This standard applies to the safety design process for plants. This standard provides a process for establishing top-level safety criteria (TLSC); safety functions; top-level design criteria (TLDC); licensing-basis events (LBEs); design-basis accidents(DBAs); safety classification of systems, structures, and components (SSCs); safety analyses; defense-in-depth(DID); and adequate assurance of special treatment requirements for safety-related SSCs throughout the operating life of the plant. This standard does not provide detailed guidance for design; other existing standards cover that.

Plants maintain radioactive releases within public health and safety requirements by passive SSCs and0or inherent characteristics by design. They rely on intrinsic physical characteristics and specific design requirements to do this. While it is beyond the scope of this standard to explain how, the following description of MHR design characteristics provides the basis for this attribute claim. Plants have one or more standard helium-cooled reactor modules, where each module has the following fundamental design characteristics:

• helium primary coolant; graphite moderator;

• high integrity, ceramic coated particle nuclear fuel enclosed in a graphite matrix;

• passive decay heat removal capability under extreme undercooling accident conditions;

• defined core geometry that ensures adequate core cooling maintained under all conditions;

• core contained within a metallic reactor pressure vessel (RPV);

• RPV contained within a robust building structure;

• negative temperature coefficient of reactivity.

MHR modules consist of standard nuclear reactor configurations coupled to a direct or indirect power conversion system and0or a process heat utilization system. MHR modules allow replication to produce the required plant output. An MHR direct power conversion system employs a gas turbine in the primary coolant system to convert the thermal energy carried in the primary coolant into electrical energy. An MHR indirect cycle employs an intermediate heat exchanger to transfer thermal energy from the primary coolant to a secondary coolant0 working fluid used for power generation. An MHR process heat utilization system employs the thermal energy produced in the reactor core as the energy input to a process for production, such as hydrogen. An MHR module providing process heat may be configured to also co-generate electricity using either a direct or an indirect cycle.

Purpose

The purpose of this standard¹⁾is to define the process for specifying criteria to assure that modular helium-cooled reactor (MHR) nuclear power plants²⁾ are designed so that they can be constructed and operated safely without undue risk to public health and safety. This purpose is achieved through the identification of applicable safety requirements from the national nuclear regulator, industrial codes and standards, and other published guidance and professional engineering practices. The MHR plant designer³⁾ is responsible for conformance to the criteria defined in this standard and supporting the design bases and expected operational characteristics by design analyses, experimental models, conformance with applicable standards, and comparisons with accepted designs or experience gained from similar designs.

The designer may consider additional or alternate requirements to accommodate specific site0 design characteristics not covered (or referenced) by this standard or related document guidance. For licensing in the United States, this standard presumes that license applicants will meet all applicable U.S. Nuclear Regulatory Commission (NRC) licensing requirements.

¹⁾ The current standard, ANSI0ANS-53.1-2011, will hereinafter be referred to as "this standard."

²⁾ Modular helium-cooled reactor nuclear power plant(s) will hereinafter be referred to as "plant(s)."

³⁾ MHR plant designer(s) will hereinafter be referred to as "designer(s)."