Plasma destructor28 July 2004
The Institute of Nuclear Energy Research in Taiwan has initiated a plasma melting research programme as part of a drive to improve treatment processes for low-level wastes. By Chin-Ching Tzeng, Tsung-Min Hung and Li-Fu Lin
Low-level radioactive wastes in Taiwan come from nuclear power plants, following normal operation and maintenance, and from hospitals and research institutes due to isotope applications.
In order to safely and cost-effectively dispose of the wastes, they must be transformed into physically and chemically stable forms capable of immobilising radioactive nuclides with maximum volume reduction. Historically, wastes have been sorted into combustibles, non-combustibles and metals in a pretreatment step. Combustibles are normally treated by incineration and non-combustibles are either compacted or directly converted into a solid matrix. Metals are cast into ingots. All these conventional process technologies have the following disadvantages:
• Different types of waste have to be separated, manipulated and treated with different equipment.
• Volume reduction and vitrification are not achieved within one step.
Fortunately, plasma technology is able to reduce volume, incinerate the combustibles in radioactive wastes and vitrify the non-combustibles simultaneously into slag or metal ingots using the same equipment and with limited pretreatment steps.
The Institute of Nuclear Energy Research (INER) has been commissioned by the Atomic Energy Council of the Republic of China (ROC-AEC) to be responsible for the treatment of all domestic radioactive wastes except those from nuclear power stations since 1978. Combustible radioactive wastes are treated in an incinerator with a capacity of 40kg/h, with the residual ash temporarily stored in steel drums. The non-combustibles are either compacted, or directly solidified into a cement matrix.
In order to offer an effective alternative to the treatment and storage of these radioactive wastes, a plasma-melting programme (INER-PMP) is in operation. Since 1994 it has passed through several developmental phases:
• Development of a non-transferred DC plasma torch.
• Establishment of a lab-scale plasma system with an in-house 100kW non-transferred DC plasma torch.
• Testing of plasma vitrification of simulated radioactive wastes.
• Establishment of a transferred DC plasma torch delivering an output power of more than 800kW.
• Development of an NOx reduction process for plasma furnaces.
• Development of a pilot-scale plasma melting furnace to verify the vitrification processes of simulated wastes.
• Establishing a LLW plasma melting facility.
PLASMA TORCH SYSTEM
The plasma torch technology has been successively studied at INER since 1993. Firstly, a Direct Current - Radio Frequency (DC-RF) hybrid plasma torch with 20kW output power was designed to study the characteristics of inductively coupled plasma (ICP), DC plasma, and DC-RF hybrid plasma, and to evaluate the simplicity and economics for construction, operation and maintenance. The DC plasma torch was selected for further development. Secondly, the INER-100NT plasma torch was developed as an arc heater for the lab-scale plasma furnace system. The output power of INER-100NT plasma torch in straight polarity is about 100kW. Finally, the INER-1200T plasma torch is developed to serve as an arc heater for a pilot-scale plasma furnace and the LLW plasma processing plant. The INER-1200T plasma torch can be operated in either the transferred mode or in the non-transferred mode, and can deliver output power of 1200kW with thermal energy conversion efficiency greater than 90%.
LLW PLASMA PROCESSING PLANT
The low-level waste plasma processing plant is in the restricted area of INER. The plasma melting facility is beside the existing incineration facility. The Figure (above right) shows the process flow diagram for the radioactive plasma melting and the incineration facilities. The existing incinerator, with a capacity of 40kg/h, was already commissioned for the treatment of combustible low-level waste. The new plasma furnace and the existing incinerator use the same off-gas treatment system. The heat source for the plasma furnace is an INER-1200T plasma torch using nitrogen as its major working gas. The specified processing rate is 250kg/h for non-combustible waste and up to 40kg/h for combustible waste. The maximum operating temperature of the primary chamber is 1650˚C, while the normal operating temperature of the secondary combustion chamber is about 1100˚C. The pressure of the plasma facility is maintained between -20 and -50mm WG. The feed system can manipulate 55-gallon waste drums with a ball-screw ram in a semi-continuous operating mode. Molten slag is discharged into a 45-gallon carbon steel drum, which is cooled by a water jacket. The vitreous slag in the 45-gallon drum then passes through a cooling tunnel until the temperature is below 60˚C. Finally, the 45-gallon drums are repacked in 55-gallon drums and sent to a temporary storage site.
As shown in the Figure, a spray dryer was added to the existing off-gas treatment system of the incinerator to remove salt generated in the scrubber. It aims to eliminate the secondary liquid waste from the process. Additionally, an electric heater and a NOx removal unit are installed behind high efficiency particulate air filters to remove the thermal nitrogen oxides from the high temperature plasma processes.
