United States Nuclear Regulatory Commission - Protecting People and the Environment

Computer Codes

The U.S. Nuclear Regulatory Commission (NRC) uses computer codes to model and evaluate fuel behavior, reactor kinetics, thermal-hydraulic conditions, severe accident progression, time-dependent dose for design-basis accidents, emergency preparedness and response, health effects, and radionuclide transport, during various operating and postulated accident conditions. Results from applying the codes support decisionmaking for risk-informed activities, review of licensees' codes and performance of audit calculations, and resolution of other technical issues. Code development is directed toward improving the realism and reliability of code results and making the codes easier to use. For more information, see the following code categories on this page:

Further information about obtaining these computer codes can be found by following the "Obtaining the Codes" link (also on the left side of this webpage).

Probabilistic Risk Assessment Codes

  • SAPHIRE: Systems Analysis Programs for Hands-on Integrated Reliability (SAPHIRE) is used for performing probabilistic risk assessments.

Fuel Behavior Codes

Fuel behavior codes are used to evaluate fuel behavior under various reactor operating conditions:

  • FRAPCON-3 is a computer code used for steady-state and mild transient analysis of the behavior of a single fuel rod under near-normal reactor operating conditions.

  • FRAPTRAN is a computer code used for transient and design basis accident analysis of the behavior of a single fuel rod under off-normal reactor operation conditions.

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Reactor Kinetics Codes

Reactor kinetics are used to obtain reactor transient neutron flux distributions:

  • PARCS: The Purdue Advanced Reactor Core Simulator (PARCS) is a computer code that solves the time-dependent two-group neutron diffusion equation in three-dimensional Cartesian geometry using nodal methods to obtain the transient neutron flux distribution. The code may be used in the analysis of reactivity-initiated accidents in light-water reactors where spatial effects may be important. It may be run in the stand-alone mode or coupled to other NRC thermal-hydraulic codes such as RELAP5.

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Thermal-Hydraulics Codes

Advanced computing plays a critical role in the design, licensing and operation of nuclear power plants. The modern nuclear reactor system operates at a level of sophistication whereby human reasoning and simple theoretical models are simply not capable of bringing to light full understanding of a system's response to some proposed perturbation, and yet, there is an inherent need to acquire such understanding. Over the last 30 years or so, there has been a concerted effort on the part of the power utilities, the NRC, and foreign organizations to develop advanced computational tools for simulating reactor system thermal-hydraulic behavior during real and hypothetical transient scenarios. In particular, thermal hydraulics codes are used to analyze loss of coolant accidents (LOCAs) and system transients in light-water nuclear reactors. The lessons learned from simulations carried out with these tools help form the basis for decisions made concerning plant design, operation, and safety.

The NRC and other countries in the international nuclear community have agreed to exchange technical information on thermal-hydraulic safety issues related to reactor and plant systems. Under the terms of their agreements, the NRC provides these member countries the latest versions of its thermal-hydraulic systems analysis computer codes to help evaluate the safety of planned or operating plants in each member's country. To help ensure these analysis tools are of the highest quality and can be used with confidence, the international partners perform and document assessments of the codes for a wide range of applications, including identification of code improvements and error corrections.

The thermal-hydraulics codes developed by the NRC include the following:

  • TRACE: The TRAC/RELAP Advanced Computational Engine. A modernized thermal-hydraulics code designed to consolidate and extend the capabilities of NRC's 3 legacy safety codes - TRAC-P, TRAC-B and RELAP. It is able to analyze large/small break LOCAs and system transients in both pressurized- and boiling-water reactors (PWRs and BWRs). The capability exists to model thermal hydraulic phenomena in both one-dimensional (1-D) and three-dimensional (3-D) space. This is the NRC's flagship thermal-hydraulics analysis tool.

  • SNAP: The Symbolic Nuclear Analysis Package is a graphical user interface with pre-processor and post-processor capabilities, which assists users in developing TRACE and RELAP5 input decks and running the codes.

  • RELAP5: The Reactor Excursion and Leak Analysis Program is a tool for analyzing small-break LOCAs and system transients in PWRs or BWRs. It has the capability to model thermal-hydraulic phenomena in 1-D volumes. While this code still enjoys widespread use in the nuclear community, active maintenance will be phased out in the next few years as usage of TRACE grows.

  • Legacy tools that are no longer actively supported include the following thermal-hydraulics codes:
    • TRAC-P: Large-break LOCA and system transient analysis tool for PWRs. Capability to model thermal hydraulic phenomena in 1-D or 3-D components.
    • TRAC-B: Large- and small-break LOCA and system transient analysis tool for BWRs. Capability to model thermal hydraulic phenomena in 1-D or 3-D components.
    • CONTAIN: Containment transient analysis tool for PWRs or BWRs. Capability to model thermal hydraulic phenomena (within a lumped-parameter framework) for existing containment designs.

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Severe Accident Codes

Severe accident codes are used to model the progression of accidents in light-water reactor nuclear power plants:

  • MELCOR: Integral Severe Accident Analysis Code: Fast-Running, parametric models.

  • MACCS2: Accident Consequence Analysis Code: The computer code used to calculate dispersion of radioactive material to the environment and the population. The MACCS2 code uses a dose-response model to determine the health consequences of a severe accident in terms of early fatalities (how many people in a population would die in the weeks or months following exposure) and latent cancer risk (how many people in a population would contract a fatal cancer as a result of exposure). MACCS2 originated as an acronym for the MELCOR Accident Consequence Code System, but is now commonly known simply as the MACCS2 Accident Consequence Analysis Code.

