Plasmarc
The Plasmarc plasma furnace in the Würenlingen interim storage facility (Switzerland) is a facility in which slightly radioactive waste is thermally decomposed and evaporated using a high-performance plasma burner at a few thousand degrees Celsius . The residues are merged with waste glass in a melting process and are thus to be chemically immobilized. The aim of the process is to produce a waste body which is difficult to chemically leach and which is suitable for later storage in geological formations. In addition, the volume of waste can be reduced by a factor of five to 20 on average.
introduction
In Switzerland , the polluters of radioactive waste are responsible for their disposal. The law prescribes permanent and safe disposal through final storage. The processing ( conditioning ) of the waste into a form suitable for disposal is carried out by the person who caused it.
The four nuclear power plant operators (BKW, KKG, KKL, NOK) founded Zwilag for this purpose .
The ZZL distinguishes between highly radioactive and medium to low-level radioactive waste.
The storage facility for the highly radioactive waste has space for 200 containers, each weighing around 100 tons. It currently contains 20 casks, 13 with spent fuel, 7 with vitrified waste from reprocessing abroad.
In addition to high-level waste, there are around 300 barrels of low-level to medium-level waste from nuclear power plants and from research and medicine every year . In addition, there is a store for 1,000 barrels of low-level radioactive waste, which is currently occupied with 600 pieces.
Plant description
Since Switzerland does not yet have a repository and the capacity at ZWILAG is finite, the sponsors decided to build a conditioning plant and a combined incineration and melting plant.
After a six-year approval process, the building permit was granted at the end of August 1996 . The first phase of construction was completed in spring 2000.
On March 6, 2000, the then Federal President Adolf Ogi issued the operating license for the conditioning and V&S system. Everything worked practically straight away, only the globally unique high-tech device suffered from one teething problem after another. The plasma furnace could only be put into operation after making subsequent investments worth millions. To make matters worse, the delivery company went bankrupt and the automatic keg loading system had to be completely renewed.
In the following, this work will only focus on the V&S system.
Goal of the plant
The intention for the construction and operation of the incineration and smelting plant is analogous to that of a conventional waste incineration plant . This is a thermal process that incinerates garbage. This greatly reduces the volume of waste. Furthermore, the mass is also reduced, since a not insignificant part of the waste is emitted as exhaust gas.
The V&S system involves pyrolysis . A plasma torch is also used as the primary energy source. This is an “electrically stabilized flash” in a gas atmosphere that is generated between two electrodes, one of which can be the item to be treated.
A plasma flame reaches temperatures of 15,000 to 20,000 ° C. The water-cooled 1200 kW plasma torch creates an arc surrounded by a 5 to 15 cm thick flame with a temperature of approx. 5,000 ° C. Nitrogen is used as the plasma gas .
It should not be forgotten that radioactivity cannot be destroyed. The resulting waste ash or slag / melt remains radioactive. It is immobilized in the glass matrix and poured into steel molds, which are then transferred to final storage drums.
Primarily, a volume reduction of up to 80% is achieved. In the last campaign, 191 barrels were reduced to just 31 barrels.
The incineration and melting plant was designed for a throughput of around 200 kg / h of combustible or 300 kg / h of meltable waste. Two campaigns of approx. 600 hours each in four-shift operation are planned. The expected annual volume of waste is around 150 t of combustible and 50 t of meltable waste.
Procedure
The heart of the PLASMARC® incineration and melting system (V&S system) is a rotary hearth furnace with a plasma torch and a downstream exhaust air purification system. This embodiment allows combustible and liquid waste to be thermally decomposed and metals to be melted in a single system. The furnace consists of a 4 m high cylindrical structure with a diameter of 3 m, with the two main components: a lid and a rotary hearth.
On the cover there are openings for the charging device (sluice barrel rotating device), the flux charging, the plasma burner and the auxiliary light oil burner, the cleaning lance manipulator for the pouring opening, the cameras and the oxygen jets. An additional opening serves as a manhole . The cover is lined with an insulating layer of sintered corundum -chromium oxide- high- fire brick to prevent melting or corrosion of the boiler.
The rotary hearth is rotated around its axis in order to bring the waste under the stationary plasma torch and to move the melt from the centrally located pouring opening to the centrifuge wall before it is poured.
Furnace operation
The furnace system is only put into operation when the flue gas cleaning system is ready for operation. The furnace is slowly brought to temperature with the auxiliary burner in order to protect the lining. After the central opening in the bottom has been closed, the aggregates are placed in the furnace. For the vitrification of the combustion residues, the additives silicon dioxide and calcium oxide are brought into the rotary hearth from 200 liter drums through the horizontal drum feeder and melted with the plasma torch. Small amounts of sodium oxide can be added as a flux , the metering is done through a rotary valve .
