Proceedings of the National Seminar & Exhibition on Non-Destructive Evaluation NDE 2011, December 8-10, 2011 LEAKAGE MONITORING OF SEAL PLUG FOR INDIAN PRESSURIZED HEAVY WATER REACTOR BY PSYCHOMETRICS ANALYSIS METHOD K. K. Verma and R. J. Patel Refuelling Technology Division, Engg. Hall-7, Bhabha Atomic Research Center, Trombay, Mumbai-400085. ABSTRACT In operating Indian Pressurized Heavy Water Reactors (PHWRs), on power refueling is a very important operation. There are total 306 coolant channels in a typical 220 MWe PHWR. Seal plugs are mounted on either ends of the coolant channel. During re-fuelling, fuelling machine (FM) removes seal plugs and puts back after completion of the operation. FM is unclamped from the channel when acceptable leakage 20cc/min is achieved. The leakage reduces to 10cc/day within 30-40 hrs. The low level leakage helps in controlling tritium level and reduces load on heavy water recovery system. Presently no method is available to confirm the leak rate near coolant channel ends. If identified, the leaky plugs can be replaced with good one to stop the leakage. In view of this, a system has been designed to identify leaky seal plugs and monitor the leakage rate. A system has been developed and tested for on line measurement of leak rate of seal plugs. In this system, FM holds the especially designed bung against coolant channel end at the suspected leaky seal plug. Steam leakage gets trapped between the seal plug and the bung cavity. To measure the leak rate known quantities of dry air circulate through cavity. This dry air picks up the moisture which increases the %RH of the air. A high sensitive hygrometer measures the psychometry of air from the bung in terms of %RH. The parameters are analyzed by software and exact amount of leakage rate of steam can be determined in terms of gram/hr or gram/day. This system is capable to measure minimum leakage of 0.10 cc/hr. The sensitivity of system is +/- 1%RH. A special method was established to calibrate the system for minute leakage. This system was installed on KG-5 fuelling machine at Hall 7 and tested successfully. The system can be used at any of the operating plants. This paper describes the leakage monitoring of seal plug by using hygrometer method and experimental programme carried out in prototype system. It is useful for leak detection of steam from leaky components in running plant. INTRODUCTION PHWR is a horizontal pressure tube type reactor. The pressure tube houses the fuel bundles and coolant flows over them to extract heat generated in the fuel bundles. At both end of the pressure tube end fitting are provided. These end fitting are blocked at both ends using seal plugs. The end fittings also house the shielding plug to retain the fuel bundles in the core and provide shielding. The entire assembly of pressure tube, end fitting, seal plug and shielding plug is called coolant channel assembly. In standard Indian PHWR design of 220 MWe, 306 such channels are provided. One of the important features of PHWR is on power refueling at regular interval. Specially designed fuelling machines are used for this purpose. The fuelling machines remove seal plug and shielding plug at both ends and replace the spent fuel assembly with the new one. Subsequently seal plug and shielding plugs are reinstalled and channel is normalized. The seal plug is expected to seal the channel after every refueling. The acceptable value for the leakage is less than 20cc/min immediately after refueling. This reduces to 10cc/ day after about 30 to 40 hours after refueling. These stringent leakage requirements are due to rise in tritium level in the vault and also due to economic penalty imposed by the leakage of costly heavy water. To limit the tritium activity level, a D 2 O vapor recovery system is installed in the Fuelling Machine vault. On occasions the collection of D 2 O is seen significantly more than expected limit indicating leakage in the system and requiring corrective action. One of the potential sources of leakage is seal plugs. In view of this, a concept has been worked out to identify leaking seal plugs. A specially designed bung is clamped on the fuelling machine. This bung is butt against the end fitting and creates a sealed cavity with the channel. Dry air (with known hygrometric properties) is circulated through the cavity. Any leaking steam gets trapped in the cavity and mixes with the circulating air. This results in changes in the relative humidity of the circulating air. A hygrometer provided in the circuit measures the variation of the humidity with time. Increase of relative humidity with time is indicative of leaking
