Answer to Question #14047 Submitted to "Ask the Experts"

The in situ disposal of a facility is known as entombment. Decommissioning of the IRT reactor near Tbilisi, Georgia, is an example of entombment. During the first stage of decommissioning, the reactor core together with some comparably highly radioactive waste was covered with special concrete. The International Atomic Energy Agency (IAEA) Technical Report Series No 463 mentions that entombment is best suited where the facility is situated far from populated locations. Populations generally grow as time progresses. How should one justify the distance between the entombed site and surrounding populations?

Before addressing the crux of your question, let's briefly discuss options for reactor decommissioning. There are three decommissioning strategies: immediate dismantlement, deferred dismantlement, and entombment. Each option carries associated costs and risks. You may find the additional information on these strategies presented in the IAEA Safety Reports Series N. 50 useful, particularly on the topic of entombment. Generally, selecting a decommissioning strategy is based on a balance of financial cost and risk. Total risk must be considered for each strategy, and there may be different risk categories for each strategy. Risks for general demolition activities include potential injuries to the workers who are dismantling the reactor and supporting systems, and subsequent waste loading for disposal. There is also potential exposure to the truck drivers and general public as wastes are transported to an off-site disposal facility. Further, there is risk to the disposal facility workers as they offload material and place it in the disposal cell, along with risk to the future public and/or environment from the disposal facility itself. All of these risks are mitigated with an in situ disposal (ISD) strategy via entombment, with the exception that the ISD site now takes the place of the off-site disposal facility.

Because the entombed facility essentially becomes the disposal facility, there is considerable overlap between the technical considerations for a near surface disposal facility and the facility subject to ISD. An additional reference you may find useful is IAEA's Specific Safety Guide No. SSG-29. Certainly regulators and the general public would prefer that ISD sites are located away from population centers. In addition to the human health considerations, the surrounding environs must be protected, and such measures are independent of population centers. The subject of the question is specific to reactor entombment but let’s not forget there are hundreds of other land disposal facilities for hazardous or industrial waste, construction debris, or simply household trash, which most certainly were also established away from population centers. Therefore, the main point of your question, which related to the changes in populations that may encroach on disposal sites, is not just a concern for ISD sites. As you can imagine, the regulatory process for entombing a facility is quite rigorous.

The permitting process for ISD is quite rigorous and requires demonstrating protection of public health and the environment. Additionally, the process requires input from multiple stakeholders before regulatory approval. This process generally begins with preparation of safety case involving extensive, complex environmental transport modeling to estimate potential future risk assuming extremely conservative, yet unlikely material release scenarios and evaluates various future land use and receptor scenarios, including unintended intruder scenarios. Receptor dose for multiple land use scenarios are modeled and might include recreational, residential, industrial, or agricultural use and considers. Additionally the hypothetical receptor is typically the maximally exposed individual under the various scenarios, which is generally independent of population density. Besides the theoretical modeling, the engineering of the ISD facility itself goes through intense scrutiny and requires robust design and safety features to demonstrate containment will not fail under extreme conditions, such as earthquakes. The aforementioned technical consideration doesn't include the political climate in the surrounding communtiy. For example, local municipalities may not accept the presence of an entombed reactor due to the perceived risk.

The safety case will describe and justify a number of active and passive barriers to prevent and reduce environmental exposure. Active barriers include a period of institutional control, whereby the facility operator maintains control of the site to prevent unauthorized use; implements an environmental monitoring program; and continually assesses the performance of engineered barriers. Successful implementation of these institutional controls often requires the coordination of multiple organizations, such as local and federal level government agencies in addition to the facility operator. The safety case will likely examine loss of institutional controls and the associated consequence. Institutional controls are typically implemented on scale of hundreds of years, after which exposure mitigation is achieved by passive controls.

Beyond the period of institutional controls, passive barriers become the primary method for protection of human and environmental health. Examples of passive barriers include proper final site grading and the instillation of groundwater diversion or collection systems, both of which prevent water from infiltrating the containment and mobilizing contaminants. Arguably, the most important passive barrier is the entombment matrix itself, which consists of a stable concrete or grout type material. Prior to grouting the facility, the reactor fuel, which contains the long-lived radioactive material, is removed. Entombment is not a suitable disposal method for long-lived radioactive materials as the passive barriers are generally not robust enough to allow sufficient time for radioactive decay. The reactor vessel and associated components contain the greatest percentage of remaining radioactivity. The radionuclides involved are primarily, if not entirely, neutron activation products, by which elements of the structural components themselves have become radioactive—via neutron activity—during operations, meaning the activity is bound in the matrix of the components. In addition to being bound in the materials themselves, the entombment further isolates the radioactive materials from the environment. Another passive barrier we must not overlook is the relevant knowledge transfer to future generations. Sturdy physical makers at the disposal site and preservation of administrative documentation by local governments are means of knowledge transfer.

In summary, no one can predict the future, nor can all risk be eliminated, but what can be done is to make plausible assumptions, then identify control risks. At this point it should be clear that there are numerous factors related to the surrounding population that one has to consider when choosing entombment as a decommissioning strategy. As a final thought, remember that the activation products generally have shorter half-lives, and natural decay will continuously reduce the amount of activity present, to the point that if there is population encroachment hundreds of years in the future, the disposed radioactivity will have decayed considerably and eventually all that will be left at the site will be a concrete monolith encasing antique piece of research equipment.

We hope this information is helpful to you.

Nick Altic, CHP
Tim Vitkus, CHP