0952-4746/45/1/014001

In this submission we opine on India adopting a rather stringent maximum single year dose limit, instead of harmonizing with international standards. We explore how dose limits evolved, why India has opted for a lower maximum effective dose limit of 30 mSv for a single year and argue that raising this limit to at least 50 mSv, in line with International Commission on Radiological Protection (ICRP) recommendations, would not only contribute to upcoming revised ICRP publications but also support the realization of India's nuclear ambitions.

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India aims to achieve 100 GW of nuclear power by 2047 [1], a goal that addresses growing domestic energy needs with reduced carbon emissions. Among the many hurdles that may arise in this pursuit, overly cautious radiation dose limits for nuclear industry workers, stricter than international standards, are a particular concern. Dose limits are crucial for ensuring the safety and health of workers in the nuclear industry but enhancing its stringency in the interest of conservativeness will only pose challenges to reactor safety design, operational efficiency, and the overall cost of nuclear projects.

Globally and historically, the International Commission on Radiological Protection (ICRP), a global not-for-profit charity organisation, develops recommendations to ensure the safety of workers, society, and the environment from ionising radiation. Their comprehensive system of radiation protection, adopted globally, is based on three principles [2]: justification (activities must provide more benefit than harm), optimisation (protection should keep exposure as low as reasonably achievable, considering economic and social factors), and dose limits (controlling individual exposures to prevent undue and inequitable risk).

Defining and choosing dose limits involves social judgments to ensure that any radiation exposure just above these limits results in 'unacceptable' additional risks [3]. This becomes challenging when there is no clear cutoff between acceptable and unacceptable risk levels, as per the prevailing linear no-threshold (LNT) radiation risk hypothesis, which suggests that no level of ionising radiation is entirely risk-free [4]. This complexity means that setting safe exposure limits requires balancing health considerations with practical operational needs.

Over the years, the ICRP has attempted to address this issue through several publications like Publication 26 (1977) [5], Publication 60 (1990) [3], and Publication 103 (2007) [2], and recommend appropriate dose limits among other radiation protection measures. Currently, their recommendation for occupational workers in planned exposure situations is an average effective dose of 20 mSv per year over a sliding 5 year period, during which the total dose cannot exceed 100 mSv, and a provision that the maximum effective dose should not exceed 50 mSv in any single year [2]. Their recommendations have been widely adopted in nuclear industry regulations worldwide (see table 1), but with some notable deviations.

Table 1. Listing the effective dose limits for occupational workers adopted by various organisations and countries and how they relate to the ICRP recommendations.

ICRP [2]

5 year cumulative effective dose limit = 100 mSv

Max effective dose in a single year = 50 mSv

Average annual effective dose limit = 20 mSv

Countries with all dose limits SAME as the ICRP recommendationsRussia [6], European Union [7], Japan [8]Countries with the SAME 5 year cumulative dose limit and maximum single year dose limit as ICRP, but HIGHER (relaxed) annual dose limit (50 mSv).Canada [9], China [10]Countries which DO NOT ADOPT a 5 year cumulative dose limit and use a HIGHER (relaxed) annual dose limit (50 mSv)United States [11]Countries which DO NOT USE a 5 year cumulative dose limit and adopt the SAME annual dose limit as ICRP.France [12], United Kingdom [13]Countries which use the SAME 5 year cumulative dose limit and same annual dose limit as ICRP, but use a LOWER (stricter) maximum single year dose limit (30 mSv).India [14]

Most countries, including Russia, Japan, Canada, and China, fully or partially subscribe to ICRP's guidelines. The USA, however, adopts a more relaxed annual limit of 50 mSv and does not implement the 5 year sliding scale for cumulative doses. France and the UK also do not adopt the 5 year sliding scale concept; their regulations align with the ICRP's average annual limit of 20 mSv but do not specify a maximum dose limit for a single year. In contrast, India, while faithfully following the recommended values for 5 year cumulative dose limit and the consequent average annual dose limit, had decided years earlier to adopt a lower, stricter, maximum single year dose limit of 30 mSv, instead of the recommended 50 mSv [14].

Relaxed dose limits, such as those in the US, compared with ICRP recommendations, provide several advantages, including greater operational flexibility, economic benefits, and increased productivity through extended work hours, all while maintaining the safety of occupational workers. ICRP too recognises the virtues of flexibility and the fact that while their guidelines are robust and have successfully met protection objectives, they must evolve to incorporate advancements in science and societal changes to stay relevant and effective [15]. Consequently, they are in the process of developing revised general recommendations to replace ICRP Publication 103 (2007).

