Monitoring Intakes of Pu/Am by External Counting: Current Status in India

Transcription

1 Monitoring Intakes of Pu/Am by External Counting: Current Status in India R.C.Sharma, T.Surendran and T.K.Haridasan Internal Dosimetry Division Bhabha Atomic Research Centre Mumbai , India. INTRODUCTION The current practice of internal dosimetry for occupational workers handling actinides, in particular Pu/Am, involves monitoring of intakes. This, in turn, requires assessments of internal actinide contamination of the relevant body organs. Such assessments are needed: a) more often and almost on a routine basis for lungs for the recent as well as old exposures; b) comparatively less frequently for skeleton and liver for older exposures and c) those of cuts and wounds as and when encountered. Therefore, direct assessments of organ (lung/liver/skeleton) burdens of actinides have formed an important part of the on-going internal dosimetry programmes at BARC. The results of direct assessments when used in conjunction with dosimetric and metabolic models of ICRP lead to the evaluation of intakes and hence the internal doses. Once embedded under tissue or tissue-like media, the actinides, essentially, constitute sources of low energy photons (LEPs; energy < 200 kev). Therefore, the most reliable direct measurements of lung/organ burdens of the actinides in-vivo are only possible by the external detection of LEPs accompanying their nuclear decays: for example for 239 Pu -ULX-rays (~ 17 kev) with 4.6% yield and 241 Am - NpLX-Rays ( ~ 18 kev) with yield 37.6% and 59.6 kev γ - rays with yield 35.9%. Highly sensitive in-vivo monitoring facilities are required in practice in order to measure low values of lung/organ burdens through the detection of low yield, low energy X/γ - rays. The external counting techniques for monitoring intakes of actinides ( 239 Pu/ 241 Am) thus, entail substantial developmental efforts and huge costs. This paper attempts to describe the current status of the direct methods of internal dosimetry of Pu/Am in India. It begins with the description of facilities established for monitoring intakes of actinides and then covers the procedures employed, monitoring philosophy, methodology for calculation of committed effective doses and lastly, gives results from the monitoring programmes as well as from follow-up studies which highlight new findings and some interpretational problems. IN VIVO MONITORING FACILITIES Shield :The shield consists of a totally shielded steel room with 20 cm thick mild steel all around and is divided into two compartments separated by a 15 cm thick steel wall. The large compartment (inner dimensions 3.8x1.8x1.9 m 3 ) is provided with 3 mm Pb lining inside and accommodates a linear scanning arrangement for the measurements of body/organ burdens of high-energy (E > 100 kev) gamma- emitters. Such measurements facilitate appropriate corrections to be made in the LEP spectral region. The small compartment (inner dimensions 2.4 x 1.8 x 1.9 m 3 ) is specially designed for in-vivo detection of LEP emitters and is provided with a graded lining inside (Pb 3 mm + Cd 2 mm + Cu 0.5mm) in order to achieve further background reduction in the low energy regions. The schematic diagram of the steel room shield is depicted in Fig. 1. Figure 1. The schematic diagram of the totally shielded steel room chambers at BARC showing the shield, location of the detectors and subject in measuring positions. Radiation Detectors: A variety of detection systems has been optimised and is in operation inside small steel room compartment. Old Phoswich Detector: This is a sandwich of NaI(Tl)-(200 mm dia x 3 mm thick) and CsI(Tl)-(200 mm dia x 50 mm thick) scintillators mounted on a single EMI-9623 B photomultiplier (model 203 YBE of Quartz and Silice). It has a radiation entrance window of 1 mm thick Be. The phoswich is currently being operated with pulse-shape discrimination (PSD) electronics built around ORTEC pulse-shape analyzer, model 1

