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New ANS VCS.

Portion of 1992 ANS-2.8 VCS that addressed 

all potential flooding insults to nuclear power 

plant sites.

Reason for separating ANS-2.31 from 

revision to ANS-2.8: Make it applicable to all 

sites, even “dry sites” where facility grade is 

well above any external flood level.

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Revision 1 comments were resolved at 6 

December 12 WG meeting in Las Vegas 

leading to Revision 2. 

Revision 3 under preparation to resolve 

outstanding WG meeting items and additional 

NRC comments on Revision 1 received after 

WG meeting.

Targeting NFSC SC-25 review this Spring.

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 ANS-2.31 based on probabilistically-defined 

on-site precipitation.

 Mean return period in years or mean 

probability of exceedence/year. 

 Ground level snow load and rain or 

combination of snow followed by rain within 

a 48-hr period. 

 48-hr period selected as longest period 

before mitigation action should be taken.

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Two different approaches are defined:

 REGIONAL APPROACH

 LOCAL APPROACH

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 Snow loads: Use existing CONUS maps for 

50-yr mean return period where ground snow 

load defined (ASCE 7-10(1) Fig. 7.1). 

 Rain loads: Use existing CONUS maps for 50-

yr mean return period preceded by 48-hr 

antecedent rain (Technical Paper 49 Fig. 16 

US Dept of Commercial Weather Bureau(2))

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1. ASCE 7-10, “Minimum Design Loads for 

Buildings and Other Structures,” American 

Society of Civil Engineers, 2010.

2. Technical Paper No. 49, US Dept of 

Commerce Weather Bureau, “Two to 10 day 

Precipitation for Return Periods of 2 to 100 

Years in the Contiguous U.S.,” 1964.

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Need: Extend 50-yr mean return period (or 

mean 2x10-2/yr probability of exceedence) to 

100,000-yr mean return period (or mean 10-

5/year probability of exceedence) for a range 

of return periods

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 Approach: Use NOAA Atlas 14, Vol. 2 Appendix 9 

500-yr and 1,000-yr mean return period 

predictions and increase parameters for 48-hr 

periods of precipitation for 86 homogenous 

regions of Central and Eastern US. 

 Data sampled to develop a multiplication factor 

as a function of doubling of return period. 

 Applying multiplication factors: ANS-2.31 Table 

5-1 presents a 0.22 increase for each return 

period doubling from 50-400 yr and a 0.16 

increase from 400-100,000 yr.

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Issue: There are geographical areas of US 

where regional ground snow load contours do 

not exist where local precipitation 

measurement station data can be used. 

Resolution: NOAA Atlas 14, Vol. 2 Appendix 

A-6 identifies such measuring stations and 

their locations for Central and Eastern US. 

This data is assembled into 86 homogenous 

regions as shown in NOAA Atlas 14, Vol. 2 

Appendix A-9.

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Table 7-1:

Return Period (years) for Design Basis Flooding Caused by Precipitation - from 

DOE Std. 1020-2012 Categories PDC-1 and PDC-2 are based on the ASCE 7-10 

Standard Requirements.

SSC Category PDC-3 PDC-4 PDC-5

Return Period (Years) 10,000 25,000 *

Table 7-2:

Return Period (Years) for Design Precipitation Structural Loads – from DOE Std. 

1020-2012

SSC Category PDC-3 PDC-4 PDC-5

Return Period (Years) 2,500 6,250 *

*Use return periods the same as those used by the NRC for commercial nuclear power 

plant design.
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Table 5-1 of Draft ANS 2.31

Multiplication Factor to be Applied to Design Basis Ground Precipitation Load(2)

as a Function of Mean Return Period

Mean Return Period

(Years)

Multiplication

Factor(1)

50 1.0

100 1.22

200 1.44

400 1.66

800 1.82

1600 1.98

3200 2.14

6400 2.30

12800 2.46

25600 2.62

51200 2.78

102400 2.94

(1) Multiplication factors to be applied to Figures 5.1 and 5.2 50-year mean return period 

snow or rain loads.

(2) The factor of 0.22 increase in load for every doubling of the return period up to the 

400 years based on ASCE 7-10 Standard. Slope of the hazard thereafter is increased

to a factor of 0.16 based on the Appendix 9 data from NOAA Atlas 14, Vol. 2.
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Fig.5.1
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Fig. 5.2 Rain Loading