One of the most costly aspects of decommissioning a nuclear licensed site is the management and interim storage of radioactive waste until ultimate disposal. How best to meet these medium-long term storage requirements (including safety, security, transport and final disposability) within limited budgets is an on-going challenge.
Radioactive waste is required to be stored in passively safe (physically and chemically stable) conditions in transportable and ultimately disposable packages, while ideally minimising the need for control and safety systems, maintenance, monitoring and human intervention. The construction of new waste treatment facilities or shielded stores is costly at a time of restricted budgets and expenditure constraints, and in the on-going drive to find the most cost-efficient and effective means of interim storage (generally assuming a storage period of up to 100 years) for Intermediate Level Waste (ILW), in line with all regulatory guidance, self-shielded packages are now becoming a popular concept.
Self-shielded packages, also known as 'ministores', offer a number of benefits, being suitable for interim storage in an unshielded building allowing man-entry and providing weather-proof cover - requiring considerably less capital expenditure than building and storing waste in conventional remote-handled stores. There may still be a need for some environmental control, but the self-shielded approach certainly simplifies storage requirements and, by avoiding the need to build a shielded store, is said to provide greater flexibility in the decommissioning programme. However, the wide and expanding range of self-shielded options, with widely differing capacities, construction materials and costs, and each with their own advantages and disadvantages, makes selection of the optimum design challenging.
Factors to consider
There are various considerations to take into account. Factors such as assured containment of the waste for the required lifetime, the handlability, transportability, and suitability for ultimate geological disposal (including mass and dimensional limits) are clearly fundamental requirements of any radioactive waste packaging. Added considerations include the amount of size reduction required to package the waste in the containers. Minimising the need for waste size reduction of decommissioning wastes will decrease worker dose exposure and associated risk, so the size of the waste package internal volume and the container lid opening are important.
Whether or not the waste is to be encapsulated is a further consideration which will influence the container selection. A non-encapsulation strategy will be more suitable for some wastes than others, depending on how passively safe the unencapsulated waste is. Solid activated materials which only corrode very slowly (bulk steel or graphite for example) are potentially suitable for non-encapsulation, whereas surface-contaminated or mobile materials such as plutonium-contaminated material or sludges are less so. The ability to store solid unencapsulated ILW without requiring a shielded store is seen as a potential advantage of self-shielded packages, retaining flexibility (with the potential to re-sort, re-pack or encapsulate in future, allowing for changes in storage and disposal options), and saving the immediate cost of an encapsulation plant. On the other hand, encapsulating the waste avoids the need for double handling (reducing worker exposure), provides added shielding, simplifies the transport safety case, can improve or simplify security during storage, and is generally ultimately needed to achieve the passively safe waste package expected to be a requirement for geological disposal. Moreover, in a decommissioning programme encapsulation at a later date may be more complex and costly if much of the infrastructure of the site needed to encapsulate or process the waste has been removed.
Ultimately cost (based on total cost of procurement and storage for 100 years) is a primary consideration in reviewing package options. Minimising cost while ensuring cost-effectiveness and/or value is one of the key challenges in selecting the optimum waste package option.
Shielded package designs available include 2m or 4m boxes, Ductile Cast Iron Containers (DCIC), Tru-Shield drums, and WAGR boxes, as well as overpacking options such as the ModuCube.
The 2m and 4m boxes (with wasteform volumes of 4.9m3 and 10.9m3 respectively, with 200mm concrete shielding included) are essentially robust freight containers, and can be used for encapsulated or unencapsulated waste. Made of austenitic stainless steel, the cost of both initial procurement and storage needs to be considered (stainless steel being susceptible to localised corrosion so a ventilation system is usually required to maintain a set humidity and minimise chloride contamination). The large opening lid minimises the waste reduction needed to pack these containers, although care must be taken, particularly of the 4m box, with dense metallic waste that the total container mass does not exceed permissible transport weight limits.
