It is unlikely that cement will be able to effectively immobilise the whole range of elements present in this waste stream as has been discussed in Section 2.1.2 [235]. A cementitious wasteform could be subject to increased dissolution and release rates due to its inherent porosity and increased internal surface area. Radioisotopes sorb to the surface of cements and can thus be more readily removed from the disposal matrix waste through changes in ground water chemistry. The high solubility and potential for removal of many of these waste elements; especially Cs, which by activity makes up over 60% of the radioactive inventory, is of significant concern [146]. These factors necessitate treatment of this waste using the most robust methodology possible to minimise potential radioisotope migration during interim or GDF storage. Vitrification is likely to offer an environmentally improved solution over the current baseline.
This study has demonstrated that a high quality product may be obtained by vitrification of PFR raffinate using G73 barium silicate glass. The benefits of vitrification go beyond the improvements in wasteform quality described in Section 5.4 and may also offer fiscal incentives for implementation. If this PFR raffinate waste were to be calcined and treated via vitrification, a substantial waste volume reduction would be achieved.
Current NDA plans are to cement this raffinate in 500 L drums with a target waste loading of 0.305 m3 per drum. With 212.1 m3 of raffinate to process this would result in 397 m3 of packaged waste for disposal (696 x 500 L drums with a displacement volume of 0.57 m3 each) [146]. If vitrification, at only 15 wt% loading were utilised for the same purpose, the volume of waste produced would be reduced to below 14.4 m³ of glass. Conceivably, this could be processed inside refractory lined 3m3 boxes, using in container vitrification technology, such as that marketed by Kurion. Assuming 70% of the box capacity (2.57 m3) could be filled, each 3m3 box would hold 1.8 m3 of vitrified product. This means the waste could be fully conditioned using just eight 3 m3 boxes, producing a total waste volume for disposal of
28.6 m3. This treatment methodology would therefore eliminate >368 m3 of ILW from the UK waste disposal inventory. The heat generation, surface activity limits and containment limits for impact of this hypothetical G73-15 waste stream have been calculated to be within NDA guidelines for a 3 m³ ILW box2 [236].
If the assumption is made that a small scale modular vitrification plant could be constructed at a similar cost to that of the cementation plant, an estimate of the potential financial savings for treatment of ILW via vitrification can be deduced. At the time of writing disposal of UK ILW had a projected disposal cost of £25.9k m-3 [237]. This alone would equate to a minimum cost saving of over £9.5 M on the GDF disposal of this waste stream alone. This price does not include any of the associated costs of interim storage, waste packaging or transport to the GDF facility. The following assumptions have been made in the production of the total cost savings provided in Table 5.6:
Cost of Interim Storage: Estimated at £10.7k based on the projected cost of £321M for an protected EPS2 type ILW store holding 30,000 m3 of waste [238, 239].
Cost of 500 L drum: £4 k based on estimated materials and manufacturing costs.
Cost of refractory lined 3 m3 box: £40k based on the assumption of comparable costings for the provision of the refractory lined 3 m3 box with that of the advanced double skinned 3 m3 box [240].
Transport costs: Assumed to be £1.25 k/m3 costs provided in [241].
The decreased risk to public health, improved quality of final wasteform, improved long term stability, smaller footprint on the Dounreay ILW stores and the reduced waste management cost, combine to provide a compelling case for treatment of these wastes using vitrification when compared with cementation.
It is recommended that further review of the cemented wasteform should be made available in order to allow a more detailed comparison of each treatment methodology. Data already exists detailing vitrification of PFR raffinate into both G73 ILW glass and standard HLW compositions. Equivalent data on the cementitious wasteform, especially concerning its waste retention and stability in aqueous environments should be produced in order to
2 Calculation based upon reported inventory of radioisotopes for this waste stream and accounting
for the concentration of activity achieved by vitrification. This packaged waste will meet specifications imposed for a square corner 3 m3 box.
provide a valid comparison. The necessity for this review has been acknowledged by Dounreay Ltd and should be completed before cementation of this material is begun. This will provide confidence in the treatment methodology selected and reduce the potential for an expensive and complicated reworking of unsuitable waste packages in the future [149- 151].
Cementation Vitrification
G73-15
Projected savings from use
of vitrification technology Waste container 500 L drum Refractory lined
3 m3 box N/A
Waste volume (m3) 347 14.4 332.6
Packaged waste volume (m-3) 397 28.6 368.4
Cost of packaging materials £2.78M £0.32M £2.46M
Cost of Interim storage £3.18M £0.23 £2.95M
Cost of GDF disposal* £10.28M £0.74M £9.54M
Cost of Transport* £0.50M £0.04M £0.46M
Expected saving
(GDF relocation) £15.41M
Expected Saving
(Long-term near surface storage)
£5.25M
Table 5.6 – Projected lifetime waste management volumes and costs for PFR raffinate as both cemented and vitrified products. *As the Scottish government has a policy of long term near surface storage for these higher activity radioactive wastes, costs associated with GDF and transport may not be relevant to this assessment, although they should not be excluded from consideration.