Radiological safety during sodium diuranate dissolution process: A radiological data study

M Shailesh; SK Suman; PK Malti; V Pol; S Manna; RV Kolekar; SK Satpati

doi:10.4103/rpe.rpe_12_22

ORIGINAL ARTICLE

Year : 2022 | Volume : 45 | Issue : 2 | Page : 99-103

Radiological safety during sodium diuranate dissolution process: A radiological data study

M Shailesh¹, SK Suman¹, PK Malti¹, V Pol¹, S Manna², RV Kolekar¹, SK Satpati²
¹ Health Physics Division, Bhabha Atomic Research Centre, Mumbai, Maharashtra, India
² Uranium Extraction Division, Bhabha Atomic Research Centre, Mumbai, Maharashtra, India

Date of Submission	07-Apr-2022
Date of Acceptance	07-Oct-2022
Date of Web Publication	20-Dec-2022

Correspondence Address:
M Shailesh
Health Physics Division, Bhabha Atomic Research Centre, Trombay, Mumbai - 000 085, Maharashtra
India

Source of Support: None, Conflict of Interest: None

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DOI: 10.4103/rpe.rpe_12_22

Abstract

The article presents an overview of the occupational radiation protection aspects of experimental sodium diuranate transferring and handling processes required for refining utilization. The health physics aspects and associated monitoring programs necessary to adequately measure and control radiological exposures to workers during the process is described here. A particle size distribution study was also carried out and estimated the activity median aerodynamic diameter (AMAD) for the process. The AMAD varied from 4.6 to 7.7 μm and geometric standard deviation ranged from 1.9 to 2.8. Obtained data serve the purpose of comparison for formulating a detailed radiological safety protocol during regular operation.

Keywords: Activity median aerodynamic diameter, dissolution process, sodium diuranate, uranium

How to cite this article:
Shailesh M, Suman S K, Malti P K, Pol V, Manna S, Kolekar R V, Satpati S K. Radiological safety during sodium diuranate dissolution process: A radiological data study. Radiat Prot Environ 2022;45:99-103

How to cite this URL:
Shailesh M, Suman S K, Malti P K, Pol V, Manna S, Kolekar R V, Satpati S K. Radiological safety during sodium diuranate dissolution process: A radiological data study. Radiat Prot Environ [serial online] 2022 [cited 2023 Mar 28];45:99-103. Available from: https://www.rpe.org.in/text.asp?2022/45/2/99/364553

Introduction

To boost nuclear research and to meet the nuclear energy demand of India, adequate quantities of natural uranium are processed in different units of the Department of Atomic Energy, as most nuclear reactors are fueled by natural uranium base materials.^[1] Uranium extraction and purification from concentrated crude uranium are processed on a regular basis to produce nuclear-grade uranium metal. This is required for the fabrication of nuclear fuel for reactors in India.^[2] India's known conventional uranium resources are estimated to be approximately 2 lakh Tons U.^[3] In the last six decades, uranium concentrate in the form of magnesium diuranate has been produced by acidic leaching in India. A novel alkaline leaching route has been developed to treat and concentrate uranium from carbonate ore in the form of sodium diuranate (SDU). This SDU contains 55%–65% natural uranium.^{[4],[5],[6],[7]} For nuclear-grade uranium (more than 99.85% pure U), the SDU is been further processed to get pure ammonium diuranate (ADU). This involves the dissolution of SDU powder to produce impure uranyl nitrate solution, and further purification of the solution by solvent extraction using 30% Tri butyl phosphate (TBP) to get pure uranyl nitrate solution and finally produce pure ADU after precipitation. During the handling and processing of crude SDU powder, airborne uranium aerosols are the main hazards concerning radiation and chemical toxicity due to inhalation.

The present work is focused on radiation protection monitoring programs during the dissolution process of SDU. Results obtained from data collected as part of the radiation protection study were used to examine potential radiation exposures and related health risks to workers.

Before initiating the dissolution process, SDU powder is fed to the dissolution tank using a pneumatic conveying system. A particle size distribution study was carried out during the process and estimated the activity median aerodynamic diameter (AMAD) for the process. This is an important parameter for determining the fractional deposition of inhaled particles in the respiratory tract and the corresponding doses.^[8],[9] Aerosol characterization in various work locations involving dry operation stages of uranium processing was already estimated.^[10] It is always better to determine the site-specific particle size to reduce the bias obtained from the median values obtained by pooling data from other locations.^[11] Hence, location-specific values of AMAD and geometric standard deviation (GSD) for the dissolution process need to be measured for precise estimation of internal contamination. AMAD was obtained by assuming the particle size distribution to be log-normal.^[12]

Experimental process description

In dissolution process, SDU is dissolved in nitric acid in a closed tank to produce a uranyl nitrate solution. It is a batch process in which approximately one-ton quantity of SDU is dissolved in nitric acid. Ten batches (named 1-B to 10-B) were conducted on experimental basis. The transfer and addition process of SDU powder from drum to dissolution tank is split up into three steps during the study and collection of the radiological data in batch wise.

