**Abstract**

Since becoming a national user facility in 2007, the type of irradiation campaigns the Advanced Test Reactor (ATR) supports has become much more diverse and complex. In prior years, test complexity was limited by the computational ability to analyze the tests' influence on the fuel. Large volume tests are irradiated in flux traps which are designed to receive excess neutrons from the surrounding fuel elements. Typically, fuel elements drive the test conditions, not vice versa. The computational tool, PDQ, was used for core physics analysis for decades. The PDQ code was adequate so long as the diffusion approximation between test and fuel element remained valid. This paradigm changed with the introduction of the Ki-Jang Research Reactor—Fuel Assembly Irradiation (KJRR-FAI) in 2015. The KJRR-FAI was a prototypic fuel element for the KJRR research reactor project in the Republic of Korea. The KJRR-FAI irradiation presented multiple modeling and simulation challenges for which PDQ was ill suited. To demonstrate that the KJRR-FAI could be irradiated and meet safety requirements, the modern neutron transport codes, HELIOS and MCNP, were extensively verified and validated to replace PDQ. The hybrid 3D/2D methodology devised with these codes made analysis of the ATR with KJRR-FAI possible. The KJRR-FAI was irradiated in 2015-2016.

**Keywords:** advanced test reactor, Ki-Jang Research Reactor, HELIOS, MCNP, 3D/2D methods
