Storage of radioactive waste

It should be noted that when spent fuel (used or irradiated) is removed from the reactor, only 5% of the energy initially contained has been used, and it consists of 95% uranium, 1% plutonium, and the rest minor actinides, long-lived products, short-lived products, and stable fission products. If the reuse of the remaining U-235 and the Pu-239 generated is considered, the fuel is reprocessed or recycled for use in other types of nuclear power plants. This operation separates these two elements from the fission products, which constitute high-level waste. This option, in which the fuel is reused, is known as the closed cycle.

If the decision is made not to reuse the energy resources contained in the irradiated fuel, it is managed as high-level radioactive waste, since the fission products remain confined within it. This option is known as the open cycle.

Spain maintains the so-called ‘open cycle’ as its spent fuel management strategy, i.e., it does not contemplate reprocessing but rather the storage of waste generated by the plants.

Each country establishes its own method of managing and storing radioactive waste, so what is described below are some of the most common types of storage (depending on the country, they may have different names, although they function similarly). In this case, the classification takes into account the activity of the waste (very low, low, medium, and high activity) and the temporary or permanent nature of the storage.

Very low, low, and medium-level waste

Surface or shallow storage facilities can be used for these types of waste. The best way to explain this is with an example, such as the El Cabril Storage Centre in Hornachuelos, Córdoba (Spain).

It began operating in 1992, after almost three years of construction, and has two distinct areas for its functions:

• The building area. This area comprises the processing buildings, where waste treatment activities occur, and the control room, where operations, monitoring, and supervision of the facility are conducted. It also has laboratories for verifying the quality of the waste and buildings that provide auxiliary services to support the facility.
• The storage area. There are two distinct areas: the low and medium-level waste area (RBMA), which consists of two platforms: the north platform, consisting of 16 storage structures or cells, and the south platform, consisting of 12; and the very low-level waste area (RBBA), which consists of a platform with four structures that will be built according to storage needs. Currently, two have been built.

The facilities at El Cabril feature:

• Automation: Operated from a control room, minimising exposure to operators.
• Solidification: Waste is stored in a solid form, ensuring that there are no liquid spills or gas leaks from the stored material.
• Seismic resistance: Capable of withstanding the largest earthquake foreseeable in the area.
• Recoverability: Waste storage containers could be removed in the future if desired.

The US Nuclear Regulatory Commission (NRC) has described the El Cabril facility in a public document as one of the best in the world for the treatment and storage of low- and medium-level radioactive waste.

How is very low-level waste managed?

Very low-level waste arrives at the facility in bags, drums or containers and is stored directly in the specific storage structure or cell. If treatment is necessary, it is sent to the area designated for this purpose.

When the structures or cells reach their capacity limit, they will be covered with different layers, the last of which will be topsoil to integrate them into the environment. At that point, the 60-year monitoring and control phase of the site will begin.

How is low-level waste managed?

The storage system is based primarily on the interposition of engineered barriers and natural barriers that safely isolate the stored materials, ensuring the protection of people and the environment.

Low- and medium-level waste arrives at El Cabril in specialised transport and is unloaded in the conditioning area or in one of the temporary storage facilities. Most of the waste generated in nuclear power plants arrives conditioned in drums. Waste from hospitals, research centres, or industries is treated and conditioned at the El Cabril facility itself.

The packages received are placed in concrete containers until they reach maximum capacity. At that point, each one is immobilised using injected mortar, forming compact blocks that are placed in the storage cell until it is full and then closed with a reinforced concrete slab and waterproofed.

When all the structures on a platform are complete, it will be covered with a final layer consisting of different drainage and waterproofing layers, topped with a final layer of topsoil to blend in with the surrounding environment. At this point, the site monitoring and control phase will begin, with an estimated duration of 300 years.

How is radioactive waste treated and conditioned?

From the moment it is generated until it is stored, radioactive waste undergoes a conditioning process that depends mainly on its physical, chemical and radiological characteristics:

• Liquid waste. This is separated according to whether it is aqueous or organic and then treated using physical methods (filtration, centrifugation, evaporation, etc.) and chemical methods (precipitation, ion exchange, etc.) in order to reduce its contamination and volume. Finally, they must be solidified because this is the safest way to transport and store them, so they are mixed evenly with concrete, mortar or cement.
• Solid waste. This is segregated according to its contamination and physical-chemical properties to reduce the volume to be treated. To do this, decontamination, cutting, crushing and compaction techniques are used, and the waste is immobilised by creating a block with cement.

Organic waste is incinerated in order to solidify it, subsequently immobilising the ashes with mortar.

Spent fuel, high-level waste and special waste

High-level waste contains significant amounts of radioactive products (long-lived alpha and beta-gamma emitters) that are highly radioactive and generate significant heat. It basically consists of spent or irradiated nuclear fuel from nuclear power plants and vitrified waste produced during the reprocessing of small amounts of used fuel (also called spent or irradiated fuel).

Special waste is long-lived waste with significant activity, whose temporary and final management will be similar to that of high-level waste.

If reprocessing is not considered, as is the case in Spain, the facilities for this type of waste are:

Sistemas de almacenamiento temporal

Temporary storage is necessary to manage the second part of the nuclear fuel cycle. A temporary intermediate facility is required where spent fuel can lose some of its residual energy before being deposited in a final storage facility.

A common procedure, although not the only option, is to store the spent fuel in pools and, after a few years, during which the level of radioactivity is reduced (each year it is reduced to one hundredth), these elements are generally transferred to an Individualised Temporary Storage Facility (ATI), where they are kept until their final disposal. The aim is that by the time final storage takes place, the level of radioactivity will have been reduced to one thousandth.

To this end, spent fuel, once removed from the nuclear reactor, must always be stored under water for cooling in the pools at the nuclear power plant.

