A Continuous Air Monitoring Sampler for 125 I - RIS-125

Transcription

1 A Continuous Air Monitoring Sampler for 125 I - RIS-125 S.Levinson, I.Belaish, T.Kravchik, U.German, O.Pelled, Y. Laichter, Y.Mazor, U.Wengrowicz, E.Dolev, H.Assido, D.Tirosh Nuclear Research Center Negev, P.O.B. 9001, Beer-Sheva, Israel INTRODUCTION Radioactive iodine is a typical fission product. Of the many iodine isotopes that can be generated in nuclear reactors only four are considered as radiobiologically significant. These are: 125 I (T 1/2 =60d), 131 I (T 1/2 =8d), 133 I (T 1/2 =21h) and 135 I (T 1/2 =7h). The main chemical forms that have been identified in reactors are I 2 (elemental), organic iodides (CH 3 I), and inorganic iodides (HOI, HI). Radioiodine is generally released as a gas, but can be adsorbed on air particulates to form radioiodine containing aerosols. Therefore, its monitoring has to include both gas and aerosol sampling. A new monitoring system, "RIS-125" (Radioactive Iodine Sampler for 125 I), has been developed to monitor 125 I (gas and aerosol) on-line in workplaces. The system samples the air containing the radioiodine through a transport line connected to an activated charcoal cartridge holder. A counting unit consisting of a 2- inch diameter x 1-mm thick NaI(Tl) detector coupled to a photomultiplier tube is positioned in front of the cartridge and detects the radiation from the accumulated 125 I. The information is processed, and the results are displayed continuously. The system was designed and manufactured at Nuclear Research Center Negev (NRCN). It was checked according to the ANSI-N42.17B standard (1) and the Minimum Detectable Activity (MDA) and Decision Limit (Lc) of the system were determined according to ANSI-N13.30 (2). The present work describes the system, its characteristics, and the check results of the last prototype. The system is routinely employed at NRCN. It can be easily switched to 131 I continuous air monitoring by replacing the detector with a thick NaI(Tl). SYSTEM DESCRIPTION GENERAL The "RIS-125" is a single channel monitoring system. It samples, measures, calculates and displays the 125 I radioactive aerosol concentration in ambient air. The system can be operated stand-alone or connected to a computer or recorder. It provides alarms when the concentration values exceed predetermined threshold levels. The system consists of the following main parts: A NaI(Tl) low energy scintillation assembly, including a suitable photomultiplier which is optically coupled to the scintillation detector. An electronic unit containing the pulse processing elements, the display module, a control module and a communication module. An air sampling unit which provides stabilized flow rate through a 1-inch diameter polished stainless steel sampling pipe to a filter cartridge which collects the Iodine aerosols or gases. The block diagram of the system is shown in figure 1. 1

