RADIOACTIVE WASTE CHARACTERIZATION IN BELGIUM Serge Vanderperre, Christine Vanhaeverbeek, Koen Mannaerts, Baudouin Centner, Philippe Beguin and Michel Detilleux Tractebel Ariane Avenue, 7 B-1200, Brussels, Belgium ABSTRACT Radioactive waste producers are nowadays increasingly required to characterize the nuclide specific activity content of the waste generated by their facilities, to demonstrate full compliance with the waste acceptance criteria for interim storage and/or final disposal. In case of surface or near-surface disposal, the long-lived critical radionuclides are the radiological source term for the long-term risk assessment of the repository. Most of them are difficult to measure and require sophisticated, time consuming and expensive analytical techniques, difficult to be implemented by the producer on a routine basis for normal radwaste productionthe LLWAA code developed by Tractebel enables to predict activity inventory for the various types of waste according to actual design characteristics and operating conditions of a nuclear power plant. This code is coupled to the DECL software that allows, based on simple waste package radiological measurements, to determine their critical isotopes inventory content. LLWAA and DECL are implemented as an integrated software package at Doel and Tihange NPP sites to generate the consignment form for each waste package in complete agreement with the requirements set by the Belgian Agency for Radioactive Materials ONDRAF/NIRAS.LLWAA code was extended for the determination of critical nuclides activities in ashes produced by LLW incineration (LLWAA-ASHES) and for the assessment of the critical nuclides activities deposited on equipment of the nuclear systems of a NPP (LLWAA-DECOM). This latter has been used for the decommissioning cost estimates of the Belgian NPPs. Keywords: radwaste, characterization, PWR, decommissioning, incineration. I. INTRODUCTION In order to determine the critical nuclides inventories, different approaches exist and the methodologies currently in use in most EU-countries are the following : 1. Simplified calculation approach based on engineering judgement and extrapolation of international experience in similar types of LWR; 2. Selection of ratios between the activity inventory of an easy to measure key nuclide and the activity inventory of a critical nuclide (or spectra), specific to each type of waste and each type of plant (BWR, PWR), from experimental databases; 3. Predictive calculation codes taking into account the plant s characteristics and operating conditions. Belgium uses the LLWAA (Low Level Waste Activity Assessment) code that enables to predict the global inventories and/or the scaling factors of the critical nuclides in the conditioned and in the non-conditioned waste generated by the operation of a nuclear power plant. Based on a required input data sheet, the LLWAA code model can be tailored to the specific characteristics of a given plant. One significant benefit of the code results from it enabling to easily perform sensitivity analyses to assess the effect on the nuclide inventories due e.g. to the change of some materials, of operating procedures or operating conditions and to uncertainties related to some model parameters. Considerable gains in time, money and operator exposure can be achieved using this predictive code.
II. CODES DESCRIPTION Further description refers to the Belgian situation where the plant process waste (ion-exchange resins, evaporators concentrates, spent filters) is conditioned at the plant while the technological waste is only pretreated. This waste is transported on the Belgoprocess site for further processing in the CILVA facility, the centralised facility for treatment and conditioning of Belgian low level waste. The conditioned waste is stored in dedicated buildings located on the same site. LLWAA Code. LLWAA is a full featured user friendly computer program, enabling the waste producer to determine the critical nuclides inventories in nuclear power plant radwaste streams and to declare the isotopic content of each waste package. Code Functions. The LLWAA code calculates the radionuclide inventories in the process and technological nuclear power plant (NPP) waste on the basis of characteristics and operating conditions of