Feasibility of measurement of N-16 in Apsara-U research reactor

Apsara-U is a swimming pool type Research Reactor having maximum capacity of 2 MWth. This reactor is used for production of radioisotopes, material shielding experiments, neutron activation analysis and scattering experiments. As part of core management, many a times configuration of the reactor core is changed leading to a change in the overall neutron flux pattern. This necessitates for calibrating the nuclear channels in accordance to thermal power of the reactor. Conventionally in a pool type research reactor, for measurement of thermal power, two methodologies are adopted. The first method is based on heat balance wherein the heat transferred to different components of the reactor is estimated using associated temperature changes with time. This method requires shutting down of secondary cooling water system and hence puts a limitation on the reactor power level and measurement time. The second method makes use of the formation of N-16 by the neutron activation of O-16 contained in the primary coolant. The N-16 content in the coolant is a direct measure of fission rate and thereby a direct measure of the total power generated. The decay of N-16 produces 6 MeV gamma rays which can easily be measured outside the coolant pipes. An N-16 power monitoring has several advantages over the ΔT and ex core power measurements; one of them being faster time response. In view of the above, an N-16 based reactor power monitoring system is being attempted in Apsara-U. Two Gamma Ion Chambers (GIC) (GIC-1 and GIC-2) with nominal sensitivity of 1.25 x 10^-09 A/R/hr, have been mounted on the coolant outlet header at suitable locations. The HV module and electrometer and log amplifiers for the measurement of the signal current are located in accessible area. The outputs of detectors are measured with operating voltage of 600 V DC applied to chambers at various power levels during reactor operation and shut down. The GIC outputs are connected to 6 decade log amplifiers and are interfaced to COIS for further analysis. The log-amplifiers outputs are measured and compared with outputs of online neutronic channels. A correlation will be established between the difference (to account for the background) of GIC outputs and reactor power, which are expected to be repeatable for various reactor operating conditions