508 Verma and Patel : Proceedings of the National Seminar & Exhibition on Non-Destructive Evaluation seal plug. Calibration of changes in humidity with time can establish the leak rates from the channel. A system is developed and qualified at BARC. Simulated leakages of known quantity were generated. These leakages were verified using KGS-5 fuelling machine, then available for testing. The results were found to be in good agreement. This paper gives details of the system, working principal, experimental work carried out to qualify the system and its potential to use it in reactor enabling minimizing the leakages arising out of seal plugs. As fuelling machine is used as carrier for the bung, it is possible to locate the leakages when reactor is on power. Reduction in leakages eliminates economic penalty and being remotely operated technique offers the advantage of reducing man-rem consumption. The technique also has the potential for use in identifying the leakages in equipments using steam/water as working media. THEORY The system works on Psychometric properties of air. The main psychometric property of air is percentage of Relative Humidity (% RH) and temperature. The percentage of RH determine the water amount contain in air at a particular temperature and pressure. The RH value 0% indicate dry air without water particles, while 100% RH shows saturated air and it cannot hold more water at the same temperature and pressure. If the dry air is mixed with saturated air then the RH value of saturated air goes down as the amount of dry air increase in saturated air. These phenomena adopted in leakage measurement where dry air picks up water and it relative humidity increases. The properties of air get changed due to pickup of water. The known amount of air is entering in the enclosed system and coming out from the system with higher humidity. The humidity varies with change of pressure and temperature. Theoretical analysis Theoretical calculation of air and steam mixture based on psychometric properties has been worked out with respect of time, airflow, temperature and moisture. p v & p a are partial pressure of vapor and p is barometric b pressure in mm of Hg m a, m v and m s are air, vapor, saturated air mass in kg (If humidity 100% p v = p a ) t d = Dry Bulb Temperature, t w = Wet Bulb Temperature w = Humidity ratio or specific humidity mass in Kg of water vapor Relative humidity ( Ø RH) It is the ratio of water vapor in a given volume at a given temperature of the mass of water vapor contained in the same temperature when the air is saturated Example: 1 Liter of dry air = 0.001 m 3 and @ 1 LPM =1.440 m 3 air per day, and weight of air per day = 1.293kg/ m 3 x1.440 m 3 = 1.862 kg air for one-day supply Let, t d1 = 27 C and RH ø 1 = 30% specific humidity =.0066329 kg/kg. dry air or 6.632 gm/kg. dry air, Moisture in 1.44 m 3 of air = 6.6328 x 1.862 gm =12.35 gm per day Let, RH increase from 30% to64% and Temp. & Partial pressure of air remain is constant p r = 760 mm of Hg; p vs = 26.73 mm of Hg; So, p v2 = 17.1072 mm Hg. Similarly w 2 = 0.014323 kg/kg = 14.323 gm/kg. dry air = 14.323x1.862 gm per day = 23.387 gm per day moisture pickup by air @ 1LPM per day is = w 2 - w 1 =23.387-12.350 = 11.03 gm. Per day DATA ANALYSIS A. If, humidity vary from 30% to 64% at constant air flow @ 1 LPM Moisture gain by air =11.03 grams water/day. B. If, humidity vary from 30% to 64% and airflow is 5 LPM rest of the data is constant Moisture gain by air =11.03 x 5=55.15 grams water/day. (Humidity varies from 30% to 64% and moisture gain per liter air is constant). C. If outlet air RH increase from 64% to 70% i.e. 6% When RH 70%, w 3 =15.552 gm/kg.dry air per day w 3 =15.552x1.862 gms per day = 28.957 gm per day at outlet x 1 = w 3 w 1 = 28.957-12.350 gms per day =16.60 gm per day amount of water gain due to 6% RH increase = 5.57 gm. D. If temperature increase from 27 0 C to 28 0 C i.e. 1 C and rest of the data is constant Moisture gain by air = 0.89 gm/kg dry air (air flow is 1 LPM)
NDE 2011, December 8-10, 2011 509 Table 1 : Summary of above data EXP. Air flow Inlet Inlet Inlet Outlet Outlet Moisture Remarks in LPM Temp. C %RH Moisture %RH Moisture gain gm/kg air gm/kg air gm./day A. 1 27 30 12.350 64 23.387 11.03 %RH CHANGED B. 5 27 30 61.75 64 116.93 55.15 FLOW CHANGED C. 1 27 30 12.350 70 28.957 16.60 %RH INCREASED D. 1 28 30 12.350 30 13.240 0.890 +1 C INCREASED DESIGN REQUIREMENT System shall meet following design requirements. On power leakage monitoring of seal plugs shall be possible. The system should be compatible with Fuelling Machine. The system should be able to detect a minimum leakage of 1 cc/hr. The measurement accuracy should be ± 3%. DESIGN PARAMETERS Ambient temperature : 40 0 C - 70 0 C End fitting face temperature : ~ 200 0 C Pressure : Atmosphere Medium : Air/Steam Environment : Radioactive SYSTEM DESCRIPTION The leakage monitoring system consists of a bung, air flow meter, hygrometric instrument unit & Psychometric properties calculator. Bung A specially designed fuelling machine compatible bung is fabricated to generate cavity in front of seal plug to trap the leaking steam from the seal plug. The bung is aligned, advanced and butt against coolant channel by fuelling machine shown in figure-1, it makes a leak tight cavity to trap the steam from channel thus preventing the escape of leaky steam to the vault atmosphere. Ports for incoming air and outgoing air are provided to enable air circulation through the bung. Air form instrument air line is injected in bung cavity and comes out after picking up the steam from the leaky seal plug. The psychometric parameters of the incoming and outgoing air are monitored and leakage rate can be found out by analyzing the psychometric parameters of the air. Air flow meter Air flow meter in system is provided to monitor air flow rate at inlet and outlet of the bung. The air flow rate in the cavity can be varied between 1 liter per minute to 50 liter per minute using a flow meter provided in the system. To measure the flow rate, two air flow meters with ranges of 1 LPM to 5 LPM and 5 LPM to 50 LPM have been used in the system. Hygrometric instrument unit The injected air picks up leakage of steam inside the