Water was chosen as the host medium due to its high heat transfer coefficient, which allows for cooling, its good shielding properties, its transparency and its manageability.

The fuel storage capacity of Spanish nuclear power plants in pools has been expanded in recent years (taking into account the current duration of operating cycles and the legal safety requirement to leave a reserve capacity equal to a complete core), although they have now reached the saturation point, leading to the search for the option of Individualised Temporary Storage Facilities (ATI).

At some plants, the pools have reached or are reaching their maximum storage capacity, or there is a need to remove the fuel from them to begin dismantling. To do this, the fuel elements are placed in containers that are stored for a certain period of time in an appropriate facility on the plant site called an Individualized Temporary Storage Facility (ATI).

There are different types of containers for temporary storage. For example, dual-purpose metal containers (storage and transport) or welded metal capsules stored in concrete-metal modules and transportable in metal containers.

The use of ITFs for the temporary storage of spent fuel is a common practice in several countries around the world with nuclear programmes.

In Spain, there is another type of storage facility called a ‘Decentralized Temporary Storage Facility’ (ATD), which consists of its ATIs plus a new complementary facility or additional measures that allow for the maintenance and repair of its containers (if necessary) to ensure recoverability at the container level.

The ATDs, including their complementary facilities, will be operational before the dismantling of the fuel pool begins and will remain in service until all spent fuel has been transferred to the Deep Geological Repository (AGP).

Deep Geological Repository (DGR)

It is a definitive storage system that aims to store high-level waste in deep geological formations to prevent the radioactive substances it contains from reaching the human environment in concentrations that could harm the environment and, consequently, human health.

To achieve this, it is necessary to isolate the waste for long periods of time so that the activity of the various radioactive elements it contains decays to sufficiently low levels that the natural background radiation is not altered and normal doses to humans are not increased.

The safety of AGP is based on the so-called ‘multi-barrier’ principle, which consists of interposing a series of artificial and natural barriers between the waste and the biosphere, together with the isolation and confinement capacity of geological formations, provided that these meet certain characteristics of stability, thickness, absence of preferential effluent migration pathways and retention capacity. The aim is to ensure that any deficiencies that may occur in the performance of a barrier over time do not compromise the overall safety of the system.

These barriers act in two different ways:

They contain radioactive materials.

They delay and dilute potential releases into the biosphere in the ecosystems that will be potentially impacted by the repository (soil, water, living beings, etc.).

There are two types of barriers or components in this concept:

Artificial or engineered barriers are designed, constructed and placed in accordance with the design of the repository, the specific function or functions assigned to them and the conditions imposed in the short and long term by the other artificial and natural barriers in the system. Their components are:

• The chemical form of the waste itself.
• The metal storage canisters.
• The backfill and sealing materials.

Artificial barriers play a decisive role in short-term safety due to their containment and retardation capacity.

Natural barriers are not specified or constructed by humans, but must be characterised and selected according to functional criteria or requirements that make them suitable for the proper functioning of the artificial barriers and the system as a whole. Their components are:

• Geosphere. Geological formations where the storage facility is located and the water and gases it contains.
• Biosphere. A set of ecosystems (soils, water, living beings, etc.) that would be impacted by the storage facility.

A geological barrier refers to the geological formation in which the repository is located, consisting mainly of a solid part, made up of rocks and minerals, and a fluid part, made up of water and gases.

The natural barrier is responsible for the long-term safety of the system, delaying the release of radionuclides into the human environment and controlling their dispersion and dilution.

The design, construction, operation and licensing of a deep geological repository or AGP follows a series of stages, each of which is conditional on the next:

• First: Selection of the technology that must demonstrate the availability and knowledge necessary to undertake the generic characterisation work of a site and the ability to assess the long-term safety of the repository.
• Second: Selection of a site, combining technical criteria and public participation. Specific data will be obtained from the surface, and its analysis will determine whether or not it is advisable to proceed.
• Third: Detailed characterisation of the site. This involves the construction of an underground laboratory to obtain specific data on the site, which is necessary both to adjust the design and to assess its safety. The characterisation techniques and long-term performance of the barriers will be verified, and a specific design for the repository will be developed in accordance with the characteristics of the site. During this phase, the operational feasibility of the repository and closure methods will be demonstrated, as well as the ‘recoverability’ of the waste.
• Fourth: Construction of the repository, safety assessment and completion of the licensing process. Once obtained, the operation will proceed.

Separation and transmutation

This option is for when a country opts for the closed fuel cycle.

It consists of recovering uranium and plutonium from spent fuel through reprocessing, separation and transmutation for subsequent energy use, leaving fission products and unrecovered actinides as waste, which are the only highly radioactive materials.

Separation consists of a series of chemical operations, either wet (hydrometallurgical) or dry (pyrometallurgical), to extract minor actinides and some long-lived fission products, converting them to a state that allows their transmutation.

The transmutation of actinides is generally carried out through fission reactions, while that of fission products is carried out through neutron capture reactions.

The separation and transmutation of radionuclides contained in high-level waste reduce the radiotoxic inventory, i.e. radiotoxicity and consequently the time during which it has high values, and reduce the volume of high-level waste. However, even though it is reduced, there will still be high-level waste that will require a final solution.

Interest in these techniques has been revived in some countries such as China, France, India, Japan, the United Kingdom, and Russia, etc.

In the United States, there has been renewed interest in reprocessing. In fact, the Global Nuclear Energy Partnership (GNEP) was created to work with other countries such as France, Japan and Russia on the development of new methods for reusing spent fuel. In addition to working on new fuel recycling technologies and the construction of new reactors in the country, this programme will also continue with the development of new reactors that can use reprocessed fuel that still contains a high percentage of the energy it had when it was first loaded into the reactor.