Assessment of Occupational Radiation Exposures at Ghana Research Reactor-1 Facility

Abstract

The annual occupational doses for workers at the Ghana Research Reactor-1 facility were assessed for the period 2018–2021. The dose records of monitored staff were retrieved and analysis done for dose distribution and collective effective doses. Thermoluminiscent dosimeters were used to monitor the occupational exposures. The dosimeters were evaluated for the cumulative radiation dose levels using the Harshaw 6600 TLD reader system. Annual dose of 1.52 mSv/year was the maximum acquired by an individual. An annual average effective dose range of 0.20–1.36 mSv was determined for all workers. The annual total collective effective dose was established to be in the range of 0.40–10.08 man-Sv. The 20 mSv annual limit for occupational exposure was not exceeded for monitored workers. The assessment shows that the GHARR-1 facility, in terms of radiation health effects, is a favorable environment for workers since exposures are mostly below occupational exposure limit.

Keywords

collective effective dose thermoluminiscent dosimeter effective dose personnel monitoring occupational exposure

Introduction

The Ghana Research Reactor-1 (GHARR-1) is a miniature neutron source reactor (MNSR) of the research reactor category. The GHARR-1 is located in Kwabenya in Accra, Ghana. It is owned by the Ghana Atomic Energy Commission (GAEC) and operated by the National Nuclear Research Institute of GAEC. GHARR-1 is a low power research reactor similar in design to the Canadian SLOWPOKE reactor. MNSR reactors are operated in many other countries, including China, Iran, Nigeria, Pakistan and Syria, and are often used as a tool for neutron activation analysis (NAA) and capacity building. The operation of GHARR-1 is regulated by the Ghana Nuclear Regulatory Authority to ensure a high level of operational safety of the reactor.

GHARR-1 is of Chinese origin, and it belongs to the class of tank-in-pool reactors.¹ GHARR-1 uses an auxiliary Pneumatic Transfer System (PTS) for the transfer of sample capsules in and out of the irradiation sites. The 90.2 percent highly enriched uranium (HEU) core of GHARR-1 was recently converted to a 13 percent low enriched uranium (LEU) core. The conversion resulted in the thermal power of GHARR-1 increased from 30 kW to 34 kW while maintaining the neutron flux at 1 × 10¹² n/cm²s. The cold clean excess reactivity for fresh core is 4 mk.^2,3

The core region of GHARR-1 is located 4.7 m under water close to the bottom of a watertight reactor vessel. The water in the vessel serves the purpose of radiation shielding, moderation and as well as primary heat transfer medium. Moderation is achieved by natural convection using water. In addition, heat can be extracted from the water in the vessel by means of a water-cooling coil located near the top of the vessel.^2,3

The core (fuel) of the GHARR-1 is the main source of ionizing radiation at the facility. Other sources of ionizing radiation at the GHARR-1 facility include irradiated samples, detector calibration sources and non-destructive testing radioactive sources. These sources contribute to the occupational exposures of the workers at the GHARR-1 facility because of their use in the utilization, practices, and processes carried out at the facility.

The operations or activities that have the potential to cause exposure at the GHARR-1 facility include but not limited to the following: unpacking irradiated samples for gamma spectrometry, performing transmission experiments using radioactive sources, radiation monitoring exercise during reactor operation, calibration of detectors, maintenance works on top of the reactor, etc. However, there are prevention measures in place to avoid exposure to the workers. These measures include the use of real time radiation monitors, the use of lead aprons and gloves, the use of the principles of time and distance, access control among others.

Occupational exposure monitoring of the workers of GHARR-1 is done in compliance to the regulatory requirement by the Ghana Nuclear Regulatory Authority. Pursuant to this requirement, the occupational exposure assessment was done by the Radiation Protection Officers of the GHARR-1 facility and the Personal Dosimetry Laboratory to determine if occupationally exposed workers at the GHARR-1 facility had their annual doses to be within the acceptable dose limits of 20 mSv/yr.

