**1. Introduction**

In 2015, the advanced test reactor (ATR) began irradiations of the Ki-Jang Research Reactor—Fuel Assembly Irradiation (KJRR-FAI) test. Concurrent with the KJRR-FAI experiment program, the ATR was in the process of software quality assurance (SQA) for a more robust transport-based code, the Studsvik-Scandpower HELIOS code. The use of HELIOS enabled high quality (i.e., NQA-1) core reload and safety analysis of the ATR cycles for which irradiated the KJRR-FAI test.

The neutronic communication between the KJRR-FAI and the ATR fuel elements required 3D analysis. However, HELIOS is a 2D code. At the time, high fidelity 3D transport simulation of the ATR was too computationally expensive to be used for fuel reload and safety analysis. The solution of intra-plate power peaking in the ATR fuel elements was particularly challenging as this requires a significant number of particle histories in a Monte Carlo method and excessive

mesh density in a deterministic transport method. As a workaround, the wellknown Monte Carlo *N*th Particle (MCNP) code was used to provide the axial peak-to-average power peaking factors which allowed for computationally efficient calculation of new core reload patterns that would satisfy the irradiation needs of the KJRR-FAI while ensuring safe operation of the ATR.
