1 Copyright © β017 by ASME Proceedings of the 25 th International Conference on Nuclear Engineering ICONE25 July 2-6, 2017, Shanghai, China ICONE25-67765 ANT COLONY OPTIMIZATION OF MULTILAYER SHIELDING FOR MIXED NEUTRON AND GAMMA RADIATIONS: A PRELIMINARY STUDY Muhammad Arif Bin Sazali Universiti Teknologi Malaysia Johor Bahru, Johor, Malaysia Nahrul Khair Alang Md Rashid Universiti Teknologi Malaysia Johor Bahru, Johor, Malaysia Khaidzir Hamzah Universiti Teknologi Malaysia Johor Bahru, Johor, Malaysia ABSTRACT Mixed neutron and gamma radiations require different shielding materials as their interaction with materials is different. Composites were developed in order to combine the shielding capabilities of different materials. However, their homogeneity is difficult to be assured which can lead to pinholes where radiation can penetrate. To avoid this problem, several materials arranged in layers can be used to shield against mixed radiations. Since the multilayer shielding can be made from any material in many configurations, the ant colony optimization (ACO) is a promising method because it deals with combinatorial optimization problems. The candidate materials are HDPE, boron, cadmium, gadolinium, tungsten, bismuth, and iron. Preliminary MCNP simulations were done to observe the effect of arrangements, thicknesses, and types of materials on the radiation spectrum. It was found that: (1) the final layer should be made of high density material, (β) an increase beyond certain thicknesses did not result in a significant increase in attenuation, and (γ) there should be an optimum combination of material that can effectively shield against both neutrons and gamma rays. Keywords: Ant colony optimization, gamma rays, Monte Carlo, neutrons, shielding. INTRODUCTION Mixed neutron and gamma radiations require different materials for shielding. This is because each of them has different mechanisms by which they interact with matter. The attenuation of radiation is proportional to the density of the shielding. This is the reason why heavy materials such as lead and concrete are popular. However, although they are easy to produce, they are restricted by their weight. Lead is also toxic to workers and harmful to the environment. Concrete, on the other hand, is prone to radiation damage and it can be activated by thermal and epithermal neutrons [1,β]. Therefore, new materials are needed to replace these materials. One possible approach is by using composite materials. They are made of matrix material embedded with micro or nano particles. Their advantage is that the shielding capabilities of the materials can be combined into a single material, reducing the weight of the shielding as seen in polymer composites [γ]. However, the difficulty in producing the material is in ensuring uniformity of the particles in the polymer substrate. The lack of uniformity in the composites could cause variations between simulated and experimental results [4−6]. Non-uniform composites may also result in pin holes, pure polymer areas which radiation can penetrate through [7]. Therefore, multilayer shielding made up of layers of different pure materials is proposed to avoid the stated problems. In a multilayer shielding, several materials are arranged against the radiation source. It was proven that bilayer radiation shielding can provide approximately 75 % weight reduction of lead based materials while providing the same shielding capabilities [8]. This is useful in mobile applications such as modular dry storage spent nuclear fuel and spacecraft structure that demand efficient and cost effective shielding. However, there are many parameters that need to be considered. With various combinations and arrangements of materials available to the designer, there is a need to ensure that they are optimized based on their desired application. Hu et al. had optimized a shielding against mixed radiation using genetic algorithm optimization coupled with MCNP [9]. With the algorithm, the authors could identify the optimum thickness of pure material shielding and the optimum weight percentages of elements to be included in polyimide composite. The author also stated that there is a need for a comprehensive design which not only considers the shielding performance but