the NPP. modeling of the Reactor Coolant System (RCS), the Nuclear Auxiliary Systems (NAS) and the Liquid Waste Processing System (LWPS). Code Parameters. Basically, the code parameters are classified into three categories : Category 1 : parameters related to the operating conditions which can vary from one cycle to another. The values of these parameters are the main input data of the code and are based on the routine reactor coolant measurement results ( 60 Co, 134 Cs, 137 Cs, 131 I, 133 I, 134 I activities and Cl - concentration). The type of fuel (MOX or 235 U enriched fuel), fuel cycle length or rated plant power are additional input values; Category 2 : parameters pertaining to the plant basic characteristics and operating procedures. Among those we find RCS, NAS and LWPS flow rates, boron content in the RCS and evaporators, ; Category 3 : core activity inventories (calculated by means of the ORIGEN-2 code), chemistry of the primary coolant, equipment materials and surface areas, equipment corrosion rates, decontamination factors of the ion-exchange resins and filters, physical-chemical forms of the activity in the RCS (fraction of activity present in soluble and in insoluble forms) during normal operation and refueling outage, diffusion coefficients through fuel cladding defects. As long as the plant design characteristics and operating conditions remain unchanged, category 2 and 3 parameters are not to be modified. The code output consists of : FIGURE 1. Input Window for the Plant Operating Conditions LLWAA output deals with : The activities of the critical nuclides in the Reactor Coolant System and in the Nuclear Auxiliary Systems; The activity inventories of those nuclides in the on-site conditioned process waste streams and in the nonconditioned technological waste streams; The scaling factors in the on-site conditioned and nonconditioned waste streams. The LLWAA code is site-specific as it takes into account the design characteristics and operating conditions of the different units of the site with a centralized waste reatment facility. The calculation of the activity inventories for each waste package requires, for each unit, the proper The specific activities (or activity inventories) of the critical nuclides in the RCS, ion-exchange resins, filters, evaporator concentrates and miscellaneous onsite non-conditioned waste (combustible waste, non combustible waste, compactable waste, ventilation filters, ); The scaling factors of the critical nuclides in the same waste streams and waste packages. Code Validation. The predicted values were compared to the values derived from specific measurements performed by the Belgian Nuclear Research Center at Mol (SCK CEN). There is good agreement between predicted and measured values for most of the critical nuclides. The code is qualified and accepted by ONDRAF/NIRAS, the Belgian Agency for Radioactive Materials, after an in-depth critical review performed by AVN, an organisation licensed by the Belgian Government in the field of Nuclear Safety. DECL Code. The LLWAA code can be coupled with DECL (Declaration) as shown in Fig. 3.
TABLE 1. Typical Activity Inventory given by the Code for a Mix of Spent Ion-Exchange Resins (Tihange NPP, 2001) Isotopes MBq/m³ Isotopes MBq/m³ Isotopes MBq/m³ C-14 2.04E+03 Sr-90 8.38E+02 U-238 8.56E-04 Cl-36 2.56E+00 Tc-99 2.41E+01 Pu-238 7.63E+00 Mn-54 2.53E+05 I-129 3.61E-01 Pu-239 9.52E-01 Fe-55 4.70E+05 Cs-134 6.73E+04 Pu-240 1.34E+00 Co-58 2.46E+06 Cs-135 2.64E-01 Pu-241 3.31E+02 Ni-59 1.76E+03 Cs-137 6.14E+04 Am-241 2.72E+00 Co-60 5.74E+05 U-234 3.83E-03 Pu-242 4.76E-03 Ni-63 2.05E+05 U-235 1.04E-04 Am-243 2.97E-01 Nb-94 5.72E+02 Np-237 8.01E-04 Cm-244 7.25E+00 Code Structure. The LLWAA code structure is the following : Core activities invenstory Measured I-131, I-133, I-134 Cs-134 and Cs137 activities in the RCS Corrosion rates of incore and out-of-core equipment in contact with the primary coolant. Water and chemical additive activation products H-3, C-14, Cl- 36 Cs-137 cladding defects and recoil FP, TRU-recoil FP, TRU cladding defects Activation of corrosion products Activities in RCS and in nuclear auxiliary systems Activities build-up and inventory in On site conditioned waste Non-conditioned waste Correlating (scaling) factors FIGURE 2. Structure of