2 Alarm Computer Electronic Unit PM-10 Detector Recorder Flow Meter Pump Air Manifold Air outlet Filter Cartridge Air Inlet Figure 1. Block diagram of RIS-125. THE DETECTION UNIT The PM-10 detection assembly contains a nominal 2-inch diameter x 1-mm thick NaI (Tl) scintillator detector coupled to a photomultiplier tube (PMT). The surface sensitivity of the detector is 440 cpm/(bq/cm 2 ) for the gamma energy emitted by 125 I (27 KeV). The detection unit includes also a single channel analyzer (SCA), which selects output pulses from the photomultiplier of energies corresponding to 27 ± 5 kev, a scaler, a built in test circuitry and TTL level drivers. THE ELECTRONICS The electronic unit handles the output pulses from the detection unit and samples the airflow through the filter cartridge. The electronic unit is based on 4 standard modules developed at NRCN. These modules are a processing and display module, a DC power supply module, a communication control module and a solid state relays module. All the modules include an internal front panel and a rear connector to the electronic unit motherboard. They are interchangeable with similar modules without any need for calibration or maintenance changes. The front panel of the electronic cabinet includes a dot matrix LCD with internal backlight, digit switches for operating function selection and operation buttons. The electronic unit provides also outputs to a serial line printer or RS232 PC type computer port and a selectable range analog output to a recorder. Serial communication to RS485 based net is also available. In normal operation mode the electronic unit constantly displays the radioactive iodine aerosol concentration and measuring time. The instrument enables on-line selection of the measuring unit on the display : cpm, nci/m3, Bq/m3 or DAC (as defined by ICRP 54 (3) ). In case of instrument malfunction or concentration higher than a pre-selected threshold, or a fast gradient of the measured count rate as a function of time, an alarm is activated. The system is designed to a maximum concentration value of about 100 DAC. THE MECHANICS The detector assembly and the filter housing are mounted in a 50 mm. thick cylindrical lead structure with copper lining. A drawer can be conveniently pulled out for filter changing. When closing it back, the filter is pressed against the detector chamber to force the air flow to pass through the filter. The whole mechanism is locked in place by a half turn handle. The radioiodine filter cartridge has a diameter of 2 1 / 4 -inch and a height of 1-inch. The adsorbent is activated charcoal (coconut shell carbon type) with 5% (by weight) TEDA impregnation and has 30x50 mesh size. The pumping unit consists of a 60-lit/min. air pump, a flow rate regulation valve and an electronic flow meter. The pump is mounted on shock absorbers under the shielded assembly. The air sampled is directed through a polished stainless steal pipe (1-inch diameter, including a 90 0 bend) towards a chamber in front of the filter. From here, the air flows through the filter cartridge, where aerosols are collected and counted by the detector. The chamber creates a uniform air flow and an homogeneous aerosol collection on the filter surface. The sampling line characteristics were chosen to minimize aerosols' deposition during their transport. Static charge effects were minimized by using a grounded stainless steel tube. The line's geometric dimensions 2

3 were selected to achieve an efficiency greater than 50% for respirable aerosols. The transport efficiency was calculated using the DEPOSITION software (4), which is recommended by the USNRC (5) as an acceptable means of calculating aerosol's transport efficiencies. Based on these calculations, the diameter of the sampling line was chosen to be 1-inch, with polished inside surface. The chamber s dimensions were chosen to have the highest aerosol's transport efficiency, on one hand, and the shortest distance between the detector and filter cartridge (in order to achieve the lowest detection limit), on the other hand. The sampling line angle was designed with a curvature radius 5 times greater than the inner line diameter, in order to minimize aerosol's inertial deposition. The sampling line and filter cartridge can be easily removed from the system for decontamination and cleaning. A view of the transport and the collection part of the system is given in figure 2. A general view of the whole "RIS-125" is given in figure 3. Figure 2. The "RIS-125" transport and collection assembly. 3

4 Figure 3. The "RIS-125" - general view. THE CALCULATIONS ALGORITHM The "RIS-125" allows a direct display of the measured radioiodine concentration in air. The filter accumulates the radioiodine present in the air stream, and the count rate increases accordingly. In order to calculate the activity concentration in air we define first the concentration in the n th minute as: C(n)=[cpm(n)- cpm(n-1)]/(vη) Where V is the air flow rate through the filter cartridge, η is the detection efficiency and cpm(n) is the count rate at the n th minute. If we average the concentration over the last k minutes we get: C(n)=(cpm(n)-cpm(n-k))/(Vkη) The average count rate for the period of k minutes ending at minute n is found by : cpm(n,k)=[counts(n)-counts(n-k)]/k 4

