Luminescent Oxygen Sensor for Monitoring of Nuclear Primary Water Cycles

Transcription

1 Luminescent Oxygen Sensor for Monitoring of Nuclear Primary Water Cycles Frank A. Dunand Nicolas Ledermann Serge Hediger Max Haller Christoph Weber ABSTRACT When we introduced a luminescent sensor to measure dissolved oxygen in water and steam cycles at the end of last year, a number of questions regarding its use in nuclear power plant applications remained open. The issues to be investigated were the behavior in the presence of hydrogen and the effect of radiation on the luminescent sensor. In collaboration with the Mühleberg Nuclear Power Plant in Switzerland a series of tests on the reactor water and the feedwater were performed to demonstrate the performance of the sensor in typical nuclear applications. In addition the system was exposed to high radiation intensities to validate the robustness of the sensor in a nuclear power plant environment. INTRODUCTION In a recent paper, we presented the first luminescent oxygen sensor applicable to monitoring the water and steam cycle []. The use of luminescent technology brings significant improvements in terms of frequency and complexity of maintenance compared with commonly used electrochemical (EC) sensors. Furthermore no particular attention has to be paid to the sample flow, and abnormally high oxygen levels have no influence on the maintenance frequency. The simplification of the maintenance and sensor preparation combined with the automation of the single point calibration renders the accuracy of the system literally operator independent. Whereas the current accuracy and detection limit of this luminescent sensor are probably not yet ideal for process monitoring requiring sub µg kg accuracy at low levels (< 5 µg kg ), it appears to be the best solution for oxygen process monitoring in plants running oxygen treatment 2007 by PowerPlantChemistry GmbH. All rights reserved. (OT) chemistry and a very good compromise of accuracy, reliability, and maintenance requirements for plants running all-volatile treatment (AVT) and working shifts with frequent contact with high oxygen containing samples. Dissolved oxygen monitoring in nuclear power plants (NPP) has the same purpose as in fossil-fueled power plants, but some applications are slightly different in essence. In this paper we will only consider the most common light water reactors (LWR): pressurized water reactors (PWR or VVER) and boiling water reactors (BWR). We will not discuss here the evolution of the different types of chemistries used in PWR or BWR reactors as this has been described in detail elsewhere [2 4]. In a PWR we have to distinguish between the primary circuit or reactor coolant system (RCS) and the secondary loop. In the primary circuit hydrogen is added, which maintains reductive conditions leading to oxygen concentrations far below 0 µg kg. Dissolved oxygen is monitored continuously in order to quickly detect any changes in the cooling system chemistry. The secondary circuit can be compared to a standard fossil power plant running AVT chemistry. Typical oxygen monitoring applications in the secondary loop include the feedwater (< 0 µg kg ), the condensate (< 30 µg kg ) and the deaerator inlet and outlet (0 30 µg kg ) [3,5]. In BWR reactors the water/steam circuit that generates power is in direct contact with the reactor. The original chemistry used in these reactors is the so-called normal water chemistry (NWC). This is very pure, slightly oxygenated water ( µg kg ) at neutral ph. As intergranular stress corrosion cracking (IGSCC) was observed under these conditions, hydrogen water chemistry (HWC) was then developed. In this chemistry, µg kg hydrogen is added to the feedwater to change the oxidizing nature of the coolant that causes IGSCC. The drawback of this method is the increase in the main steam radiation levels. However, this can be mitigated by the addition of a catalyst that helps the recombination of hydrogen with oxygen and oxygen peroxide in the reactor. This technique is called nobel metal chemical addition (NMCA). With this method, the hydrogen content of the feedwater can be limited to around µg kg whereas the hydrogen and oxygen contents in the reactor water are very low (dissolved H 2 < 50 µg kg and dissolved oxygen < µg kg ) [4,6 8]. Independent of the application, nuclear power plants do of course require very accurate monitoring of the oxygen levels all along the water cycle, but one of the most important 58 PowerPlant Chemistry 2007, 9(9)

