Method II: Segment-by-Segment Categorization

DOE-STD-1021-93

component design is evaluated to be inadequate for the preliminary hazard level, or if

the preliminary hazard level leads to an expensive design or requires a design change,

a less conservative categorization method (Method II or III, see below) should be used.

(ii)

When available data/information from system safety and accident analyses or from

other sources are not adequate for Methods II and III, and generating additional data is

not cost-effective.

(iii)

When system configuration is such that designing its constituent components to

unequal NPH levels is not cost-effective, or can cause significant uncertainty in the

functional integrity of a higher category component. For example, consider a length of

piping that does not perform any safety functions, but is attached to a PC-4 tank or

pressure vessel (say, separated by an isolation valve). If the configuration of its

interface (penetration or attachment) with the vessel is such that the failure of the pipe

increases the uncertainty in the functional integrity (say, leak-tightness) of the vessel, it

may be more cost-effective to design this piping system up to the isolation valve or

possibly to the first anchor point as a PC-4 system, than to demonstrate that the piping

failure would not result in the local failure of the vessel (or to evaluate system

interaction effects necessary to assign a lower performance category to the piping

system).

(b)

Method II: Segment-by-Segment Categorization

A segment of a system is defined here as a group of components (mechanical, electrical,

piping, structural, etc.) that perform one of the safety functions listed in Table 3-1 (or any

other distinct subfunction). It may also be defined as a group of components belonging to a

system that are physically located in a certain area, room, or building, or whose functions are

such that it may be more cost-effective, or programmatically more desirable, to design or

procure them using the same NPH design criterion.

In Method II, all components belonging to a segment of an NPH-related safety system (i.e., a

system for which there is an entry in the safety system-function matrix, see Table 3-1) are

placed in the highest performance category of any component in that segment, as applicable,

according to Subsection 2.4 guidelines. The use of Method II is illustrated through the

following example.

System B in Table 3-1 has two segments, B2 and B3, that constitute the safety part of the

system (the components of the rest of System B are non-safety components). If categorization

Method II is used, all components of Segment B2 will be placed in one performance category

that corresponds to the safe shutdown or safe-state maintenance function. Similarly, B3

components will be placed in one performance category that corresponds to the nuclear

criticality maintenance function. Components common to both segments will be placed in the

higher category.

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