The development of new practices in nuclear research reactor safety aspects and optimization of recent nuclear reactors needs knowledge on forced convective heat transfer within sub-channels formed between several nuclear fuel rods or heat exchanger tubes, not only in the fully developed regime but also in the developing regime or laminar flow regime. The main objective of this research was to find a new correlation equation for calculating the convective heat transfer coefficient in the vertical square sub-channels. Recently, a simulation study was conducted to find a new heat transfer correlation equation for calculating the convective heat transfer coefficient within a vertical square sub-channel in the developing regime or laminar flow regime for Reynolds number range 400 ≤ Re ≤ 1700. Simulations were carried out using a computational fluid dynamics (CFD) code and modeling already defined in the software. The novelty of the research lies in the analysis of the entrance effect for the sub-channel by proposing a new empirical correlation that can then be inserted into the STAT computer code. The surface temperature distribution around the tangential direction of the active cylinders shows that the implementation of active and dummy cylinders in the current study can simulate sub-channels that exist in a real nuclear reactor core. The current study shows that the flow simulated in this study is in its developing condition (entrance region). A new forced convective heat transfer correlation for the developing region in the form of Nu = 2.094(Gz)^0.329 for the Graetz number range 161 ≤ Gz ≤ 2429 was obtained from the current study.

A security issue on nuclear facilities will give a significant effect on national security, economy, public health, or the combination among them. The evaluation of physical security effectiveness is done by considering it as a probability that the system will defeat an adversary at a particular time. This research examined the effectiveness of Physical Protection Systems (PPS) of a nuclear research reactor in Indonesia, as a role model facility. Three possible adversary scenarios were assessed. Estimation of Adversary Sequence Interruption model was implemented to assess the physical security system. The estimation of the effectiveness of physical-cybersecurity combination was done by Bounding Analysis Approach. The effectiveness of security system with cyber system working properly was assessed and compared with the system receiving cyber attacked. The results showed that the system effectiveness is high (0.839, 0.949, and 0.837, respectively). The results showed that the assessment method is effective to examine whether a security element is working effectively or not in such facility. This work will be useful as base guidelines for nuclear regulatory body and licensee for the application and evaluation of PPS and provision of countermeasure strategies in RSG GAS, and for further benefit, it can be used by nuclear facilities.