Noble Gas Control Room Accident Filtration System for Severe Accident Conditions (N-CRAFT)

Transcription

1 E-Journal of Advanced Maintenance Vol.7-1 (2015) Japan Society of Maintenology Noble Gas Control Room Accident Filtration System for Severe Accident Conditions (N-CRAFT) Axel HILL *1, Dr. Cristoph STIEPANI 2, Michael DRECHSLER 2 1 AREVA GmbH, Kaiserleistr. 29, Offenbach, Germany 2 AREVA GmbH, Paul-Gossen-Str. 100, Erlangen, Germany ABSTRACT Severe accidents might cause the release of airborne radioactive substances to the environment of the NPP either due to containment leakages or due to intentional filtered containment venting. In the latter case aerosols and iodine are retained, however noble gases are not retainable by the FCVS or by conventional air filtration systems like HEPA filters and iodine absorbers. Radioactive noble gases nevertheless dominate the activity release depending on the venting procedure and the weather conditions. To prevent unacceptable contamination of the control room atmosphere by noble gases, AREVA GmbH has developed a noble gas control room accident filtration system (CRAFT) which can supply purified fresh air to the control room without time limitation. The retention process is based on dynamic adsorption of noble gases on activated carbon. The system consists of delay lines (carbon columns) which are operated by a continuous and simultaneous adsorption and desorption process. CRAFT allows minimization of the dose rate inside the control room and ensures low radiation exposure to the staff by maintaining the control room environment suitable for prolonged occupancy throughout the duration of the accident. CRAFT consists of a proven modular design either transportable or permanently installed. KEYWORDS Severe Accident Filtration System, CRAFT, Noble Gas Retention ARTICLE INFORMATION Article history: Received 28 October 2014 Accepted 10 February Introduction The Control Room Accident Filtration System (CRAFT) is designed for accident conditions with increased activity concentration in the plant environment e.g. during complex accident scenarios in combination with containment bypasses, leakages and/or during Containment Filtered Venting operation. It is usually in standby and planned to put into operation during accident conditions as described before. Thus, the habitability of the operator room will be maintained. The airborne activity unit of the CRAFT system has been installed already in several NPPs worldwide. Based on the current evolution in Japanese regulator s requirements an additional Noble Gas Retention Unit which needs to be used in combination with the existing airborne activity unit or also as self-standing unit has been developed by AREVA GmbH. This Noble Gas Retention Unit enables an uninterrupted supply of the Control Room with fresh outside air. The design is based on activated carbon delay beds which prevent an intake of noble gases from the plant environment during above mentioned complex accident scenarios. The CRAFT system s Noble Gas Retention Unit is based on proven technologies since the technology for noble gas delay is used in more than 50 NPP with AREVA built noble gas retention systems (off gas systems). Since the 1968 AREVA performs intensive R&D on the investigation of the behavior of activated carbon to delay radioactive noble gases. Furthermore, AREVA owns the complete process qualification, design and manufacturing process up to the commissioning. Moreover, AREVA-owned test facilities allow fast test performance in case of additional licensing requirements. * Corresponding author, Axel.Hill@areva.com; Phone ISSN / JSM and the authors. All rights reserved. 34

