Radiation Protection and Environment

ORIGINAL ARTICLE
Year
: 2022  |  Volume : 45  |  Issue : 1  |  Page : 16--21

Application of thermoelectric cooling module for sampling of tritium in air


Lokesh Kumar, V Shreenivas, Saurav Sood, P Ashokkumar, Ranjit Sharma, MS Kulkarni 
 Health Physics Division, Bhabha Atomic Research Centre, Mumbai, Maharashtra, India

Correspondence Address:
Lokesh Kumar
Health Physics Division, Bhabha Atomic Research Centre, Mumbai - 400 085, Maharashtra
India

Abstract

The sampling and measurement of airborne tritium is an essential component of workplace monitoring at heavy water handling facilities and nuclear reactors using heavy water as coolant and moderator. Tritium being an internal hazard, its workplace monitoring and assessment of internal exposure of workers is a regulatory requirement for the facility. The conventional tritium air sample collection methods are condensation, bubbling, and trapping with appropriate media such as dry ice, water, and desiccants, respectively. A novel method for rapid collection of moisture in the air for the estimation of tritium is presented and discussed in this article. It involves condensation of tritium oxide in the air using a commercially available thermoelectric cooling module which has removed uncertainty in the availability of dry ice or desiccant. The instrument is capable of collecting 2–3 ml of sample in 30 min at a relative humidity level of about 60% and temperature of about 25.5°C. The quantity of sample collected is sufficient for the estimation of tritium concentration in air. The Peltier module-based low-cost, simple, and reliable system has been successfully implemented for workplace tritium in air sampling at radiological facilities.



How to cite this article:
Kumar L, Shreenivas V, Sood S, Ashokkumar P, Sharma R, Kulkarni M S. Application of thermoelectric cooling module for sampling of tritium in air.Radiat Prot Environ 2022;45:16-21


How to cite this URL:
Kumar L, Shreenivas V, Sood S, Ashokkumar P, Sharma R, Kulkarni M S. Application of thermoelectric cooling module for sampling of tritium in air. Radiat Prot Environ [serial online] 2022 [cited 2022 Nov 27 ];45:16-21
Available from: https://www.rpe.org.in/text.asp?2022/45/1/16/348724


Full Text



 Introduction



Tritium (T) is a radioactive isotope of hydrogen and it is generally present as tritiated water molecule (HTO) or T as a gas in the surrounding air. The higher concentration of T is prevalent at heavy water (D2O)-based nuclear reactors and also it is being increasingly used at certain accelerators, research, and industry. Tritium sampling and assessment of its radioactivity levels at the workplace and environment is an essential component of radiation protection as well as a regulatory requirement. The continuous air monitoring program is implemented to monitor airborne tritium at the workplace to assess the need for appropriate personnel protective equipment such as fresh airline respirators and plastic suits. The quick detection and measurement of unexpected airborne contamination in case of tritiated heavy water spillage in the area can help in introducing timely intervention and protective action for minimizing internal exposure. The radiation workers are regularly monitored for internal exposure to assess committed effective dose due to tritium intake leading to appropriate cautioning or removal from radioactive work.[1] Health physicists collect the HTO vapor from the air especially at plant occupancy areas of reactors and radioactive facilities that handle tritium as well as in the environment at the vicinity of all such facilities.[2],[3] The sampling is carried out by applying convenient methods[4] such as (i) passing the air through a desiccant, (ii) bubbling the air through nontritiated water or other appropriate solvents, and (iii) condensing or freezing. After sample collection, the specific activity of T in the moisture condensate is analyzed using liquid scintillation spectrometer (LSS) for the estimation of tritium concentration in air.

Sampling by passing the air through desiccation agents such as silica gels or molecular sieves involves the retention of moisture present in the air on their surface. The water is recovered by evaporation and condensation of water from the desiccation agent to assay the T in the sample. The requirement of the air sampling pump and a longer sampling period due to the low water retention capacity of desiccation agents is a disadvantage. Moreover, it requires a bulky installation to recover tritium from the desiccation agents. Another sampling method consists of passing HTO vapor through water, which is free from tritium contamination. The HTO vapor present in the air is exchanged with the tritium-free water in the bubbler bottles. Tritium concentration in the water of bubbler bottles is measured with LSS. A constant sample flow through the bubbler has to be maintained using a pump. This sampling method has drawback that it requires significant sampling time due to the required low airflow in bubbler bottles. The cold trap method consists of condensing or freezing the HTO vapor on cold surfaces. Cold traps are formed by the continuously cooling surface of copper strips or aluminum plates or glass beakers with the use of dry ice or liquid nitrogen. The moisture containing HTO vapor condensed on the surface is recovered manually for further analysis. The tritium concentration in the collected water is measured using LSS. The cold trap requires the constant availability of dry ice or liquid nitrogen for sample collection. The nonavailability of dry ice due to breakdown in supply from respective agencies and the problem of long-term storage may affect the sample collection during any accidental heavy water spillage leading to an increase in airborne tritium.

