Safety-Significant Structures, Systems, and Components

DOE Order 420.1, Facility Safety, requires the detailed application of that order s requirements to be guided by safety analyses that establish the identification and functions of safety (safety class and safety significant) structures, systems, and components (SSCs) for a facility and establish the significance of safety functions performed by those SSCs. It specifies that nuclear facilities shall be designed with the objective of providing multiple layers of protection to prevent or mitigate the unintended release of radioactive materials to the environment. The safety analyses must consider facility hazards, natural phenomena hazards, and external man-induced hazards. Paragraph 4.4.1 requires safety analyses for hazardous facilities to include the ability of SSCs and personnel to perform their intended safety functions under the effects of natural phenomena. DOE O 420.1 (DOE 1995) incorporates requirements from the cancelled DOE Orders 5480.28, 5480.7A, and 6430.1A(DOE 1993). 

Firefighting systems shall be designed to ensure that their rupture or operation does not significantly impair the safety capability of these structures, systems, and components. 

Criterion 3 - Fire protection. Structures, systems, and components important to safety shall be designed and located to minimize, consistent with other safety requirements, the probability and effect of fires and explosions. Noncombustible and heat resistant materials shall be used wherever practical throughout the unit, particularly in locations such as the containment and control room. Fire detection and fighting systems of appropriate capacity and capability shall be provided and designed to minimize the adverse effects of fires on structures, systems, and components important to safety. Firefighting systems shall be designed to assure that their rupture or inadvertent operation does not significantly impair the safety capability of these structures, systems, and components. 

Criterion 5 - Sharing of structures, systems, and components. Structures, systems, and components important to safety shall not be shared among nuclear power units unless it can be shown that such sharing will not significantly impair their ability to perform their safety functions, including, in the event of an accident in one unit, an orderly shutdown and cooldown of the remaining units. 

The MR also requires that before performing maintenance activities the licensee shall assess and manage the increase in risk that may result from the proposed maintenance activities. The scope of the assessment may be limited to those structures, systems, and components that a risk-informed evaluation process has shown to be significant to public health and safety. 

In this context, PSA shall also supplement deterministic safety demonstrations to assess the safety significance of modifications of Structures, Systems and Components (SSC), measures or procedures as well as of findings that have become known from safety-relevant events or phenomena that have occurred and which can be applied to the nuclear power plants in Germany that are referred to in the scope of application of (BMU 2013a), for which a significant influence on the results of PSA can be expected. 

Fire fighting systems shall be automatically initiated where necessary, and systems shall be designed and located so as to ensure that their rapture or spurious or inadvertent operation does not significantly impair the capability of structures, systems and components important to safety, and does not simultaneously affect redundant safety groups, thereby rendering ineffective the measures taken to comply with the single failure criterion. 

The occurrence of external events significant to plant safety should be documented and reported. An extensive plant inspection after the occurrence of an external event either close to the design basis external event or significant to plant safety should be carried out to assess the behaviour and consequences on structures, systems and components against their safety classification, accessibility and representativeness for all items of the external event category. 

It is required (Ref. [1], para. 5.1) that All structures, systems and components, including software for instrumentation and control (I C), that are items important to safety shall be first identified and then classified on the basis of their function and significance with regard to safety. They shall be... 

The functions and safety significance of at least those structures, systems and components (SSCs) in the RCSASs performing the following safety functions should be classified ... 

The method for classifying the safety significance of a structure, system or component shall primarily be based on deterministic methods, complemented where appropriate by probabilistic methods and engineering judgement, with account taken of factors such as ... 

In accordance with DOE-STD-3009-94, (DOE 1994) safety SSCs are divided Into two categories (1) safety-class and (2) safety-significant. DOE-STD-3009-94 defines safety-class SSCs (SCSSCs) as those SSCs, including environmental monitors and portions of process systems, whose failure could adversely affect the environment or safety and health of the public as identified by safety analysis. The phrase adversely affect refers to exceeding offsite EGs (i.e., a whole-body dose of 25 rem to the nearest located member of the public). SCSSCs are systems, structures, or components whose preventive or mitigative function is necessary to keep hazardous material exposure to the public below the EGs. 

Additionally, components, structures and systems need to be classified on the basis of their safety significance and to be designed, manufactured and installed to a level of quality conunensurate with that classification. 

The structures, and components are classified according to their safety significance using the classification system in Reg. Guide 1.26 and ANSI/ANS 51.1. 

The fully developed fire is affected by (a) the size and shape of the enclosure, (b) the amount, distribution and type of fuel in the enclosure, (c) the amount, distribution and form of ventilation of the enclosure and (d) the form and type of construction materials comprising the roof (or ceiling), walls and floor of the enclosure. The significance of each phase of an enclosure fire depends on the fire safety system component under consideration. For components such as detectors or sprinklers, the fire development phase will have a great influence on the time at which they activate. The fully developed fire and its decay phase are significant for the integrity of the structural elements. 

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