Kozloduy’s plasma plant4 July 2018
Plasma technology can be used to treat a variety of radioactive waste streams. Jan Deckers and Mieke Roos explain how a new plant is gearing up to treat waste from Kozloduy in Bulgaria.
At the beginning of 2009 Kozloduy, in Bulgaria, awarded a joint venture of Iberdrola Ingeniería y Construcción (Spain) and Belgoprocess (Belgium) the contract to supply a facility for treating and conditioning solid radioactive waste and reducing its volume, using plasma technology. The contract is funded by the Bulgarian Government (30%) and the EBRD’s Kozloduy International Decommissioning Support Fund (70%).
Iberdrola as an engineering company is the leader of the joint venture. It provides detailed engineering, procurement of the main components, site management, the safety assessment report and licensing. Belgoprocess is the process provider and is responsible for basic design and nuclearisation of the facility, as well as reviewing engineering documents, developing the environmental impact assessment, commissioning and startup, and training operations staff.
The Kozloduy plasma melting facility (PMF) consists of a tilting plasma furnace with a 500kW torch as heat source. It will treat 250t of low- and intermediate-level radioactive waste per year, over 40 operational weeks. In the process organic waste is gasified and inorganic waste is melted, reducing the volume of waste and immobilising it. The end product is a robust slag. The flue gases undergo treatment similar to that used in conventional radwaste incinerators (there is one at the Belgoprocess site). The facility has an operational life of 40 years.
Factory acceptance tests have been competed, and Iberdrola-Belgoprocess JV has sent all the major equipment components to Kozloduy.
Onsite construction started in 2015 and posed a challenge, as an existing building (AB-2) had to be enlarged to allow components to be installed inside. In October 2016 commissioning began. Integrated and safety tests with a hot furnace were performed in early 2017. They were followed by ’72-hour’ site acceptance tests in which all aspects of the installation underwent three days of continuous operation with simulated radioactive waste. Different pours of hot liquid slag at a temperature of about 1300°C were carried out successfully, as well as ultimate safety tests.
In May 2018, the JV will perform a 120-hour test, during which nuclear waste will be processed in 120 hours of continuous operation.
One single process for various waste streams
The PMF will treat and condition 250t of waste per year. The waste is characterised as Category 2a under Bulgarian law and consists of:
- Organic untreated waste packed in bags with a volume reduction factor (VRF) of 80;
- 200L drums containing compacted organic and inorganic waste (VRF = 20);
- supercompacted metallic 200L drums containing mixtures of concrete, wood and other organic material
- (VRF = 2); and
- liquid waste such as oils and drummed spent ion exchange resins.
Typical specific radioactivity of the waste is approximately 5x105 Bq/kg.
The PMF will treat these various radioactive waste streams is a single process. Waste packages will be fed to the installation unopened, without a need for pre-treatment or sorting. This eliminates contamination risk and strongly limits dose uptake in accordance with ALARA principles.
The PMF has the following components:
- A robust dual shredder with extruder feeder with a nitrogen blanketing system;
- a primary treatment chamber, with the torch, and a sealed slag collection chamber;
- a secondary combustion chamber in which the syngases are mixed with air to complete oxidation to primary combustion components;
- a boiler to cool the off-gases;
- off-gas filtration and radiological purification, comprising bag filters and high efficiency particulate air (HEPA) redundant filters;
- a wet off-gas scrubbing system, consisting of a quench tower and a counter current scrubbing tower to remove HCl and SO2;
- redundant flue gas extraction fans to keep continuous negative pressure in the system and evacuate the flue gases;
- a catalyst system to reduce the NOx; and
- continuous emissions monitoring and radiation monitoring systems.
The PMF tilting furnace is designed to pour the slag in a controlled way into a mould.
It is a closed system so there is no release of radioactive or hazardous substances. There is no plug or stop in the tapping hole, which eases accessibility. The plasma furnace has few moving parts which could become contaminated and need maintenance. It provides flexible treatment of glass-like or metal-like slag. A patent on the system is currently pending.
The PMF uses a plasma arc of 5000°C as heat source. The torch contains two metallic tubular electrodes (upstream and downstream with respect to the plasma flow direction) separated by a gas injection chamber. An electrical arc flows between the negative and positive electrodes and therefore the gas flow injected into the chamber is ionised. The result is a high-temperature gas flow coming from the downstream electrode in a plasma jet.
Power ratings typically range from 100kW to several MW. Process conditions can be varied from inert (eg Ar or N2) to oxidising status (air or pure oxygen). The torch applied for the PMF has a power rating of 500kW and compressed air as torch gas.
The PMF uses a continuous feeding system through a two-stage shredder. A continuous feed smooths and reduces peak off-gas flow rates.
The shredded waste is transferred to the feeder tube which has a rotating connection so that the tube of the shredder is fixed-mounted in relation to the tilting furnace. On the opposite side of the furnace the contaminated hot gases, at 1300°C, are diverted away to the afterburner chamber.
Slag collection chamber
In the plasma, metals are melted and oxidised. Concrete debris, sand, inorganic granulates, insulation material such as mineral rock wool and asbestos are melted. They are transformed into a chemically inert and amorphous glassy slag. Liquids and organic materials are vaporised so the residue is organics-free.
When 200L of slag is produced, it is poured into the slag mould. Some 50L of slag remains in the furnace and is used as a thermal flywheel for the next waste batch. The remaining slag also protects the refractory against the high temperatures.
The slag is tapped into the mould in a confinement to stop contamination spreading into the normal work area and environment.
The slag contains the concentrated radioactivity. It has to remain in the low-level waste category. Medium-level waste would need additional precautions and be more expensive to store, so the radioactivity content of the incoming waste will be monitored.
As with conventional radioactive waste incinerators, the flue gases are passed through a thermal oxidiser chamber and are treated. Hydrocarbons are oxidised to primary components such as CO2, H2O, HCl and SO2.
The off-gas cleaning is a multi-step procedure to eliminate chemical compounds such as fly ash, HCl, SO2 and radioactivity. The fly ash can be sent back to the plasma furnace.
As noted above, with plasma technology virtually all types of radioactive waste can be treated, which makes the technology especially suitable for the treatment of historical waste.
The waste can be treated with a high VRF and with limited preparation and minimum risk of radioactive contamination. As the final waste form is free from organics and liquids, it will meet strict quality and stability requirements for long-term storage or final disposal. So even historical conditioned waste in a bituminous or concrete matrix, which does not meet current acceptance criteria for conditioned waste, can be retreated in a plasma facility and produce an acceptable conditioned product.
Author information: Jan Deckers, Technical business manager at Belgoprocess; Mieke Roos, International business development at Belgoprocess.