2 458 (1). The detector has been operated continuously for the past 24 years and its initial PSD electronics was indigenously developed. Fig. 2 is the block diagram of the current PSD electronics in use with the phoswich and Table below brings out its performance. Figure 2. Schematic diagram of the pulse - shape discrimination electronics currently in use with phoswich. The logic is based on pulse-shape analyser, ORTEC model No.458. TABLE Background counting rates in low energy bands for two different types of phoswich detectors operated inside a totally shielded steel room with graded Z lining. ORTEC PSD electronics was used and PSD settings were optimised separately for each one of them. Detector background Old phoswich detector New phoswich detector 200 mm dia x 3 mm thick 200 mm dia x 12.7 mm thick Energy NaI(Tl) as primary NaI(Tl) as primary kev (cpm) (cpm) ± ± ± ± ± ± ± ± ± ± 0.5 Detection efficiency at 10 cm from window on axis (1) Old Phoswich : 241 Am in kev at 0% PSD loss = cpm / kbq. (2) New Phoswich : 241 Am in kev at 0% PSD loss = cpm / kbq. High Purity (HP)Ge Detector:It is the ORTEC model LO-AX /20 co-axial detector having 51 mm dia and 20 mm sensitive depth. The radiation entrance window of the detector is 0.5 mm thick Be. It has a portable 1.2 l liquid nitrogen dewar and is operated with ORTEC electronics (1). Cadmium Telluride Detector: It is a miniature semiconductor detector, model B-101 of Radiation Monitoring Devices (USA) and has 10 mm diameter and 2 mm thickness. It is operated at room temperature and is ideal for localised wound monitoring (1). Twin NaI(Tl) Detector System: Each NaI(Tl) (BICRON make) is 127 mm in dia x 12.7 mm thickness and is mounted in a one over each lung configuration. This is particularly useful for monitoring thorax burdens of uranium. New Phoswich Detector : A new 200 mm dia phoswich detector, model 203 YBEA 12.7 W 51, 2

3 selected on the basis of performance evaluation studies of four detectors, has been recently installed. It consists of 12.7 mm thick NaI(Tl) as the primary and 50 mm thick CsI(Na) as the secondary detectors. Its radiation entrance window is of MIB material. The detector is coupled to three (75 mm dia each) low noise photomultipliers type EMI 9765 Bo3. This phoswich is operated with the PSD set up given earlier in Fig. 2. It offers a figure of merit for detection of low energy photons which is comparable to that of the old phoswich. A comparison of the performance of this phoswich detector with the old one can be seen in the Table given above. Electronic Instrumentation: The entire electronic instrumentation (pulse-shape discrimination electronics, data acquisition and recording equipment and other electronic units) except the detector preamplifiers, is located outside the steel room shield. The data acquisition and recording equipment are common for all detector systems and consist of micro-processor based 4 K multichannel pulse height analysers (BARC model HPD 4K) linked with an IBM compatible personal computer ( PC Pentium MMX 233 MHz ) for on line data transfer, analysis, storage and retrieval. Recently, with the installation of a MCA Add - on Card (ECIL make) in the PC, a computer based multichannel analyser has also become operational. In addition, a closed circuit TV and music system have been installed inside the steel room as anti-claustrophobic measures. Calibration and Counting Geometries: Although initially, the phoswich detector was calibrated by an in-vivo calibration technique which involved inhalation of 103 Pd- 51 Cr labelled polystyrene aerosols by human volunteers, the current calibration factors for assessments of actinide lung burdens are based on measurements on the internationally accepted standard, namely, the realistic thorax phantom, designed and developed by Lawrence Livermore National Laboratory, (LLNL), (USA), and made available to us by the International Atomic Energy Agency(IAEA) as a part of an international intercalibration exercise ( 2,3). The calibration factors were obtained for 239 Pu and 241 Am sources uniformly distributed in the lungs of the phantom for different thicknesses of chest wall of three different tissue compositions - muscle (M), 50% muscle + 50% fat (MF) and 13% muscle + 87% fat (F). Whereas the calibration factors for 239 Pu showed prominent dependence on both, the thickness and composition of the