Abstract Due to the planned first criticality start-up of the Nuclear Power Plant (NPP) Mochovce units 3 and 4 in the near future, detailed analyses of core parameters are required to support safe operation of the nuclear facility. Since the previous calculations were performed by the deterministic nuclear codes, the stochastic approach was used to produce the independent results. The paper introduces determination of temperature feedback reactivity coefficients, especially summary (isothermal) and moderator (density) reactivity coefficients between 200 °C and 260 °C with step of 2 °C. The evaluation of the critical boron acid (H 3 BO 3 ) concentrations for different positions of the 6th control assembly group was performed for the given coolant temperatures as a precondition of the thermal coefficients calculation. All calculations were performed by computational code MCNP5 1.60 supported by NJOY99.364 microscopic cross sections processing system and our control scripts. For the validation of the criticality calculations (critical boron acid concentration), the experimental data based on the first critical conditions of the NPP Mochovce 2 start-up were used.