The ESS is being constructed in Lund, Sweden, to be the world brightness cold ( <1eV) pulsed spallation neutron source. The facility uses a 2GeV proton beam hitting a tungsten target to produce neutrons. To construct this place it is important to model and optimise the neutron transport system because it is necessary that the whole research lab will be safe from the radiological point of view. Simulate the entire project and to assess the radiological risk it is an extremely challenging computation task, for this reason it is needed to use extensive variance reduction techniques, developing codes, and proper algorithm depending on the problem. I will talk about the past and future challenges we face in designing this facility. Bio of speaker I am now a scientist at European Spallation Source in Lund. I work in simulation and data analysis for the design of this facility.I previously I worked in High Energy Physics experiments in California and in China.