In this work, we examine how dose limits evolved, why India has opted for a lower maximum effective dose limit of 30 mSv for a single year and argue that raising this limit to at least 50 mSv, in line with ICRP recommendations or the USA's standards, would not only contribute to upcoming revised ICRP publications but also support the realisation of India's nuclear ambitions.

The ICRP bases its dose limit recommendations on a comprehensive evaluation of the detriments to human health associated with ionising radiation exposure.

Initially, in its Publication 26 (1977) the ICRP compared radiation risks with industrial accident mortality rates in non-radiation industries to set occupational dose limits. However, this approach had several limitations, such as variability in global industrial safety standards, the fact that dose limits apply to individuals while industrial data often reflect industry averages, and the consideration of mortality data alone, excluding non-fatal health conditions which could have led to less restrictive limits [5]. This publication also introduced the concept of 'detriment' to quantify the harmful probabilistic effects of low-level radiation exposure on humans.

In its 1990 publication (Publication 60), the ICRP adopted a more comprehensive approach to radiation exposure by considering various attributes of detriment, in the form of both mortality and morbidity. For mortality, these attributes included the lifetime attributable probability of death, time lost due to death, reduction in life expectancy, annual distribution of death probability, and increases in age-specific mortality rates. For morbidity, the ICRP considered non-fatal cancers and hereditary disorders and weighted them for severity and life impairment [3].

To convey the degree of risk tolerability to radiation exposures, the ICRP used certain subjective terms—'Unacceptable' exposures were those deemed unacceptable on any reasonable basis in normal practice, although they might be necessary in abnormal situations such as accidents; 'Tolerable' exposures were those that are not welcome but can be reasonably tolerated, while 'Acceptable' exposures were those that are acceptable without further improvement once protection has been optimised. Within this framework, the ICRP stated that a dose limit represents a boundary between 'unacceptable' and 'tolerable' exposures for controlling practices [3].

To establish a dose limit, the ICRP evaluated potential dose values by considering annual effective doses over a 47 year working lifetime (now considered 50 years) and the total accumulated dose, particularly for external and short-lived internal radioactive sources. They reviewed test values of annual effective doses—10 mSv, 20 mSv, 30 mSv, and 50 mSv—corresponding to lifetime doses of approximately 0.5 Sv, 1.0 Sv, 1.4 Sv, and 2.4 Sv, respectively, assuming constant annual doses over a working lifetime [3]. Their goal was to assess the consequences of continued exposure to these various effective dose values and identify the dose that results in consequences deemed just short of unacceptable, thereby defining that as the dose limit.

Based on this risk modelling in Publication 60, the ICRP determined that a regular annual effective dose of 50 mSv (a lifetime dose of 2.4 Sv) was too high and excessive and would not set a good standard for the relatively new nuclear industry [3]. Instead, they recommended setting dose limits to prevent the total effective dose from exceeding about 1 Sv over a working lifetime, with the application of radiological protection systems to ensure this figure is rarely approached. Dividing the 1 Sv lifetime dose over 50 years corresponds to an annual dose limit of 20 mSv.

However, the ICRP noted practical difficulties with enforcing lifetime dose limits. These included challenges in interpreting limits for workers with varied exposure periods, making long-term employment decisions for those exceeding lifetime limits, and the risk that long control periods might lead to rapid dose accumulation early on, weakening the emphasis on design-based exposure control. Therefore, the ICRP recommended controlling effective doses annually and introduced a flexible limit over a few years, while retaining annual limits. This approach was deemed to pose lesser practical problems in administering the limits, as compared to the lifetime limit.

To enable this flexibility, they proposed a 5 year moving period or sliding scale, averaging 20 mSv per year (cumulatively 100 mSv in 5 years), with a maximum of 50 mSv in any single year. The ICRP reasoned that a 5 year period would adequately limit these difficulties and provide sufficient flexibility, with the annual constraint for optimisation not exceeding 20 mSv [3]. A maximum value of 50 mSv in a single year was considered reasonable as long as it is not regularly incurred and the cumulative dose over 5 years remains below 100 mSv. The intention was to reserve this high value as a planning constraint for specific activities involving higher exposures. The ICRP's latest publication 103 (2007) [2] only reinforces the above considerations at least as far as the effective dose limits are concerned.

Reviewing the history of Indian nuclear regulations reveals that domestic dosimetric standards have closely followed ICRP guidelines. When the Atomic Energy Regulatory Board (AERB), the nuclear regulatory body of India, was established in 1983, it relied on ICRP Publication 26 (1977) [5] for guidance on dose limits for occupational workers. Consequently, the initial annual effective dose limit for Indian workers was set at 50 mSv, in line with ICRP recommendations at the time. There were no adjustments made to these limits based on India's specific reactor operations or socio-cultural context. But then again this was a time when India had limited experience, with only four operational nuclear power plants and a small occupational worker cohort for study, compared with the extensive global data available to the ICRP for forming their recommendations.

