Criticality Analysis of Thorium Element (ThO2) Insertion at Various Location in the Kartini Reactor Core

The criticality analysis of Kartini reactor by inserting of thorium element contained in ThO2 has been done. The main objective of this research was to observe the effect of thorium element insertion to the criticality level of Kartini reactor. The analysis method was a neutronic computation using MCNP computer code. The criticality analysis was performed by inserting a thorium element of several different masses at several locations in the reactor core. The analyses were performed for the insertion of ThO2 at several different positions from ring B to ring F of the reactor core. The result showed that in the general, by inserting the thorium elements on the reactor core would generate a smaller k-inf. It was because thorium would produce a negative reactivity in the reactor core. This analysis result showed that the reactor is on the subcritical level (k-eff. < 1) when 3000 g of ThO2 was inserted in the ring B, C and D. Meanwhile, it still on the critical level (k-eff. ∼ 1) when ThO2 element was inserted in the ring E and F. However, to ensure the safety of the Kartini reactor during this experiment, it was necessary to calculate the shutdown margin (SDM) and also radial power peaking factor. The analysis showed that the calculated SDM was 2.5$ and the radial power peaking factor was 1.5. These values still meet the required value for the Kartini reactor i.e. SDM should be higher than 0.5$ and the radial power peaking factor should be lower than 1.75.