2 Luminescent Oxygen Sensor for Monitoring of Nuclear Primary Water Cycles requirements in this industry is the As Low As Reasonably Achievable (ALARA) principle for operator exposure to radiation. Although our luminescent oxygen system has proven its capability in fossil power plants where the operator intervention is limited to a 5 min service every 8 months, there was still a question mark regarding the influence of radiation and hydrogen on the measurement using luminescent technology. The goal of this paper is to demonstrate whether our luminescent dissolved oxygen analyzer can be used in nuclear applications. THE BWR OF MÜHLEBERG NUCLEAR POWER PLANT All on-site tests were performed in the nuclear power plant Mühleberg (KKM) close to Bern, Switzerland. Mühleberg nuclear reactor is of type GE BWR 4 and was commissioned in 972. The installed power is 355 MW and the annual energy produced is GWh. The plant runs under hydrogen water chemistry (HWC) conditions with NMCA [6]. The 4 m 3 h (STP) of H 2 injected into the feedwater is generated by a polymer electrolyte membrane (PEM) that electrochemically produces H 2 and O 2 from water [9]. Dissolved oxygen is continuously monitored in the reactor water and the feedwater. During these tests, the luminescent oxygen analyzer was installed at both the reactor water and feedwater sampling points as shown in Figure. OXYGEN MONITORING OF THE REACTOR WATER The ORBISPHERE G200 luminescent oxygen sensor was first installed for more than 0 days on the reactor water sampling line near an originally installed ORBISPHERE electrochemical (EC) sensor. Under NWC conditions, the oxygen level in the reactor water would rise up to µg kg due to the radiolysis of the water. In order to limit the oxygen concentration to below µg kg, hydrogen is injected into the feedwater. The hydrogen level in the reactor water is around 2 µg kg. The radiation level at the reactor water sampling point is about 60 µsv h. To start the test, a zero calibration with pure nitrogen gas was performed and the auto-calibration feature was programmed to run a zero calibration every 5 days in order to observe the reproducibility of the calibration procedure in those conditions. The system required no intervention after initial installation. Figure 2 shows an example of the dissolved oxygen data recorded by the G200 system between day #55 and #6 (6 days). Over the full test duration the G200 successfully measured the process at low dissolved oxygen concentrations with expected ±2 µg kg accuracy. On day #58, one of the programmed auto-calibrations took place, confirming the effectiveness of the calibration procedure and validating the accuracy of this low level Figure : KKM reactor with oxygen sampling point positions. The first sampling point is the reactor water (< µg kg dissolved oxygen); the second measures the feedwater (oxygen content 50 µg kg ). Both water samples are exposed to radiation and contain H 2. PowerPlant Chemistry 2007, 9(9) 59

