Document: NUREG-0800
Document ID: 643ab154-682e-480f-a57b-a5fd7390a34e
Document Type: srp
Title: REVIEW OF TRANSIENT AND ACCIDENT ANALYSIS METHODS
Source: NUREG-0800
Source URL: https://www.nrc.gov/docs/ML0535/ML053550265.pdf
Revision Date: 2023-06
Chapter: 15
Section ID: 15.0.2
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CFR Title: 

Content:
ill have a set of governing physical phenomena that drive the results of the calculation. The key physical phenomena, including constitutive equations needed for model closure, must be defined for the calculation being performed by the code. Physical phenomena that are important for one accident scenario may not be important for a different accident scenario. The key physical phenomena can also be specific to a particular plant design. The mathematical equations that comprise an evaluation model can be characterized as being either a field equation or as a constitutive or closure relationship. Field equations are a set of rigorously derived equations that contain no approximations other than the initial assumptions used in deriving the equations. The range of applicability of the field equations is limited only by the validity of the assumptions used in their derivation. An example of a set of field equations is the set of fluid transport equations for mass, momentum, and energy that are derived from macroscopic balances of these quantities. Although these equations are mathematically exact, the equations contain more unknown quantities than there are equations. Some of these unknown quantities are the equation of state, the stress tensor, and the heat flux. In order to be able to solve the field equations the unknown quantities must be expressed in terms of the known quantities from the field equations. The equations that relate the unknown quantities to the known quantities are constitutive or closure relationships. These equations are often models or approximations that are much more restrictive in their range of validity than the set of field equations that they are used with. For example, using Newton’s law of cooling as a closure relationship for heat flux 15.0.2-9 Initial Issuance - December 2005 from a heat structure to a fluid will limit the application to model situations where radiation heat transfer is not significant even though the field equations