the LLWAA code Code Functions. DECL allows to: Produce and manage a complete database containing the data related to each produced waste package (physical form, weight, volume, specific weight, contact dose rate, specific activities of the key nuclides 60 Co and 137 Cs); Issue the consignment form required for the waste evacuation (evacuation campaign number, waste package identification number, conditioner, installation used, type of waste treated, all characteristics on the waste package, i.e. dose rate, inventories of critical nuclides, results of gamma spectrometric analyses, residual surface contamination, ). Code Calculation Methodologies. Apart from the physical data of the waste packages which are constant, the activity inventories to be mentioned on the official forms for request of transfer to the Belgoprocess site are based upon :
For the on-site conditioned waste : gammaspectrometric measurements of 60 Co and/or 137 Cs in the waste packages as these are considered as the keynuclides. These measurements, together with the scaling factors computed by the LLWAA code, are entered in the DECL code which calculates the inventories of the critical nuclides. Should the 60 Co and/or 137 Cs activity not be detectable in the conditioned waste, the DECL code uses the LLWAA computed activities of these nuclides. Dose rate measurements can also be used; For the non-conditioned waste : the nuclide inventories are calculated by the LLWAA code based on dose rate measurements. LLWAA Code Extensions. LLWAA-ASHES and LLWAA-DECOM codes are based on the LLWAA code calculating the required specific activities as input data. The interrelationship of these codes is given in Fig. 3. LLWAA-ASHES Code. Objectives of the Code. The LLWAA-ASHES code calculates the specific activities and the scaling factors of the critical nuclides in the ashes produced by the incineration of the combustible waste generated by nuclear installations. This code has been validated against measurements for the major critical nuclides ( 14 C, 54 Mn, 58 Co, 60 Co, 59 Ni, 63 Ni, 94 Nb, 90 Sr, 134 Cs, 137 Cs, U- and Puisotopes). The structure of the code is given in Fig. 3. Code Parameters. This code takes into account : The characteristics of the combustible waste : basic materials, specific weight and specific activities of the critical nuclides (calculated by LLWAA code); The operating conditions of the incinerator: temperature, waste mass reduction factors and critical nuclides volatility. Code Output. The code output consists of: The specific activities (or activity inventories) of the critical nuclides in the ashes and fly-ashes; The scaling factors of the critical nuclides in the ashes and fly-ashes. Code Calculation Methodology. The code takes into account all related incineration process parameters; for each type of waste to be incinerated a mass balance has to be established. The solid or liquid waste entering the incinerator will leave the equipment under different chemical and/or physical forms i.e. a solid part (ashes and fly-ashes) and a gaseous part (flue gas). The activity of each isotope, present in the ashes is derived from : Category 1 parameters LLWAA calculated specific activities scaling factors The corresponding activity of the isotope in the combustible waste, entering the incinerator (this activity can be calculated by the LLWAA code or is simply a measured value); measurements gamma spectrometry contact dose rates inventories non conditioned waste packages combustible waste noncombustible waste DECL inventories conditioned waste packages LLWAA-DECOM deposited activities scaling factors input number of cycles, cycle length particle properties piping properties thermal hydraulic conditions measurements gamma spectrometry Co-60, Cs-137 contact dose rates LLWAA-ASHES scaling factors ashes Inventories ashes Co-60, Cs-137 spectrometric measurements deposited activities inventories Figure 3. Interrelationship between the Different Codes The mass reduction factor depending on the type of waste to be incinerated and the operating conditions of the incinerator (the most important parameter is the combustion temperature); The activity distribution factor between the ashes and volatile materials; for a given type of waste this parameter depends on the considered isotope.
