CONTROL ROD CALIBRATION OF NIGERIA RESEARCH REACTOR -1 (NIRR-1) USING POSITIVE PERIOD METHOD
Calibration of control rod has become an essential tool for measuring the reactivity changes in a reactor. The Positive Period Method of control rod calibration was shown to be a relatively independent, more accurate and well suited to the core configuration and control rod mechanism of low power reactors. The Nigeria Research Reactor-1 (NIRR-1) is a low power Miniature Neutron Source Reactor (MNSR). In this work, NIRR-1 control rod was calibrated from 107mm to 177mm withdrawal length at a low flux of 5 x 109 ncm2s-1. This flux and length was chosen to contain effects on reactivity due to temperature variation. Doubling times measurements were carried out in conjunction with the “Inhour” data to obtain reactor periods and their corresponding reactivities. The integral reactivities obtained were plotted against control rod withdrawal positions. The plot was compared with the s-shape integral curve earlier obtained for NIRR-1 during the zero power experiments in China and the two figures were found to be in agreement. The integral rod worth of (3.34) mk obtained here was also compared with the in-built full rod length total reactivity worth of 7mk and was found to have accounted for 47.8% of the length. This result is an indication that 10mm withdrawal length is approximately equivalent to 0.56mk reactivity insertion and the length used here is the most reactive portion of the control rod. It also shows that only 30% of the control rod length is required to achieve the characteristic s-shape curve using the positive period method. The uncertainty of 3.29% obtained in the integral rod worth is found to be within the margin of experimental results. This has satisfied the safety requirements of MNSR for shutdown margin. Our work has demonstrated that routine calibration of the MNSR control rod via positive period method is possible and satisfied all safety requirements, and thus recommended.
1.1 BACKGROUND TO THE STUDY
The central characteristics of a nuclear reactor are its controllable, self-sustaining fission chain reaction which releases useful neutrons and energy (CNSC, 2003). This controllability was shown to be greatly achieved via control rod motion and as such the knowledge of a reactor‟s response to specific control rod motions is essential for the safety, efficiency, operation and durability of a nuclear research reactor (Souza and Mesquita, 2010). Research reactors are nuclear reactors that are majorly neutron source. Also called non-power reactors, the major contrasts between them and power reactors are that power reactors are utilized for electricity production, heat generation, or maritime propulsion while the neutrons produced by a research reactor are used for several purpose which include scattering, non-destructive testing, analysis and testing of materials, production of radioisotopes, research and public outreach and education.
Research reactors comprise a wide range of civil and commercial nuclear reactors which are generally not used for power generation (Ahmed et al, 2006). Over 270 research reactors are currently operating in more than 50 countries (Parrish and Nicholas, 2007). Research reactors are separated into various classes of which one of such is the Miniature Neutron Source Reactor (MNSR) which the Nigeria Research Reactor-1 (NIRR-1) fall under. The Nigeria Research Reactor-1 (NIRR-1), is the 8th commercial Miniature Neutron Source Reactor (MNSR) designed by China Institute of Atomic Energy (CIAE). First criticality was achieved on 03 February 2004. It is particularly designed for use in neutron activation analysis (NAA) and limited radioisotope production.