chest wall tissue, those for 241 Am (using 60 kev photopeak) were comparatively much less affected by the tissue composition. Using the standard calibration curves drawn by measurements on the realistic phantom, appropriate calibration factor for any subject of known chest wall thickness and composition can be obtained. Calibration factors thus derived are used at BARC to get the lung burden values of the internally contaminated subjects (2,3). Three counting geometries with 200 mm dia phoswich were calibrated for assessment of Pu/Am lung burdens with the aid of realistic thorax phantom. The first is a single phoswich with its window horizontal, placed centrally over the chest of a supine subject and designated as Trombay Standard Geometry (TSG).The second geometry consisted of two phoswiches in one over each lung configuration with the detector windows horizontal. In the third geometry, phoswich placement was similar to that in TSG but its window was angled to follow body contour. For all geometries, the distance between the chest surface and detector window was kept as minimum as possible ( ~ 1 cm ). The first geometry is most commonly employed for conducting routine lung monitoring programmes for occupational workers. Figs. 3 and 4 depict the variations of calibration factors of 239 Pu and 241 Am respectively with the chest wall thickness and its composition for TSG. The calibration curves of these figures can be used to obtain calibration factors for lung counting of 239 Pu and 241 Am for any subject e.g. for a subject of mm chest wall thickness with 26.5 % adipose content in it, these work out to be 6.6 x 10-4 counts photon -1 and 0.83 cpm Bq -1 respectively. 3

4 Figure 3. Counting efficiency Vs. chest wall thickness for a single (200 mm) dia phoswich positioned centrally over the chest of LLNL phantom containing Pu impregnated lungs and other organs inactive. The two curves correspond to the overlays of 100 % muscle and 13 % muscle + 87 % adipose tissue materials respectively. The equations representing the two curves are given in the inset. Figure 4. Variation of counting efficiency for 241 Am with the chest wall thickness of three compositions M, MF and F for activity uniformly distributed in lungs of LLNL phantom.phoswich in Trombay Standard geometry. Full peak energy band (43-76 kev). Fitted least-squares equations for the three data sets are given in the inset. Data Analysis and Computational Techniques: Various γ - spectrometric data analysis techniques are incorporated in the software which is used to calculate and report the monitoring results in proper formats to several agencies/plants. Besides this, PC versions of linear least-squares code for analysing complex phoswich spectra, SAMPO code for analysing HPGe data and lung model codes for acute and chronic intakes using ICRP- 30 model (INDOS) as well as ICRP-66 model ( LUDEP 2.05 ) are also available for use as and when required. Quality Assurance: The Quality Assurance (QA) programme of an in-vivo counting laboratory for 4

5 actinides provides the continuing evidence that the lung/organ burdens being assessed have the desired accuracy which in turn, will ensure the correctness of internal exposure evaluations. Therefore, at BARC due emphasis has been given to QA programmes, in particular, regular checks are carried out on the performance of equipment and methods used for gamma/x-ray spectrometry and participations in intra and interlaboratory comparisons are encouraged. Over the past many years, BARC laboratory has participated in the following intercomparisons: i) Five intercomparison experiments between BARC and AERE, Harwell(U.K.) were conducted spread over a period of about two years. Each experiment involved the measurements on a human volunteer who had been administered via inhalation known quantities of 103 Pd- 51 Cr labelled polystyrene aerosols. ii) Participated in an IAEA sponsored Co-ordinated Research Programme (CRP) which was completed in 1988 and a report titled as ` International Calibration of Detector Systems for the Measurement of Low Energy Photon Emitters In-Vivo', was issued by IAEA in This CRP made use of LLNL realistic chest phantom (3). iii) Participated in another IAEA sponsored CRP on `International Intercomparison of Lung Counting Techniques for Actinides ( 238 Pu, 241 Am, Nat. & 3 % Enriched U and 232 Th) using a Reference Asian Phantom'. The final meeting of this CRP was held recently (4). IAEA is likely to issue the report of the CRP soon. MONITORING PHILOSOPHY, INTAKE EVALUATION AND CALCULATION OF INTERNAL DOSE Monitoring Philosophy: Assessment of Pu/Am in lungs by external counting requires the accurate determinations of many important variables. Among them are : a subject's natural background in low energy bands, spectral interferences from other incorporated radionuclides, thickness of overlying tissue (chest wall thickness) and its composition (adipose content), precise calibration factor for the subject being monitored, isotopic composition of the contaminant and its distribution (uniform or non-uniform) in lungs as well as skeleton, particularly, rib bones. Suitable methods have been devised at Trombay to take into account the effects of all these variables, so that an accurate estimate of an assessed lung burden of Pu/Am could be obtained. Usually, phoswich positioned centrally over the chest of a subject (geometry TSG) is employed for routine measurements of actinide lung contamination. Phoswich is also used for assessments of liver and skull burdens. Measurements with the HPGe detector occasionally supplement phoswich findings. For ascertaining Pu-intakes, all measurements of organ (lung, liver and skull) burdens, generally assume 241 Am as a tracer for Pu. Pu lung burden can, then be obtained from that of 241 Am and the ratio of Pu to Am in the contaminant aerosol. For observed cases of lung contamination, the validity of 241 Am tracer assumption or otherwise is always established. The philosophy has been that all workers handling type S Pu compounds are monitored for lung activity under routine monitoring programme once a year. Over and above this, any operational or accidental intake of type M or S Pu is monitored as soon as possible or within 10 days following the incident. Monitoring Programmes: In order to readily obtain actinide intake values corresponding to the results of direct measurements, monitoring programmes are conducted in accordance with the principles described in the publication 54 of International Commission of Radiological Protection (ICRP) (5). These fall under four categories: routine, operational / special, confirmatory and follow-up. Further details regarding these programmes can be found in Paragraphs 71 to 84 of Reference ( 5 ). Evaluation of Intake and Internal Dose Earlier, internal doses were evaluated by the following two methods a) Using ICRP 54 methodology (5) and b) Using ICRP 30 methodology (6). However, ICRP has adopted a new model for the human respiratory tract recently (1994), details of which are given in ICRP- 66 ( 7 ). The new model represents more realistically the deposition of inhaled particles in the respiratory tract and biokinetic behaviour of inhaled radionuclides than does the 1979 model. ICRP in publication 68 has brought out a compendium of committed effective doses per unit intake (dose coefficients) for about 800 nuclides based on new lung model and on some new information on systemic models. ICRP as well as International Atomic Energy agency s ( IAEA ) document on Basic Safety Standard ( BSS ) have recommended the immediate use of new lung model and the latest dose coefficients for internal dose calculations for workers of nuclear industry. Therefore, we obtained the licenced version of software package LUDEP 2.05 which is a user-friendly personal computer program for calculating internal doses using the ICRP-66 respiratory tract model (8). The use of LUDEP (Lung Dose Evaluation Program) has been standardised by running a set of bench mark problems and scrutinising the results. These included whole body counting and bioassay data of some radionuclides and calculations of dose coefficients for M and S types of Pu for inhalation of 1 and 5 µm AMAD aerosols. The LUDEP has now been put to routine use and has also been used to re-evaluate the internal doses for the selected cases of Pu intakes. Following example illustrates the evaluation of internal doses with the methods mentioned above for one case of Pu intake discovered through the lung counting of 241 Am. i). ICRP - 54 Methodology 5