Concerning the volatility, the code takes into account the different specific behavior mechanisms of certain isotopes during incineration; four main volatility groups are considered (very volatile: Cs, Te, I, C; average: Co, Mn, Nb, Ni, Tc; low: Sr, Ba and very low volatility: U, TRU). Scaling (correlation) factors address only isotopes having the same generation mechanism and behaviour in RCS. Therefore the reference isotope in the ashes for fission products is 137 Cs, while for activation products it is 60 Co. The use of these calculated scaling factors and gammaspectrometric measurements of 60 Co and/or 137 Cs on the waste packages containing the ashes, allows to calculate the absolute inventories of the critical nuclides. Should the 60 Co and/or 137 Cs activity not be detectable in the incinerated waste, the code uses the LLWAA values. Code Validation. The code has been validated comparing to measurements, performed by Belgoprocess, for the major critical nuclides ( 14 C, 54 Mn, 58 Co, 59 Ni, 63 Ni, 94 Nb, 50 Sr, 134 Cs, 137 Cs, U and Pu-isotopes). Generally there is good agreement between the calculated and measured values. LLWAA-DECOM Code The costs of the future decommissioning of the NPPs depend on the equipment contamination (pipe-work, valves, heat exchangers, ). Therefore a software has been developed to assess the critical nuclides activities deposited on the equipment of the nuclear auxiliary systems. This software takes into account the contamination in the streams of the systems (calculated by LLWAA), the operating conditions (fluid velocity, ph, temperature), the corrosion products characteristics (particulate diameter distribution) and the nuclides deposition/release rates on the equipment. The main goal of the program is to estimate, by using correlation factors, the deposited activity on the piping in contact with radioactive liquid or gaseous fluids. Scaling (correlation) factors address only isotopes having the same generation mechanism and behavior in the reactor coolant system. For fission products the reference isotope is 137 Cs, for activation products 60 Co is the reference. Specific activities of the considered isotopes Cv (calculated by the LLWAA code for the considered nuclear auxiliary circuit); Time elapsed between the reactor final shutdown and the decontamination or dismantling. TABLE 2. Example of the Input Table of the LLWAA- DECOM Code Hot Part of the Steam Generator Tubes Cl-36 2.30E+02 Mn-54 1.70E+06 NPP'S information Fe-55 5.40E+06 Number of operation cycles 20 Co-58 8.80E+05 Cycle length (months) 10 Ni-59 2.40E+03 t (t_decom. - t_shutdown)(years) 40 Co-60 5.50E+05 t (t_decont. - t_shutdown)(years) 2 Ni-63 2.90E+05 Nb-94 6.10E+00 Sr-90 8.90E+05 Particle properties Tc-99 1.70E+04 Particle density (kg/m³) 2000 I-129 1.40E+02 Cs-134 2.00E+07 Pipwork properties Cs-135 1.50E+02 Pipe diameter (m) 0.0132 Cs-137 4.50E+07 Wall tickness (m) 0.0014 U-234 3.30E+01 Pipe roughness (mm) 0.0015 U-235 1.10E+00 Density material (Kg/m³) 7860 Np-237 1.30E+00 Distance Detector - pipe (mm) 22 U-238 6.60E+00 Pu-238 3.20E+03 Thermal and hydraulic conditions Pu-239 4.70E+03 Absolute temperature ( K) 574.15 Pu-240 2.90E+03 Average fluid veloctiy (m/s) 2.57 Pu-241 5.00E+05 Am-241 6.10E+02 Pu-242 1.90E+00 Am-243 7.90E+01 Cm-244 9.00E+02 Fe-59 2.80E+05 Code Output. The code main output consists of : The particle deposition and release rates; The deposited activities and the scaling factors at any given time. Code Input. A number of parameters are considered : Characteristics of the equipment to be dismantled (piping diameter, pipe rugosity,..); Operating conditions (temperature, average fluid velocity, ph, number of cycles, cycle life, ); The corrosion products characteristics (particle density, particle diameter distribution, ); Physical and chemical characteristics of the isotopes (decay rate λ i, decay energy photon conversion factors he, );