2 In order to develop and to design the CRAFT system, an AREVA R&D project has been launched. This development includes tests in an AREVA radiochemical laboratory, the optimization of a technical process of the CRAFT system, the design of a prototype as well as an intensive design study for two NPPs. This flexibility already shows that the CRAFT system can be individually scaled to the needs of the customer since the fundamental research on the process itself allows for a scaling of the system from small devices for only a few people up to a completely operational and manned control room with around 100 persons and an exchange rate of fresh air of m³/h per person. Furthermore, the required operating time of the CRAFT system during a severe accident is difficult to predict and depends strongly on the scenario of the accident. Hence, the CRAFT system is designed such that no obvious limitation in operating time is present. The very reverse is the case: The CRAFT system operates continuously without a limitation in time or consumption of any operating resources and ensures a radiation-protected stay inside the control room during a severe accident. In the following, the basic principle of the noble gas retention as well as the basic design of CRAFT will be presented and the advantages of the system will be discussed. 2. CRAFT Description 2.1. Conceptual design Figure 1 shows a schematic sketch of the CRAFT system. On the left hand side, the control room is shown while the contaminated air supply is on the right hand side. After passing an aerosols and iodine filter, the airflow is guided via pipes and pneumatic valves to a vessel that is filled with a delay bed based on activated carbon. In general, a multiple of a double unit of delay beds needs to be installed. This means that at least two delay beds (one double unit) are necessary. While the fresh air is pumped into the control room via the first column which is loaded with noble gas, the second column is desorbed with the consumed air leaving the control room. The retention effect is based on the fact that the velocity of gas components inside the activated carbon depends strongly on its individual mass. Hence, nitrogen, oxygen, and carbon dioxide are not affected significantly, but heavy nuclides like heavy noble gases such as Kr and Xe. These components are delayed compared to light components due to physical temporary adsorption. From this it follows that no radioactive noble gases will pass the delay bed within a certain time window that in turn depends on several aspects like the dimensioning of the delay bed, pressure, temperature, humidity, etc. Exhaust air Noble gas delay unit Delay beds Airborne activity unit Control room Iodine Aerosols Outside supply air Control EPS Fig. 1. Schematic Sketch of the CRAFT System Since these time windows are well studied by AREVA GmbH, the use of the first delay bed is limited to a time within the discussed time window before the noble gases are able to break through. Now, the delay beds have to be switched and the fresh air is guided through the second delay bed while the consumed air from the control room is guided to the first delay bed containing the retained radioactive noble gases. With the consumed air stemming from the control room, the noble gases inside the first delay bed are desorbed and flushed back to the environment. Since both delay beds are 35

3 of the same dimensions, the time for the back flushing process is equal to the previous flushing process. The following experimental results will show that the retained noble gases can be flushed back very efficiently by applying a certain process (fractionized desorption) and only a very small amount of noble gases remain inside the activated carbon and most of the radioactivity is flushed back into the environmental atmosphere. From this it follows that the first column is almost completely refreshed for the next cycle. Within the scope of an experimental study the behavior of the process has been optimized and experimental knowledge has been gained about the remaining activity inside the delay bed, the activity that is finally penetrating the column and entering the control room, and the proof that no accumulation effect of the activity occurs has been obtained. Finally, a decontamination factor can be extracted and especially the long-term operability of the system is proven. Especially the latter feature is one of the main advantages of the system in comparison to alternative systems. The CRAFT system can be used as long as needed, independent of the accident scenario, multiple accidents on a NPP site or in case of several venting procedures with large waiting times in between Basic principles of adsorption / desorption Within this section, the adsorption and desorption effects on activated carbon will be discussed. Most attention will be drawn on the effects that influence the adsorption coefficient that represents the properties of the adsorption such as temperature, pressure, humidity, velocity, etc. In general, adsorption describes the effect of short-term concentration of molecules or atoms on a surface. In the present case, the activated carbon serves as a molecular sponge with a large phenomenal surface area made up of millions of pores. These pores can be categorized into: Micropores (< 2 nm) Mesopores (2 50 nm) Macropores (> 50 nm) While the macropores mainly serve as access ways for the fluids to enter the inner structure of the activated carbon, the main adsorption effects are achieved in the small structures of the micropores. Since both the activated carbon as well as the adsorbate, here noble gases, are electrically uncharged particles, only weak forces known as Van-Der-Waals forces serve as a basis for the fact that noble gases are held by the carbon s surface. Due to its size and atomic number, Xe has a lower ionization energy than Xr and therefore, the adsorption properties for Xe are strongly enhanced. In a reverse conclusion, the inverse process of the adsorption, the desorption process describes the overcoming of the interaction and the release of the surface-bound atoms or molecules. Therefore, effects that increase the adsorption process will lead to a decrease of desorption properties and vice versa. These effects are of manifold nature. Several parameters like temperature, pressure, humidity, velocity, etc. influence the adsorption properties. In general, the K value is usually introduced that describes the adsorption coefficient as follows: K = t / m * dv / dt (1) Here, t is the retention or breakthrough time, m is the mass of activated carbon and dv/dt is the flow rate. However, K is not a constant but is a function of the already mentioned parameters. The time t results from the dynamic adsorption and from the delay that is caused by the activated carbon. If a gas flows through the activated carbon, the gas components will pass the carbon bed in different times since the adsorption will cause a delay of the larger atoms or complex molecules as discussed above. Therefore, radioactive noble gases will be delayed compared to oxygen, nitrogen, etc. that composes fresh air. However, after a certain time t, the noble gases will break through the delay bed of activated carbon. 36

