Turning up the heat on waste30 October 2000
Modern incinerating equipment can reduce the hazards of treating radioactive waste.
The Siebersdorf Centre, 35km south of Vienna, was where Austria’s first steps were taken towards developing a nuclear power industry. The first step was a research reactor at the site which went into operation in 1960. A few years later two other research reactors were built, in Vienna and Graz. During the 1970s, a power reactor was built but was abandoned after a referendum.
During the same period, work was conducted to establish a central facility to treat, condition and store radioactive waste. This was finally built on the ARCS site. The facilities included an incinerator for the treatment of burnable radioactive waste, which began test operation in 1980, and was commissioned for radioactive material in 1983.
In the absence of a nuclear industry in Austria, only low and intermediate level radioactive waste, collected from medical, research, and metallurgical industrial applications of radioactive material is now treated, conditioned and stored at the facility. The facility has collected and treated only Austrian waste, with the exception of a project to treat contaminated ion-exchange resins from a power plant in Italy. After treatment and conditioning, this waste was sent back to Italy for interim storage.
The shaft incinerator is of the Karlsruhe type: an excess air unit with a single chamber. It has a capacity of about 40kg/hr and a combustion volume of 1m diameter and 5m height. The off-gas cleaning system comprises a set of ceramic hot gas filters, a quench, a two-stage wet scrubber and HEPA-filters (see panel).
Over the years, a number of modifications to the original design have been carried out to improve safety, keep up-to-date with technical standards and meet the changing requirements of regulations. For example, the off-gas cleaning system has had to be upgraded several times over the years. Modifications to the shaft have also been carried out, introducing additional openings to facilitate the incineration of powdery material.
About 1000t of ion-exchange resins has now been successfully incinerated: during this process dried material was transferred into the combustion chamber via a screw and a blowing system.
Maintenance and repairs to the facility have been significant, including: removal of an electro-static filter installed at the off-gas cleaning system; renewal of the refractory-brick lining of the lower half of the combustion chamber after 12 years of operation and a part of the refractory-brick lining in the hot gas filter boxes; renewal of HEPA-filter boxes; plus replacement of parts of piping and other equipment.
In its first 20 years of operation the following amount of waste was treated:
•Liquid waste 41430kg (41.4m3).
•Solid waste 695,576kg (4200m3).
•Ion exchange resins 534,882kg (1600m3) after drying.
By the end of 1999 the plant had incinerated a total of about 1300t (5841m3), and had operated for some 43,000 hours. It is operated in two shifts a day, from 6.00am till 10.00pm six days a week, with two operators in each shift.
The treatment of the waste resulted in a volume reduction of about 50:1 comparing raw material to ashes. But operating the facility creates secondary waste, changing the picture of volume-reduction significantly.
The secondary waste comprised:
•Ashes – 78,000kg, 112m3.
•Hot gas filter candles – 11,200kg, 20m3.
•HEPA filters – 23,000kg, 125m3.
•Replacements for refractory bricks – 14,000kg, 8m3.
•Equipment replacements – 10m3.
•Solid waste arising from precipitation of 15,000m3 of waste water – 25m3.
Different techniques were used to treat and condition radioactive secondary waste. Crushing and cementing ceramic filters produced 140 200-litre drums. Supercompaction HEPA filters produced 25 200-litre drums. Crushing and cementing refractory bricks filled 90 200-litre drums and supercompaction or cementing of other equipment produced 80 200-litre drums.
During the first 12 years of incinerator operation the resulting ashes (some 72m3) were cemented. Later, supercompaction was applied, and in the end some 40m3 was supercompacted.
Comparing the raw waste of 5,841m3 fed into the incinerator to the 368m3 it has created, a total volume reduction of 16:1has been achieved during during 20 years of operation. This includes treatment and conditioning, but not final decommissioning.
The 1,843 drums containing conditioned waste from incinerator operation (cemented and super-compacted) are part of a total of about 8,500 drums stored in interim storage at Seibersdorf.
Radioactive releases to the atmosphere are checked by analyses of samples collected in a sampling system connected to the stack. Release limits are radionuclide specific and set by the authority. At no time during 20 years of operation has an activity release in excess of the limits set by the authority been observed. Most of the time, far less than 1% of the limit has been measured. Total releases over 20 years of operation have been: 11.4MBq of alpha releases; 77.4MBq of beta releases; less than 24.8MBq of Co-60; less than 24.7MBq of Cs-137.
Routine measurements and reported activity values of institutional radwaste are very inaccurate, as its activity is very low. It has not been possible to obtain an activity balance or a decontamination factor when treating institutional wastes (by taking activity measurements for the ashes, the hot-gas filter, the HEPA filter and wastewater). This has only been possible in specific experiments, when waste with known radionuclides and activity was fed to the incinerator to ascertain retention factors.
Cross contamination within the incinerator unit causes an additional problem, when the surface of the plant exposed to the off-gas absorbs and simultaneously releases radioactive particles into it. These factors combine to make activity balancing nearly impossible.
An incinerator is not an appropriate tool for treating C-14 or tritium. These contaminants aside, Co-60 is the dominant radionuclide measured at all contaminated parts of the plant and in the ashes. Due to the high gamma energy it emits, it can easily be detected and its activity determined.
