At UKAEA, Harwell they have 100m³ of intermediate and low level radioactive waste safely stored in liquid and sludge forms. Over a three-year campaign, this waste is being immobilised by encapsulation in a cementitious matrix capped off with a cement grout, all within 50 litre stainless steel drums.

Customer Requirements

Liquid waste is pumped from sorage tanks wo a stock vessel. A batch of waste is then charged to a drum. A premix of Blast Furnace Slagand Ordinary Portland Cement (BFS/OPC) is then added to the drum up to a capacity of 390 litres when combined with the waste. Each drumis equipped with an internal paddle driven by an external drive to ensure a homogeneous, monolithic matrix or waste and cement. The paddle is sacrificed and becomes encpsulated within the matrix, which takes about 24 hours to cure.

Once the matrix has cured, 50 litres of cement grout, a mix of Pulverised Fuel Ash and Ordinary Portland Cement (PFA/OPC), is charged to the drum to cap the matrix and to reduce the residual space at the top of the drum to less than 15 litres. The drum is then fitted with a stainless steel lid.

 A 136 litres capacity bol mixer, located on a weighing plaform, is used to prepare the required batch of capping grout. UKAEA required a reliable and accurate means of conveying the PFA/OPC powder mix from its storage siko to the bowl mixer.

Spiroflow Solutions

A 'Spiroflow' Flexible Screw Conveyor is used to transfer a precise weight of PFA/OPC premix from the storage silo to the mixer bowl - which is pre-charged with water. The precise weight is achieved by a control signal from the weighing platform. So precise is the conveyor and so consistent is hte powser fed rate from batch to batch (+/- 1.5%) that UKAEA do not sue the trickle feed facility.

UKAEA's conveyor is unusual on two counts: firstly it is driven from the bottom not the top and secondly the conveying tube is stainless steel, not the traditional flexible UHMWPE plastic. The majority of 'Spiroflow' conveyors are top driven but the layout and modus operandi made it impossible at UKAEA. The stainless steel tube was used because no support cold be provided at the top otherwise the weight of the conveyor would have affected the mixer weigh platform.

As it was impossible to anticipate the final radial position of the conveyor, the 'round to square' transition piece, which connects the conveyor to the silo, incorporated a slip ring trapped beween flanges to accomodate this.

The conveyor feed hopper incorporates an agitator to ensure no bridging of cement over the inlet to the spiral.