Monte Carlo N-Particle (MCNP) transport code accelerated by AutomateD VAriaNce reducTion Generator (ADVANTG) code was used to simulate neutron and prompt gamma particles emitted from TRIGA research reactor during operation. Firstly, the method was validated by measuring dose rates around open beam port number 5 was unplugged. Neutron and gamma dose rates inside the reactor hall in the vicinity of the beam port were calculated and compared to the measurements. Due to the satisfactory agreement, the method was later used to design external shielding for the same beam port when it was upgraded – special mechanism was installed that allows irradiation of larger samples. Computational analysis of the proposed shielding configuration provided acceptable dose rate levels inside the reactor hall. When the shield was constructed, calculated dose rates were confirmed by the actual measurements. No modifications were needed. •For the first time neutron and gamma ray transport outside JSI TRIGA shielding body was simulated using Monte Carlo Method•Method was validated by comparing calculated and measured dose rates around beam port no. 5 during reactor operation.•Removable shield was placed in front of the beam tube opening in order to study backscattered radiation.•It is demonstrated that developed method can be used in designing appropriate shielding for the future experiments.