TABLE 3. Example of Output Data of the LLWAA- DECOM Code - Deposited Activities and Scaling Factors on the Hot Part of the Steam Generator Tubes at Reactor Final Shutdown and 40 years after Reactor Final Shutdown Deposited activities (Bq/m²) Correlation factors Isotope t = 0 t = 40y t = 0 t = 40y C-14 2.34E+06 2.33E+06 C-14/Co-60 2.82E-02 5.36E+00 Cl-36 4.90E+00 4.90E+00 Cl-36/Co-60 5.90E-08 1.13E-05 Mn-54 1.03E+07 8.43E-08 Mn-54/Co-60 1.24E-01 1.94E-13 Fe-55 6.37E+08 2.14E+04 Fe-55/Co-60 7.67E+00 4.92E-02 Co-58 2.44E+07 2.75E-55 Co-58/Co-60 2.94E-01 6.32E-61 Ni-59 5.11E+05 5.11E+05 Ni-59/Co-60 6.16E-03 1.17E+00 Co-60 8.30E+07 4.35E+05 Ni-63 6.04E+07 4.58E+07 Ni-63/Co-60 7.28E-01 1.05E+02 Nb-94 1.30E+03 1.30E+03 Nb-94/Co-60 1.57E-05 2.99E-03 Sr-90 1.76E+04 6.69E+03 Sr-90/Cs-137 1.97E-02 1.88E-02 Tc-99 3.63E+02 3.62E+02 Tc-99/Cs-137 4.06E-04 1.02E-03 I-129 2.99E+00 2.99E+00 I-129/Cs-137 3.34E-06 8.40E-06 Cs-134 2.08E+05 2.83E-01 Cs-134/Cs-137 2.32E-01 7.95E-07 Cs-135 3.20E+00 3.20E+00 Cs-135/Cs-137 3.58E-06 8.99E-06 Cs-137 8.95E+05 3.56E+05 U-234 2.11E+04 2.11E+04 U-234/Cs-137 2.36E-02 5.93E-02 U-235 7.03E+02 7.03E+02 U-235/Cs-137 7.85E-04 1.97E-03 Np-237 8.31E+02 8.31E+02 Np-237/Cs-137 9.28E-04 2.33E-03 U-238 4.22E+03 4.22E+03 U-238/Cs-137 4.72E-03 1.19E-02 Pu-238 2.00E+06 1.45E+06 Pu-238/Cs-137 2.23E+00 4.07E+00 Pu-239 3.00E+06 3.00E+06 Pu-239/Cs-137 3.35E+00 8.43E+00 Pu-240 1.85E+06 1.84E+06 Pu-240/Cs-137 2.07E+00 5.17E+00 Pu-241 2.78E+08 4.02E+07 Pu-241/Cs-137 3.11E+02 1.13E+02 Am-241 3.88E+05 7.92E+06 Am-241/Cs-137 4.34E-01 2.22E+01 Pu-242 1.21E+03 1.21E+03 Pu-242/Cs-137 1.35E-03 3.40E-03 Am-243 5.05E+04 5.03E+04 Am-243/Cs-137 5.64E-02 1.41E-01 Cm-244 5.13E+05 1.10E+05 Cm-244/Cs-137 5.73E-01 3.09E-01 Fe-59 3.38E+06 7.10E-93 Fe-59/Co-60 4.07E-02 1.63E-98 Code Calculation Methodology. Based on the input data, the code, using different modeling equations, calculates : The Brownian diffusion velocity (V b ); The particle velocity due to the fluid movement (V f ); The nuclides deposition and release rates. The latter are depending of V b, V f, of probabilistic settling of particles and of transport phenomena. Factors affecting the deposition and release rates are mainly the particle diameter distribution, the temperature of the fluid, the flow pattern (turbulent, laminar,..) and the geometry (diameter, surface-to-volume ratio). The evolution of the deposited activity depends mainly on : There is a general good agreement between predicted and measured contact dose rates. The LLWAA-DECOM code allows the calculation of the scaling factors within the range excellent to good. There is also a good consistency between the measured and predicted values of the deposited activities on the portions of the systems when they were measured, namely during the steam generator replacement programmes. Some precautions have to be taken in order to obtain valid measurements : The measuring point should be representative for the equipment to be considered; Dead zones or drain tie-ins should be avoided for measurements (risk of hot spots ); Back-scattering effects of other equipment in the neighborhood of the equipment to be measured should also be avoided; Accurate knowledge of the distance between equipment and detector is required. III. CONCLUSIONS The above codes are site specific and provide accurate figures increasing confidence in the source terms to be used for surface or near surface LLW disposal long-term safety risk assessment. LLWAA and DECL codes are written using Microsoft Visual Basic language and full featured computer programs for use in radiological characterization of materials, packages and shipments. These programs operate under Windows TM environment and are systematically used at the Belgian NPPs, being approved by ONDRAF/NIRAS after an in-depth review by AVN. They were developed to provide an easy to operate system for retrieval of all information related to the waste packages. Significant savings in time, money and operator exposure can be achieved by using these predictive codes. The nuclides deposition and release rates; The cycle life time and the number of cycles; The initial activity of the deposited materials at the start of the cycle; The specific activity of the considered isotope in the circulating fluid which is given by the LLWAA code. Code Validation. Direct measurements of deposited activities present a number of difficulties. Therefore, to validate the code, a dose rate model, based on the principle of consistency between calculated and measured dose rates values, has been coupled to the calculated deposited activities. The predicted values of the scaling factors are compared to the derived values from specific dose rate measurements performed during NPP shutdown and decontamination operations.