Dose and risk criteria for protection of people following the closure of a disposal facility for radioactive waste

Advisory Note

May 2019

Summary

  • Disposal of solid radioactive waste in a purpose-built disposal facility is a planned exposure situation. It is subject to requirements for protection of people from the harmful effects of radiation as specified in the Planned Exposure Code.
  • A safety assessment is required to demonstrate that a disposal facility is designed to meet short- and long-term protection standards.
  • When the site is released from regulatory control and active institutional control terminated, its safety will be entirely dependent on natural and engineered features that will not be actively managed or monitored.
  • Dose constraints are a regulatory requirement that guide optimisation of protection.
  • Risk targets promote safety and guide implementation of best available technique (BAT) in the siting, construction, operation and closure of a disposal facility.
  • Dose constraints and risk targets are complementary and can be used separately or in combination depending on the circumstances, the type of information available and the uncertainties involved.
  • Dose and risk estimates in safety assessments assist decision makers and stakeholders, including members of the public, in comparing exposures and risks from waste disposal with risks from other activities.
  • After closure of a disposal facility, potential radiation exposures caused by a disruptive event, such as a future seismic or flooding event, may or may not occur. The nature and progression of such events are uncertain. Therefore, estimates of future exposures and risks must be interpreted with caution.

Note to readers

This advisory note provides an explanation of the radiation dose and health risk criteria for protection of members of the public in RPS C-3: Code for Disposal Facilities for Solid Radioactive Waste (ARPANSA 2018), herein referred to as the Disposal Facilities Code. These criteria are particularly aimed at protection and safety beyond closure of a disposal facility, including beyond the period of surveillance and active institutional control. RPS C-3 also includes requirements for protection of the environment that must be met. The scope of RPS C-3 provides additional details on the types of waste disposal facilities that are covered by this Code. The target audience are regulators, licence holders, licence applicants and interest groups.

Terms used in this advisory note are explained in the Planned Exposure Code (ARPANSA 2016) and in the Disposal Facilities Code (ARPANSA 2018).

How is protection and safety achieved and demonstrated?

Disposal of solid radioactive waste in a disposal facility is a planned exposure situation. It is subject to the requirements, including dose limits, for protection of people from the harmful effects of radiation specified in the Planned Exposure Code (ARPANSA 2016). Additionally, protection must be optimised to achieve the highest level of protection under the prevailing circumstances, including consideration of economic and societal factors. Optimisation is an ongoing, iterative process that includes evaluation of the exposure situation, evaluation of potential exposures, selection of a dose constraint and/or risk target below which protection is optimised, and the identification, selection and implementation of the preferred optimisation option (ICRP 2007).

For protection in the long-term, including beyond surveillance and active institutional control, uncertainties in estimates of exposures and risk generally increase, including estimates regarding the demographics, habits, land use and economics of the public to be protected (the representative individual). The Disposal Facilities Code (ARPANSA 2018) stipulates that best available technique (BAT) must be considered in parallel with optimisation. BAT gives priority to the choice of proven, effective, advanced and appropriately conservative design of systems, structures and components that will ensure stability and integrity of the disposal facility. Optimisation and BAT reinforce each other by strengthening the protection outcomes.

A safety assessment[1] is required to demonstrate safety and the level of protection of people and the environment provided by the disposal facility from the radiation hazards posed by the disposed radioactive waste. The safety assessment is developed by the applicant/operator and must demonstrate to the regulator and interested parties that the facility meets agreed standards and achieves the fundamental safety objective, which is to protect people and the environment from the harmful effects of radiation (ARPANSA 2014).

Multiple safety functions must be in place, where containment and isolation of the waste is achieved by engineered barriers and the host environment. The safety functions are intended to prevent, or delay and reduce, radiation exposures of members of the public from the radioactive waste. Safety must not be unduly dependent on a single safety function. Post-closure safety must be considered throughout the life-time of the facility, as decisions on siting, design, waste acceptance criteria and operations all influence post-closure protection and safety.

The performance of the barriers must be well understood so that their protective capability over time can be assessed with reasonable certainty. If the barriers provide the intended containment and isolation, exposures to the public will be virtually zero. However, the safety assessment must also consider what could happen if the barriers do not perform as expected.

What about disruptive events?

Some seismic events and severe weather conditions including heavy rainfall and flooding will likely occur during the life-time of a disposal facility. These events occur with a certain frequency, although it cannot be predicted with certainty when the events will occur. A disposal facility must be designed to withstand reasonably foreseeable disruptive events without significant loss of protective capability.

Post-closure protection and safety concerns must also relate to severely disruptive events that may or may not occur following closure of the facility, including beyond the period of surveillance and active institutional control.

"Disruptive events from natural processes on Earth as well as human errors, inadvertent actions or acts with malicious intent cannot easily be assigned a likelihood of occurrence, particularly when the institutional control period of a facility has passed."