Furnace loading
The barrels are then placed individually into the vertical loading lock. There they are pierced and then turned horizontally and pushed towards the oven. The propane burner cuts the barrels into slices, and their contents fall in portions into the rotating glass melt. The waste gets under the plasma torch. The waste decomposes through pyrolysis and splits up into residue and gas. The gases are fed to the exhaust air treatment system.
Barrels with liquid contents are connected to a pump device outside the loading lock and injected into the rotary hearth. The procedure for the empty barrel is exactly the same as for a full barrel.
Melt emptying
As soon as the furnace has reached a content of 800–850 liters, pouring is started. The speed of the rotary stove is increased significantly and the bottom lock is opened. The flow of the melt is initiated and regulated by reducing the speed. While the melt flows through the chamber into the mold , the pyrolysis gases are drawn off from the side and fed to the afterburning chamber. The casting chamber has water-cooled double walls. Furthermore, the casting chamber has two locks to prevent contamination when changing the mold . When a mold is full (approx. 140 liters) the speed is increased and a new mold is introduced. The filled mold is brought into the exit lock and, after a certain flushing time, after a rigid layer has formed on the melt, transported into the shielded cooling tunnel. After staying in the tunnel for around 24 hours, the temperature of the cast has dropped to below 50 ° C due to air cooling.
After cooling, the molds are pulled out of the cooling vessels and placed in 200-liter final storage containers lined with concrete . In the sand filling station, the remaining cavities in the disposal container are filled with sand. The containers are closed automatically and, after an automatic dose rate and contamination measurement, are sent to the keg output warehouse.
Flue gas cleaning
The plant for treating the flue gas is comparable to that from conventional waste incineration plants .
The flue gases from the incinerator are first completely oxidized in the afterburner chamber. They then pass through the waste heat boiler. The energy gained is used to heat other components.
Quench
The component called quench, which is used to absorb harmful substances and heat, consists of a vertical steel cylinder. The hot flue gases are introduced in the upper area and further cooled with atomized water. The quench water absorbs active substances such as cesium , but also pollutants such as zinc , chlorine and fluorine compounds.
Acid scrubber
In the acid scrubber, a packed column , the exhaust gases are introduced in countercurrent, where the acid-soluble inactive and active pollutants are separated.
Dedusting
In the electrostatic filter are aerosols and dust particles which are still present in the exhaust gases deposited. The finest particles that still flow through the wet electrostatic precipitator are separated out in two HEPA filters . After these purification stages - apart from radioactive carbon - there should no longer be any radioactive substances in the exhaust air. The following components are only used for the separation or conversion of inactive pollutants.
Desulfurization
Sulfur dioxide is separated from the exhaust gas in the alkaline gas scrubber . The exhaust gas is passed through a washing stage in countercurrent. The sulfur dioxide reacts with slightly basic water. This process represents a flue gas desulphurization . The following blowers generate a negative pressure in the plasma furnace.
DeNOx
After the fan, the exhaust gases are heated to 320 ° C in a pipe heat exchanger . From there, the exhaust gases pass into a mixing chamber, in which the exhaust gases are mixed with ammonia. In the DeNOx plant, nitrogen oxides are reduced to nitrogen in the presence of ammonia and a catalyst . The exhaust gas is then discharged through a 35 m high chimney .
Waste products and residues
The waste water resulting from the treatment of exhaust air is collected in a buffer tank. There part of the radioactive cesium is bound by adding zeolites. The liquid phase is pumped into a second buffer tank and neutralized with sodium hydroxide . Both containers are equipped with dosing devices to precipitate heavy metals. The wastewater is then filtered. The resulting filter cake is returned to the incinerator. The filtrate is passed through an ion exchange column and then evaporated in a distillation column . The loaded ion exchange resins are also treated again in the V&S plant. The resulting distillate is used as process water in the incineration plant. The HEPA filter elements are filled into 200 liter drums and also fed to the V&S system.
In addition, approx. 50 m 3 a −1 sodium chloride solution is produced, which after renewed purification (centrifugation, evaporation) and control measurement for radioactivity in the conditioning system is released into the PSI waste water system via receiving water .
criticism
The regional association of the southern Upper Rhine of the BUND criticizes the procedure and refers to a report in the Tages-Anzeiger from February 24, 2005.
literature
- J. Vigfusson: HSK report on the application from Zwilag Zwischenlager Würenlingen AG for the issuing of an operating license for the conditioning plant and the incineration and melting plant of the central interim storage facility for radioactive waste in Würenlingen [HSK27 / 45] [KSA 27/99]. 1999.
- The Swiss Federal Council: Order regarding the operating license for the conditioning plant and the incineration and melting plant of the central interim storage facility for radioactive waste in Würenlingen. March 6, 2000.
- H. Lüthi: This is how you pour radiation into barrels. In: Mittelland newspaper. April 16, 2005, p. 22.