3 measurement. On day #59, a small oxygen peak was generated manually by a slight oxygen injection on the sampling line in order to test whether the system would react properly. Figure 3 shows that the luminescent oxygen sensor successfully tracked this slight oxygen concentration change up to 60 µg kg. The accuracy of the luminescent sensor is excellent as the difference between the EC and the G200 is less than 2 3 µg kg oxygen on average. OXYGEN MONITORING OF THE FEEDWATER The same luminescent oxygen analyzer was plugged into the feedwater sampling line for 40 days. As depicted in Figure, oxygen treatment (OT) is applied just after the condensate pump. The average dissolved oxygen level is about 50 µg kg but can range between 30 and 200 µg kg. The OT process is identical to that found in fossil power plants, where the ORBI- SPHERE G00 system has proven its capability. It is of primary interest to see if radiation and hydrogen have an influence on the measurements of the luminescent sensor. In the feedwater, the radiation level is lower, with a measured value of 55 µsv h near the sampling point. The H 2 set point is 80 µg kg, corresponding to 4 m 3 h (STP) hydrogen gas injection. Figure 4 shows the feedwater sampling panel with the luminescent oxygen sensor, the existing ORBISPHERE oxygen electrochemical (EC) sensor and a hydrogen thermal-conductivity (TC) sensor. Figure 2: ORBISPHERE G200 data recorded during 6 days of a 0-day trial in the reactor water with dissolved oxygen concentration of < 2 µg kg. Figure 5 shows the results of the ORBISPHERE 3600 EC and the ORBISPHERE G200 recorded during the last week of a 40-day trial Figure 3: on the feedwater line. During this test again no intervention was necessary and all calibrations were done on automatic mode. On days #33 and #37, the luminescent sensor successfully detected two small and brief oxygen peaks that occurred due to startup of a precoat filter. Over the 39 days of the test, we programmed 9 auto-calibrations to verify the calibration reproducibility. All calibrations successfully took place, taking only 3.7 min on average. Their reproducibility was within the expected ±2 µg kg oxygen accuracy. HIGH RADIATION LEVEL RESISTANCE After 0 days of operation in the reactor water (at 60 µsv h ) and 40 days in the feedwater (at Comparison between an ORBISPHERE 3600 electrochemical (EC) sensor and the ORBISPHERE G200 dissolved oxygen measurements during small oxygen peak. 55 µsv h ), the radiation resistance of the G200 sensor was further tested by exposing the probe to a high intensity radioactive source (up to Gy h = Sv h with quality factor = ) for h. Data were recorded by the G200 sensor, while measuring pure gaseous nitrogen. Neither degradation nor influence of radiation was observed on the auto-calibration process, on the sensor repeatability (± µg kg ) or on the fluorescence signal intensity (Table ). Based on the radiation level in the KKM reactor water (60 µsv h ) and in the feedwater (55 µsv h ), this high 520 PowerPlant Chemistry 2007, 9(9)

4 Heft :59 Uhr Seite 52 Luminescent Oxygen Sensor for Monitoring of Nuclear Primary Water Cycles Radiation Test Zero DO repeat- Fluorescence level duration calibration ability intensity (Gy h )* (h) (µg kg ) (V) OK.74 OK.70 Table : G200 functional testing under high radiation levels. * = Sv h with quality factor = DO dissolved oxygen radiation test is equivalent, in terms of cumulated dose, to a system running for 9 and 27 months respectively. Over the complete trial, the G200 luminescent sensor was exposed to a cumulated radiation exposure of about 2 Gy without any loss of sensor performance. EFFECT OF HYDROGEN No measurable effect of hydrogen was observed during the above described tests at the Mühleberg NPP. Additional laboratory tests performed on 00 % hydrogen samples have shown no interference on the luminescent sensor signal. This has been confirmed by the measurement on stator cooling water, which is normally saturated with hydrogen (i.e., dissolved hydrogen levels in the mg kg range). Figure 6 shows a 0-day data extract from a 5-month test on a stator cooling water loop from a UK power plant. The G200 sensor was very consistent and stable in measuring the stator water with typical dissolved oxygen concentration below 0 µg kg, giving further proof of its insensitivity to hydrogen. Figure 4: Feedwater sampling panel, originally equipped with O2 ORBISPHERE EC sensor and H2 ORBISPHERE TC sensor, with ORBISPHERE G200 sensor at the bottom right. Sampling point Feedwater on Figure. CONCLUSION