EVOLUTION OF WASTE MANAGEMENT TECHNOLOGIES FOR REPROCESSING IN BNFL
Dr L Edmiston and Dr G A Fairhall
British Nuclear Fuels plc, Research and Technology, Sellafield, Seascale, Cumbria,
CA20 1PG, United Kingdom
ABSTRACT
BNFL has been operating its Sellafield site for almost fifty years. Activities on the site during this time include: the operation of three commercial reprocessing facilities, reactor operations and, in more recent years, decommissioning. As a result of these activities at Sellafield, a wide range of radioactive waste streams have been generated.
BNFL's approach to the treatment of these waste streams has developed over the last half century resulting in the Waste Management Strategy, the key drivers of which are to :
This paper looks at how BNFL's approach to the management of reprocessing wastes has developed and demonstrates how the experience gained is now being used to optimise waste management for the future.
INTRODUCTION
BNFL has been operating its Sellafield site for almost fifty years. Activities on the site during this time include: the operation of three commercial reprocessing and fuel fabrication facilities, reactor operations and, in more recent years, decommissioning. As a result of these activities, a wide range of radioactive waste streams have been generated in each of the UK radioactive waste categories :-
BNFL's approach to the treatment of these waste streams has developed over the last half century resulting in the Waste Management Strategy, the key drivers of which are to:
This paper looks at how BNFL's approach to the management of reprocessing wastes has developed and demonstrates how the experience gained is now being used to optimise waste management for the future.
FIRST GENERATION WASTE MANAGEMENT
Until the end of the 1970s, only low level waste was disposed of. This was, as today, consigned to BNFL's Drigg site. All other wastes were stored (in an unconditioned state), in purpose built stores, silos, tanks and ponds on the Sellafield site. During this time, little was done to develop the final disposal route for the waste (1).
From the late 1950's, LLW has been disposed of at the Drigg near surface disposal site, close to Sellafield. Waste consigned to this site includes that from the UK nuclear industry, British Universities and hospitals. Developments in waste management practices both at Drigg and in the nuclear industry have ensured this facility will be available well into the next century.
High and Intermediate level waste was consigned to purpose built stores and silos. There were few programmes of work focused at developing long term solutions for the treatment and final disposal of these wastes.
SECOND GENERATION WASTE MANAGEMENT
In the early 1980's, BNFL, therefore, embarked on developing its Waste Management Strategy covering historic, current and future waste arisings.
Detailed technical studies by BNFL in the early 1980's highlighted that storage of waste in an immobilised form represented a reduction in the risk level and led to lifetime cost savings and a reduced dose uptake to operators (4).
Since 1990 HLW has been vitrified at Sellafield in the Waste Vitrification plant (WVP) (2). The vitrification process at Sellafield involved two steps: calcination of high active (HA) liquor and vitrification. The HA liquor is a suspension of metal nitrates which are introduced into a rotating hot tube enabling the evaporation and partial denitration of the material to form a reactive and friable calcine.
The calcine is then fed into the vitrification melter along with glass frit. The current objective is to produce a product having a waste loading of around 25 %. This process was developed for the treatment of liquors arising from Magnox and THORP reprocessing and was underpinned by an extensive development programme, including full scale test facilities. The glass products that have been generated are in an engineered store pending ultimate disposal in the UK or return to customers overseas.
Following an extensive R&D programme covering a range of encapsulation options and a systematic evaluation of these options against a number of criteria (3,4,5), cementation was selected as the best option for the immobilisation of ILW.
Supported by an extensive R&D programme, the ILW strategy was implemented ensuring treatment facilities are available to condition and immobilise current and future wastes as they arise. Historic wastes are to be retrieved and conditioned as the treatment plants are brought on line.
BNFL now has 4 operational plants for the cementation of ILW. These facilities encapsulate all current arisings of ILW from Magnox and THORP reprocessing and since 1993 historic waste has been immobilised. A fifth plant is due to be built and will be operational within the next five years (see table 1). With the exception of the Box Encapsulation plant, the plants are designed to treat both historic and current/future arisings.
Table I. Encapsulation Plants at Sellafield
|
Plant |
Operational |
Waste |
|
Magnox Encapsulation Plant |
1990 |
Magnox Cladding |
|
Waste Encapsulation Plant |
1994 |
THORP wastes, retrieved solid/sludge wastes |
|
Waste Packaging and Encapsulation Plant |
1994 |
Flocs, sludges |
|
Waste Treatment Complex |
1996 |
Plutonium contaminated material |
|
Box Encapsulation Plant |
2001 |
Retrieved solid wastes |
THIRD GENERATION WASTE MANAGEMENT
Since its introduction in the early 1980s there has been continuous optimisation of the Waste Management Strategy. The primary objectives for BNFL are now waste reduction and reduced life cycle costs.