  • SCDAP/RELAP5: Integral Severe Accident Analysis Code: Uses detailed mechanistic models.

  • CONTAIN: Integral Containment Analysis Code: uses detailed mechanistic models. (CONTAIN severe accident model development was terminated in the mid-1990s.) The MELCOR code has similar containment capabilities (but less detailed in some areas) and should generally be used instead of CONTAIN.

  • IFCI: Integral Fuel-Coolant Interactions Code.

  • VICTORIA: Radionuclide Transport and Decommissioning Codes: Radionuclide transport and decommissioning codes provide dose analyses in support of license termination and decommissioning.

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Design-Basis Accident (DBA) Codes

DBA codes are used to determine the time-dependent dose at a specified location for a given accident scenario:

  • RADTRAD: A simplified model for RADionuclide Transport and Removal And Dose Estimation. The RADTRAD code uses a combination of tables and numerical models of source term reduction phenomena to determine the time-dependent dose at specified locations for a given accident scenario. The RADTRAD code can be used to assess occupational radiation exposures, typically in the control room; to estimate site boundary doses; and to estimate dose attenuation due to modification of a facility or accident sequence. RADTRAD 3.03 is available from the Radiation Safety Information Computational Center (RSICC)

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Emergency Preparedness & Response (EPR)

CodesEPR codes compute power reactor source terms, airborne transport of activity, and the resulting doses to allow easy comparison to EPA protective action guidelines:

  • RASCAL: Radiological Assessment Systems for Consequence AnaLysis. The RASCAL code evaluates releases from nuclear power plants, spent fuel storage pools and casks, fuel cycle facilities, and radioactive material handling facilities and is designed for use by the NRC in the independent assessment of dose projections during response to radiological emergencies. Obtain the latest information on RASCAL here

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Health Effects/Dose Calculation Codes

Health effects/dose calculation codes are used to model and assess the health implications of radioactive exposure and contamination.

  • VARSKIN: The VARSKIN computer code is used by NRC, Agreement States, licensees, vendors and International Partners to calculate radiation dose to the skin resulting from exposure to radiation emitted from hot particles or other contamination on or near the skin. These dose assessments are required by 10 CFR 20.1201(c) in which the assigned shallow dose equivalent is to the part of the body receiving the highest exposure over a contiguous 10 cm2 of skin at a tissue depth of 0.007 centimeters (7 mg/cm2).

    Five different predefined source configurations are available in VARSKIN that allow simulations of point, disk, cylinder, sphere, and slab sources on the skin. Recent improvements to earlier VARSKIN versions include an enhanced photon dosimetry model, as well as models to account for air gap and cover materials for photon dosimetry. Additionally, VARSKIN has been updated to better predict beta dosimetry in shallow skin targets. Although the user can choose any dose-averaging area, the default area for skin dose calculations in VARSKIN is 10 square centimeters, to conform to regulatory requirements pursuant to Title 10 of the Code of Federal Regulations, Section 20.1201(c). Data entry is condensed to a single screen, a variety of unit options are provided (including both British and International System (SI) units), and the source strength can be entered in units of total activity or distributed in units of activity per unit area or activity per unit volume. The output page and the user's ability to add radionuclides to the library are greatly simplified. A library file contains data on gamma rays, X rays, beta particles, internal conversion electrons, and Auger electrons. VARSKIN allows the user to build a customized library of exposure radionuclides. Finally, an extensive, context-sensitive help file is made available for VARSKIN to provide guidance and to offer new users a tutorial in the use of the skin dosimetry software.

    The enhanced photon model accounts for photon attenuation, charged particle buildup, and electron scatter at all depths in skin. The model allows for volumetric sources and clothing/air gaps between source and skin. The beta dosimetry model has been upgraded to better account for beta energy loss and particle scatter. Dose point kernels are now Monte Carlo based and the code agrees very well with the EGSnrc Monte Carlo code.

    See directions on how to obtain the latest VARSKIN code.

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Radionuclide Transport Codes (for License Termination and Decommissioning)

Radionuclide transport and decommissioning codes provide dose analyses in support of license termination and decommissioning:

  • DandD: A code for screening analyses for license termination and decommissioning. The DandD software automates the definition and development of the scenarios, exposure pathways, models, mathematical formulations, assumptions, and justifications of parameter selections documented in Volumes 1 and 3 of NUREG/CR-5512.

  • Probabilistic RESRAD 6.0 and RESRAD-BUILD 3.0 Codes: The existing deterministic RESRAD 6.0 and RESRAD-BUILD 3.0 codes for site-specific modeling applications were adapted by Argonne National Laboratory (ANL) for NRC regulatory applications for probabilistic dose analysis to demonstrate compliance with the NRC's license termination rule (10 CFR Part 20, Subpart E) according to the guidance developed for the Standard Review Plan (SRP) for Decommissioning. (The deterministic RESRAD and RESRAD-BUILD codes are part of the family of codes developed by the U.S. Department of Energy. The RESRAD code applies to the cleanup of sites and the RESRAD-BUILD code applies to the cleanup of buildings and structures.)

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Page Last Reviewed/Updated Thursday, January 29, 2015