When ICRP Publication 60 came out in 1990 [3], the Indian nuclear regulator, just like other regulators around the world, were now faced with the problem of transitioning from a singular annual effective dose limit to a pair of annual and cumulative dose limits with a 5 year sliding scale, along with the need to define maximum dose allowable in a single year for special circumstances of planned exposure situations.

In response, AERB gradually reduced the annual effective dose limits for radiation workers over the period of 1991–1993 [16]: From 50 mSv in 1990 to setting the limit at 40 mSv in 1991, reducing it to 35 mSv in 1992, and further lowering it to 30 mSv in 1993. This gradual reduction aimed to help the Indian nuclear industry adjust to the stricter standards without causing operational disruptions. To keep in-line with the newly introduced concept of a 5 year sliding scale, AERB implemented a 5 year cumulative dose limit starting from 1 January 1994. For the period from 1994 to 1998, the cumulative effective dose for individual radiation workers was capped at 100 mSv. Furthermore, during this 5 year block, the annual effective dose for any worker was not to exceed 30 mSv. AERB also established stringent dose constraints for future radiation installations, including those still under design. The effective dose limit was set at 20 mSv per year for occupational workers and 1 mSv per year for the public. Starting from 1 January 1999, the 5 year cumulative dose constraint of 100 mSv was applied to consecutive 5 year blocks. These dose limits still apply.

While the ICRP suggested a maximum annual effective dose limit of 50 mSv to the occupational worker in special circumstances of planned exposures, AERB maintained a stricter limit of 30 mSv. To paraphrase the AERB Safety Directive 6/94 [17], 'AERB's implementation of these standards was in a way more conservative than the ICRP recommendations'. As far as we know, there does not appear to be any particular technical or scientific or even socio-cultural reason for opting for such conservatism.

To address the somewhat arbitrary nature of the current conservativeness, we advocate for harmonising with the ICRP-recommended value of 50 mSv as the maximum single-year dose limit in India. Several compelling arguments support this proposal. While increasing the maximum annual dose limit from 30 mSv to 50 mSv could yield operational benefits, its practical relevance largely depends on specific scenarios where the 30 mSv limit has posed significant constraints. Below, we outline examples and considerations illustrating how a higher limit could be advantageous, particularly in decommissioning and nuclear medicine, while adhering to the cumulative 5 year limit of 100 mSv.

4.1.  Decommissioning of aging nuclear facilities

The decommissioning of aging facilities, such as the CIRUS reactor at BARC or components of India's PHWR fleet, involves work in high-radiation zones [18]. Tasks like dismantling, cutting, and removing highly radioactive components result in rapid dose accumulation for workers. The current 30 mSv limit often necessitates frequent personnel rotation, interrupting workflow, increasing manpower requirements, and extending project timelines.

Adopting a 50 mSv limit would allow experienced and skilled workers to spend additional time on critical tasks without compromising long-term dose safety under the 100 mSv cap over 5 years. This adjustment could reduce manpower requirements, improve task continuity and enhance efficiency for specialised response teams.

4.2.  Reactor maintenance and life extension projects

Maintenance tasks, such as en-masse coolant channel replacement in PHWR reactors (e.g., KAPS-1) or steam generator repairs, often require proximity to high-radiation sources [19]. Under the 30 mSv limit, workers quickly reach their annual dose cap, necessitating large teams and extended project durations to distribute exposure [20]. This leads to increased operational costs and logistical challenges. Raising the limit to 50 mSv would:

(i)  

Enable highly skilled workers to complete high-radiation tasks more efficiently.

(ii)  

Reduce the need for frequent personnel rotations.

(iii)  

Shorten task timelines, such as those involving coolant channel replacements, without breaching cumulative dose safety limits.

4.3.  Nuclear medicine and radiopharmaceutical handling

In therapeutic nuclear medicine, such as the handling of high-dose radioiodine or lutetium-177 for cancer treatments, healthcare workers can accumulate doses near the annual limit during periods of high patient demand. This can restrict patient interactions and delay critical treatments [21]. For example, technologists handling PET-CT scans or SPECT imaging in high-volume hospitals often approach the dose threshold due to frequent exposure to high-activity radiopharmaceuticals. Increasing the limit to 50 mSv would provide greater flexibility for technologists and radiologists to manage higher patient volumes. It would also reduce dependency on additional staff or rescheduling, while also enhancing the capacity to deliver timely treatments.

