Community groups and contamination experts have attacked the plan to store 350,000 tonnes of spent pot-linings, contaminated soils and demolition waste on site.

Hydro managing director Richard Brown rejected a suggestion that the company had scrapped the controversial plan.

‘‘We were hoping to have it in by now, but the process of completing all of the specific studies has taken longer than we thought,’’ he said.

‘‘We are hoping to submit it to the Department of Planning by the end of the year.’’

It is proposed to build the containment on the 2000 hectare former smelter site before it is sold for private development.

A metre of clay, sealed with plastic, would form the cell’s floor.

The cell would be capped with 60 centimetres of compacted clay and another plastic liner.

Senior research associate at the University of Newcastle’s centre for excellence for geotechnical science and engineering Brett Turner said history had shown that all permeable containment cells eventually failed. 

‘‘They are usually designed to last for about 100 years but some fail after 20 or 30 years,’’ he said.

Dr Turner said the  geological volatility of the coalfields, which frequently experiences earth tremors, increased the risk that the cell would break.

‘‘There will be low pH on the outside of the cell and high pH on the inside. Both of these environments will be attacking the cell wall,’’ he said.

The company has rejected the option of incinerating the waste at1500  celsius, known as vitrification. Although more expensive the incineration process, considered to be world’s best practice, reduces the waste to an inert, glass-like material.

‘‘There are a range of factors, not just cost, why we are using a containment cell,’’ Mr Brown said.

‘‘There is a mix of material involved, some of which is not suitable for incineration.’’