Hee-Chul Yang, Won-Zin Oh and Joon-Hyung Kim
Radwaste Treatment Team, Nuclear Fuel Cycle R&D Group, KAERI
150 Toekjindong Yusongku Taejon, 305-353, Korea

Youn-Keun Lee, Sung-Ho Jo, Hong-Tae Kim and Tae-Won Suk
Radwaste Safety Assessment Department, KINS

After 5-year technology demonstration and safety review, the Demonstration-Scale Incineration Plant (DSIP) at Korea Atomic Energy Research Institute (KAERI) has finally received a permit to treat burnable low-level waste at the site. Radiation dose assessments for routine releases on annual basis as well as several severe accidental releases on short-term basis (2h) revealed that there will be no significant environmental impact when the low-level waste is incinerated in DSIP. For semivolatile radioactive cesium, expected emission concentrations are shown to be in conflict with 10% of maximal permissible concentration. In this reason, an extensive investigation on the control capability of utilized low-temperature dry off-gas system is required on the stage of permitting. Additional trial burns using inactive cesium proves that the control capability of low-temperature dry off-gas system are enough to treat low-level dry active waste. It is expected that condensation by means of dry off-gas cooling system and collection of condensed particulate cesium species are effective enough to treat volatilized cesium species when low-level dry active waste are treated.

In 1991, the demonstration-scale incineration plant (DSIP) was designed and installed at Korea Atomic Energy Research Institute (KAERI) for the purpose of technology demonstration [1]. A step-by-step approach to demonstrate developed incineration technology to treat low-level burnable radioactive waste has been made during last 5 years. After the change of governmental policy on the radwaste management in 1996, KAERI has planned to utilize the DSIP as a conventional incineration plant to treat burnable dry active waste (DAW) which is generated at the site. After radiological risk assessments and a little modification, the DSIP has finally received a permit to treat DAW. In the absence of directly concerning regulations and technical guidance on the radwaste thermal process in Korea, applicable other regulations and guidance on domestic nuclear fuel cycle facilities and those in other nations were applied to get a permit.

In the first part of this paper, the results of radiological risk assessment on DSIP are described. Emission source terms were decided on the basis of trial burn results and analyzed distributions and concentrations of nuclides in the waste. Expected emission concentrations of typical radionuclides under normal operation are compared with maximal permissible concentrations (MPCs) in air. Radiological risk assessments under abnormal operations with potential accidental scenarios are also described. For cesium species, which were determined as dominant emission sources, additional investigations on their behavior in the plant were required to prove enough decontamination by means of the low-temperature dry off-gas system. Recent trial burns using inactive cesium tracer were focussed on the condensation of volatilized cesium species and the removal of particulate cesium by bag filter and the results are also discussed in the second part of this presentation.

The schematic diagram of the demonstration-scale incineration plant is shown in Fig. 1. The plant with a capacity of 20-25 kg/h consists of two incineration chambers and an off-gas treatment system. The off-gas treatment system includes a heat exchanger, an air dilution cooler, a bag filter, a wet scrubber, and HEPA filters. The after-burning chamber has a structure that functions as a cyclone to remove some coarse particles in the off-gas. Only when the release of some amount of HCl gas is anticipated due to PVC content in the wastes, a wet scrubber will be utilized. The operation of the incineration plant can be described as follows; The properly repacked waste is fed into the incinerator using a set of the two sliding gates. Some amounts of incompletely burned materials like soot and hydrocarbons in the leaving off-gas from the incinerator are burnt in the secondary chamber (after-burning chamber). The gas exiting from the secondary chamber with a temperature is cooled down by passing through a heat exchanger and an air dilution mixer in series. Used primary filtering device is a bag filter and a set of HEPA filters is used as a final filtering device.

After the installation of described DSIP in 1991, a step-by-step approach to demonstrate developed incineration technology had been made until 1995. This demonstration procedure is summarized in TABLE I. As a first step, the trial burns of inactive simulated waste had been made in 1992. In 1993, trial burns of simulated wastes including inactive metal compounds such as As, Cd, Co, Cr, Cu, Hg, Mn and Pb with chemical forms of oxides, chlorides and elements had been made in order to investigate the general behavior of hazardous metals and radionuclides in the plant [2-4]. In 1994, permission to use typical radionuclides such as Co-60, Cs-137, Mn-54 and I-131 was obtained based on the results of previous trial burns of simulated waste containing inactive tracers. In 1995, fifteen drums of DAW from Nuclear Power Station at Kori site were burnt [5].