Such events may potentially cause degradation of safety functions resulting in potential exposures that may be greater than those incurred with all safety functions intact. These potential exposures must be considered in the safety assessment.

Example: Understanding disruptive events

There are many different types of disruptive events that have the potential to influence the safety of a disposal facility.  A comprehensive analysis of potential features, events and processes (FEPs) should be used to identify the relevant factors that may impact the long term performance of a disposal facility. A FEPs analysis assists in identifying the main characteristics of the site and facility (features), events that could occur and ongoing processes that need to be considered. This allows appropriate scenarios to be developed to assess potential releases of radionuclides from the disposed waste and subsequent transport to the human environment.

Features include the waste, cover, soil and rock types, atmosphere, water bodies, human, flora and fauna populations and engineered barriers. Events would include things that may occur in the future like fire, drought, earthquakes and faults. Human activity events include deliberate intrusion.  Processes are things that are ongoing, such as weathering, erosion, water seepage, groundwater flow and transfer to plants and animals. Human activity processes include farming, irrigation habitation, traditional use and salvage.

Are there specific dose criteria and risk targets for optimisation and safety following the closure of a disposal facility?

Yes, dose constraints are a regulatory requirement that guide optimisation of protection. Additionally, the Disposal Facilities Code (ARPANSA 2018) stipulates that optimisation and application of BAT must aim at reducing annual risks for detrimental health effects to a risk target in the range 10-5 to 10-6 per year or lower, for normal evolution and reasonably foreseeable disruptive events, and allowing for different assumptions regarding the representative individual (see the section “Can we link dose to risk?” for information on how the risk is calculated). Risk estimates can be compared to the risk target to inform decisions on siting, facility design, waste acceptance criteria, operations and closure. The risk estimate can be informed by a probabilistic[2] assessment. Optimisation and BAT is applied at all stages. This provides for a very high level of protection and the Disposal Facilities Code (ARPANSA 2018) is well aligned with the international framework for safety in this regard (IAEA 2011).

The Disposal Facilities Code (ARPANSA 2018) further specifies that assessments should be made for severely disruptive events, to identify events that may result in an annual effective dose above 1 mSv per year (ie above the public dose limit stipulated in the Planned Exposure Code). These events must be assessed deterministically[3].

Human intrusion scenarios are not amenable to strict assessments of likelihood, and must also be assessed deterministically. Reasonable efforts should be expended on optimisation, BAT and on reducing the likelihood of inadvertent intrusion if it can be demonstrated that doses above 1 mSv per year can be anticipated for those living near the site. If doses higher than 10 mSv per year to those living around the site can be anticipated from plausible inadvertent intrusion scenarios, re-design or other options may have to be considered.

diagram describing the probability of certain events occuring and the corresponding protective approaches

Figure 1: The Framework for protection in the Disposal Facilities Code.

Can we link risk to dose?

The risk can be formulated as the product of probability of the exposure (ie how likely it is that an exposure occurs in a given time period), and resulting harm should that exposure occur. Doses, quantified using effective dose (expressed in sievert, Sv), provide a means of estimating the amount of energy imparted in the human body by radiation, the ability of different types of ionising radiation to cause biological effects, and the relative sensitivity of different body organs to radiation.

For planning purposes, the International Commission on Radiological Protection (ICRP) recommends using a nominal risk coefficient of about 0.05% per mSv for stochastic health effects[4] (ARPANSA 2016, ICRP 2007) to convert an effective dose to an estimated risk of stochastic health effects in a population.

This conversion between dose and risk is used for radiation protection purposes in planned exposure situations. It should not be used for attributing cancers based on past exposure because it does not represent the actual risk to an individual person or population. This coefficient is primarily based on observations of cancer in populations that have been exposed to radiation at levels orders of magnitude above the effective dose limit (1 mSv) for members of the public.  Exposures at the public dose limit do not lead to discernible health effects in the whole population, and health effects among individuals in a population exposed at such levels cannot be unequivocally attributed to radiation. Health risks can, under these circumstances, only be inferred (UNSCEAR 2015).

What is the benefit of a risk target?

Dose constraints and risk targets are complementary and can be used separately or in combination depending on the situation, the type of information available and the uncertainties involved.

A risk target provides a measure against which probabilistic and deterministic risk estimates for different scenarios can be compared to allow a demonstration of the possible health impact of the facility and its host environment during operation and after closure, so that a sufficient level of confidence in safety can be achieved. The risk estimates made by the applicant/operator will be carefully reviewed by the regulator and assist decision makers and stakeholders in comparing radiation risks to other risks. This is because the radiation risks are able to be presented in the same way as other risks associated with the site.