bung cavity and passes through hygrometric instrument unit. This unit consists of an inbuilt probe-in instrument for measuring the relative humidity and temperature of the injected air and also at outlet. Psychometric data Calculator A computer based programme is used to calculate thermodynamic properties of air used in system. These data are analyzed by Psychometric Calculator to know the correct amount of water hold by air at exit. Hygrometric instrument unit, air flow meter and data calculator are mounted on a panel. Flexible tubing is used for carrying dry air to the bung and wet air back from the bung. The panel is mounted near the valve station of the fuelling machine away from the fuelling machine vault and enables the data collection remotely. Fig. 1 : Seal plug location and fuelling machine operated bung align with coolant channel in 3D model measurement TEST SET UP FOR QUALIFYING THE SYSTEM To qualify the prototype system, a calibration setup consisting of micro steam generator was used. Leaks were generated and measured using change in weight of the micro steam generator and hygrometric unit. A micro steam generator is designed and fabricated to generate small amount of steam to simulate leaky seal plug leakage. In this steam generator a very low amount of steam is produced
510 Verma and Patel : Proceedings of the National Seminar & Exhibition on Non-Destructive Evaluation Fig. 2 : Relative humidity and temperature variation during calibration data collection for 15 minutes experiment Fig. 3 : Bung circled white advance by Fuelling Machine against channel to create leak tight cavity in front of seal plug. 3-D models showing aligning of bung with coolant channel in figure (a) and (b). Table 2 : Summary of setup and experiment data SETUP RANGE EXPERIMENTAL RANGE Moisture pickup Maximum Minimum Maximum Minimum Air at Inlet Outlet Inlet Outlet Inlet Outlet Inlet Outlet Air flow 50 LPM 50 LPM 1 LPM 1 LPM 24 LPM 24 LPM 2 LPM 2 LPM % RH 7 100 (sat.) 7 12 12 36 12 92 Temperature 0 C 32 32 32 32 30 29.5 27.5 27.6 Absolute humidity gm/m 3 2.37 33.79 2.37 4.05 3.64 10.63 3.17 24.49 Moisture cc/hr 7.11 101.37 0.1422 0.243 5.24 15.30 0.38 2.938 Leak rate cc/hr 94.26 cc/hr 0.10 cc/hr 10.06 cc/hr 2.558 cc/hr or 2.40 cc/day
NDE 2011, December 8-10, 2011 511 by using 50 watts variable electrical heating device. This steam generator contains a chamber with regulator heating system. In this chamber known amount of water is kept where heat converts the water into steam at constant rate. This known amount of steam produced by the system is used for calibration of the hygrometric properties. To know the exact amount of water being vaporized during process of steam generation in a specific time, a precision weighing machine is used to measure vaporizing water. During the experiment, water was heated for 15 minutes in the steam generator and resulting vapor produced getting mixed with the injected air. This caused change in relative humidity. The temperature and relative humidity of the air were measured at an interval of one minute. Fig -2 shows the plot of data collected. It was found that 2.705 gm of water was vaporized whereas 2.650 gm was picked up by the air injected as measured using hygrometer. The agreement between vapor generated in steam generator and picked by air injected is found to be within 2%. TESTING AND OBSERVATION ON FUELLING MACHINE During the testing, the hygrometric prototype setup was installed near the 220 MWe PHWRs fuelling machine test facility. The bung for collection of leakage steam was clamped in the snout of the FM shown in figure-3. During the testing Integrated Thermal Facility (ITF) available at Hall-7 was used to maintain the coolant channel in reactor simulated condition. The FM was aligned with the ITF coolant channel having a leaky seal plug. Inlet and outlet lines of the bung were connected with the hygrometric setup as described earlier. Instrumental quality air was supplied to the test setup and variation of flow was maintained by adjustment of the control valve mounted on the prototype setup. Data like hygrometric parameters of inlet and outlet air, temperature and flow were recorded from the setup. The data obtained during the experiment was analyzed by computer based psychometric calculator to know the actual leakage. At the time of installation, seal plugs had leakage of about 10 cc/hr which reduced to 2.5 cc/hr in 50 hrs. Data for leakage was collected every hour. Initially air flow was maintained in the range of 50 LPM and was reduced gradually as leakage was not enough to laden the air with vapor giving measurable change in relative humidity. RESULTS The maximum leak rate that can be picked up by the system is ~94cc/hr for 50 LPM air flow and minimum leak rate 0.10cc/ hr for 1 LPM air flow. The experiment conducted in FM with available leaky seal plugs the maximum and minimum leakage could be achieved by the seal plugs are 10.06 cc/hr and 2.558cc/hr. Corresponding psychometric data tabulated as shown in Table 2. CONCLUSION The proposed system offers a simple and easy to install equipment to identify leaky seal plugs operating units of PHWRs. The system offers a powerful tool in managing the tritium level to a minimum in the vaults of Nuclear Power Plant. As it is carried by the fuelling machine, it offers the advantage of monitoring the leakage remotely and reducing the economic penalty. REFERENCE P. L. Ballaney, Thermal Engineering.