Decontamination & decommissioning

Burning down the waste

13 November 2009

A consortium has begun a two-year, EUR13 million project to apply a wet oxidation process to nuclear decommissioning at Italy’s shut down Trino plant.

The process will be used to treat 100m3 of spent ion-exchange waste from the operation of the Enrico Fermi nuclear power plant in Trino, near Turin, and decommissioning of its steam generators. The reactor, Italy’s first, was a single-unit 260MWe PWR shut down in 1990. The spent resins represent almost a quarter of the total waste currently present on site that need to be treated and conditioned.

The resins are currently stored in 106 stainless steel vessels (about 1m3 capacity each) in a temporary storage on site. The estimated total activity was about 26TBq at the end of 2005. Due to the high activity of the vessels, operation on them must be carried out remotely with shielding devices. Italian state decommissioning agency Sogin will develop and carry out the treatment, with Ansaldo Nucleare and Swiss firm Granit Technologies.

Granit’s proprietary wet oxidation process consumes the organic material in liquid sludges produced by municipal waste water treatment plants. The process will reduce the volume of waste by 85%, help reduce its inflammability and make it more chemically stable.

wet oxidation process
Diagram of the wet oxidation process. A heat exchanger recovers steam heat to preheat the liquid at the inlet

The process is based on a chemical reaction that takes place in the liquid state, consisting of the oxidation of organic and inorganic substances through the use of oxygen (or air) at high temperature (200-300°C) and a pressure between 35 and 200 bar.

The reaction, which does not require catalysts, transforms organic material into water and carbon oxide and the inorganic component into mineral oxides without producing toxic gases, sulphur dioxide, nitrogen oxides or carbon monoxide.

Intermediate decomposition stages in the reaction

The oxidation reaction is exothermic and, if the quantity of organic material is sufficient, there is no need for external heat source. Residence time in the reactor varies between 30 and 300 minutes, depending on the operational conditions. The installation is automated and works continuously, requiring only supervision by an operator from a remote monitoring station.

In order to obtain good performance, organic matter has to be diluted in water to obtain a suspension chemical oxygen demand between 40 and 120g/l. The maximal diameter of solid particles suspended in liquid should be 500µm. Before treatment, NaOH (30%) is added to the suspension to avoid corrosion problems caused by the presence of acids.

Once reaching the optimal initial conditions, the suspension to be treated is mixed with the oxidant gas and preheated. The reaction takes place in a tubular vertical reactor.

A water suspension containing soluble and insoluble mineral matter and the gas steam are separated in the phase separator. The two phases are then treated independently. The treated effluent, an aqueous solution containing insoluble metal oxides and soluble salts, will be filtered and concentrated through evaporation, conditioned and put into containers certified for final disposal.

Ansaldo will be responsible for adapting the technology to the nuclear field. In particular, adaptation will include:

• Use of few flanged connections, seamless piping and seamless reactor vessel to reduce the potential for leaks.

• Suction from the upper part for all the hold-up and mixing tanks to minimise the loss of process liquids.

• Use of self-priming pumps installed above the tanks to avoid release of the process liquid from the tank.

• Installation of drains in the process piping to the hold-up tanks in case the process stops.

• Installation of liners underneath the process components to contain process liquids in case of component rupture.

• Installation of connections to demineralised water and to compressed air to clean all the process lines.

• Application of appropriate redundancy for safety-critical operations.

• Dynamic confinement of the process rooms through a dedicated ventilation system.

• Adoption of a segregation policy to permit operability and maintainability of process components.

• Installation of proper shielding to minimise operator doses.

• Continuous monitoring of the gases and liquid treated and produced in the process before release.

Plant commissioning is expected in 2011 and the Trino resins treatment campaign should last about six months.

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