Zachry Nuclear Engineering
RADTRAD-NAI™ is a versatile code for radionuclide transport, removal, and dose estimation. The code calculates transport of radioactive isotopes between compartments while accounting for removal mechanisms such as filters, pipe deposition, natural deposition, spray removal, and pool decontamination factors. RADTRAD-NAI uses these calculations to determine the dose to personnel based on occupancy, breathing rate, and atmospheric dispersion.

RADTRAD-NAI is Zachry's version of RADionuclide Transport, Removal, And Dose (RADTRAD), a computer code developed by the NRC for radiological dose analysis. The code is suitable for dose analysis under the new Alternative Source Term (AST) Guidelines (NUREG-1465) as well as the TID-14844 Guidelines.
Simplified RADTRAD-NAI Compartment Model
The RADTRAD-NAI code is designed to estimate radiological doses at offsite locations and in the control room of a nuclear power plant as consequences of certain postulated accidents. The potential radiological consequences of a postulated nuclear power reactor accident depend on the timing, quantity, physical form (i.e., vapor or aerosol) and chemical species of the radioactive material released into the environment. Another important application of RADTRAD-NAI is the estimation of radiation exposure as part of Environmental Qualification of equipment. In addition, RADTRAD-NAI may be used to estimate the effect of facility modifications and alternative accident management strategies on release of predicted source terms to the environment. These estimates may be used to support probabilistic safety assessment (PSA) and licensing studies.

RADTRAD-NAI is designed to model two types of releases from the reactor coolant system into the containment. Instantaneous releases are specified in Calculation of Distance Factors for Power and Test Reactor Sites, TID-14844 and in Regulatory Guides 1.3 and 1.4. More protracted and realistic source terms are specified for boiling water reactors (BWRs) and pressurized water reactors (PWRs) in Accident Source Terms for Light-Water Nuclear Power Plants, NUREG-1465. These instantaneous and protracted releases are represented through default input files that are included for convenience with RADTRAD-NAI. In practice, plant-specific source term data is required. Such data can be implemented by making consistent modifications to a set of the source term files.

RADTRAD-NAI includes models for a variety of processes that can attenuate and/or transport radionuclides. It can model sprays, filtered flow and natural deposition that reduce the quantity of radionuclides suspended in the containment or other compartments. It can model the flow of radionuclides between compartments within a building, from compartments within buildings into the environment compartment and from the environment compartment into a control room compartment. Flows between compartments can be through piping, filters, a suppression pool or convection pathways. For piping, filters and suppression pool pathways, models in RADTRAD-NAI can be used to account for the removal of radionuclides from the flow between compartments. The convection pathway provides the ability to model flow between compartments without removal of radionuclides.

Other features in RADTRAD-NAI are available to model various influences on the distribution of radionuclides. Among these are radioactive decay and in-growth of daughters, and moderation of the source term by radionuclide removal in an overlying pool during the ex-vessel phase of an event.

Given an input description of the power plant under consideration, a description of the source term and a description of the removal mechanisms, RADTRAD-NAI calculates the transient distribution of radionuclides throughout the model. RADTRAD-NAI can then use the radionuclide concentrations in conjunction with user-defined atmospheric dispersion factor (X/Q) data, breathing rates, and occupancy factors to calculate estimated dose at user-selected locations. All dose is due to either inhalation or submersion in a contaminated cloud; there is no direct shine or streaming. Also, RADTRAD-NAI does not perform shielding calculations.