On the basis of determined decontamination factors (DFs) of the plant and maximal permissible concentrations (MPCs) of major radionuclides in the waste, the acceptable limit of nuclide contents per unit mass of DAW, which can be referred to as maximal specific activity (MSA), were determined by following equation;

Eq 1

where MSA is the maximal specific waste of a nuclide in mCi/kg, Q_n is the volumetric flow rate of total off-gas in Nm³/h, MPC is maximal permissible concentration of a nuclide in air in mCi/Nm³, W is the feeding rate of the waste in kg/h, SF which is regarded as 0.1 is safety factor of the process, and DF_i is decontamination factor of the process unit.

The estimated MSAs for major radionuclides are shown in TABLE II. Applied specific activity was based on the average values for DAW generated from Korea Nuclear Power Station (KNPS) at Kori site. Nuclide distributions were the measured values of the sample of DAW transported from the same power reactor. Assuming that Ru and Nb have the same DF for semivolatile cesium, their MSAs were estimated. MSAs for most nuclides except for cesium species is lower than average concentrations in the DAW. However, estimated emission concentrations of Cs-137 and Cs-134 are in the range between 10% and 30% of MPCs. Estimated MSAs for Cs-137 and Cs-134 slightly exceed average concentrations of those in DAW from KNPS if the safety factors of 0.1 is considered. Considering that the estimated emission concentrations of Cs-137 and Cs-134 are in conflict with 10% of MPCs, extensive investigations on the removal characteristics of volatilized cesium species by the off-gas system were required. The results of these investigations are discussed in the second part of this study.

On the basis of the estimated release concentrations in TABLE II, which can be accepted as emission source term, radiological impacts from routine operational release of radionuclides were estimated. Used analytical tool was GASDOS, which was developed computer program based on the model in Regulatory Guide 1.109 by USNRC and also has a regulatory compliance in Korea. Wind speeds and directions measured from Jan. 11. 1995 to Dec. 31. 1995 were applied to determine dispersion factors. Considering low height of the stack (15 m), ground release model was used. The calculated results, as shown in TABLE III, do not exceed the dose limit for individuals under regulation (500 mR/yr) [6].

Like other conventional incineration facilities, some considerations to prevent or mitigate accidents and abnormal operations, such as alarming systems, pressure relief valves, automatic power generator and stand-by draft fans, etc. were included in the DSIP. It can be, therefore, understood that potential radiological hazards will not be associated with severe accidents such as explosion, fire and loss of power failure but rather with operational mistakes occurring in sorting and handling of waste and incinerator ash. However, considering lack of design considerations against acts of earthquakes, risk assessments for several kinds of severe accident were required in the stage of licensing. On the assumption that earthquakes can cause severe accident like explosion, fire, failure of the filter and rupture of the off-gas pipe, radiological risks were assessed. Used analytical tool for the determination of dispersion factor was PAVAN, which was developed computer program based on the model in Regulatory Guides 1.145 and 1.111 by USNRC and also has a regulatory compliance in Korea. The same atmospheric transport data used in the assessment of normal operational risk were used and the same ground release model was applied. In TABLE IV, the results of radiological risk assessments for considering four cases of accidents are shown. Calculated public doses are much lower than the dose limit (5mSv/yr) notified by Ministry of Science and Technology (MOST) [6].

As shown in previous part of this presentation, the dominant emission source of DSIP is determined as Cs-137 and Cs-134, due to its high volatility and relatively high concentrations in the waste. Also expected their emission concentrations slightly exceed 10% of MPCs. In this reason, it was required to confirm the control capability of those emissions by utilizing low-temperature dry off-gas system in DSIP. The removal efficiency of volatilized cesium species by means of low-temperature dry off-gas system, which utilizs heat-exchanger and air diluter as a dry cooling device and bagfilter as a primary filter, are investigated by the trial burns of simulated waste containing inactive cesium.

The gas flow patterns of the dry off-gas system in DSIP are described in Fig. 2. The off-gas leaving afterburner is cooled down to about 450 - 500° C by vertical air to gas heat exchanger. About 35% of heated cooling air at 250° C is used as combustion air for incinerator. About 65% of the heated air is used to heat off-gas leaving the HEPA filter in order to prevent water vapor from condensing in stack. Being mixed with air, off-gas leaving heat exchanger is further cooled down to about 160 - 200° C for the safe operation of following the PTFE bag filter. Relatively clean gas leaving bag filter is further filtered by a HEPA filter before emitting from the stack.

Ten packages of plastic waste (PE) were prepared for cesium-containing simulated waste. The 10 g of inactive CsCl powder was contained in each waste package. The incinerator was initially heated up to 450° C by a gas burner. Some waste packages without tracer metals were fed into the incinerator until it reached 900° C. The first simulated waste package with tracers was then fed and the desired temperature was maintained with fluctuations of 30-80° C during burning of each tracer-containing waste package.