"A risk target can be particularly useful when evaluating post-closure exposure scenarios where it is difficult to determine the precise probability of the disruptive event occurring, or when the event is very unlikely to occur."

In these cases, it can also be beneficial to evaluate the risk estimate and radiological significance of each event (or scenario) individually. This approach allows a more informed consideration of potential exposures from events where it is difficult to determine the probability of the event occurring.

Example: Future land use

There are large uncertainties associated with identifying likely land uses hundreds of years into the future. To address this uncertainty the applicant provided risk estimates for a range of land use scenarios including recreation, agricultural use, permanent occupancy and nomadic occupancy. The risk estimates for each scenario are then compared to the risk target to provide an understanding of the range of possible future health risks due to potential radiation exposure.

How confident can we be in dose and risk estimates?

A dose or risk estimate will provide information on possible health impacts in a population from potential radiation exposure caused by a disruptive event, some of which may or may not occur during the time the radioactivity of the waste remain a concern. Dose and risk estimates must be interpreted with caution as there are uncertainties associated with estimates of dose, health risk and the likelihood of events. Estimates may vary with time as the impact of events may increase in the long-term due to barrier degradation and the likelihood of events occurring may change due to factors such as climate change. Additionally, the likelihood of health consequences of an event may decrease with time because of decay of radioactive substances. The variability of such estimates over time, and the degree of certainty in predictions of disruptive events, must be considered in the safety assessment.

It is important to recognise that the uncertainties associated with dose and risk forecasts made for hundreds of years into the future increase as the time period over which the forecast is made increases, therefore they should not be used as measures or predictions of future health detriment. Instead, estimates of dose and risk over long time periods should be subject to optimisation criteria and consideration of BAT (ICRP 1998, ICRP 2013).

References

ARPANSA 2014. Australian Radiation Protection and Nuclear Safety Agency 2014. Fundamentals for Protection Against Ionising Radiation. Radiation Protection Series F-1. [https://www.arpansa.gov.au/regulation-and-licensing/regulatory-publications/radiation-protection-series/fundamentals/rpsf-1]

ARPANSA 2016. Australian Radiation Protection and Nuclear Safety Agency 2016. Radiation Protection in Planned Exposure Situations. Radiation Protection Series C-1. [https://www.arpansa.gov.au/regulation-and-licensing/regulatory-publications/radiation-protection-series/codes-and-standards/rpsc-1]

ARPANSA 2018. Australian Radiation Protection and Nuclear Safety Agency 2018. Code for Disposal Facilities for Solid Radioactive Waste. Radiation Protection Series C-3. [https://www.arpansa.gov.au/sites/default/files/rpsc3.pdf]

IAEA 2011. International Atomic Energy Agency 2011. Disposal of Radioactive Waste. Specific Safety Requirements No. SSR-5. [http://www-pub.iaea.org/MTCD/publications/PDF/Pub1449_web.pdf]

ICRP 1998. International Commission of Radiological Protection 1998. Radiation Protection Recommendations as Applied to the Disposal of Long-lived Solid Radioactive Waste. ICRP Publication 81. [http://www.icrp.org/publication.asp?id=ICRP%20Publication%2081]

ICRP 2007. International Commission on Radiological Protection 2007. The 2007 Recommendations of the International Commission on Radiological Protection. ICRP Publication 103. [http://www.icrp.org/publication.asp?id=ICRP%20Publication%20103]

ICRP 2013. International Commission on Radiological Protection 2013. Radiological Protection in Geological Disposal of Long-lived Solid Radioactive Waste. ICRP Publication 122. [http://www.icrp.org/publication.asp?id=icrp%20publication%20122]

UNSCEAR 2015. United Nations Scientific Committee on the Effects of Atomic Radiation 2015. Sources Effects and Risks of Ionizing Radiation, 2012 Report to the UN General Assembly, Scientific Annex A: Attributing Heath Effects to Ionizing Radiation Exposure and Inferring Risks. [http://www.unscear.org/unscear/en/publications/2012.html]

[1] The safety assessment supports the safety case, which is the collection of scientific, technical, administrative and managerial arguments and evidence in support of the safety of the facility, covering the suitability of the site and the design, construction and operation, the assessment of radiation risks, and the assurance of the adequacy and quality of all of the safety related work that is associated with the facility.

[2] A probabilistic risk assessment includes multiple computational realisations of scenarios where values for safety significant input parameters have been sampled based on their distributions.

[3] A deterministic assessment should use cautious but not overly conservative assumptions when determining the values of the input parameters.

[4] Stochastic effects of radiation are those whose frequency in a population is related to the level of exposure. The nominal risk coefficient applies to cancer and heritable disease (both of which are stochastic effects) in the whole population exposed to low linear energy transfer (LET) ionising radiation at low dose rates. It is gender- and age-averaged and adjusted for ‘detriment’ such as years of life loss and loss of quality of life.