6 Measurement Result - Lung burden of 241 Am on 5th day post intake = 200 Bq DRL s = 3.1 Bq ; ALI ( 241 Am class Y ) = 600 Bq Intake of 241 Am = I s = (1/30) x 600 x (200/3.1) = 1.29 KBq Total intake of alpha activity (class Y)= (3.8x )KBq = 6.19 KBq Commited Effective Dose Equivalent (CEDE) = 6.19 x 10 3 x 8.1 x 10-5 Sv = 500 msv Here, Pu: Am Ratio = 3.8 in the inhaled aerosol. The CEDE result obtained from the INDOS code was the same. ii). ICRP - 30 Methodology Commited Dose Equivalent ( CDE ) to lung (mass=807g)=1.6x10-10 x1.67x10 11 x Sv = 3.46 Sv CEDE = 0.12 x 3.46 Sv = 415 msv iii). ICRP 66 Methodology LUDEP results gave Intake of Type S- 241 Am (5µm AMAD)=3.29 KBq. Pu:Am Ratio = 3.8; So, intake of Pu = 3.29 x 3.8 = KBq Total Alpha Activity Intake (Type S) = = KBq Now, Dose Coefficient - 5 µm AMAD=8.647 µ Sv/Bq (Type S) Committed Effective Dose = x 10 3 x x 10-6 Sv = msv Thus, the calculated CED is almost a factor of 4 less than that obtained from the ICRP - 54 methodology. Currently, at BARC, ICRP-66 methodology is being used for internal dose evaluations. IN-VIVO MONITORING RESULTS FOR OCCUPATIONAL WORKERS OF FUEL REPROCESSING PLANTS The presently achievable minimum detectable activity for lung counting at BARC for a subject of 20 mm chest wall thickness and counting time of 50 min is about 6 Bq for 241 Am. Although this may be inadequate to identify inhalation exposures at the derived recording levels ( DRLs) of actinides, yet the direct methods have proved very useful in conducting the mentioned types of monitoring programmes rigorously. However, the only detectable lung contamination cases due to Pu/Am have been observed in special/operational monitorings and not in routine, despite the fact that about 300 subjects have been monitored every year for the past many years. In fact, the results of routine lung counting for 241 Am, recorded as net count-rates in the energy band (43-76 kev) from the phoswich used in standard lung counting geometry (TSG) showed log-normal distributions for control subjects (geometric mean 48 cpm) as well as for the workers population handling Pu (geometric mean cpm). Thus, the routine counting data of workers year after year have not shown any significant departure whatsoever from the data typical of control subjects. Some results of follow-up studies on selected cases of internal contamination due to Pu/Am are given in the next section. FOLLOW-UP STUDIES AND SOME INTERPRETATIONAL PROBLEMS Once a case of detectable inhalation intake is identified, the first estimate of internal dose can be obtained by using ICRP biokinetic models. However, initial dose estimates can be refined and replaced with the individual specific estimates only after obtaining a set of follow-up measurements which could last for years for intakes of both M and S types of plutonium compounds, since not only the lung burden would be required to be assessed but also the skeletal and liver burdens. This clearly underscores the importance of long-term follow-up studies on the cases of detectable intakes of Pu/Am. However, the data obtained from actinide lung counters could have several interpretational problems. The following examples taken from our studies serve to illustrate some typical situations one may encounter while using direct methods of internal dosimetry of actinides. 6

7 Type S Behaviour of 241 AmO 2 in Lungs: For workers of fuel reprocessing plants, inhalation exposures can occur either to Pu aerosols or to a physical mixture of Pu and 241 Am aerosols or to aerosols of pure 241 Am compounds. Most often, Pu & Am are found to be in nitrate or oxide form, although their other compounds may also be involved. For exposures to all the three types of aerosols, exposure evaluation is possible by lung counting of 241 Am. However, if the origin of 241 Am is from the decay of 241 Pu present alongwith other isotopes of Pu, then 241 Am being embedded in the larger crystal structure of Pu, tends to follow the