Safety-Class Structures, Systems, and Components

The design of safety class structures, systems and components (SSCs) shall provide defense-in-depth features against the uncontrolled release of radioactive materials to the environment under normal conditions, AOEs, and DBA conditions. 

Quality Standards. Safety class structures, systems, and components shall be designed, fabricated, erected, and tested to quality standards commensurate with the importance of the safety functions to be performed. 

Fire Protection. The probability and effect of fires, explosions, and related perils at DOE facilities shall be minimized. Safety class structures, systems, and components shall be designed and located to minimize, consistent with other safety requirements, the probability and effect of fires and exp I os i ons. Noncombust i bIe and heat res i stant mater i a Is sha11 be used whenever practical throughout the unit, particularly in areas vital to the control of hazardous materials and maintenance of safety functions. Eire detection and fighting systems shall be designed and provided with sufficient capacity and capability to minimize the adverse effects of fires and explosion on safety class structures, systems, and components. 

Sharing of Structures. Systems, and Components. Safety class structures, systems, and components shall not be shared among nuclear facilities unless it can be shown that such sharing will not impair their ability to perform their safety functions, including, in the event of an accident in one nuclear facility, an orderly shutdown and cooldown of the remaining nuclear reactor fac i I i t i es. 

Support Systems. Support systems (e.g., electrical power, cooling) required to ensure that safety class structures, systems and components can provide their required safety function shall also be considered safety class systems. 

Safety class structures, systems, and components should be designed to withstand the effects of natural phenomena, without loss of capability to perform their safety functions. 

A system to provide heat removal capability for all safety class structures, systems, and components. This system should have the capability to remove the total heat-load during all normal conditions and AOEs and transfer this heat to the ultimate heat sink (described be Iow). 

Off-site consequences >25 rem from operational accidents must be protected with Safety Class Structures, Systems, and Components (SSCs) independent of frequency. Follow DOE-STD-3009 for manmade external and natural phenomenon events. 

Single Fai lure. Safety class SSCs shall be able to accommodate a single failure and still meet their intended safety function, as required, to ensure compliance with the facility acceptance criterion, A "single failure" means an occurrence which results in the loss of capability of a safety class structure, system or component to accomplish its required safety functions. Multiple failures resulting from a single occurrence are considered to be a single fai lure. 

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). 

This section presents a derivation of the HCF TSRs. Because of the hazard classification of the HCF (i.e., HC-2) and the results of the accident analysis, no safety-class structures, systems, or components (SSCs) have been identified. That is, there are no SSCs vt/hich are needed to maintain off-site consequences within the 25 rem off-site Evaluation Guideline. Therefore, the TSRs consist of Limiting Conditions for Operation, Surveillance Requirements, and Administrative Controls as shown in Table 5.3-1. 

The hazard classification of the facility dictates the QA levels that are assigned to the facility, structures, systems, and components (SSCs), based on the consequence of failure. The extent of requirements and depth of QA activity are commensurate with the scope, complexity, importance, and degree of risk of a SSC. Through the use of quality levels, the stringency of the requirements is applied in a graded approach to all SSCs within the facility. The most stringent requirements (Quality Level I) apply, by default, to all safety- class SSCs, which are specified in accordance with DOE Orders and Standards [i.e., DOE Order 5480.23 (DOE 1992) and DOE-STD-3009-94 (DOE 1994a). As described in Chapter 3, no safety class SSCs are identified for the HCF. 

From previous discussions it is seen that there are safety categories and classes as defined by lEC standards. lEC 61226 defines safety categories for nuclear safety 1. C functions (category A, B, C). lEC 61513 also defines three safety classes for 1. C systems (Class 1, Class 2, Class 3). A nonsafety class (NS) can be added to this classification. Based on the safety analysis, all safety I C subsystems will be assigned to a safety class on a case-by-case basis. Safety importance class (SIC) describes a classification scheme for structures, systems, and components that perform a safety function and contribute to the general safety objectives at ITER during incident/accident situations [12]. In the case of fusion technology, these classifications are termed SICl, 2, and SR. The relationship between these is shown in Table XII/3.3.M. 

A listing of safety categories and classes associated with normal operation and DB fault conditions for all plant SSCs are presented in the APIOOO UK Safety Categorisation and Classification of Structures, Systems and Components report (Reference 4.2). 

A design class for structures, systems and components can be defined as the level of safety margin (i.e. the inverse of the reliability to perform the assigned safety function) that can be used in the design/qualification of a structure, system or component. It can be evaluated according to the external event probabihty of exceedance and the performance goal associated with the structure, system or component. 

For hazard category 1 and 2 research reactors, the design basis wind can be evaluated on the basis of the selected probability of exceedance for the external event hazard according to the performance goal assigned to their safety class 1 and 2 structures, systems and components. For more sophisticated investigations and analysis, further guidance is provided in Ref. [22],... 

In some Member States there is a need to evaluate the adequacy of the design of structures, systems and components of safety classes 1 to 3 in hazard category 1 to 3 facilities. This is usually done by installing physical barriers to vehicle intrusion, and providing sufficient stand-off distance to ensure the required performance. Reference [28] provides practical examples of this. 

Due to the frequent subcontracting of the different tasks to different contractors, a unique project, plant or facility-specific QA system is put in place for hazard category 1 and hazard category 2 facUity projects, as well as for hazard category 3 facilities if such facilities contain structures, systems and components that are of a safety class higher than 3. The requirements set forth in Refs [7,48] may be used for the QA system for a research reactor. 

Safety Class SSCs - Systems, Structures or Components including primary 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. 

Modifications. Modifications to Safety class systems, structures, and components in existing facilities, or changes to the operations, to comply with the requirements of this Order shall be considered in accordance with Attachment 1, paragraph 4f(9) of DOE 5480.23. This provision in DOE 5480.23 will be supplemented by using the procedures provided in DOE N 5480.5, IMPOSITION OE PROPOSED NUCEEAR SAEETY REQUIREMENTS, This process should ensure cost effective modifications that maximize the safety benefit while avoiding unnecessary or unproductive expense in retrofitting existing fac i I i t i es. 

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