Opening of sealed SDU drum (Step A)
Pneumatic transfer of SDU powder to the hopper (Step B)
Powder addition from hopper to dissolution tank (Step C).

Materials and Methods

Size-selective sampling and estimation of activity median aerodynamic diameter

For characterizing the uranium aerosol, an ingeniously made cascade impactor named Particle Aerodynamic Size Separator was used. It is a seven-stage impactor with progressively decreasing cutoff diameters so that as the air stream passes through these stages, smaller and smaller particles are deposited on them. It has a backup filter paper to collect the residual small particles. The principles and operation of this instrument are described by Singh et al.^[13] The impactor is loaded with glass fiber filter paper up to the last stage. The samples were collected as near as possible to worker locations and the height of the inlet nozzle of the impactor was set at a height of 1.5 m. Sample collection time was so controlled that there was no re-entrance of particles in between the stages. At the end of sampling, the impactor was disassembled, and the samples from each stage were counted for alpha activity using ZnS (Ag) scintillation detector. The counts obtained on each stage were fitted to a log-normal distribution. From this, the distribution parameters, the AMAD and GSD, were evaluated.^[14]

Air activity sampling

Air samples were collected by a glass fiber filter having 99.9% trapping efficiency using a high-volume air sampler (F and J-T8400M series) and a continuous air monitoring (CAM) system. To get a representative sample of worker's occupancy during normal working condition, the appropriate sampling locations were selected. The sampling time selected was between 5 and 10 min and for CAM approximately 120 min during each experimental batch. A personnel air sampler (Gilian 12 from Sensidyne, USA made) was used for accurate assessment of the exposure to the worker due to airborne activity. It was worn by the worker during the entire operation. Alpha activity measurements of collected samples were carried out using ZnS (Ag) detector. It was calibrated using the standard source. The average efficiency observed was 23%. The minimum detectable activity for a sampling time of 60 min and a counting time of 3600 s is 1.8 mBq/m³. The counting was carried out after sufficient delay time in order to allow complete decay of the short-lived decay product of radon and thoron progeny. Portable Radon Monitor (SMART RnDuo) is used for build-up radon measurements from SDU drum. It is based on the principle of detection of alpha particles by scintillations with ZnS (Ag). The details of instrument are discussed elsewhere.^[15]

Area monitoring and surface contamination

Measurement of gamma and beta dose was carried out by using the RadEye B20-survey meter. The instrument is based on a pancake GM-tube detector with a window diameter of 30 mm. Measuring range is from 0.02 μSv/h to 100 mSv/h with a display resolution of 0.01 μSv/h. It can work in the energy range of 17 keV–3 MeV. The detector has a filter for beta dose rate measurement. The instruments were calibrated using different standard sources at BRIT, Vashi, BARC.

Results and Discussion

Airborne monitoring for long-lived radioactive dust

During the operation, the dry powder is pneumatically transferred to the dissolution tank from a stored drum. This leads to re-suspended dust which is a potential inhalation hazard for radiation workers. Airborne monitoring in process area was carried out based on related radiological and work condition during the operations. Applicable monitoring techniques includes personal air sampling, high volume air sampling and continuous air monitoring system in the work area.

Since the radiological composition of SDU is around 60% natural uranium, gross alpha analysis of air samples is appropriate for determining uranium activity in the air. The average airborne uranium activity level (in derived air concentration [DAC]) with different sampling methods is shown in [Table 1].

Table 1: Average airborne activity level during different steps of the dissolution process

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Percentage airborne uranium activity distribution during each step is given in [Figure 1]. Variable airborne uranium activity concentration was observed during the different steps of operation. The highest concentration has observed during Step A. Possible causes are fineness of power and mechanical movements involved. In addition to that, the presence of radon activity was also observed. Radon concentration increased from a background of 25 Bq/m³ to a maximum of 300 Bq/m³. Apply of local exhaust reduced the airborne activity and untrap radon gas immediately from the working zone during Step A and B process. It has reduced the airborne activity in further experiment runs, as shown in [Figure 2]. Radon concentration also came down to a background level of 25–30 Bq/m³.^[16]

Figure 1: Percentage uranium activity during different steps of dissolution for each batch

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Figure 2: Uranium activity distribution during Step A of dissolution for each batch

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Breathing zone sampling activity has been observed above one DAC (1 DAC of natural uranium for the medium class compound is 4 Bq/m³)^[17],[18] for most of the batch. Hence, a proper fit respirator (Honeywell 7580 P100) was the only feasible option to reduce the inhalation hazard. Hence, respirator was made compulsory for this operation.