4 In the following, the dependencies of the adsorption coefficient will be discussed Temperature In general, increasing temperature leads to a decrease of the K value. This can be expressed by the Antoine equation: K = exp( A + B /( C + T ( C))) (2) K is given in units of cm³/g. Various experimental data sets have been analyzed in the past. An overview is given in [1]. In this reference, a fitting of the parameters A, B, and C on the experimental data is presented Pressure Another strong effect on the K value results from the pressure. In general, an increase of pressure will reduce the K value itself. However, due to the pressure dependence of the gas volume that depends on the equations for ideal gases, an increase of pressure has a positive effect on the final noble gas retention. The introduction of a pressure coefficient α allows for a description of the K dependence on the pressure as follows: K p = K 1 (1 + α *ln( p )) (3) * 2 In this equation, K 1 is the K value at atmospheric pressure, p 2 is the pressure of interest and K p is the product of the K value and the pressure at p = p 2. Furthermore, the pressure coefficient is a function of temperature. An overview of several data sets investigating this phenomenon is published in [1] Humidity and moisture Experimental tests have shown that the humidity of the gas itself does not affect the adsorption coefficient significantly and not directly. However, the humidity may cause condensation in the activated carbon and may generate moisture inside the delay bed. This moisture has an important effect that will be discussed in this section. Several experimental tests have been performed in the past showing a decrease of the K value with increasing moisture. This effect is strong at low moisture of a few mass-% and the change of the K value gets less at higher values as described by exponential functions. A detailed investigation of this behavior has been done and is property of AREVA. The experimental data shows that both the adsorption of Kr and Xe are affected by moisture in the same way up to about 10 mass-%. At higher values the Xe effect is more pronounced. The effect of moisture is of great importance even at low values. Hence, it has to be ensured that the delay bed stays dry in order to ensure an efficient retention of noble gases Velocity As described in [1], the velocity of the carrier gas does not play a significant role on the adsorption coefficient. All discussed values as well as dependencies are valid below a velocity of ca. 3 m/s. Hence, within this velocity range, the dependence on the velocity can be neglected Experimental Tests Experimental setup 37

5 In order to optimize the process and to determine the properties of the activated carbon, various experimental tests have been performed in the radiochemical laboratories at the AREVA research center in Erlangen, Germany. Figure 2 shows the test setup that consists of the following main components: Three differently sized columns for activated carbon A radiation monitor sensitive to beta radiation Heating system for temperature controlling of the columns Humidity device for selecting a certain humidity of the gas Nitrogen supply serves as carrier gas Inlet for radioactive noble gas Vacuum pump Measurement devices for temperature, flow rate, pressure, humidity I&C interface for data acquisition and parameter settings Using this setup, different amounts of radioactive noble gases, in this case 85 Kr, can be added to the carrier gas nitrogen and flushed on the delay beds. Depending on the characteristics of the carbon and environmental properties, the Kr will be delayed compared to the nitrogen due to dynamic adsorption on the activated carbon. However, after a certain time, the Kr will break through. This time represents an important observable for further calculations and the design of the CRAFT system. Furthermore, the variation of parameters allowed for an optimization of the process itself in order to ensure an efficient desorption process. It has to be emphasized that the time for the adsorption and for the desorption process are equal. Hence, the radioactive noble gases in the column need to be flushed back to the environment quickly and efficiently in order to clean the activated carbon within the available time window. The results will be presented in the following sections Experimental results Fig. 2. Experimental Setup of Test Device The optimized process for the CRAFT system is a pressure swing procedure that includes a loading of the columns at high pressure of about 7-10 bar(abs) and a back flushing starting with a depressurizing to 1 bar(abs). In order to support the back flushing further, the column can be evacuated. 38

6 Furthermore, the CRAFT system consists of a cooling and drying device in order to ensure that no condensation occurs during the air suctioning from the environment. Figure 3 shows an example of the loading with 85 Kr at 8 bar(abs), the break through, and the back flushing at atmospheric pressure. The counts per seconds measured in the beta sensitive detector are shown as a function of time. It is clearly visible that the Kr peak left the column during the back flushing within the available time window. This proves the successful optimization of the process. According to equation (1) the K value can be determined based on the break through time measured within those process cycles. As a result of various experimental tests, the K value for the CRAFT system can be averaged to K = cm³/g for Kr isotopes. For comparison, the K value for Xe is expected to be about cm³/g on average in this case and the ratio of both K values results to K(Xe)/K(Kr) = From this it follows that the retention of Xe is about 15 times more effective and the CRAFT system needs to be designed for the limiting vales of Kr. Nevertheless, experimental tests using stable Xe isotopes and a mass spectrometer for detection have been performed and have proven the assumption that no significant amount of Xe will pass the system. After five full cycles no Xe was visible in the spectrum in a first test campaign. Fig. 3: Example for a Process Cycle Figure 3 already indicates that a small amount of radioactive Kr remains inside the column and can be flushed into the control room within the following cycles. Multi-cycle tests have shown that the increased background stays constant and does not increase with the number of cycles, see Fig. 4. Detailed analyses have been performed in order to determine a decontamination factor (DF) that is defined as the ratio of the totally injected activity and the activity entering the control room. 39