The usage of ionizing radiation sources at the facility has been ongoing safely since the last 2–3 decades due to their importance in the practices and processes carried out at the facility.^4–6 Much effort has been given to the advancement of radiation protection and safety of people, systems and environment concerned with ionizing radiation by the International Commission on Radiation Protection (ICRP) and International Atomic Energy Agency (IAEA).^7,8

Due to the importance and health concerns of occupational exposures of radiation workers, radiation monitoring, record keeping and assessment are enforced by authorities and regulatory bodies in the nuclear and radiological fields.^9–11

Personnel radiation exposure monitoring has been ongoing as part of the radiological protection and safety program of the GHARR-1 facility since commissioning of GHARR-1 in 1995. Personnel radiation exposure monitoring is a requirement by the Nuclear Regulatory Authority, Ghana, for all institutions involved in radiation related activities.¹²

Ionizing radiation is potentially harmful if one is exposed to significant doses. Notwithstanding the health effects, ionizing radiation has several helpful uses in scientific research, agriculture, industrial applications, and medicine.¹³ The radiation exposure risks to staff, public and environment arising from the application of ionizing radiation have to be assessed and controlled.¹⁴ A standard radiation protection and safety program should be available at the workplace to ensure that absorbed dose—amount of energy per unit mass deposited into tissues and organs of the body—limits of staff at nuclear and radiological installations do not exceed minimum thresholds, beyond which deterministic effects may occur. Deterministic effects of ionizing radiation are related directly to the absorbed radiation dose and the severity of the effect increases as the dose increases. A deterministic effect typically has a threshold below which the effect does not occur.

Ionizing radiation can trigger both deterministic and stochastic effects. Deterministic effects occur only above the threshold value of a dose. There is no threshold value for stochastic effects. Dose limits are recommended for managing exposures to ionizing radiation and protecting humans from its adverse effects. In line with radiation protection, these dose limits are set to prevent acute and chronic radiation-induced tissue reactions (deterministic effects) and to reduce the probability of cancer (stochastic effect) to a reasonably achievable level while maintaining the benefits to people and society from activities that generate radiation exposures.

The dose limit values are set so that deterministic effects are ruled out. To keep the risk of stochastic effects from ionizing radiation as low as possible, three general principles have been set out in radiation protection for dealing with ionizing radiation. These principles are Justification of practice, Dose limitation, and Optimization of safety and protection. These principles are based on recommendations from the International Commission on Radiological Protection (ICRP).

Radiation protection and safety programs are established to ensure effective management of radiation exposure to staff and public, release of ionizing radiation to the environment, compliance with regulatory requirements, and optimization of operational practices.¹⁵ There is an established Operational Radiation Protection Program (ORPP) to help achieve the objectives of radiation protection and safety program at the GHARR-1 facility. The ORPP covers risk assessment activities, including personnel monitoring aimed at preventing over exposure and avoiding unnecessary exposure of personnel working with various sources of radiation.¹⁶ Therefore, monitoring radiation doses received by staff of the GHARR-1 facility is mandatory to assist management to protect all staff and the public from over exposure and undue effects of radiation.

Radiation monitoring of GHARR-1 workers is done with the technical support of the Radiation Protection Institute which is the technical service provider for all the occupationally exposed persons within the GAEC. The Radiation Protection Institute also provides personnel radiation exposure monitoring for occupationally exposed workers of other facilities in Ghana whose practices include industrial radiography, nuclear gauge, well logging, gamma irradiation, diagnostic radiography, radiotherapy, nuclear medicine, among many others. The Radiation Protection Officer of the GHARR-1 facility is responsible for putting in place measures to minimize exposures of staff so that the annual dose limit of 50 mSv per a single year or 20 mSv average over five consecutive years is not exceeded. These dose limits are adhered to by scrutinizing each monitoring report received after the assessment period. All doses that will exceed recommended levels shall be investigated thoroughly.

The aim of this study was to assess if occupationally exposed workers at the GHARR-1 facility were monitored for their radiation exposure doses and whether their doses were within allowed limits. For the purposes of this assessment, dose records of staff of GHARR-1 facility from 2018–2021 were retrieved from the dose management system (DMS) and analysis done for annual dose, dose distribution, and collective effective doses.

Materials and Methods

Materials

Thermoluminiscent dosimeter. The Personal Dosimetry Laboratory (PDL) of the Radiation Protection Institute (RPI), GAEC issues thermoluminiscent dosimeters (TLD) to workers at the GHARR-1 facility on monthly intervals. The TLDs are passive radiation dosimeters, which detect ionizing radiation exposure by indicating the intensity of visible light emitted from a sensitive crystal in the detector when heated. Figure 1 shows a picture of a TLD card without a holder.

