N-16 power measuring system

The thermal power output of a reactor can be measured by monitoring the total amount of heat removed from the reactor core per unit of time. Other methods of measuring the power level indirectly, as for instance, utilizing the neutron flux outside the core, have the disadvantage of heavy influences on the accuracy due to changes of the neutron flux distribution in the core caused by control rod operation and xenon buildup effects. Heat balance (ΔT) measurements in the primary loop are used to measure reactor power but require hot leg RTD bypass loops and have slow time response. An N-16 system has been developed which shows several advantages over the ΔT and excore power measurements. The Nitrogen-16 (N-16) activity in the primary coolant water of a PWR has long been considered as a possible parameter for continuous measurement of reactor power level. The N-16 activity is formed by fast neutron activation of Oxygen-16 contained in the water. Activation results from a threshold reaction requiring >10 MeV neutrons. It can be shown that the N-16 content in the coolant is a direct measure of the integrated fast flux throughout the core and is thereby a direct measure of the fission rate and total power generated. Decay of the N-16 produces 6 MeV gamma rays which readily penetrate the wall of the high pressure piping. Thus, the N-16 concentration in the coolant can easily be monitored by measuring the gamma radiation outside the pipe--for instance, at the hot leg of the coolant loop. The N-16 power monitor is calibrated against secondary heat balance measurements, since these are assumed to give the most accurate information on the absolute thermal power level