The salt powders from the spray dryer and fly ash from the bag-filter are collected in 55-gallon drums, and could be recycled back into the plasma furnace for vitrification or be disposed of after solidification. The picture on the right shows the INER-PF250R LLW plasma furnace. The INER-1200T plasma torch is mounted on the top of the furnace with a three axis operating mechanism. A closed-circuit television system enables operators to monitor the waste melting operation from the control room.
PLASMA VITRIFICATION TESTS
The INER-100NT plasma torch is used as a heat source for the crucible-type plasma melter and the 10kg/h plasma furnace. Several types of surrogate wastes have been processed by these plasma systems. The volume reduction ratios (VRR) defined as the quotient of initial waste volume and slag volume ranged from 2 for cement solidified wastes to 100 for combustible wastes.
The densities of slag are between 2.4 and 3.2g/cm3. The compressive strength ranged from 400 to 3000kg/cm2. The leaching indices of tracing elements are between 8 and 15. The quality of vitreous slag from the small scale vitrification testing is superior to the acceptance criteria for low-level radioactive waste forms for final disposal.
In addition to small-scale plasma vitrification testing, full-scale tests of various 55-gallon drum surrogate wastes were performed by the non-radioactive INER-PF250 and the radioactive INER-PF250R plasma facilities. In the early development stage, 55-gallon drum cemented surrogate wastes were studied at full scale using the Plasma Hearth Process (PHP) non-radioactive pilot-scale plasma system through a cooperation contract between INER and Science Applications International Corporation (SAIC). The PHP plasma furnace, owned by SAIC, also uses a 1200kW transferred arc torch as the heat source. Boric acid waste surrogate is used to simulate cement-solidified waste from PWR plants.
The waste forms consisted of four types of cemented surrogate waste, two drums of each type. A total of eight drums were tested in a batch mode. The testing results show that the overall VRR is about 1.76. The densities of the slag are between 2.85 and 3.2g/cm3. Compressive strengths of the slag samples taken from vitreous slag drums vary from 700 to 3000kg/cm2. Leaching indices of slag samples measured using the ANSI 16.1 standard test method are shown in Table 1. In these vitrification tests, Co and Cs are purposely added in the surrogate waste as tracers. In comparison with other elements, the leaching index of Cs is relatively low. However, it is still beyond the lower limit for disposal (6 for the Republic of China).
The low-level radioactive wastes to be treated by INER-PF250R plasma facility are listed in Table 2. Surrogate wastes to simulate the waste category were prepared for cold testing. Concrete, soil, thermal lagging, bottom ash and fly ash surrogate wastes were successfully processed by INER-PF250R and converted into high quality vitreous slag. Photographs, below left, show the fly ash drums from the municipal solid waste (MSW) incinerator before and after vitrification into slag drums by INER-PF250R. The slag densities are between 2.68 to 2.93g/cm3. The volume reduction ratio is about 4.2. Compressive strengths of the slag samples taken from vitreous slag drums are between 1300 and 2780kg/cm2. Besides the characterisation of the slag samples, the off-gas was continuously monitored during the plasma vitrification testing to see if the emissions meet regulation limits. The experimental results show that the concentrations of SOx, NOx, HCl, Co, particulates, heavy metals, dioxins and furans in off-gas are all below limits set by regulators in the Republic of China.
From the vitrification tests of full drums of surrogate wastes, high-quality vitreous slag with a compressive strength ranging from 400 to 3000kg/cm2 and leaching indices from 8 to 17 was created. The volume reduction ratio is about 2 for cement solidified waste and greater than 100 for combustible waste.
The promising results show that thermal plasma processing is an innovative technology for the treatment of low-level wastes. At the INER, the design and construction technology for high-power plasma torches has been established, a LLW plasma processing plant with a capacity of 250kg/h is under cold testing and several hundred hours of continuous plasma vitrification tests have been performed to verify the reliability and availability of INER-PF250R.
Hot testing of the plasma facility is scheduled for the last quarter of 2004 after system repairs and adjustments. The commissioning of INER-PF250R is expected in the middle of 2005. At that time, the INER should be able to offer a more efficient service for the treatment of the low-level radioactive wastes generated from domestic isotope applications.
Chin-Ching Tzeng, Tsung-Min Hung and Li-Fu Lin, Institute of Nuclear Energy Research, 1000 Wenhua Road, Chiaan Village, Lungtan, Taoyuan, 325 Taiwan, Republic of ChinaTablesTable 1. Average leaching indices of vitreous slag produced from PHP plasma furnace treatment of cement-solidified waste surrogates Table 2. Radioactive wastes to be treated at INER.