7 Fig. 4: Background Behavior in Case of Multiple Cycles Since the remaining activity is slightly above the background, the statistical uncertainties of the experimental tests are rather large. Depending on the amount of activity loaded on the carbon, a DF of about 400 with a statistical uncertainty of about 7% has been determined for high activity loads. In case of small activity loads the statistical uncertainty was much larger and results to about 80% at a DF of at least 280. In order to design the CRAFT system on a conservative basis, the DF of 280 has been chosen as design parameter Radiological model In case of a severe accident a release of activity to the environment can occur. Considering a scenario, the dose exposure to the staff on shift inside the control room can be estimated and the effect of the CRAFT system can be studied. Assuming a 100% core melting and containment filtered venting due to an increase of pressure inside the containment, a release of activity to the environment between 1E+14 Bq/s and 1E+15 Bq/s can occur for a typical BWR type power plant. Depending on the location of the release and the inlet of the control room, different distribution factors including different weather conditions can be present. From these distribution factors the activity at the inlet at the CRAFT system can be determined. Based on a study for a control room located at a distance of 50 m to 2000 m, and an effective height between the effluent outlet and the CRAFT inlet of 40 m to 100 m, a covering distribution factor of 1E-04 s/m³ has been determined. From this it follows that a conservative assumption of the activity at the CRAFT inlet is 1E+11 Bq/m³. Applying the decontamination factor of the CRAFT system, an activity concentration inside the control room would result to 3.6E+08 Bq/m³. In a final step, the exposure dose can be calculated. Without using the CRAFT system, an exposure dose of 1600 msv/day as to be assumed for the staff. Using the CRAFT system, the exposure dose would be at 5.7 msv/day. This clearly shows that the CRAFT system is able to protect the staff on shift and decreases the exposure dose from a lethal dose to an uncritical value. 40

8 2.5. Customized design Based on the design parameters determined from the experimental tests a basic design for the CRAFT system has been developed. Since the CRAFT system shall be flexible and modular, standard 20 foot containers shall be used. Depending on the requirements of the customer, different flow rates and exchange rates of fresh air can be reached by adding a corresponding amount of columns. Figure 5 shows an example of a CRAFT system with four columns and standard containers with the following dimensions each: L x W x H: 5.71 m x m x m Included are all main components like the columns, the compressors, piping and valves, exhaust air blower, and the cooling device. The total dimensions for this example system is 10.71m x m x 5.10 m and is designed for an exchange rate of 3500 m³/h. Assuming an air exchange rate of 35 m³/h per person, this system is able to supply fresh air for 100 persons inside the control room. 3. Conclusion Fig. 5: Example for a Basic Design of the CRAFT System In order to protect the staff on shift in the control room of a nuclear power plant in case of a severe accident and the release of radioactivity to the environment, the CRAFT system has been developed. This system is able to supply fresh air to the control room by applying an effective noble gas retention mechanism based on activated carbon and a pressure swing procedure. As a result, the exposure dose on the staff can be reduced in case of containment filtered venting and conservative 41

9 assumptions from a lethal dose of 1600 msv/day to an uncritical value of 5.7 msv/day. The successfully performed experimental tests at the Areva GmbH research center allowed for a basic design study of the CRAFT system. It has also been proven that the CRAFT system is in operation without any obvious limitation in time and is therefore able to protect the staff inside the control room during the entire severe accident. Acknowledgement The author acknowledges the help of colleagues within the AREVA GmbH. References [1] D. W. Underhill, D.W. Moeller, The Effects of Temperature, Moisture, Concentration, Pressure, and Mass Transfer on the Adsorption of Krypton and Xenon on Activated Carbon, Washington, D.C, Advisory Committee on Reactor Safeguards, U.S. Nuclear Regulatory Commission, NUREG-0678,