Figure 1.

A picture showing TLD card.

The badges/cards are placed in holders as shown in Figure 2 before issuing it to the workers. The type of holder used is 8814. The TLD-100 (LiF:Mg,Ti) that was used has the ability to measure both the deep and skin dose as it is equipped with the appropriate filter for such.

Figure 2.

A picture showing TLD card holder.

Every TLD assigned to a worker has been coded to match the details of the wearer.^9,17 The deep dose, [Hp (10)] measured the effective dose if the exposure is uniform over the worker's body,^18,19 and is the dose received by tissue (effective dose) at a 10-mm depth from the skin surface.²⁰ The interest of the study is on the deep dose, [Hp (10)] records of the working staff since it is the operational quantity recommended for radiation protection for personal monitoring.²¹

The Harshaw 6600 TLD reader system. The TLD cards were analyzed with the Harshaw 6600 TLD reader system. Figure 3 shows a picture of the Harshaw 6600 TLD reader system. It is enabled with an automatic calibration capability, and it can read a maximum of 200 dosimeters at a time. The Harshaw TLD reader is connected to an external personal computer (PC) and is operated through installed menu driven WinREMS software.²² The DMS is then employed in the dose assessment, dosimetry report generation and the ensuing electronic storage of dose records. The DMS estimates the “year-to-date” dose, corresponding to the summation of the doses from the start to the end of a specific year.⁹

Figure 3.

A picture showing Harshaw 6600 PLUS TLD reader.

Methods

For monitoring occupational exposures of staff to ionizing radiations, each radiation worker at the GHARR-1 facility is issued with a specific TLD to record radiation dose received in a given period. The TLDs were tagged with the worker's identification information. This assessment was done for staff of the facility for the years 2018–2021. At the end of each given period, the TLDs were collected and sent to RPI for evaluation. The dosimeters were evaluated for the cumulative radiation dose levels using the Harshaw 6600 TLD reader system. The DMS further manages the data and generates the required measurements.^23,24

The TLDs had sensitive lithium fluoride crystals or chips, which absorbed the ionizing radiation for storage during the period of measurement. Readout instruments extracted the stored data on heating the chips to measure the amount of emitted light, which corresponds to the dose of radiation absorbed by an individual.

The doses measured by the TLDs were the effective dose of an individual. The two dose quantities, deep dose [Hp(10)] and surface or skin dose [Hp(0.07)] were respectively reported for each staff member. The mean annual dose and the annual collective dose were further estimated from the measured doses by the TLDs as given in equations 1 and 2.⁹

The mean annual dose,

E_{m} = \frac{\sum W_{T} H_{T}}{N}

(1)

Where H_T is the equivalent dose in tissue T and W_T is the tissue weighting factor for tissue T

N = number of measurements in a year.

The annual collective dose, S, is given by⁹

S = \sum_{j = 1}^{r} N_{j} E_{j i}

(2)

Where E_ji is the annual dose calculated for the jth reading of the ith worker and N is the number of workers in a facility and r is the number of measurements in a year.

Results and Discussion

The results presented show the distribution of annual occupational doses for the various categories of workers at the GHARR-1 facility who are classified as occupationally exposed workers due to their involvement in practices employing ionizing radiation for the period 2018–2021. These working group categories include the Reactor Operation and Maintenance Group, Analytical Laboratory Group, Radiation Protection Group, and Administration Group. The Administration Group is not typically occupationally exposed. In this study, it was considered as occupationally exposed group as a precautionary measure. This is because they share the same building that houses the reactor and the other laboratories. Their offices are not so distant from the reactor hall. Again, once in a while they assist in housekeeping activities performed at the supervised areas. The basic data used were doses measured; deep dose—Hp (10), representing the radiation which penetrated 10 mm into the skin; and weak dose—Hp (0.07), referred to as the skin dose. The minimum detectable limit for deep dose and the skin is 0.1 mSv. The annual dose limit of 20 mSv averaged over five years is used as the standard but also the 50 mSv as the maximum dose for a single year could be applicable in some extremities. From Table 1, it can be observed that no worker received annual dose above the annual dose limit of 20 mSv. Most of the workers had exposures not more than 1 mSv/year with just a little of the workers having exposures in the range of 1 to 1.52 mSv. The recorded values did not exceed the ICRP recommended limit of 20 mSv per year.²⁵

Table 1.