The Peltier effect is the cooling of one junction and heating of the other one when an electric current is maintained in a thermocouple of two dissimilar conductors. This effect is the reverse of Seebeck effect, where two dissimilar materials are joined together and their junctions are kept at different temperatures. The voltage difference is developed across the junctions which are proportional to the temperature difference of the two junctions. The Peltier effect is described as the heat extraction or absorption at the contact of two dissimilar metals when a direct electric current flows through it. If the hot junction is kept outside the insulated area, the cold junction can be used to cool the specified region. Peltier elements or thermoelectric cooling (TEC) modules are available in various forms and shapes. Typically, they consist of a larger amount of thermocouples arranged in rectangular form and packaged between two thin ceramic plates. Later, it has been evolved that the most prospective materials for Peltier effect were semiconductors. A Peltier device is generally fabricated using semiconductor material such as bismuth telluride (Bi2Te3), lead telluride (PbTe), and bismuth antimony telluride (PbSbTe).[5],[6],[7] A Peltier material exhibits very high electrical conductivity and relatively low thermal conductivity, in contrast to normal metals which have both high electrical and thermal conductivity. A thermoelectric element is formed by making junction between P- and N-type semiconductor pallets using copper tabs.[8] When an electric current is passed in the appropriate direction through the junction, both types of charge carriers, electrons in N-type and holes in P-type semiconductor pallet, move away from the junction and convey heat away, thus cooling the junction. Several such elements are arranged between two rectangular plates with connecting wires to form a TEC module which connects the thermoelectric elements electrically in series and thermally in parallel.

The Peltier effect has been utilized in this work to produce cooling for moisture condensation. Peltier modules form the electricity-driven cold traps. We have developed a novel device to quickly collect the T activity sample in air using commercially available TEC or Peltier cooling modules. The design, development, and implementation of the TEC-based T in air sampling device have been described, along with the implementation of the device and measurement results.

 Materials and Methods



An air sampling unit has been developed utilizing the Peltier effect, which will be used for the moisture condensation from the air using electricity instead of dry ice/liquid nitrogen. To achieve the condensation of moisture in the air, a suitable TEC module is selected to develop the tritium in air sampling device.

Fabrication and assembly of T in air sampling system

The T in air sampling system to condense moisture from air is a portable device which has been constructed using a TEC module with an air-cooled heat sink made of copper connected at its hot surface. The air sampling system design has been optimized to collect the required amount of condensed moisture from the air in minimum sampling time. The fabrication and assembly of various parts of the Peltier effect-based T in air sampling system are discussed here.

The sampling device consists of four blocks, namely power supply, TEC module, heat sink attached with fan for heat removal and moisture sample collection surface. [Figure 1] gives the TEC-based T in air sampling device. A 12V direct current power supply having 15 amperes current rating which operates with 230V mains, has been used to power the system. The efficiency of moisture sample collection depends on the effectiveness of heat removal mechanism from the hot ceramic side of the TEC module. The required cooling on the moisture condensing surface could be achieved with an efficient heat dissipation unit. We used a high performance, compact-sized, heat removal system with a precise combination of airflow cooling coils and heat sink having aluminum fins fitted mechanically to small (92 mm × 92 mm × 22 mm) high speed (2000 rpm) fans connected to a 40 mm × 40 mm metallic plate. To enhance the heat removal efficiency, two fans are utilized. A commercially available Peltier module TEC1-12715[9] is used as the cooling device [Figure 2].{Figure 1}{Figure 2}

This is a standard-sized (40 mm × 40 mm × 3.6 mm), single-stage thermoelectric device having 127 numbers of thermocouples with a rating current of 15 amperes for operation. [Table 1] gives the specifications of TEC1-12715.{Table 1}

The hot junction side of the TEC module is contact coupled with the metallic surface of the fan/heat sink combination with external fins for conduction cooling. A conical-shaped sample collection device is fabricated using copper material to have an optimized and efficient cooling surface. The schematic diagram of the sample collection device is shown in [Figure 3].{Figure 3}

This sample collection device is mechanically fitted with the cold junction side of the TEC module. The moisture condensate from the copper sample collecting surface is collected in a sample holder kept below it. All the parts of the system are assembled within an aluminum frame. This frame has been designed open to ambient air to have no restriction on air movement through the moisture condensing surface fitted to the TEC module. This ensures the appropriate collection of a representative sample from the surrounding air to assess T concentration in the work environment.

Tritium sample collection method with thermoelectric cooling module

When the power supply to the device is switched on, the cooling is sensed on the cold ceramic side of the TEC module instantaneously and it is followed by condensation of moisture over the moisture collection surface [Figure 1]. The condensed moisture over the moisture collection surface is collected in a glass Petri dish by placing it below the moisture collection surface. The temperature, humidity, and moisture content in the air were measured using the dry- and wet-bulb temperatures of a sling psychrometer having two conventional side-by-side thermometers.

