In Korea, preparations for the dismantling of Kori Unit 1 are underway. Except for the main components, the treatment and disposal of contaminated concrete are major issues due to the very large quantities. In particular, the bio-shield surrounding the reactor core has the highest neutron flux, such that the bio-shield is the first thing to be considered in terms of activated concrete dismantling. Instead of fully demolishing a contaminated concrete structure, surface removal techniques can be used to separate the contaminated concrete from uncontaminated concrete, and thus the volume of material requiring disposal as a radioactive material would be significantly reduced. When we remove contaminated concrete with various depths and shapes, using tools to predict the decay gamma source distribution of radioactive concrete structure would be very useful. The rigorous two-step (R2S) method, which has been mainly used to calculate the shutdown dose rate in the International Thermonuclear Experimental Reactor (ITER) calculation, was applied to nuclear facility dismantling. The R2S method consists of two-steps. The neutron transport calculation is the first step and the activation calculation is the second step. In addition, to get decay gamma dose distribution, additional gamma transport calculation is required. In this research, the AETIUS, in-house code, deals with neutron/gamma transport calculation and the FISPACT-2007 code performs the activation calculation. The difference from the conventional R2S method was that the neutron/gamma transport calculation was performed using a deterministic method that uses an unstructured tetrahedral mesh instead of using a Monte Carlo method. This is expected to be more useful in a radioactive structure dismantling applications in view of providing a high-resolution activation map and a change of decay gamma dose distribution while removing radioactive concrete immediately. In addition, by using a high-resolution activation map, we could reduce the volume of radioactive concrete to be removed significantly.