Distribution of Worker Occupational Exposures in GHARR-1 Facility From 2018 to 2021.

Year	Total No. of workers	No. of workers in effective dose interval/year
Year	Total No. of workers	≤ 1 mSv	> 1 < 20 mSv	≥ 20 mSv
2018	13	13	0	0
2019	17	16	1	0
2020	16	14	2	0
2021	22	7	15	0

Table 2 shows that the annual average effective dose for all the categories of workers at the GHARR-1 facility ranges from 0.20 to 1.36 mSv. The Radiation Protection/QAQC group in the year 2021 recorded the highest annual average effective dose 1.36 mSv while the minimum annual average effective dose of 0.20 mSv was recorded by the Administration group in the year 2018. The annual total collective effective dose was determined to be in the range of 0.40–10.08 man-Sv. The analytical laboratory and the administration groups recorded the maximum and the minimum annual total collective effective dose respectively.

Table 2.

Film Badge Dosimetry Results of GHARR-1 Workers.

Occupational group	Year	Total No. of workers	Annual total collective effective dose (man-Sv)	Annual average effective dose (mSv)
Reactor operation and maintenance	2018	6	1.62	0.27
	2019	8	6.64	0.83
	2020	7	6.23	0.89
	2021	7	9.17	1.31
Analytical laboratory	2018	3	0.75	0.25
	2019	5	3.25	0.65
	2020	5	4.3	0.86
	2021	9	10.08	1.12
Radiation Protection/QAQC	2018	2	0.48	0.24
	2019	2	1.62	0.81
	2020	2	1.48	0.74
	2021	2	2.72	1.36
Administration	2018	2	0.4	0.20
	2019	2	1.38	0.69
	2020	2	1.5	0.75
	2021	4	2.52	0.63

It can be observed that the total number of workers kept increasing over the years as seen in Table 1. The annual average effective dose mostly increases from 2018 to 2021 for all the working groups as observed in Table 2. The trend of levels of the annual average effective dose could be attributed to several factors at the facility, such as staff workload and schedule, reactor operating period and job assignment for each year. Specifically, the duration of reactor operation time; types of materials undergoing irradiation (for instance, geological materials typically release more radiation than biological materials); number of operational activities within each year; and number of reactor annual maintenance and modification works are the main factors that can cause variation in the annual average effective dose. The increase in the number of workers over the years is a reason for the observed trend. The other major factor is that the utilization or operational periods of the reactor kept increasing gradually by the years after the core conversion project. This is also contributed to the observed trend in the annual average effective dose. For a particular year, the annual average effective dose varied slightly among the different groups. Notwithstanding, the mean annual dose for all workers for the four (4) year period under review is lower than 1 mSv which indicates a conducive environment for workers, especially female workers to work as reactor operators, neutron activation analysts or radiation protection officers in the sensitive zone of GHARR-1 facility. However, due to the increasing trend observed in the annual average effective dose, the fetal dose limit of 1 mSv per year could be exceeded during the period of pregnancy. Therefore, it is recommended that pregnant women be assigned to administration at no loss of pay for the duration of their pregnancies as is the best practice.

The analysis of the dose data for the period 2018 to 2021 indicates that none of the staff's annual dose exceeded the limit. All the categories of workers had their doses within the annual dose limit with the highest being 1.52 mSv/year as seen in Figure 4. The analytical laboratory and the reactor operation and maintenance groups form the greater portion of workers in the GHARR-1 facility.

Figure 4.

GHARR-1 facility personnel dosimetry results (2018–2021).

Figure 5 graphically shows the results of the average collective effective doses for all the worker category. The highest average collective effective dose was recorded for the reactor operation and maintenance group followed by analytical laboratory group, Radiation Protection/QAQC and administration groups which are 5.92, 4.60, 1.58, and 1.45 man-Sv respectively. For other research reactors of almost the same or similar power, these exposure levels measured for workers of the GHARR-1 facility compare well with monitoring data that is available.^26,27 This assessment gives a better understanding of the exposure dynamics, the suitability of working environment and the compliance to the regulatory requirements of the Ghana Nuclear Regulatory Authority. The results indicate adherence to radiation protection principles by the staff of the GHARR-1 facility.

