PLASMA ARC MELTING TREATMENT PROCESS OF LOW LEVEL DRY ACTIVE WASTE

Katsuo Yamazaki, Satoshi Karigome, Yoshihiro Akagawa
The Japan Atomic Power Company

Yasuo Dougaki, Yoshihisa Nakayama, Keishi Obara, Akio Tsuchiya, Yukito Tsuji
Toyo Engineering Corporation

ABSTRACT

The Japan Atomic Power Company (JAPC) and Toyo Engineering Corporation (TEC) have developed an incinerating/melting process using the Plasma Arc Centrifugal Treatment process (PACT) for Low Level Dry Active (Miscellaneous Solid) Waste (DAW). We had supported tests of melting incombustible waste, tests treating organics which have high chlorine/sulfur content with controlling, together with incombustible waste, and tests of certifying the proper characteristics of solidified slag and metal.

JAPC placed an order with TEC for constructing an incinerating/melting facility using Plasma Arc Centrifugal Treatment process ( PACT developed by Retech ,USA ) in Tsuruga Nuclear Power Station because of it's advantage in progress of development and the highest treatment efficiency at the result of comparison with other plasma technologies in 1995. This paper shows the results of incineration/melting tests and explains the concept of our Plasma Arc Waste Volume Reduction Facility, for which engineering is on-going.

INTRODUCTION

In Japan, Nuclear Power Stations have begun to transport for a shallow land disposal to Low Level Radioactive Waste Disposal Center (Rokkasyo, Aomori) since December, 1992. In the first phase operation of the Center, the target waste is the solidified liquid waste, but they are planning to start the disposal of DAW consist of concrete, insulation materials, filters and metal wastes in the second phase from around 2000.

Japan is evaluating some specified ways in DAW treatment for final disposal. One is the mortar filling solidification process which pours mortar into 55gal drum containing pre-placed wastes. This process needs to sort the wastes according to the shape and the strength and to remove Aluminum, Combustible Materials etc. from wastes in advance of solidification for the purpose of the stable waste form. Another is an incineration /melting process which has the advantage of reducing the pretreatment work removing them.

JAPC selected the latter one: incineration/melting and chose a Plasma Arc Melting technology as the treatment process to minimize the overall cost of treatment and disposal, to reduce those volume and to convert wastes to a stable waste form. In addition, the Plasma Arc Melting technology has a large treatment capacity and has a capability of processing a variety of wastes including organics in a single step.

Plasma Arc Melting technology , however, was not established as a treatment process for DAW in Japan except municipal waste incineration ash melting. Then JAPC planned to carry out some certification tests to show whether plasma technology is suitable for radioactive waste treatment or not.

CERTIFICATION TEST

Purpose of Test

We performed the test according to the waste component of Table I.

Tsuruga Station wastes include PVC and Rubbers and we want to treat those organics in mixture with other inorganic and metal wastes. Some plasma scientist believed that when treating organic waste, plasma arc may be cut off because decomposed organic gas would disturb normal arc operation. If we can not treat organic waste in plasma furnace, we must prepare the other treatment technology for organics in addition to inorganic/ metal plasma melting furnace.

The behavior of the radioactive nuclides is very important for the high temperature treatment of radioactive waste because it affects the method of measurement/evaluation of radioactive nuclides for final disposal. Especially Cesium is the key nuclide in the evaluation of emitters and it is required to remain in the slag layer.

Test Procedure

We performed the test with PACT-2 and PACT-8 of MGC-Plasma, Muttenz, Switzerland. The PACT-2 has 2 feet I.D. furnace and 150kW torch and the PCAT-8 has 8 feet I.D. furnace and 1,200kW torch. We used the PACT-2 as bench scale test apparatus and the PACT-8 as performance test apparatus. Table II shows the contents for all the test run.

• Surrogate waste:
  We prepared the surrogate waste as follows.
  PVC sheet, Rubber, Activated Carbon, Ion exchange resin, Concrete, Metal (stainless steel, carbon steel)

• Surrogate nuclides:
  We prepared the surrogate nuclides as follows.
  Co₃O₄, Cs₂CO₃

• Measurement and analysis
  We analyzed the following items; off gas component (O₂, CO, CO₂, SO₂, NO, NO₂, HCl, HC, dust),slag component, Carbon and Sulfur content in the slag layer and Cs & Co content in the slag layer and metal layer.

• Bench scale test(PACT-2)
  Test run No.1 - No.10 were performed with the PACT-2. We got the basic knowledge in these bench scale tests and then verified those condition with the PACT-8.

• Performance test(PACT-8)
  Test run No. 11- No. 12 were done in the PACT-8. We prepared concrete blocks which were half the size of a 55 gallon drum, and 55 gallon drum filled with DAW.

Test Result

Test results showed that the Plasma Arc Melting technology could efficiently melt incombustible waste with equality heating, with metal and organic.