Laboratory and field tests of the ORBISPHERE G200 luminescent dissolved oxygen sensor have shown that the sensor is not influenced by the presence of radiation or hydrogen in the sample. As the maintenance of this analyzer being limited to the annual exchange of the active luminescent spot, the operator will be able to drastically reduce his exposure to radiation and therefore his annual cumulated dose. Based on its 2 µg kg accuracy and reproducibility, this system is perfectly suited to monitoring the water and steam cycle in BWR plants running NWC, HWC, or NMCA chemistries as well as the condensate in PWR plants. The use of the analyzer on the reactor water of BWRs, RCS of PWRs and feedwater of PWR PowerPlant Chemistry 2007, 9(9) Figure 5: Data recorded by the G200 and EC sensors during the last 8 days of a 40-day trial in the FW. The difference between the EC and luminescent oxygen sensors is lower than 5 µg kg. The small oxygen peaks on days #33 and #37 were due to startup of a precoat filter. 52

5 plants where the dissolved oxygen level is typically below 5 µg kg will depend on the level of oxygen at which an alarm is released and an action should be taken. The system is clearly not applicable if the alarm level is at 5 µg kg of dissolved oxygen but can be effectively used if the alarm is set at a dissolved oxygen concentration of 0 µg kg or above. ACKNOWLEDGMENTS The authors warmly thank the Gx00 development team, T. Chatelain, D. Rosset, P. Vauclin, A. Doux, S. Orlandini and Y. Kestin for their efforts and enthusiasm. REFERENCES [] Dunand, F., Ledermann, N., Hediger, S., PowerPlant Chemistry 2006, 8(0) 2. [2] Bain, D. I., Bates, J., Ultrapure Water online 2004, May/June, UP20449, Part. [3] Bain, D. I., Bates, J., Ultrapure Water online 2004, September, UP20449, Part 2. [4] Fruzzetti, K., Wood, C. J., PowerPlant Chemistry 2007, 9(4) 209. [5] Böttcher, F., CD-ROM Proc., International Conference Instrumentation for PowerPlant Chemistry, 2006 (Zurich, Switzerland). PowerPlant Chemistry GmbH, Neulussheim, Germany. [6] Sarott, F. A., CHIMIA 2005, 59(2), 923. [7] Seifert, H. P., Ritter, S., CHIMIA 2005, 59(2), 944. [8] Stellwag, B., Staudt, U., PowerPlant Chemistry 2005, 7(2), 95. [9] Puthawala, A., Henzel, N., PowerPlant Chemistry 2000, 2(3), 72. THE AUTHORS Figure 6: Frank A. Dunand (Ph.D., Chemistry, Swiss Federal Institute of Technology, Lausanne, Switzerland; M.S., Chemistry, University of Fribourg, Switzerland) authored 5 technical papers in the analytical chemistry field before joining Orbisphere in He currently holds a position as senior product manager at Hach Ultra, heading the team managing all Orbisphere and Polymetron products. Nicolas Ledermann (Ph.D. and M.S., Materials Science, Swiss Federal Institute of Technology, Lausanne, Switzerland) has authored and co-authored 25 scientific 0 days of stator water monitoring from a UK power plant with the G200 sensor. papers in the field of applied materials science, MEMS and sensor design. He is presently R&D manager at Hach Ultra Analytics, Geneva, heading the sensor development group. He is a member of IEEE and the Materials Research Society. Serge Hediger (Ph.D., Micro-engineering, Swiss Federal Institute of Technology, Lausanne, Switzerland; M.S., Micro-engineering, Swiss Federal Institute of Technology, Lausanne, Switzerland) authored 6 technical papers in the micro-engineering field before joining Orbisphere in He currently holds a position as R&D engineer at Hach Ultra in the sensor development group. Christoph Weber (Ph.D., Chemistry, Ruprecht Karls Universität Heidelberg, Germany) is the division manager for chemistry at the Nuclear Power Plant Mühleberg, Switzerland. Max Haller (Chemical Engineer) is a senior laboratory engineer with more than 36 years of experience in onlineinstrumentation and analysis at the Nuclear Power Plant Mühleberg. CONTACT Frank A. Dunand Hach Ultra Analytics SA 6, Rte de Compois CP 22 CH-222 Vésenaz Switzerland fdunand@hachultra.com 522 PowerPlant Chemistry 2007, 9(9)