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RELAP5 Analysis

RELAP5 Analysis


BWXT provides transient analysis of nuclear power plants using the RELAP5TM/MOD3.3. The RELAP5TM code has been extensively validated and developed to provide the best estimate prediction of thermal hydraulic systems normal operation and dynamic response. RELAP5TM is a computer code developed by Idaho National Labs and recognized by the Nuclear Regulatory Commission (NRC) for the thermal-hydraulic analysis of transients and small and large-break type accidents in light-water nuclear power plants. It uses a one-dimensional, two-fluid model consisting of steam and water. BWXT has long standing experience in providing services for its customers to predict thermal hydraulic performance and structural loads on components and internals using RELAP5TM.


RELAP5TM is capable of modeling a wide variety of system configurations by linking its many generic component models, such as:

  • Pumps
  • Valves
  • Pipes
  • Heat releasing or heat absorbing structures
  • Reactor point kinetics
  • Electric heaters
  • Jet pumps
  • Turbines
  • Separators
  • Accumulators
  • Controls system components

RELAP5TM can model a wide range of transient conditions, including transient water level control, small and large pipe breaks, loss of feedwater, loss of offsite power, and station blackout.

Due to its flexibility and generic structure, RELAP5TM can be also be used to model transients in balance of plant (BOP) and non-nuclear systems.


The RELAP5TM code has been developed for best estimate transient simulation of light water reactor coolant systems during postulated accidents. The code models the coupled behavior of the reactor coolant system and the core for loss-of-coolant accidents, and operational transients, such as anticipated transient without scram, loss of offsite power, loss of feedwater, and loss of flow. A generic modeling approach is used that permits simulating a variety of thermal hydraulic systems. Control system and secondary system components are included to permit modeling of plant controls, turbines, condensers, and secondary feedwater systems.

BWXT has used RELAP5TM for a range of analyses including:

  • Analysis of internal loads on steam generator components during a LOCA event
  • Analysis of internal loads on steam generator components during main steam line and main feedwater line breaks
  • Transient analysis of steam generator water level during a turbine trip
  • Analysis of natural recirculation in the tube side of a multi-pass vertical heat exchanger during standby

RELAP5TM is a trademark of Innovative Systems Software.

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