K.K. Rajan

Raman distributed sensor system for temperature monitoring and leak detection in sodium circuits of FBR

Leak detection in coolant loops of nuclear reactors is critical for the safety and performance of the reactors. The feasibility of using Raman distributed temperature sensor (RDTS) has been studied on a 30m test loop. Temperature in sodium circuits of fast Breeder Reactor (FBR) exceeds 550°C, gold coated fiber is chosen as sensor fibers. Leak is simulated through an artificial micro fissure integrated in the test loop with provision for controlled leak rate. The results are discussed in the paper.

Testing and Qualification of Diverse Safety Rod and Its Drive Mechanism for PFBR

PFBR, India’s first commercial fast breeder reactor employing fast fission is a challenging project from technological point of view to meet the energy security of the country. It is currently under advanced stage of construction at Kalpakkam, India. PFBR is equipped with two independent, fast acting and diverse shutdown systems. A shutdown system comprises of sensors, logic circuits, drive mechanisms and neutron absorbing rods. The absorber rods of the second shutdown system of PFBR are called as Diverse Safety rods (DSR) and their drive mechanisms are called as Diverse Safety Rod Drive Mechanisms (DSRDM). DSR are normally parked above active core by DSRDM. On receiving scram signal, Electromagnet of DSRDM is de-energised and it facilitates fast shutdown of the reactor by dropping the DSR in to the active core. For the prototype development of DSR and DSRDM, three phases of testing namely individual component testing, integrated functional testing in room temperature and endurance testing at high temperature sodium were planned and are being done. The electromagnet of DSRDM operates at high temperature sodium environment continuously. It has been separately tested at room temperature, in furnace and in sodium. Specimens simulating the contact conditions between Electromagnet and armature of DSR have been tested to rule out self welding possibility. The Dashpot provided to decelerate the DSR at the end of its free fall has been initially tested in water and then in sodium. The prototype of DSR has been tested in flowing water to determine the pressure drop and drop time. The functional testing of the integrated prototype DSRDM and DSR in aligned and misaligned conditions in air/water has been completed. The performance testing of the integrated system in sodium has been done in three campaigns. Based on the performance testing in the first two campaigns of sodium testing, design modifications and manufacturing quality improvement were done. Methods of drop time measurement based on ultrasonics and acoustics were also developed along with the first two campaigns. During the third campaign of sodium testing, the performance of the system has been verified with 30 mm misalignment at various temperatures. The third campaign has qualified the system for 10 years of operation in reactor. This paper describes the test setup for all the above mentioned testing and also gives typical test results.Copyright

Assessment of Dynamic Instability in Once Through Steam Generator

Hydrodynamic flow instability in Once Through Steam Generators (OTSG) is one of the important problems in the design and operation of Liquid Metal Fast Breeder Reactors (LMFBRs). Under certain operating conditions, water flow in OTSG is susceptible to instability due to the close coupling between the thermal and hydraulic processes. Sustained flow oscillations due to instability are undesirable since they result in flow mal-distribution among the tubes resulting in thermal stress, mechanical vibrations and system control problems. It is therefore, necessary to assess the operating conditions, under which instability occurs so that the system may be designed to operate always under stable conditions. The cause of the main type of instability, important for the design of SGs is the propagation of density waves. This type of low frequency instability is referred to in literature as parallel-channel, density wave, time delay or mass flow-void feedback instability. Dynamic instability (density wave oscillation DWO) occurs because of the phase mismatch between the primary perturbation (water flow) and the response to this perturbation (pressure drop). As many tubes are operating under essentially constant pressure heads, this mismatch can lead to sustained/diverging oscillations. Water flow oscillation in tubes manifests as oscillations in the steam temperature at the tube outlet/pressure fluctuations. However it is difficult to instrument individual tubes in SG for such measurement in an operating plant. If the flow oscillation in the tube manifests itself in the overall module flow, then fluctuation in the overall flow/flow noise could be utilized for on-line stability measurements. Towards this, experiments were conducted in the sodium heated once through steam generator in an OTSG model. To confirm the extent of oscillation in the steam temperature and in inlet water flow, 3 tubes out of 19, were monitored besides overall module flow. Main objective of the present study was to assess the occurrence of dynamic instability in SG through module inlet flow perturbations, measured by ΔP measurements across the orifice at entry to the tubes and steam temperature fluctuation measurement at the outlet of tube by bare thermocouples. This paper discusses the experiments carried out in the Steam Generator model of Prototype Fast Breeder Reactor to investigate the instability phenomenon, the instrumentation details, the results and its discussion.Copyright