Figure 5.

Average collective effective doses of the worker category.

Proper implementation of the exposure prevention measure has the potential to prevent over exposure of the workers. These findings show that the exposure prevention measures are effective in keeping exposures below the annual occupation exposure limits. The facility's policy of continuous regular monitoring of occupational radiation exposure to the workers follows regulatory requirements and best international practices. The regular monitoring is good in detecting any exposure that needs investigation and further action. These findings of below annual occupational exposure limit align with the facility's history of good safety culture and management. The ORPP that is at work at the facility is commended and needs to be continued as it has proven to ensure there is effective management of radiation exposure to staff.

Conclusion

The assessment of occupational doses for the various categories of workers at the GHARR-1 was done to ensure no worker was unduly exposed and that the levels were within acceptable limits of exposure.

The annual and average collective effective doses of the workers at the GHARR-1 facility were analyzed to estimate the total health effects associated with their practices involving ionizing radiation at the facility. The study showed that occupationally exposed workers at the GHARR-1 facility were monitored for their radiation exposure doses and their doses were within allowed limits. The 20 mSv recommended annual limit was not exceeded for the measured doses of all the workers observed during the four-year period and this can be attributed to the compliances of radiation protection principles workers. Notwithstanding, a stricter monitoring and evaluation process will be put in place to identify, and data prove the main and specific factors that contribute to the observed trend in the annual average effective dose at the GHARR-1 facility.

The assessment shows that the GHARR-1 facility, in terms of risk to radiation health effects, is a favorable environment for workers since annual radiation exposures are below the occupational exposure limit. This could be because of sensitization of workers and regular training on the importance of personal monitoring and radiation protection principles as part of the ORPP. The assessment will help in the further optimization of radiation protection at the GHARR-1 facility in its continuous utilization.

Footnotes

Acknowledgments

The authors appreciate the support of the Radiation Protection Institute of Ghana Atomic Energy Commission for their technical services in assessing the TLDs and the management and staff of the Ghana Research Reactor-1 facility for their cooperation with the research team and granting access to their dose data.

Declaration of Conflicting Interests

The authors declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The authors received no financial support for the research, authorship, and/or publication of this article.

ORCID iD

Kwame Gyamfi

Author Biographies

Dr. Kwame Gyamfi is a research scientist at the National Nuclear Research Institute, Ghana Atomic Energy Commission. He holds a Doctor of Philosophy degree in Applied Nuclear Physics from the University of Ghana. He is currently the Radiation Protection Officer responsible for the Ghana Research Reactor-1. His research works center around radiation safety, reactor safety radiological dispersion assessment, etc. He teaches and supervises both undergraduate and postgraduate students from the University of Ghana.

Philip Owusu-Manteaw is a Accomplished Radiation Protection researcher with developed skills in external dosimetry analysis and evaluation, working collaboratively to support high-performing research projects and technical services to effectively aid progress in Health Physics studies. He is committed to continuous process improvement through assessment of the effectiveness of functions and systems as well as current practices; streamlines standards and processes and develops innovative approaches to programme development and implementation; promotes collaboration with partners, colleagues across teams and stakeholders, and offers guidance and advice in his area of expertise on the application of scientific/professional methods, procedures and approaches. He holds an MPhil in Radiation Protection and BSc Chemistry, both from the University of Ghana and currently working with Ghana Atomic Energy Commission.

Dr. Edward Shitsi is a research scientist at the National Nuclear Research Institute, Ghana Atomic Energy Commission. He holds a Doctor of Philosophy degree in Nuclear Engineering from the University of Ghana. He is currently the Management System Officer responsible for the Ghana Research Reactor-1. His research works center around management systems (quality assurance and quality control) and reactor safety. He teaches and supervises postgraduate students from the University of Ghana.

Edith Amoakie Amoatey is a researcher at the Radiation Protection Institute of the Ghana Atomic Energy Commission. Over the past decade, she has been working at the Personal Dosimetry Laboratory, where she monitors occupationally exposed workers across the country. Additionally, she has been involved in the installation and operation of the National Dose Registry (NDR) and the new Dose Management System (DMS), provided by the International Atomic Energy Agency (IAEA). She also assists other African member states with the implementation of these systems. She holds an MPhil in Nuclear and Environmental Protection from the University of Ghana.

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