 Results and Discussion



T in air concentration was estimated using the sample collected with the air sampling system and the results have been compared with those obtained using the conventional cold finger method. Air samples were collected simultaneously at the same location using both the methods. The sample collection time was 30 min for each measurement. Dry-bulb and wet-bulb temperatures were noted at the same time to estimate the moisture content in the air. Tritium concentration in the air has been estimated by measuring the tritium concentration in 1 ml of collected moisture sample using a liquid scintillation counter. The dew point, relative humidity (RH) (%), and moisture content of air are also calculated using standard psychometric methods. The amount of water vapor in the atmosphere varied from 16.3 g/m3 to 17.2 g/m3, and this value has been utilized to estimate the accurate T concentration in air. The details of the measurements are given in [Table 2]. The deviations in measured tritium concentration using the Peltier sampling method are within 6% of the cold finger method, which shows that the results of the measurements are in agreement with each other.{Table 2}

The rate of sample collection depends on various factors such as temperature, humidity of ambient environment, and air movement. The sample collection efficiency of the system has been studied by collecting the samples in different environmental conditions with respect to temperature and humidity. The volume of sample collected in 30 min with respect to the RH (%) of the environment is plotted in [Figure 4]. The samples were collected in the facility environment with the temperature ranging between 25°C and 26°C. The volume of sample collected varies from 1.85 ml at 55% of RH to 3.6 ml at 92% of RH. [Figure 4] shows that the volume of the air sample collected at a specific temperature is proportional to the prevailing RH (%).{Figure 4}

Approximately 2 ml of ambient atmospheric moisture sample is collected at RH level of about 60% and temperature of ~ 25.5°C for a collection time of 30 min. The volume of sample required for analysis can be collected within 20 min at the same ambient environmental conditions, whereas at locations having higher RH values, one can collect larger volume of sample in less time.

The consistency in sample collection efficiency with respect to the operating duration of the system has been tested by measuring the sample collected in different sample collection timing in the same environmental conditions. The quantity of sample collected in 15 min–120 min of collection duration varies from 0.7 ml to 6.8 ml at temperature and humidity in the range of 24°C–25°C and 57%–62%, respectively, as shown in [Figure 5]. The linear rise in the sample quantity with the sample collection duration shows the consistency of sampling efficiency.{Figure 5}

Comparison of sampling efficiency with respect to conventional method

The condensed moisture sample from air with the conventional condensation method, cold finger method varies with the volume and surface area of the sampling beaker. However, conventionally used 250 ml of beaker filled with dry ice collets 1.65 ml of moisture on its outer surface in half an hour of sampling duration at dry-bulb temperature and RH of 25°C and 57%, respectively. The developed Peltier effect-based tritium in air sampling system also collects approximately the same amount of moisture from air in the same ambient conditions.

As compared to the conventional method of bubbler with water or cold strip condensation using dry ice, the present device is faster and gathers direct air condensation over the collection surface without using any medium such as dry ice. This sampling method is easy to implement and further this low-cost device can be fabricated using locally available components. This system was successfully employed for the sampling of T in air during nonavailability of dry ice and emergency conditions. The only drawback that can be pointed out is that it requires power supply instead of dry ice and hence cannot be used for remote environmental sampling. It is planned to employ the presently developed TEC-based T in air sampling device in various facilities handling tritiated heavy water. A battery-operated system can be implemented for environmental sampling.

 Conclusions



A low-cost, portable TEC module-based T in air sampling system has been developed and tested in elevated levels of airborne tritiated environment. The sampling and activity analysis was carried out using TEC module and dry ice-based sampling technique to have a comparison of the tritium activity measurements. The deviation in the measured tritium concentration collected with Peltier effect-based air sampling system is within 6% of cold finger method. Therefore, the tritium concentration measured with Peltier effect-based air sampling unit is in good agreement with cold finger method. The required sample for analysis can be collected in less than half an hour duration and the use of electronic cooling devices has removed the uncertainty in the availability of dry ice or desiccant.

Acknowledgments

We acknowledge the guidance and encouragement given by Dr. D K Aswal, Director, HS&E Group, BARC during this developmental work. We are immensely thankful to Dr. R. K. Gopalakrishnan, Ex-Head RHCS, HPD, BARC for encouragement and guidance to develop the sampling system. The encouragement from Shri Kunal Chakrabarty Head, Reactor Operation Division, Shri P. Sumanth, Head of Research Reactor Maintenance Division, and Shri S C Parida Head, Process Development Division is gratefully acknowledged. We thank Shri B G Avhad, Head, Engineering Service Section, RPhD, and colleagues at RC&IG workshop for the fabrication of the mechanical assembly. The valuable suggestions during the work from Shri Sajin Prasad, Shri Lalit K Vajpyee, and Shri K. S. Babu are gratefully acknowledged.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.

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