[en] Neutronic instrumentation plays a vital role in monitoring, controlling and ensuring safety of the reactor in case of any operational occurrences. In FBTR, the neutron flux measurement at the sensor location varies over 11 decades from shutdown to full power of 40 MWt. The original instrumentation was indigenously designed and developed. The system has been operational for the last 11 years. In view of certain problems faced on these channels and to overcome component obsolescence, it is planned to replace these instrument channels, with second generation of instrumentation developed indigenously. This paper details the experience with the present channels and the special features adopted in the new instrumentation

[en] The nuclear instrumentation systems of the Prototype Fast Breeder Reactor (PFBR) primarily comprise of global Neutron Flux Monitoring, Failed Fuel Detection and Location, Radiation Monitoring and Post-Accident Monitoring. High temperature fission chambers are provided at in-vessel locations for monitoring neutron flux. Failed fuel detection and location is by monitoring the cover gas for fission gases and primary sodium for delayed neutrons. Signals of the core monitoring detectors are used to initiate SCRAM (safety action) to protect the reactor from various postulated initiating events. Radiation levels in all potentially radioactive areas are monitored to act as an early warning system to keep the release of radioactivity to the environment and exposure to personnel well below the permissible limits. Fission Chambers and Gamma Ionisation Chambers are located in the reactor vault concrete for monitoring the neutron flux and gamma radiation levels during and after an accident. (authors)

[en] This paper examines the role of the k/N systems in improving the reliability of the radiation monitoring systems with β factor Common Cause Factor (CCF) model

[en] The construction of the Prototype Fast Breeder Reactor (PFBR) a 500 MWe pool type sodium cooled breeder reactor with MOX fuel has started at Kalpakkam. The Instrumentation and Control of PFBR is designed for safe, reliable and economic operation of the plant. Special feature of breeder reactors is sodium instrumentation. Leaks in sodium systems have the possibility of being exceptionally hazardous due to the reaction of liquid sodium with oxygen and water vapour in the air. In addition, leakage from primary systems can cause radioactive contamination. Potential regions of leakage are near welds and high stress areas. Sodium also reacts with concrete releasing hydrogen and leading to damage and loss of strength of concrete structures. Leaking sodium catches fire depending on its temperature. Sodium temperature in the plant ranges from 423 K at filling condition to 820 K at reactor nominal power operating condition. Leak detectors are provided on pipelines, tanks and other capacities. Sodium leak detection systems are designed to meet requirements of ASME section XI- division 3 which specifies that sodium leak at the rate of 100 g/h are to be detected in 20 h for air filled vaults and 250 h for inert vaults. Diverse leak detection methods are employed for active and non-active sodium equipment and pipes. For detection of water leaks into Sodium in steam generators, Hydrogen in Sodium Detectors (HSD) are used. Hydrogen in Argon Detectors (HAD) are used for sodium temperatures below 623 K as HSD is not effective below this temperature due to non-dissolution of hydrogen formed. Choice and challenges posed in implementation of above leak detection requirements are discussed in this paper. (authors)

[en] Full text: Prototype fast breeder reactor (PFBR) uses liquid sodium as coolant. Sodium reacts with air and catches fire and reacts with concrete and damages it. Various design provisions made to prevent sodium leak in the system and measures provided to mitigate the consequences of sodium leak and fire should a leak occur are discussed in this paper

[en] Full text: The shutdown system of PFBR is designed to assure a very high reliability by employing well known principles of redundancy, diversity and independence. The failure probability of the shutdown system limited to < 10^-6/ ry. Salient features of the shutdown system are: Two independent shutdown systems, each of them able to accommodate an additional single failure and made up of a trip system and an associated absorber rod group. Diversity between trip systems, rods and mechanisms. Initiation of SCRAM by two diverse physical parameters of the two shutdown systems for design events leading potentially to unacceptable conditions is the core. The first group of nine rods called control and safety rods (CSR) is used for both shutdown as well as power regulation. The second group consisting of three rods known as diverse safety rods (DSR) is used only for shutdown. Diversity between the two groups is ensured by varying the operating conditions of the electromagnets and the configurations of the mobile parts. The reactivity worth of the absorber rods have been chosen such that each group of rods would ensure cold shutdown on SCRAM even when the most reactive rod of the group fails to drop. Together the two groups ensure a shutdown margin of 5000 pcm. The speed and individual rod worth of the CSR is chosen from operational and safety considerations during reactor start up and raising of power. Required drop time of rods during SCRAM depends on the incident considered. For a severe reactivity incident of 3 $/s this has to be limited to 1s and is ensured by limiting electromagnet response time and facilitating drop by gravity. Design safety limits for core components have been determined and SCRAM parameters have been identified by plant dynamic analysis to restrict the temperatures of core components within the limits. The SCRAM parameters are distributed between the two systems appropriately. Fault tree analysis of the system has been carried out to determine the reliability of the system

[en] Full text: The core monitoring systems primarily comprise of global neutron flux monitoring, failed fuel detection and location, fuel subassembly temperature monitoring and core flow monitoring. High temperature fission chambers are provided at in-vessel locations for monitoring neutron flux. Failed fuel detection and location is done by monitoring the cover gas for fission gases and primary sodium for delayed neutrons. Chromel-Alumel thermocouples (TC) are provided at the outlets of the fuel subassemblies and core inlet for core temperature monitoring. For core flow monitoring, permanent magnet flowmeter and probe type Eddy current flowmeters are provided at the primary sodium pump discharge. Signals of the core monitoring detectors are used to initiate SCRAM to protect the reactor from various postulated initiating events

[en] The primary sodium circuit of Prototype Fast Breeder Reactor(PFBR) consists of intermediate heat exchangers and primary sodium pumps which are housed in the sodium pool inside the main vessel. The neutron flux around the reactor is very low as neutron shields are provided in the core, to protect the components housed inside the main vessel and to reduce the radioactivity of the secondary sodium that flows inside the intermediate heat exchangers housed in the sodium pool. Various options for neutron flux monitoring were studied. High temperature fission chambers with a sensitivity of 1 cps/n/cm²/s (²³⁵U thermal eqvt. flux) are chosen and provided in the control plug locations, symmetrically, just above the core. Originally, graphite was provided in the top axial shield of the Fuel Subassemblies, to thermalise the fast neutrons and thereby, it was possible to get a shutdown count rate of 3 cps at the detector locations. As there were possibilities of reactions between graphite and sodium in the presence of oxygen and the measures required to avoid such reactions were costly, it was decided to remove it. It was decided to provide Sb-Be neutron source subassemblies in the core, to improve the shutdown count rate. The source subassemblies are initially loaded in the core with- out any irradiation. The source strength builds up with in-situ irradiation and provides sufficient shutdown count rates at the detector locations, after 60 full power days of reactor operation. For flux monitoring during first approach to criticality, a special Instrumented Central Fuel Sub Assembly (ICFSA) that accommodates 3 high temperature fission chambers of 0.1 cps/n/cm²/s sensitivity and also fuel pins is provided at the core centre position. This paper summarises various studies carried out and options considered in finalising the Neutron Flux Monitoring System for PFBR. (author)