This activity was determined for all drums containing conditioned ashes, before they were loaded into interim storage.
At the beginning of ash conditioning in 1983, measurements were not as accurate as now, but were adequate for an estimation.
The total activity of Co-60 measured in all drums containing conditioned and unconditioned ashes reached 180GBq by the end of 1999. Since over 95% of the existing cobalt is concentrated in the ashes when it is incinerated, we know that 190GBq has been fed into the plant since it started up.
Staff working at the incinerator not only act as operators during incineration, but are also responsibile for maintenance checks and repairs of the system. They also handle and segregate the radwaste prior to combustion. No staff member involved in handling and incinerating of radioactive waste has had a radiation dose over the regulatory limits.
The first years of operation were marked by two incidents. The first was an explosion caused by burnable vapour in the ash box.
The ash box is a compartment housing a shielded drum which takes up ashes from the combustion chamber. The box is attached to the combustion chamber. The volume of the ash box is about 0.8m3. The combustion chamber is separated from the ash box by a fire resistant lid. To empty the combustion chamber of ash, the lid is opened downwards into the box, so ashes can fall into it. Since the ash is usually hot, it is cooled by steel plates before being lowered into the shielded drum.
The incident was caused by the bottom closure – or lid – of the combustion chamber, which was (and still is) leaking. Even small quantities of liquid burnable waste in polythene bottles (for example toluene), fed to the combustion chamber in the usual way from the top do not evaporate and burn immediately. So there is time for the liquid to seep through the lid into the ash box.
The temperature in this box is usually around 25°C – high enough for the continued evaporation of organic liquids. In this situation it was just a question of time before the burnable vapour reached a sufficient concentration and was ignited by the connection to the combustion chamber. The steel containment of the box was not damaged, but viewing windows were broken by the pressure and consequently the room around the ash box was contaminated.
Counter measures against the possibility of explosive vapour-air mixtures have been set in place by periodically rinsing the ash box with air. The vapour-air mixtures passes the gap of the loose lid into the combustion chamber. In addition a shelf was mounted just under the lid since, due to the design of the lid, leaking cannot be prevented.
When there are leaks, liquid collects on the shelf and burns immediately because of the high temperature there. The air rinsing of the ash box takes place from bottom to top, passing the shelf and the lid, and so taking the off-gas of burning liquids from the shelf into the combustion chamber. Temperature sensors placed close to the shelf indicate rising temperatures if burning takes place there, so the operator is warned and can act if necessary.
The second incident was a fire in the off-gas system of the incinerator plant.
At the time this happened, a tower made of mild steel was installed, serving as a cooling and washing unit for off-gas. It used water, pumped in circuit and sprayed onto the off-gas in the tower. Plastic rings were used to separate the water from the wet off-gas during combustion operation in the upper part of the tower.
At the end of a shift, the operators forgot to close the valves at the hot gas filters and so hot air from the combustion chamber could pass by natural draft into the tower, igniting the plastic materials. Since the plant was out of operation, the plastic rings were dry. In the absence of enough fresh air, mainly smoke was created. But enough heat was generated to evaporate the paint on the outside of the top of the tower and some piping. Smoke detectors placed in the building triggered an alarm and the fire-brigade was called in. The problem was solved quickly by switching on the water circuit of the equipment. The HEPA-filter was not damaged and prevented release into the environment.
The recent installation of new off-gas cleaning system and modifications at the ash box eliminated the natural draft, so heat remains in the combustion chamber, even if the valves of the hot gas filters are not closed.
Various techniques for treating organic radioactive wastes have been developed and evaluated at ARCS. Some treatments are applicable to both solid and liquid organic wastes whereas others are specific to an individual type. No one treatment appears to have an overriding advantage, although incineration is attractive because it is applicable to both solids and liquids. Organic wastes are easily combustible and high volume reduction factors can be achieved. Despite the rather complex technology of an incinerator and its high capital and operating cost, it is a proven technology.
Since acceptance requirements for long term interim storage and final repositories are becoming more stringent about untreated organic material (in the future such material may only be accepted when treated), incineration will certainly remain an option. Increasing prices for storage and disposal will also push treatment technologies with high volume reduction factors.
|Main characterisitics of the incinerator|
|• Excess air incinerator • Shaft type, single chamber • Combustion chamber: 1 m diameter, 5 m high • Combustion temperature: 1000°C • Capacity : ~ 40 kg / h solid burnable waste (calorific value: average 21x106 J/kg = 5000 kcal/kg) • Underpressure in the combustion chamber: 103 Pascal = 10 mbar • Air flow: 300-600 m3/h variable, depending on underpressure in combustion chamber • Feeding from top batchwise (2-3 kg) through airlock, liquids through burner • Feeding of powdery material by blowing system into combustion chamber • Hot gas filter, in brick-lined filter box, Silicon-carbide candles, mean porosity : 20µm • Quench, spray cooler with nozzles, decreases off-gas temperature from 700°C to 70°C • Two stage scrubber (one trickle flow, one spray) using caustic soda solution to pH 8.1 • Heater, raises off-gas temperature to ~ 100°C • HEPA filters • Off-gas draft fan, radial blower, regulated by underpressure of combustion chamber • Mixing chamber • Stack, 35m high|