Uranium aerosol size distribution

The distribution of radioactive aerosol was studied during the dissolution process and estimated the AMAD [Figure 3]. [Figure 4] shows that maximum uranium aerosol was associated with particle size higher than 5 μm.

Figure 3: Characterization of AMAD for typical sample from dissolution process. AMAD: Activity median aerodynamic diameter

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Figure 4: Typical size distribution of aerosol samples

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The mean of the AMAD and GSD for different samples of the dissolution process was calculated. The measured AMADs varied from 4.6 to 7.7 μm with a median of 5.3 μm. The GSD varied between 1.9 and 2.8.

It may be noted that there exists uncertainty in AMAD determination and air sampling due to sampling error and counting statistics of radioactivity measurement. It was estimated that this uncertainty was <10% of the reported value for all the measurements.

External exposure surveillance

External exposure monitoring is a need in the process. It is necessary when large quantities of SDU are stored and processed. This has led to need for monitoring and control of exposure during work near dissolution process, handling of SDU drums, and maintenance of these systems.

Extremity exposure from short-lived beta-emitting uranium progeny (²³⁴Th and ²³⁴Pa) is a safety concern during drum shifting and loading of SDU powder into the hopper via a pneumatic transfer system. Accordingly, gamma and beta exposure rate surveys were conducted on a batch basis to assess the degree of this potential hazard. The annual dose limit for the skin and the extremities is 0.5 Sv (50 Rem). Due to rotation of involved workers in each batch, it is unachievable for a worker to approach a dose of 10% of the annual exposure limit for skin or extremities. The collective dose incurred in this special operation was 3.1 P-mSv to the whole body and 7.8 P-mSv to the skin.

Contamination monitoring and control

The most effective measure to avoid any internal contamination is to prevent the spread of contamination. In the dissolution process, SDU is handled in both powder and an aqueous form until product drying and packaging. Potential surface contamination is one of the sources which cause inhalation via resuspension and ingestion via inadequate personal hygiene. Hence, contamination surveillance and control are necessary throughout the process. Several protective measures are taken during the process to reduce the spread of contamination such as local exhaust during all steps, areas, and personnel survey and imposing shoe barrier. As the natural uranium composition of crude SDU is about 60%, alpha contamination monitoring instrument and gross alpha analysis of swipe samples is appropriate for determining contamination levels. Within restricted areas, surface contamination levels of 0.37 Bq/cm² are usually acceptable.^[19],[20] These levels are low and ensure little contribution to airborne radioactivity yet are practical to meet. [Figure 5] shows the alpha contamination levels during the different experimental batches. Contamination level above 0.37 Bq/cm2 was observed due to SDU spillages in few cases. This is due to leakage from the joint at the junction of the pneumatic hopper and dissolution tank. The area was immediately decontaminated and reduced to well below the permissible level (<0.37 Bq/cm²). To stop recurrences of such unwanted spillage, properly fitted bellow was connected between the joints. This has reduced the spread of floor contamination in further batches, as shown in [Figure 5]. Urine analysis of involved radiation workers was carried out. It was observed no case of positive uranium internal contamination in radiation workers involved in this work.

Figure 5: Average floor contamination level (Bq/cm²) during each batch operation

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Conclusions

The present study has been confined to the evaluation of the radiological status of the dissolution process using SDU powder. The study has been based on monitoring results of radiation surveillance carried out for each step of the process during the experimental run. The same may serve the purpose of comparison for formulating a detailed radiological safety protocol during regular operation. Controlling airborne activity in Step A by implementing a local exhaust, use of a proper respirator, and optimum storage of SDU drum in the working platform has reduced the personnel exposure. No internal exposure was received by radiation workers during the experimental runs. Uranium particle size distribution studies show that AMADs varied from 4.6 to 7.7 μm with a median of 5.3 μm. The GSDs in these measurements varied between 1.9 and 2.8. According to our results, the AMAD value of 5 μm recommended in ICRP Publication 68 is a realistic value for radioactive workplace aerosols. The GSD approximation of 2.5 is also a reasonable value for these aerosols.

Acknowledgments

The authors are grateful to Dr. M.S. Kulkarni, Head, Health Physics Division, BARC, for his keen interest in this work. The authors are thankful to all UMRT teams, UED, BARC, for constant support during entire operation.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.

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