Reactor power measurement using N16 and N17 production rate in MARIA reactor

National Centre for Nuclear Research, Poland

^* justyna.cybowska@ncbj.gov.pl

Published online: 21 November 2023

Abstract

In the water surrounding the reactor core, especially in the water flowing through the cooling system of the fuel channels, the (n,p) reactions on ¹⁶O and ¹⁷O occur producing ¹⁶N and ¹⁷N isotopes. The decay products of ¹⁶N and ¹⁷N may be used to regain information about the reactor power and the neutron flux density in the core. The aim of the study was the simulation of the activity of the ¹⁶N and ¹⁷N isotopes during the water flow through the fuel channels’ cooling system in the MARIA reactor, checking the possibility of the improvement of the currently performed measurements and the determination of the most optimal placement for the activity measurements. The simulations were performed taking into account the power generated by the individual fuel rods and the information on their burnout. The calculations were made for various configurations of the reactor core over the last 3 years. To check the reliability of the simulation, the activation measurement of the high-purity Au foil was performed. The Au have been activated at the outlet pipeline of the cooling system during whole reactor cycle (4 days) and after that, the activity of the produced ¹⁹⁸Au was measured. The ¹⁹⁸Au is the product of the (n, gamma) reaction on the ¹⁹⁷Au and its activity can be the measure of the neutron flux density in the foil, which in turn gives the information of the ¹⁷N content in the cooling water. For better neutron flux density estimations, the models of the fuel element and the model of the outlet pipe with the Au foil were made in the MCNP simulation code. The Au foil activation was also simulated in the Fispact-II code system which allowed estimating the neutron flux density needed to obtain the measured foil activity. This work shows the results of the studies together with the possible sources of uncertainties and the estimation of their influence on the final result.