lung clearance pattern of Pu. For example, the results of follow-up measurements of 241 Am in lungs of a subject who accidentally inhaled PuO 2 aerosols, yielded a lung clearance half-time of 1444 days and thus, showed type S behaviour of 241 AmO 2 in lungs ( Fig.5 ) in contrast to type M listed by ICRP for pure 241 AmO 2 ( 9 ). For such exposures, 241 Am could be legitimately used as a tracer for Pu. Figure 5. Observed clearance of 241 AmO 2 in the lungs of a subject. Fitted equation is given in the inset. It is worthnoting that in case of inhalation exposures to either a physical mixture of Pu and 241 Am aerosols or to the aerosols of pure 241 Am compounds (e.g. 241 AmO 2 ), the lung clearance patterns of Pu and Am would be different. Whereas 241 Am compounds will be absorbed as type M material, Pu could be absorbed from lungs either as type M or type S depending upon its chemical form. Obviously, in such exposure cases, use of 241 Am as a tracer for Pu would not be correct. Evidence for Very Long Retention in Lungs: Sometimes the follow- up measurements of lung burdens of 241 Am for cases of inhalation exposures to Pu aerosols can reveal a growth or build-up instead of decline of 241 Am activity in lungs with passage of time. This seems to be particularly true for exposures to Pu of relatively high burn-up origin. An illustration of this is given in Fig.6 which presents the results of follow-up measurements on a case who had an accidental exposure to high burn-up Pu aerosols. The observed linear buildup of 241 Am with time was explainable from the radioactive decay of 241 Pu atoms in lungs assuming that the Pu deposit in lungs has remained immobilised over a long period of time. For such cases showing more tenacious retention in lungs, it is possible to calculate Pu: Am ratio in the contaminant from the results of the lung burden measurements and the isotopic composition of the inhaled material. In the terminology of old lung model, this type of retention of Pu in lungs has been termed as Super Class Y ( 9 ). 7

8 Figure 6. Measured 241 Am lung burdens of a subject plotted against the elapsed time since first measurement. The least-squares fitted straight line clearly shows the build-up of 241 Am in lungs. Fitted equation is given in the inset. False Indications of Actinide Lung Burdens: It should be noted that injection intakes, particularly through wounds on hands could lead to depositions of actinides elsewhere in the chest region e.g. in axillary lymph nodes. For such deposits, the usually adopted lung counting geometries could give false indications of an apparent lung burden of an actinide. For example, in one case of actinide intake, the mapping of anterior chest with a HPGe detector, revealed the presence of the contaminant ( 241 Am) in the axillary lymph nodes and not in lungs. However, a phoswich positioned centrally over chest would give a positive result which could be erroneously interpreted as lung burden and therefore, the internal dose estimates based on the phoswich observations would, indeed, be substantial overestimations. Consequently, it is important that lung monitoring programs for occupational workers handling actinides are conducted with this caution in mind and that due consideration is given to cases of minor cuts/wounds in order to rule out overestimations of internal radiation doses. Another implication of this case is that the multiple types of exposures e.g. injection + inhalation, for occupational workers should be avoided as far as possible, since in the presence of activity in the axillary lymph nodes, it would be almost impossible to quantify the lung burden of an actinide by the external counting techniques ( 10 ). Interference due to 137 Cs Body Burdens: In the workers of fuel reprocessing plants, it is not unusual to come across cases of low level internal contamination due to the long lived cesium isotope, 137 Cs,which is known to distribute uniformly in the human body. Presence of 137 Cs in the body even at levels of about 300 Bq or above can also give false indications of Pu/Am lung contamination. Therefore, it is essential that whenever an abnormal phoswich spectrum is observed, the