Simultaneous particulate samplings of both inlet and outlet off-gas of the bag filter were made as soon as the first waste package including tracer metals was fed into incinerator. Sampling trains were arranged in a series of sampling probes, 8-stage particle-sizing cascade impactors, rotameters, moisture removal systems and flow control systems. Sample flow rates were set before the sampling operations according to the isokinetic rates determined by the off-gas linear flow rates. When the flow rates were lowered to 90% of isokinetic rates by increasing the pressure drop across the impactor stages, sampling operations were finished. After weighing, quantitative analyses of each size group of particles on metal substrates and bag-up filters were obtained by Neutron Activation Analysis (NAA).

Based on the total weight and the results of NAA of each size group of particles collected on impactor substrates, the distribution of particulate matters (PMs) and the average concentration of cesium species in a discrete size interval between D_p1 and D_p2 was determined at each sampling position over sampling time. The variations of this function for particulate matters (PMs) and cesium species are plotted in the form D m/D logD_p versus logD_p, and are shown in Figures 3 and 4, respectively. In the figures, collection efficiencies are also plotted as a function of particle size against right axis. Overall particle collection efficiency for bag filter is 99.96%. For inlet cooled off-gas at about 200° C, particulate tracer metals species in a particle size range of 0.29-1.45 m m were somewhat more distributed than those by the series of laboratory experiments[2] but relatively evenly distributed with particle size. The effect of existing soot particles, which are up to 99% by mass in this size range, are found. However, the distributions of the condensed particulate cesium species in this size range, which are corresponding to the transition range from diffusional to the inertial collection, are relatively small. Less than around 5% of the particulate metal species is distributed in this size range of particulate matters. In this reason, the collection efficiencies of bag filter for condensed cecium species are not much less than those for total particulate matters. The collection efficiency of bag filter is determined to be 99.93% for condensed particulate cesium, showing enough decontamination efficiency as a primary filter for major emission source.

Radiological risk assessments to utilize the DSIP as a conventional plant reveal that there will be little impacts on environment by its routine operation to treat DAW. Also even in the case of potential severe accidents like explosion, fire and failure of power or filters, expected radiological risks are too low to be ignored. For the purpose of regulation, the estimated emission concentrations from the stack were required to meet maximal permissible concentrations, ignoring dispersion or dilution effect in the atmosphere, but rather considering safety factor of 0.1. However, the compared results based on the trial burns reveal that emission concentrations of most radionuclides meet the MPCs, except for semivolatile cesium species. The expected emission concentrations of Cs-137 and Cs-134 are in conflict with 10% of MPCs if the DAW from KNPS is treated.

An extensive investigations using inactive tracer cesium show that most vapor-phase cesium volatilized in an incinerator furnace condenses during dry off-gas cooling to 200 ° C. Cesium species are present as particulate in the off-gas before passing through low-temperature filtering devices. Distributions of condensed metal species in the transition size range between diffusional and inertial region are less than 5%. It can, therefore, be said positively that a low-temperature dry off-gas system, which composed of dry off-gas cooling and low-temperature filtering with a minimization of secondary waste, is able to effectively control vapor-phase cesium, which volatilized from an incinerator furnace.

1. J.H. KIM et al., "The Development of Radioactive Waste Treatment Technology (V)," KAERI, KAERI-NEMAC/RR-62/92: 2-1-2-10 (1992).
2. H.C. YANG, W.Z. OH, J.H. KIM, K.S. LEE and Y.C. SEO, "The behavior of arsenic, mercury and cesium in an incinerator with low-temperature dry off-gas system," Proceedings of 1997 International Conference on Incineration and Thermal Treatment Technologies: 755-759 (1997).
3. H.C. YANG, J.H. KIM, W.J. OH and H.S. PARK, "Removal Characteristics of Volatile Metals in Incinerator Off-gas with Low-Temperature Dry Off-Gas System," IchemE Symposium Series No. 143, 159 (1997).
4. J.H. KIM et al. "Radwaste management basic research and development - Technology development for radwaste volume reduction and solidification (II)," KAERI-NEMAC/RR-177/96: 2-30-2-31 (1997).
5. H.C. YANG, I.T. KIM, J.G. KIM, J.H. KIM and Y.C. SEO, "Trial burns of dry active waste at KAERI demonstration-Scale incineration plant," J. of Korean Nuclear Society, 27: 767-744 (1995).
6. Notice of MOST No. 96-5, "Regulations on Radiation and etc.," Korea Ministry of Science and Technology

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