The cuboid or cylindrical DCICs, made from ductile cast iron, were developed in Germany as ILW storage, transport and disposal containers. Although relatively high cost to procure (two or three times that of the 2m or 4m boxes) they can be stored without requiring either a shielded building or stringent environmental controls (the lid seal integrity would need to be monitored, but is said to be designed to last at least 40 years), and do not require the waste to be encapsulated. The Type VI DCIC, (commonly referred to as the 'yellow box'), with an approximate 2.9m3 internal volume, is considered particularly suitable for fuel element debris, sludges, resins, sand and gravels. Its considerable weight (18 tonnes empty) again means care is needed to avoid container mass exceeding transport limits. The cylindrical container (approximately 500 litres volume), comes with optional additional lead shielding to store higher dose items and weighs between 5-10 tonnes depending on shielding.
The WAGR box, another alternative, is a reinforced concrete box designed to meet requirements for transport of radioactive materials on public roads and rail. The boxes come in two variants, normal and high density, according to the shielding requirement, with a conditioned waste volume of 5.6m3. With a container mass of 14-20 tonnes, depending on shielding, they are typically not as heavy as the Yellow Boxes, and the lid opening is large, facilitating efficient packing (with care again needed to keep within allowable transport weight limits). The WAGR box is closed by casting a concrete lid which is integral to the strength of the package, so the waste must first be encapsulated. Significant benefits include the low procurement cost (about half that of the 2m and 4m boxes, and around a sixth of the cost of the DCIC boxes), and minimal storage control, essentially needing only a weather-proof building. The box also has a proven track record, having been used for packaging of operational and decommissioning wastes from the WAGR reactor since the late 1990s.
As a further option, an innovative approach by Babcock is driving a development of the Tru-Shield container model to meet UK requirements as a self-shielded waste package for interim storage and disposal. The design is based on the concept of a lead-lined stainless steel drum, with a lead thickness of 50-75mm as required. With a 1.75 tonne container mass, and payload volume up to 305 litres, these containers are relatively small, easily handlable, and potentially suitable for transport on public roads. Waste can be encapsulated or unencapsulated, and the containers would be particularly suitable for specific purposes such as smaller quantities of waste from, for example, a re-processing, plutonium or fuel manufacture plant where fissile content is a concern. The containers could also be used for liquid wastes, with integral mixing paddles. Waste could either be directly loaded into the Tru-shield or into a drum liner if later removal may be required.
Procurement costs are not high, with a cost per m3 of wasteform estimated in the region of just under two thirds that of a DCIC box. These self-shielded containers do not require storage in a shielded building, although some environmental control would be required. A further benefit of Tru-Shield containers is the potential to use waste lead (clean or previously surface contaminated) for the lead shielding, providing recycling opportunities for reducing or eliminating site inventories of lead waste.
A different approach is the ModuCube ministore; an 'overpack' providing a shielded enclosure able to receive four unshielded 500 litre drums or one 3m3 box. This is not designed for use as a transport or final disposal package, its main benefit being to provide re-useable shielding and interim storage of encapsulated waste in pre-existing approved disposal packages. The design is relatively simple and hence cheaper than the DCIC if the DCIC benefits are not required.
To summarise, while the benefits of avoiding the high capital expenditure associated with building and storage within shielded stores are evident, there are arguments for and against each self-shielded package option to be weighed up. Each has its benefits and relative 'best use'. While a single approach may be tempting to enable use of common infrastructure, in reality there will always be a need for a range of designs for different needs. The WAGR concrete box, for example, provides a considerably less costly alternative to the newer steel designs, with benefits such as reduced storage requirements. Unlike their steel counterparts the waste must be encapsulated, but if there is no clear benefit to delaying encapsulation then the WAGR box can provide a good, inexpensive, practical solution. For smaller volumes of waste, especially fissile contaminated materials, the Tru-Shield container not only offers a good solution but also the added advantage of re-using waste lead for shielding. Ultimately, the primary focus should always be on 'fit for purpose', along with maximising cost efficiency.
Val Drake is head of science and technical services, Babcock, London, UK. www.babcock.co.uk/nuclear