We discuss the following items:

• Stability of torch operation
  We could operate stably the plasma torch even when we treated substantial quantities of organic.

• Sufficient incinerating/ melting capacity
  We could treat organic in the capacity of 150kg/h at least and the inorganic and metal melting rate is 650kg/h at least. Remaining carbon content in the slag layer is under 0.03% and we think it certifies that the incineration is complete.
  We also treated half drum size concrete block, charged into the hearth bottom.

• Design data for off gas treatment process
  We got the component of CO, NOx. SOx, HCl and so on in off gas.

• Characteristics of solidified melt
  We got two separated layers of solidified melt: slag is upper layer and metal is lower. The slag phase was glassy and homogeneous. Average slag specific density was about 2.6 g/cm³.

• Behavior of nuclide
  Cs remained about 50% in the slag layer even after 500 minutes melting period. Almost all the Co charge was in the metal layer. Figure 1 shows Cs distribution ratio in slag.
  Those nuclides existed homogeneously in both layers: slag and metal. Fig.2 shows the result of the slag and metal homogenity

• Feeding System
  We verified the operability of the horizontal drum feeding system with 55gallon drum
  
  We are satisfied by the results of these tests and verify that the Plasma Melting technology is adequately useful for treatment of Low Level DAW.
  We are planning to evaluate the balance of materials and radioactive nuclides from melting furnace to the stack and improve upon operability of plant and maintenance method by applying automation within two years.

CONTENT OF CONCEPTUAL DESIGN

Tsuruga Station has stored many Dry Active Wastes. Some special features about its station is that there are a little of many wastes: PVC, Rubber and the other organics. We have verified that PACT can adequately treat organics/inorganic waste mixture in our tests.

The conceptual flow diagram is shown in Fig.3 and we summarize the specification of Tsuruga plant in Table III.

Feeder System

We have discussed the way to feed into the furnace the following items: the weight, the shape and the size of waste, the chemical characteristics; organic, inorganic or metal and it's chlorine content and so on. We selected the following feeding system.

• Main waste feeding
  We will feed the 55gal. drum into furnace with Horizontal Drum Feeder and cut the drum with drum cutting torch.

• Heavy waste feeding
  Heavy waste will be filled in 4 or 5drums and settled on the bottom of hearth with Vertical Drum Feeder before starting a day operation for protecting hearth refractory.

• Continuous feeding
  We are planning to feed organic waste continuously with a Screw Feeder at a constant feed rate for the purpose of reducing peaks in off gas generation.

Operation Mode

Our basic operation mode is day time operation (8hr/ day) but the plasma furnace has adequate design for 24 hr. operation. Therefore we have to prepare the storage area of feed waste and solidified waste and adjust the capacity of pretreatment operation.

Optimum Operation Condition

We are now continuing the performance test and we will be able to get the optimum operation condition.

Process Flow and Material Balance

We discussed the rational waste stream and material balance. We are now continuing to analyze the material balance and activity balance. Especially concerning with activity balance, we must get detailed data to verify that public exposure from off gas is extremely low.

Off Gas Treatment Process

In Japan, they have approximately from 25 to 30 incinerators in nuclear field. Those incinerators have almost been installed with Ceramic Filter (CF) as dry off gas treatment systems. We also decided to adopt CF as removing radioactive nuclides but we have to solve the acid gas treatment due to HCl and SOx from organic waste. We are now planning to install a caustic scrubber after removing the radioactive nuclides with CF and discharge the scrubber waste water to sea after neutralizing and monitoring.

We are now performing the decontamination factor test of the off gas treatment system with both cold tracer and R.I. tracer. We will be able to get the result in the near future.

Final Waste Form

We have to condition waste to a final disposal form. Regulation of waste form is not yet finally decided but the following items will be required. We are planning to inspect these final wastes based on transport and disposal regulation and send them to the disposal site with steel container by ship.

• Container vessel is 55gal. drum fitting with JIS
• Not heavier than 1 ton/ drum
• Solidified or filled with mortar
• Without large vacancy inside waste form
• Evaluation of radioactive nuclides content as follows; Co-60, Cs-137, Sr-90, I-129, C-14, H-3, Ni-59, Ni-63, Nb-94, Tc-99, Total alpha emitter
• Removing inhibited materials such as lead etc. for disposal.

We can solve some of those items because we adopted the melting process but we have to take care to limit the weight for example. We must adjust the feed ratio between inorganic and metal not to excess the weight of waste form over 1 ton/ drum.

Concept of Building

We discussed the basic equipment layout and the concept of building. Fig. 4 shows the appearance of our planned facility.

CONCLUSION

We got good results of our certification test. Then we have started the basic design and the second phase of performance testing.

The basic design and this performance test now going on are used for application to the approval for installation to the Regulatory Authorities and of course are used as our detail design data. We hope to apply to the Regulatory Authorities within this year and start to construct in 1998. Turn over and operation start are expected in 2001.