium.
The uptake of tritium from off-gassing materials is difficult to predict. Off-gassing tritium that is readily measured indicates contaminated equipment that should not be released for uncontrolled use.
Liquid is almost universally monitored by liquid scintillation counting. The liquid must be compatible with the cocktail. Certain chemicals can degrade the cocktail. Others may retain much of the tritium; still others result in a high degree of quenching. In addition, samples that contain peroxide or that are alkaline may result in chemiluminescence that can interfere with measuring. Such samples should first be neutralized before counting. Chemiluminescence and phosphorescence both decay with time. Phosphorescence, activated by sunlight or fluorescent lighting, decays in the dark in a few minutes (fast component) to several days (slow component). Chemiluminescence, the result of chemical interaction of sample components, may take days to decay at room temperatures, but takes only hours to decay at the cold temperatures of a refrigerated liquid scintillation spectrometer. Distillations may be necessary for some samples.
For rather "hot" samples, as may be the case for vacuum pump oils, Bremsstrahlung counting may be useful. This technique may also be useful for active monitoring of "hot" liquids. Liquids may be monitored actively with scintillation flow cells, which are often made of plastic scintillator material or of glass tubing filled with anthracene crystals. However, both types are prone to memory effects that result from tritium contamination. In addition, flow cells are also prone to contamination by algae or other foreign material that can quickly degrade their counting efficiency.
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