5 By inserting this expression for cpm in the former formula we get: C(n)=[counts(n)-2counts(n-k)+counts(n-2k)]/(Vηk2) We have chosen k=10 minutes in order to get a quick response of the system to air contamination changes with some averaging to minimize fluctuations. In order to verify the algorithm and its sensitivity to concentration changes, we used a WAVETEK 100 MHz DDS Function Generator controlled by a PC computer. A simulation of the case of a 100 DAC air contamination during 30 minutes, beginning after 20 minutes of background sampling is presented in figure 4. Figure 4. Simulation of a 125I concentration step function of 100 DAC (from minute 20 to minute 50). CHARACTERISTICS The following characteristics apply to the latest prototype of "RIS-125", which serves now at NRCN. The initial background count rate was measured several times, and its average value was found to be about 4 cpm. The background increases with time because of the environmental radioisotopes collected continuously on the filter. The efficiency was measured using calibrated point sources of 125 I. These measurements were repeated several times. The efficiency value for a point source was found to be 0.22±0.01 cpm/dpm. The efficiency of the system was determined also for a filter cartridge; it was found to be 0.02 cpm/dpm. The difference between the two values is due to geometrical and absorption factors. The MDA (minimum detectable activity) and Lc (decision limit) were calculated according to ANSI- N13.30 (2) for 125 I. The MDA is determined by the background level, the system sensitivity and the counting time. It is calculated by the expression : MDA = [(3+4.65*(B)1/2)/(ηTV)] Where B is the background count, η is the total efficiency,t is the acquisition time and V is the air volume that passed through the filter during T. The value of the minimum detectable concentration evaluated for a background level of 4 cpm is ~3.5 Bq/m 3. An estimation of the time required to pass the decision limit Lc is presented in table 1 for several 5

6 background levels. Different values are obtained for gases and aerosols, as only aerosols are deposited during transition through the system and their transmission efficiency is less than 1. It can be seen that in case of a 1 DAC air contamination and a background of up to 100 cpm, the "RIS-125" needs up to about 1/2 minute to alert for both aerosols and gases. T - Time required (sec) for Lc to reach 1 DAC - for gases. T - Time required (sec) for Lc to reach 1 DAC - for aerosols. (AMAD = 5µm) Background (cpm) Table 1. The time required to achieve Lc = 1 DAC (as defined by (6)). The ANSI N42.17B Standard (1) requires a standard deviation of less than 10% when calibrating with known sources with an activity greater than 2000 dpm, and less than 15% for sources with an activity less than 2000 dpm. The "RIS-125" checks show a standard deviation of ~3.3% for a 4000 dpm source and ~8.5% for a 400 dpm source, within the range of the requirements. Long range stability measurements were also performed using an 125 I source. Each measurement lasted 5 seconds and each result is an average over 250 seconds. Figure 5 presents the results of over 600 hours of continuos operation (corrections were made for radioactive decay). The standard deviation of the count rate average during this period is ~6%. It can be seen that the system stays stable during long periods of operation COUNTS (cpm) TIME (hours) Figure 5. Stability measurements of the "R.I.S-125". SUMMARY 6

7 A continuous air monitoring system for 125 I was designed and manufactured at NRCN. It is a single channel stationary unit which samples, measures, calculates and displays the 125 I concentration in ambient air. The transport section was optimized and the system's characteristics were determined. The "RIS-125" was checked according to the requirements of the relevant Standards and was found to comply with these requirements. BIBLIOGRAPHY 1. ANSI N42.17B Standard, Performance Specifications for Health Physics Instrumentation - Occupational Airborne Radioactivity Monitoring Instrumentation, (1989). 2. ANSI N13.30 Standard, Draft American National Standard for Performance Criteria for Radiobioassay, (1989). 3. International Commission on Radiological Protection, Individual Monitoring for Intakes of Radionuclides by Workers, ICRP Publication 54, (1987). 4. Anand, N.K., McFarland, A.R., Wong, F.S., Kocmoud, C.J., DEPOSITION: Software to Calculate Particle Penetration Through Aerosol Transport Systems. Office of Nuclear Regulatory Research, U.S. Nuclear Regulatory Commission, Washington D.C , NUREG/GR-006, (1993). 5. U.S. Nuclear Regulatory Commission, Regulatory Guide 8.25: Air Sampling in the Workplace, Draft DG-8003, Office of Nuclear Regulatory Research, (1992). 6. Safety Series No.115-I, International Basic Safety Standards for Protection against Ionizing Radiation and for the Safety of Radiation Sources, IAEA, (1994). 7