Numerical Simulation and Experimental Validation of Future FBR Surge Tank Hydraulics

Surge tank is provided in the secondary sodium circuit of Sodium cooled Fast Reactors (SFR) to protect the secondary sodium circuit components from the pressure surges due to sodium water reaction in Steam Generator (SG). The pressurized argon gas inside the surge tank above sodium will act as a cushion and absorb the pressure surges. The entrainment of argon gas into the sodium due to free level fluctuations, turbulence etc. can cause operational difficulties in reactor. It is necessary to develop effective gas entrainment mitigating devices which keeps the sodium free surface calm but the development only through experiments is difficult and time consuming. Therefore a CFD model of surge tank is developed to predict the surge tank hydraulics and it is validated through experiments. Velocity measurement in the model at different directions and different elevations has been carried out using Ultrasonic Velocity Profiler (UVP).

Experimental Qualification of Mechanical and Electrical Sub-systems of a Complex Mechanism Against Fatigue Failure

Absorber rod drive mechanisms (ARDM) play an important role in ensuring safety of a reactor by rapid insertion of an absorber rod during abnormal conditions. Various components/sub-systems of ARDMs, both mechanical and electrical, are subjected to different cyclic loadings during service life. Thus, qualifying these systems against fatigue is an important step for gaining confidence in their safe operation for the design life. ASME in Sec. III, Div. 1, Appendices (Para II—1500) provides guidelines for the experimental evaluation of the capability of components to withstand cyclic loading. These rules are developed for static components like pressure vessels. Since no such rules are available for moving components like mechanisms, the same were adopted for the ARDMs, with an understanding that the effect of inertia loads of a moving component are to be accounted in the experiments. In application of these rules to a complex mechanisms such as ARDM, various special cases arise which are not addressed explicitly in the code. The paper describes the intelligent adoption of the fatigue life rules given in ASME to various special cases and their extension to electrical systems. The paper also outlines the experiments carried out for qualifying the ARDM against fatigue.

Vibration Measurements on PFBR Fuel Subassemblies

Prototype Fast Breeder Reactor (PFBR) is a 500 MWe liquid metal cooled fast breeder reactor, which is in the final stage of construction. The core of PFBR consists of 1,758 subassemblies supported at the bottom on the grid plate sleeves. Liquid sodium is used as the coolant and flow through the maximum rated fuel subassembly is 36 kg/s. The coolant flows axially from the bottom of the subassembly to top and it is in highly turbulent regime. This turbulent flow can excite flow-induced vibration of fuel subassembly which can cause failure of the fuel pin clad tubes from fatigue, wear and vibration induced fretting. Excessive vibration of fuel subassembly can also results in reactivity noise, fatigue or rattling. Flow induced vibration studies of dummy fuel subassemblies in water were conducted in subassembly test facility and the design was qualified for PFBR. However it is planned to measure the amplitude and frequency of vibration during pre-commissioning tests of PFBR. Measurements are planned during the isothermal run of PFBR at 200 °C with dummy subassemblies loaded in the core. Since measurement has to be carried out in high temperature sodium environment, conventional contact type sensors such as accelerometers, strain gages etc. cannot be employed for vibration measurement. Non-contact measurement technique using ultrasound waves was planned to be developed for vibration measurement. Extensive experiments were carried out in various test facilities and ultrasonic vibration measurement technique was established and demonstrated. Based on the experimental results, a device named SONAR was designed and developed for PFBR. The SONAR device is equipped with ultrasonic sensors, which focuses on subassembly crown region, and is capable of movement in Z-axis (up and down) and in Theta-axis (rotation). The movement of the subassembly is detected from the train of ultrasonic pulses and echoes from the target subassembly. Time signal and frequency spectra of vibration are extracted from the ultrasonic signals using signal processing technique implemented in LabVIEW platform. This paper discusses the details of the FIV measurements on PFBR fuel subassemblies, details of ultrasonic technique and SONAR device, its testing and results and conclusion.

Acoustic Technique for Diverse Safety Rod Drop Time Measurement in PFBR

Diverse safety rods (DSRs) are used in Prototype Fast Breeder Reactor (PFBR) to shutdown the reactor during emergency conditions (SCRAM). These rods contain neutron absorber materials to reduce the reactivity for the safe shutdown of the reactor. During normal operation of the reactor, DSRs are held above the active core using electromagnets. During a SCRAM the electro-magnets are de-energized and DSR falls into the core. Since it is a safety related action, no active system is used for driving the rods into the core and the rod falls under gravity force. It is required to measure the fall time of DSR during each SCRAM in the reactor to ensure proper insertion of the DSR in the core. Experiments were carried out in various facilities to develop a measurement technique. This paper discusses the details of Diverse Safety Rod Drive Mechanism (DSRDM), acoustic technique used for fall time measurement, experiments carried out, analytical modeling details and its results and conclusion.

