Developed and tested over the past 20 years, SIAL is a new generation, geopolymer solidification technology that offers a safe and cost- effective alternative conditioning technique for solidifying and immobilising various radioactive waste streams.

SIAL can be used at room temperature and can incorporate up to about four times as much sludge or resin waste as a Portland cement matrix, depending on the waste being treated.

Compared with conventional techniques it has greater mechanical strength, lower leachability, low volatility, a low fire risk and excellent physical stability – no distortion or cracking – in the presence of frost and water.

SIAL was developed in response to the challenge of dealing with sludge waste at the Jaslovske Bohunice A1 nuclear power plant in Slovakia, when the unit was shut down for decommissioning after an INES level 4 accident in February 1977.

Fuel assemblies and fuel cladding damaged in the accident led to significant strontium-90, caesium-137 and transuranic contamination. As a consequence of the long-term corrosion of the containment materials, there was an accumulation of highly contaminated sludge, which could not be effectively immobilised using conventional methods such as cementation or bitumen treatment because of adverse physical-chemical properties and high specific activity of the caesium-137 waste.

Various goals were considered in developing and testing the SIAL matrix, solidification technology and equipment. First, the aim was to convert the quasi-liquid sludges into a form safe for transportation, temporary storage or disposition in a radioactive waste repository. The second aim was to perform the solidification local to the point of retrieval, quickly and with consideration of properties and parameters of the sludges. Use of long transfer routes and transfer containers was not possible because of the properties of the sludges and the fact that they had accumulated in inaccessible areas of the reactor.

Three-step approach

Amec Foster Wheeler has a three-step approach to implementing the SIAL waste treatment process: characterisation, pre- treatment and solidification.

The treatment/solidification process is modified according to the composition of individual radioactive waste streams. To determine the most appropriate composition, the SIAL application technology is laboratory tested on real samples of
each individual waste stream identified for solidification. Then an appropriate mixture of aluminosilicate and other inorganic compounds is defined for a given solidification application.

When determining the most appropriate mixture, other requirements taken into consideration are the maximum dose rate on the drum containing solidified product, mass, volume, alpha radionuclide content in the product, the need for dewatering sludges/ resins before solidification and the dry matter content of sludge or resin in the solidification product.

Radioactive waste for treatment from nuclear power plants is usually stored in big tanks, resulting in wasteform layers, which are metres thick. The character and properties of the stored wasteforms can change in the both the vertical and horizontal directions as no mixing system is usually installed within the tanks. To better understand this waste, standard characterisation procedures are undertaken at various locations. This involves measurement of dose rates in contact with tanks, TV inspection of tank internals, representative sampling of waste, radiochemical analyses, chemical analyses and physical-chemical analyses.

The removal of radioactive sludge/resins from the tanks is ranked among the most difficult operational stages of wasteform solidification. This is due to the remote nature of the work, the congested space, and access restrictions which were not necessarily considered during power plant design and construction.

There are two basic approaches to removing sludge/resin from tanks:

  • Mix or suspend waste in a liquid so that slurry can be flushed to a fixed location where a pump then removes it from the tank – it is necessary to place the pump or the suction piping as low as possible and to install a flushing system if this approach is used; or
  • Remove waste using a remotely operated vehicle (ROV) operating at the waste surface.

Waste is transported from the storage tank through plastic or metal tubing, normally around 10-30m into the pre-treatment facility for disintegrating or dewatering. The pre- treated waste is then measured into 200l drums (or other packages) and is solidified by adding the SIAL matrix components into the drum during mixing.

Waste solidification

The SIAL technology is deployed at room temperature. The polycondensation process during curing is a slightly exothermic reaction with the maximum temperature reached in the middle of the drum being approximately 55°C. This is the maximum temperature observed to date for the solidification of more than 1500t of actual nuclear power plant waste. No special cooling arrangements are made to manage the curing temperature.

To date the historic application of the SIAL technology has been in-situ immobilisation and solidification of medium-level waste and low-level waste (sludge, resins, other waste streams). The equipment used for the solidification process is relatively small, lightweight and designed for the specific application on site, directly in situ or local to the tanks storing the waste streams. This mitigates the problems normally experienced by transferring sticky, tight waste through long pipes. The equipment is normally tailor-made to take into account the real space conditions and other existing facility requirements. The equipment is manufactured and tested to demonstrate the safety and effectiveness of the solidification process in specific conditions for each implementation of the SIAL matrix.

Next steps

Over the past 20 years, SIAL has been used successfully to immobilise approximately 1500t of waste which includes sludge, resin and crystalline borates from Bohunice, Slovakia and approximately 250m3 of spent ion exchange resins from tanks on site at the Dukovany nuclear plant in the Czech Republic. International missions from WANO and OSART evaluated the SIAL matrix technology at Dukovany as an example of good practice.

Last year, it was announced that SIAL is being trialled for use at Fukushima in Japan. The work, carried out in partnership with Fuji Electric on behalf of the Japan Atomic Energy Agency, will test whether SIAL can be used to solidify sludge arising from the damage caused by the earthquake and resulting tsunami in March 2011.

Hiroshi Ozaki, general manager of the Nuclear Power Engineering Department of Fuji Electric, said: “SIAL has already been used successfully in commercial power stations in Europe. Fuji believes it has great potential and wishes to spread this technology in Japan together with Amec Foster Wheeler.”