4.4.  Large-scale decommissioning projects (future needs)

As India plans for the eventual decommissioning of larger nuclear reactors, such as the Tarapur reactors or fast breeder reactors, tasks like segmentation of reactor vessels, cutting activated steel, and removing neutron-activated components will pose significant challenges to the 30 mSv limit. Extensive manpower rotation or advanced shielding solutions would be required to stay within the current limit. Adopting a 50 mSv limit would:

(i)  

Simplify workforce planning for highly radioactive tasks.

(ii)  

Minimise reliance on extensive rotation strategies.

(iii)  

Ensure efficient and timely execution of decommissioning activities requiring expertise.

From the perspective of leveraging operational learning from decades of safe operations, it is possible to uphold high safety standards and safeguard worker health even under a higher limit. India's operational radiation safety record has been commendable, thanks to its robust occupational radiation protection programme. In 2022, the average annual dose for workers in the nuclear fuel cycle and other related industries ranged from 0.14 mSv to 4.94 mSv [22], well below the annual dose limit of 20 mSv. Given this track record, it is highly likely that India would continue to maintain its excellent safety performance even with a 50 mSv maximum single year effective dose limit, while also offering a greater and flexible dose margin to optimise operations. It is interesting to note that such low doses are an outcome of the investigation level of 10 mSv set by the Indian regulator for planned exposure situations against excessive exposures. This investigation level acts as a practical constraint, ensuring that exposures remain far below the annual dose limits. However, this current conservative approach, while effective for routine operations, imposes significant constraints during exceptional circumstances such as outage maintenance. Raising the single-year dose limit from 30 mSv to 50 mSv would only be impactful if the investigation level, which triggers operational reviews and resource reallocations for doses exceeding 10 mSv, is similarly increased to 20 mSv. Harmonised changes to both limits would enhance operational flexibility without compromising safety.

From an economic perspective, raising the maximum single year dose limit to 50 mSv can have several benefits for the nuclear industry, particularly in terms of cost-efficiency and resource utilisation. Firstly, higher dose limits can relax design requirements for radiation shielding and other safety measures. Designing facilities to meet stricter dose limits often necessitates more complex and costly engineering solutions, including enhanced shielding and more robust containment systems. By adopting a 50 mSv maximum dose limit, the industry could potentially reduce these stringent design criteria, leading to more cost-effective construction and retrofitting of nuclear facilities. Additionally, higher dose limits can help minimise operational costs by reducing the need for frequent downtime and the associated costs. When dose limits are set too low, operational procedures may need to be halted or modified to ensure compliance, leading to increased downtime and potential delays in maintenance activities. This can result in higher costs due to lost production and the need for additional personnel to monitor and manage radiation exposure.

This would be helpful for the future of Indian nuclear power programme as well. Many international reactor designs incorporate ICRP recommendations for radiation safety, such as radiation shielding designs. Adhering to stricter annual dose limits in India could necessitate modifications to these designs, potentially hindering the adaptability of imported technologies. Harmonising the domestic dose limits, particularly the maximum single year dose limit, with the ICRP recommendations, would simplify compliance for multinational nuclear technology corporations operating or seeking to operate in India and facilitate international cooperation and comparison of safety practices. This alignment is particularly important as India may need to import reactor technologies to meet growing power demands in the future [1].

Finally, from a health risk perspective, the increase in risk between 30 mSv and 50 mSv is relatively minimal according to the radiation risk models assessed by the ICRP. While it is acknowledged that the assumptions underlying the LNT hypothesis are subject to debate [23], and modifications to these assumptions could theoretically result in either an increase or a decrease in estimated risks, the ICRP continues to support a maximum single-year dose limit of 50 mSv. This recommendation is contingent upon the use of an annual average dose limit of 20 mSv as a constraint in optimising operations [3, 5]. Furthermore, advancements in radiation protection and monitoring technologies now enable more precise dose management, ensuring worker safety even at slightly higher exposure limits.

Given these considerations, the potential health impacts of a 50 mSv limit are well within acceptable safety margins when modern protective measures are implemented. Moreover, the biological evidence, while not definitive, suggests that cellular repair mechanisms at low to moderate doses (<100 mSv) may mitigate radiation-induced damage [24]. This further supports the notion that the operational and economic benefits of increasing the single-year dose limit to 50 mSv outweigh the relatively minor and uncertain risk increase.

In summary, after many decades of safe reactor operations and very low occupational doses to workers, along with a robust and mature operational radiation protection program, it is time to raise the maximum single-year dose limit to 50 mSv, in alignment with ICRP recommendations. This change will facilitate the integration of future reactor technologies, making it easier to add power capacity and manage operations, as well as decommissioning and maintenance tasks.

The data that support the findings of this study are available upon reasonable request from the authors.