Porous Body Model Based Parametric Study for Sodium to Air Heat Exchanger Used in Fast Reactors

Sodium to air heat exchangers (AHX) with finned tubes is used in fast breeder reactors for decay heat removal. The aim of decay heat removal is to maintain the fuel, clad, coolant, and structural temperatures within safety limits. To investigate the thermal hydraulic features of AHX, a robust porous body based computational fluid dynamics (CFD) model has been developed and validated against the experimental data obtained from a model AHX of 2 MW capacity in Steam Generator Test Facility at the Indira Gandhi Centre for Atomic Research, Kalpakkam. In the present paper, the developed porous body model is used to study the sodium and air temperature distribution and the influence of various parameters that affect the heat removal rate and sodium outlet temperature in full-size AHX used in the fast breeder reactors. The parameters include mass flow rates and inlet temperatures of sodium and air. The focus of the study has been to identify conditions that can pose the risk of sodium freezing.

Active magnetic bearings for sodium pump

Centrifugal Pumps are used in the primary and secondary heat transport systems of fast reactors for pumping liquid sodium. Lubrication oil leakage from the conventional bearings used in these pumps is a potential threat to cause reactivity changes which could result in extended reactor shut down. Actively controlled magnetic bearings which do not require lubrication is an excellent alternative to conventional bearings in overcoming these problems. The present work deals with the development of “Thrust and Radial Active” magnetic bearings for a small centrifugal sodium pump of 50 m3/h capacity, which controls both the axial and radial movements of rotor. The developed active magnetic bearing takes a thrust load of around 100kg, and a nominal radial load for the existing vertically-configured shaft system of the centrifugal pump. Performance of the active magnetic bearings has been tested successfully by running the pump up to the full operating speed of 2900 rpm and the measured vibration levels were well within the allowable limits of ISO 14839.

Experience with Material Combinations for Sliding Applications in PFBR In-Vessel Fuel Handling Machine

Prototype Fast breeder Reactor (PFBR) is a pool type reactor of 500 MW(e) capacity using mixed oxides of Uranium and Plutonium as fuel and liquid sodium as coolant, which is currently under advanced stage of construction at Kalpakkam. In-Vessel handling of the core subassemblies (SA) is carried out by an offset arm type machine called Transfer Arm (TA). Different types of material combinations are utilized for various sliding pairs used in the machine for various motions. Criteria for the selection of these material combinations are decided by the compatibility of the respective machine element with the working environment, magnitude of the contact stress, working temperature, linear speed, availability of external lubrication, required life, required tolerance etc. Transfer of a SA is achieved by gripping/ungripping of SA using fingers, raising / lowering the gripper outer tube and rotation of TA. The drive for gripper finger operation is at ambient environment and finger actuation is in liquid sodium. An inner tube links the linear actuator to the finger actuator and is housed and guided inside the gripper outer tube. Relative movement of inner tube with respect to outer tube results in open / closing of gripper fingers. Initially, combination of material pairs at five nos. of guide locations was SS 304 LN for the outer tube and hardchrome plated SS 304 LN for inner tube. During testing in air after 20 cycles, jamming of inner tube with respect to outer tube was observed. This was solved by reducing number of guides to two, by changing the surface contact to line contact and by changing the material combination to SS 304 LN against colmonoy coated SS 304 LN. Similar failure was observed for sliding movement at guide locations between the outer tube and shielding sleeve during hoisting of the gripper. Initial material combination of SS 304LN and hardchrome plated SS 304 LN was changed to colmonoy coated SS 304LN and hardchrome plated SS 304 LN. The selected material combinations were validated by testing on a separate subassembly simulating the geometry & loading before actual implementation on the machine. Guide tube, which is used to guide the gripper is raised / lowered by means of a screw-nut mechanism. Initial material pair used for the screw and nut, which are working at ambient conditions was SS 304LN and SS 410 respectively to provide corrosion & galling resistance. However during initial performance testing, this material combination failed and the nut got jammed. Subsequently the problem was studied and overcome by changing the material of nut to phosphor bronze, which is relatively softer and hence provided uniform contact across the nut surfaces. Appropriate material selection and proper design of the geometry of guiding surfaces are very essential for the smooth operation of machine elements in sliding conditions. With improvements in the material choice and geometry of the guides, qualification testing of transfer arm was successfully completed in air and hot argon. Testing in sodium is under progress and the experience at high temperature has been encouraging.