Comprehensive reviews of BNFL's waste and decommissioning strategies and associated liabilities have provided the basis for targeting development effort. Key initiatives include waste reduction and elimination at source.
Specific BNFL technology development programmes contributing to waste minimisation are being undertaken aimed at improving established waste management operations:
Free release of materials is being developed either for re-use or non- nuclear disposal, with the objective of reducing the volume for LLW disposal. A major success has already been achieved in respect of free release, for re-use, of large quantities of aluminum from a decommissioned uranium diffusion plant. Decontamination and monitoring technologies are being developed for arisings from redundant plant and equipment. Building structures, potentially forming a major volume of waste, are the subject of bio-decontamination investigations and development of both in-situ and post demolition monitoring techniques.
FOURTH GENERATION WASTE MANAGEMENT
BNFL's Waste Management Strategy has evolved from the storage of waste to the development of technologies aimed at volume reduction and minimisation of lifetime costs. BNFL is now using this wealth of experience to ensure waste management is an integral part of main process chemistry development (6).
BNFL is currently developing two options for the next generation reprocessing facility:
In developing the Advanced PUREX scenario, waste treatment technologies are designed into the process to eliminate of combine waste streams at source.
A more radical approach has been adopted for novel separations in order to make a step change in process and waste technology to achieve a cost saving of > 50 % compared to THORP.
ADVANCED PUREX
The Waste Management Strategy for the Advanced PUREX programme is aimed at developing a cost effective combination of waste treatment processes in order to ensure:
In further developing this strategy, BNFL is able to use the experience it has gained from the operations of both its Magnox and THORP reprocessing plants. BNFL is using THORP as a model to investigate where in the process the waste streams are produced and is identifying alternative technologies which could be used to eliminate or combine waste streams. These technologies will be designed into the process as an integral part of the system and thus will reduce wastes at source.
NOVEL SEPARATIONS
Potentially even greater cost savings and improved waste management could be achieved by moving completely away from PUREX-based technology. Hence, within the Novel separation project, a radical approach has been adopted in attempting to make a step change in process and waste technology to achieve cost saving of >50%.
Through reviews of earlier studies, application of non-nuclear technology and innovative research, BNFL is pursuing a number of promising alternative technologies to solvent extraction including molten salt reprocessing.
MOLTEN SALT REPROCESSING
Molten salt technology has been used in several ways in the development of purification processes for nuclear materials.
BNFL's current research and development programme is targeted at developing the waste management processes for treating chloride wastes generated by the reprocessing technology.
The waste streams generated as a result of either reprocessing options can be broadly classified as:
The primary waste stream consists of the alkali metal chloride solvent containing fission product chlorides of which caesium, strontium, barium and the lanthanides must be removed to ensure continual process purity. The waste must be treated to enable recycle of the salt and any residual actinides and produce stable wasteforms of the minimum volume possible.
In general chlorides are difficult to incorporate into glass and, hence, the straightforward vitrification of molten salt chloride wastes may not be feasible. For example, the chlorine content of borosilicate glasses does not usually exceed 0.1% and other glass systems which do contain small amounts of chlorine are usually extremely soluble.
In addition to investigating the waste treatment processes proposed by ANL and Dimitrovgrad, BNFL are investigating alternative treatment methods which are cost effective and produce a wasteform suitable for disposal. Technologies under investigation include:
Work is being carried out to determine the uptake of lanthanide, alkali and alkaline earth fission products by zeolites. Processes such as melting, sintering and pressing into ceramics are being investigated for the stabilisation of the loaded zeolite for long term storage and disposal.
Work to precipitate phosphates from molten salts are being carried out. In addition phosphate glasses along with a range of other phosphate glass systems and phosphate ceramics are being investigated with a view to immobilisation of lanthanides.
Precipitation of cadmium-containing glasses and other novel ceramic waste forms are under review.
CONCLUSION
Over the past five decades, BNFL has developed its Waste Management Strategy. This strategy, which is applicable to historic, current and future arisings, has been instrumental in building BNFL's capability to deliver fit-for-purpose, cost-effective solutions to waste management problems.
The experience gained is now being used to ensure waste management is an integral part of all future processes developed by BNFL.
REFERENCES