presence of 137 Cs should be ascertained and due corrections applied before quantifying Pu/Am in lungs. If the lung contamination due to both, 241 Am and 137 Cs is suspected, it is advisable to continue the follow-up measurements for 241 Am as well as 137 Cs body burdens. Such measurements can aid not only in identifying 241 Am lung burden but also in quantifying it more accurately. Alternatively, the measurement with a HPGe low energy photon spectrometer over the chest of the subject can help in identifying both 137 Cs and 241 Am. Interference due to 137 Cs body burdens needs to be kept in mind while implementing lung monitoring programmes for workers of reprocessing plants. Interference due to Surface Contamination over Chest: In accidental cases, sometimes surface contamination over chest due to Pu/Am results which may not get removed with the shower bath and thus may give false indications of lung contamination. However, unlike whole body counting of high energy gamma emitters, the surface or shallow contamination of LEP emitters over / in tissue can be readily distinguished from that in lungs e.g. shapes of the recorded pulse height spectra of 241 Am can clearly reveal if the deposition is under a few cm thickness of tissue (chest) or on its surface. In addition, follow-up measurements can also provide a clue to the surface contamination over chest as a very fast elimination of the contaminant from the surface could be expected. 8

9 CONCLUSIONS This paper has described the current status of direct methods of monitoring intakes of Pu/Am by external counting techniques at BARC. In particular, the facilities established, their rational utilization for conducting various monitoring programmes for workers and the methodologies for calculation of intakes and committed effective doses have been highlighted alongwith some important follow-up studies providing insight into the interpretational problems of operational health physics significance. ACKNOWLEDGMENT The authors express their appreciation to Dr. V.Venkat Raj, Director, Health, Safety and Environment Group for his support and keen interest in the work reported in this paper. REFERENCES 1. T.K..Haridasan, T.Surendran, R.C.Sharma and S.Krishnamony, Detection Systems for Monitoring Intakes of Actinides At Trombay. Radiat.Prot.Dosim. 51, No.1, (1994). 2. R.C.Sharma, T. Surendran, T.K.Haridasan, G.Krishnamachari, and S. Somasundaram, Measurements on IAEA Realistic Thorax Phantom: Evaluation and Efficacy to Investigate Cases of Internal Contamination.Final Report Submitted to IAEA, Research Agreement No.3843, (1988). 3. R.E.Toohey, D.Newton, A. Moiseev and A.Bianco, International Calibration of Detector Systems for the Measurement of Low Energy Photon Emitters In Vivo. Final Report of a Co-ordinated Research Programme ( Vienna: IAEA) (1991). 4. R.C. Sharma, T.Surendran, T.K. Haridasan and C.B.Ghatikar, Measurements on Reference Asian Phantom at BARC: Results and Preliminary Evaluation. Paper presented at the IAEA-RCM held at China Institute for Radiation Protection (CIRP), Taiyuan, China, Nov. 2-6, (1998). 5. ICRP. Individual Monitoring For Intakes Of Radionuclides by Workers:Design and Interpretation. Publication 54. Ann.ICRP 19(1/3),(Oxford:Pergamon Press) (1988). 6. ICRP. Limits for Intakes of Radionuclides by Workers. Publication 30 (Oxford:Pergamon Press) (1979). 7. ICRP. Human Respiratory Tract Model for Radiological Protection, ICRP Publication 66. Annals of the ICRP (1994). 8. N.S.Jarvis, A. Birchall, A.C.James, M.R.Bailey and M.D.Dorrian. LUDEP 2.05, A Personal Computer Program for Calculating Internal Doses Using the New ICRP Respiratory Tract Model. A Proprietary Software Developed by NRPB, Chilton, U.K. Report NRPB- SR 264 (1996). 9. T.Surendran, T.K.Haridasan, R.C. Sharma and S.Krishnamony, Experiences at Trombay In Monitoring Actinide Intakes By Occupational Workers By Direct External Counting. Radiat.Prot.Dosim. 59, No.1, 15-24(1995). 10. R.C. Sharma, T.K. Haridasan and T. Surendran, False Indications of an Actinide Lung Burden Arising from a Contaminated Finger Wound. Health Physics, 73(5), (1997). 9

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