Approach to criticality in cold core of the Unit 1 of the cycle 10 of the Laguna Verde power plant with control rods similar in design to the original ones and the advanced ones

In the Laguna Verde power plant (CLV) worn out control bars have been replacing for control rods of advanced design. At the moment of the 109 rods of similar design to the original of the Unit 1 of the CLV, 14 of them have been substituted with two types of different advanced control rods. The same as the original rods the advanced rods use boron carbide as control element for neutrons, but additionally they use hafnium, to achieve a longer useful half life. The rods of similar design to the original are homogeneous axially on the other hand the advanced presenting several zones. In principle the advanced rods were designed to be neutronically compatible to the original rods. However the differences in the design and of constituents assumed to consider a different performance during the operation. Since the one numbers of advanced bars is growing with the number of cycles of the CLV reactor has requested to the National Institute of Nuclear Research (ININ) to evaluate in detailed form the performance of the control rods. Previously the calculations and analysis of cycle design that that were carried out assumed that control rods should be of similar design as the original. Using the HELIOS/CM-LENDS system, the effect of the inclusion of advanced rods was analyzed in the approximation of the criticality in cold (20 C) in the cycle 10 of the Unit 1. Four scenarios were poposed, where the type of the rod of substitution control is varied. The value of reactivity was calculated for the rods similar in design to the original and for the advanced one. By comparison of these values, it was found that the value of reactivity of the control rods of the advanced design was superior to the value for rods similar to the original ones by 7.6% for one of the advanced designs and of 13.6% for the other one. (Author)