Design Basis vs. Design Requirements cont'd

DOESTD107393

generated by the engineering organization and approved for construction; from an operator's viewpoint,

design requirements might be a combination of these other views.

Because the DOE operational CM program focuses on the operational life-cycle phase, it is oriented

more toward the operating organizations (which includes maintenance and testing) than the facility

design or construction organizations. From this context, a DOE operational CM program defines design

requirements as the output requirements the design organization has placed on the facility

configuration. In contrast, incoming requirements to the design organization are design inputs.

Historically, changes were made to the facility hardware at some commercial nuclear facilities without a

full recognization that the design was being changed. Heightened industry awareness led to an

appreciation that hardware changes were, in fact, design changes that needed careful evaluation and

prior approval. Everything involved with the design of the physical hardware was considered to be the

basis for the physical design, including the functional requirements, the physical characteristics, and the

required performance capabilities, as well as the underlying calculations and analyses. Many nuclear

utilities developed summary design documents, which they called Design Basis Documents, describing

both what is required and why it is required. The term "design basis" was used to support the central

concepts that physical changes are design changes and that an appreciation for the basis of the

physical design is necessary. However, this usage of the term design basis lacked accuracy.

As commercial nuclear and DOE experience accumulates, it has become apparent that the physical

design does not actually conform with the design basis. Rather, the physical design matches the

design output requirements. The design basis, in turn, provides the information demonstrating that the

design output requirements are appropriate. To take advantage of this improved understanding, this

Standard adopts refined definitions for design requirements and design basis. In simple terms, the

design requirements specify what is required and the design basis explains why it is required. The total

set of design requirements and design basis is referred to as the design information.

The following example illustrates the difference between design requirements and design basis. A

design has been requested for a safety heat removal system. The design inputs might state that the

system needs to provide at least 165 gpm of light water with a temperature no greater than 80 F into a

system that is pressurized at 125 psig. The origin for this design input might have been a worst-case

accident heat removal calculation. The pump sizing calculation might be performed according to a

defined engineering procedure for pump sizing. From these design inputs, constraints, analysis and

calculations, the design outputs might specify a centrifugal pump with a 200 gpm rating at 150 psig.

For this example, the requirement for 165 gpm at 80 F into 125 psig is a design input. However, the

pump design requirement specifies a 200 gpm pump at 150 psig. The difference might have come out

of calculational uncertainties or specified design constraints, or it might be a reflection of the

conservative margin employed by the designer. Regardless, the design requirement is 200 gpm at 150

psig. In addition, the nameplate rating of the pump purchased to meet this requirement might exceed

the design output requirement, thus providing an additional margin. In a different case, the design

output might be identical to the design input. The design process, through its constraints and analysis

and calculations, does not always result in a change of the original design input.

As a matter of good practice, design requirements are best specified as the minimum acceptable value,

without excessive built-in design margin. For example, a design analysis determines that a 200 gpm

pump is needed to meet system requirements, but the vendor can only supply a 250 gpm pump. The

pump design requirement is best established at 200 gpm because this is the value the designer

determined is needed to satisfy the system design requirements. The procurement specification might

state the 200 gpm design requirement and request the 250 gpm pump that is available to satisfy it. The

200 gpm value is preferred as the specified design requirement because it tends to minimize

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