scope:

This standard¹⁾ establishes criteria for the selection and use of meteorological data and identifies supporting hydrologic information to determine whether the design water temperature and cooling capacity requirements for the Ultimate Heat Sink (UHS) at a nuclear generating facility are adequately established. This standard also describes atmospheric effects that need to be considered when designing UHSs for safety-related systems at nuclear power plants such that cooling capacity requirements are met during a licensing-basis event (LBE).

• This standard applies to the design of the UHS cooling system at existing plants that utilize external water or air sources and includes both open-cycle (e.g., rivers, lakes, oceans, atmosphere) and closed-cycle (e.g., cooling ponds, spray ponds, cooling towers-wet or dry) UHS systems. It also applies to new nuclear plants that rely on similar external UHS cooling systems.

• This standard applies to cooling systems that are part of the UHS complex that reject heat to external water or air resources and involve meteorological elements in the design and operation of a UHS.

• Accident scenarios and assumptions for UHS designs that are not directly related to design meteorological input variables are not within the scope of this standard (e.g., downstream dam failure).

• This standard does not apply to natural draft cooling towers or to reactor designs that use passive cooling systems.

The working group has incorporated risk-informed, performance-based (RIPB) recommendations into this standard. The recommendations are restricted to the assessment of the probability of peak return-water temperatures or 30-day water losses, rather than a single deterministic result that maximizes the most adverse outcome. The RIPB concepts apply to the analytical approaches used to evaluate UHS performance but not to the meteorological data input to such analyses.

The remaining conditions and input parameters to the UHS analysis, such as event combinations, plant heat loads, flow rates, and degradation states of the UHS following an LBE, are beyond the scope of this standard and remain subject (at least for commercial nuclear power generating units) to the guidance of U.S. Nuclear Regulatory Commission (NRC) Regulatory Guide 1.27 [1].²⁾ This standard does not address the coupling of safety systems in which anticipated UHS temperatures are used to analyze systems that subsequently affect the estimated UHS heat load (e.g., pressure increases in the containment at the beginning of a Loss- Of-Coolant Accident).

¹⁾ The current standard, ANSI/ANS-2.21-2022, is hereinafter referred to as "this standard."

²⁾ Numbers in brackets refer to corresponding numbers in Sec. 9, "References."