A large radiological accident will result in the contamination of surrounding people, animal, vegetation etc. In such a situation assessing of the level of contamination becomes necessary to plan for the decontamination. There are plans existing for evaluating contamination on people. However, there are limited to no plans to evaluate animals. It is the responsibility of the United States Department of Agriculture (USDA) to decontaminate animals. So the objective of this thesis work was to design a scalable gamma radiation portal monitor (RPM) which can be used to assess the level of contamination on large animals like cattle. This work employed a Monte Carlo N-Particle (MCNP) radiation transport code for the purpose. A virtual system of cow, radiation source representing the contamination, cattle chute and different detector configurations were modeled. NaI scintillation detectors were modeled for this work. To find the optimal detector size and configuration, different detector orientations were simulated for different source positions using the MCNP code. Also simulations were carried out using different number and size of the detectors. It was found that using 2" x 4" x 16" detector yielded a minimum detectable activity (MDA) value of 0.4 microCi for 137Cs source.