The riskiest part of nuclear decommissioning is the removal of the nuclear fuel from the reactor and the facility. These risks are well under control. After the removal, the risk picture changes.
All industrial facilities have to be decommissioned at the end of their useful life, and nuclear facilities are no exception. What sets nuclear decommissioning apart from decommissioning of other industrial facilities, is the presence of a considerable amount of radionuclides such as cobalt isotope Co-60 and hydrogen isotope H-3. Radionuclides are present in the nuclear fuel, they are borne out of the interaction of neutrons with the matter in equipment, structures, and liquids, they are dissolved in fluids, and physical processes transport them to surfaces that they contaminate. Radionuclides emit ionizing radiation that is, in large doses, dangerous and even lethal to humans and other lifeforms.
An understanding of the hazards and risks involved is a necessary prerequisite to managing risks in any industrial endeavour. In nuclear decommissioning, it is more important because, at worst, radiation sickness and death and substantial damage to the environment may be caused and large sums of money may be lost. Decommissioning costs of nuclear power plants vary from tens to hundreds of millions of euros. The nuclear decommissioning risk profile differs from that of conventional power plants because of radionuclides and the long-time spans that are usually involved.
A conventional nuclear decommissioning project can roughly be divided into three main stages.
- Stage one starts with the final shutdown of the reactor. Usually soon after that, spent nuclear fuel (or other primary sources of ionizing radiation), fluids (coolant, hydraulic fluids, solvents etc.), and are removed from the facility soon after that. Stage one continues with the removal of nonfixed items of plant such as process materials and rigs. The removal of spent fuel reduces the amount of radionuclides in the facility by about 99 %; this has been the case also with VTT’s FIR1 research reactor. After this, a lengthy period may follow consisting of maintenance, care, and waste removal.
- Stage 2 consists of decontamination of contaminated areas and dismantling the remaining equipment and demolition of structures, provided that their radioactivity is within prescribed limits or they are easily removable. The remaining areas with unacceptable radioactivity are sealed. After this, there may be several years with only guarding and monitoring activities.
- Stage 3 consists of removing the remaining materials, equipment and structures, handling possible contaminated landmasses, landscaping, and releasing the site to unrestricted use (for example as a park or a golf course).
The main hazards that threaten nuclear decommissioning include hazards that might lead to a nuclear accident and occupational hazards. After the removal of nuclear fuel from the facility, the risk picture changes radically, because only a small fraction of radionuclides remain. Exposure to ionizing radiation is still a major hazard. The rest of the hazards are more conventional and common in other industrial and decommissioning settings. Fires may be ignited for example by sparks produced by demolition cutting with power tools, arcs and short-circuits of electric appliances, and overheated heaters; the igniters may kindle, for example, solvents used in decontamination, cable isolation, and lubrication oils. Sparks from cutting and other igniters may kindle also explosions; the explosive material may consist of vaporized decontamination solvents, inflammable dust generated in dismantling etc.
Workers at the site may be exposed to toxic or otherwise hazardous materials (asbestos in structures, lead in paints, radiation shields, counterweights, etc.). The electrical systems of an operating nuclear facility are usually quite safe. However, in the dismantling and demolition phase, the facility’s electrical system has been taken out of use, and the power tools, spotlights and other electrical appliances needed are supplied power via extension cables and temporary electrical lines. This exposes the workers to electrical hazards. Ever-present in the demolition of structures is physical hazards: the collapse of structures and falling of heavy objects. Nuclear facilities are not safe from natural hazards – floods, strong winds and so on – and demolition exposes the remains of the structure even more to these as protective structures are removed.
Decommissioning of nuclear facilities poses risks to humans, the environment, and the companies involved in it. The humans exposed to safety risks are mainly the decommissioning workers and persons visiting the site. The companies involved are the company that owns the site (the license holder) and decommissioning contractors. Usually, the main contractor utilizes subcontractors for different tasks. The risks are related to human health and safety, loss of money (either directly or as loss of anticipated revenue), damage to the environment, and detriment to reputation.
Concerning the decommissioning personnel, primary risks of decommissioning are occupational health and safety risks: dangerous amounts of ionizing radiation, poisoning, burns, disablement, and even death. Concerning the public – mainly people living or staying close to the decommissioning site – the main risks are public health and safety risks (dangerous amounts of ionizing radiation, poisoning) that would result from a release of dangerous substances from the site to the environment. Concerning the surroundings of the facility, the main risks are environmental risks: soil or water system contamination that may be harmful to ecosystems and prevent the use of the site for intended purposes. To the companies involved in decommissioning (site owner/license holder, decommissioning contractors), the main risks are project risks: delays in schedule, cost overrun, and problems with decommissioning quality. Realization of decommissioning risks may also be detrimental to the corporate image and image of the government.
Decommissioning from the final shutdown of the facility to the release of the site for unlimited use takes many years, and during this period, the risk profile changes. Before nuclear fuel has been removed in stage 1, the risks are qualitatively the same as those of an operating facility. In this period, the dominating risks are related to core damage or fuel damage, the possibly resulting release of radionuclides from the facility, and the public health and economic risks they incur. The most critical phase from the risk point of view is the removal of nuclear fuel from the reactor and the facility. However, its risks are well under control: they are quite much like the risks that the nuclear facility has faced during its planned and nonplanned outages, and therefore the means of risk analysis and management developed for outages may usually readily be utilized. After the removal, the dominating risks are related to occupational safety – mostly concerning ordinary occupational hazards in common with the decommissioning of ordinary industrial facilities – and the project itself; analysis methods developed for these risks must be applied.
The words hazard and risk are used with varying meanings. Here we use them in the following meanings.
- Hazard: “A source or condition that alone or in combination with other factors can cause harm” (Rausand and Haugen 2020).
- Risk: “Uncertainty about and severity of the consequences (or outcomes) of an activity with respect to something that humans value” (Aven and Renn 2009).
Senior Scientist, VTT
WP2 leader (Risk-based conformity, implications for licensing)