RELAp65

RELAp65 is a thermal-hydraulic system analysis code used to simulate the transient behavior of nuclear reactor coolant systems. It represents a modern re-implementation of the RELAP5 family, with an emphasis on maintainability, portability, and extensibility while preserving the modeling concepts and input conventions familiar to RELAP5 users. The code is designed to support safety-relevant analyses of light-water reactor systems, including primary and secondary loops.

The modeling approach in RELAp65 is based on a network of one-dimensional flow channels and components. Users can assemble pipes, junctions, pumps, valves, heat exchangers, accumulators, and pressurizers to form complete coolant loop configurations. The code supports single- and two-phase flow, phase change, and the conservation equations for mass, momentum, and energy, using numerical schemes suited to stiff, transient problems often encountered in safety analyses.

RELAp65 is developed through a collaborative, community-driven effort and is distributed with a focus on open development practices. It aims to provide cross-platform support, modular architecture, and enhanced tooling for input preparation and post-processing, facilitating research, education, and regulatory applications. While it builds on RELAP5 concepts, RELAp65 seeks to improve software maintainability, documentation, and interoperability with modern computing environments, including potential parallel execution to leverage multi-core hardware.

Typical applications include loss-of-coolant accidents, depressurization scenarios, natural circulation studies, and stability analyses for reactor coolant systems. The project remains active, with ongoing updates that expand component libraries, validation cases, and usability features.