Multiphase Reactors Engineering and Applications Laboratory (mReal) at Missouri S&T has designed, developed, and tested a dual channel module. The facility represents a scaled down prismatic modular reactor to mimic pressurized conduction cooldown (PCC) accident scenario for the prismatic modular reactor with a reference to High-Temperature Test Facility at Oregon State University (OSU-HTTF). The current facility was constructed to investigate a plenum-to-plenum (P2P) natural circulation heat transfer through two channels for different coolants (working fluid) at high operating pressure of 413.7 kPa. The natural circulation heat transfer in terms of temperature fields and heat transfer coefficients across the core of current facility (i.e., channels) has been investigated at constant outer surface temperature of upper plenum and downcomer channel (278.15 K) under nonuniform heating center peaking step (approximating cosine shape) using an advanced fast response heat transfer technique. Results showed that a net inner surface temperature gain along the riser channel by 84, 95, 98 and 150K for carbon dioxide, nitrogen, argon, and helium respectively. Also, an average increasing of centerline temperature along the riser channel is observed by 110, 133, 151 and 204 K for carbon dioxide, nitrogen, argon and helium, respectively. Furthermore, the current results show a common heat transfer coefficients trend for all coolants along the riser channel; the local heat transfer coefficient decrease with axial location from the entrance (Z/L = 0.044) until a minimum value at Z/L = 0.279 and after this position, the local heat transfer coefficient starts to increase again till Z/L= 0.591 (laminarization effects). And finally, heat transfer coefficient decrease from Z/L= 0.591 till the exit into the upper plenum. However, it was observed that heat transfer coefficients for helium was higher than all other gases for the entire riser channel and remained positive for much higher heights. In the laminarization effects region (0.279