1The external zone is the area immediately surrounding a proposed site area in which population distribution and density, and land and water uses, are considered with regard to their effects on the possible implementation of emergency measures. This is the area that would be the emergency zone if the facility were in place.

2For sites at which nuclear installations of different types are collocated, particular consideration should be given to using a graded approach.

3Other meteorological phenomena which are not addressed in this Safety Guide may require consideration on a site specific basis (e.g. salt spray from seawater wind flows).

4For a description of a waterspout, see paras 4.59–4.61.

5In some States, formal elicitations are conducted to evaluate the significance of uncertainties in modelling and data uncertainties.

6A time series in this context is a chronological tabulation of values of a given variable measured continuously or at stated time intervals.

7A common assumption in many time series techniques is that the data are stationary. A stationary process is a stochastic process whose joint probability distribution does not change when shifted in time or space. Such a process has the property that parameters such as the mean and variance do not change over time or position. This stationary property means, in general terms, that there is a flat looking time series, without a trend, with constant variance over time, a constant autocorrelation structure over time and no periodic fluctuations.

8For the purpose of obtaining information on the temporal distributions of different input variables, the characterization of all input parameters as random processes, with given autocorrelation and cross-correlation functions, would be desirable. However, simplified approaches may assist in establishing adequate load combination criteria.

9For instance, for an annual frequency of exceedance of a hazard of 10^–2, typically adopted to determine the design parameters in meteorology, the minimum period of continuous observation should be at least 30 years, since the hazard cannot be estimated with sufficient accuracy for values more than three to four times the length of the sample period. Moreover, for characterizing climate variability, a reference called ‘climate normal’ is used by the WMO, which considers that 30 years is long enough to eliminate year to year variations for the purpose of obtaining an accurate average value and assessing its variability.

10Dry bulb temperature refers to the ambient air temperature.

11Wet bulb temperature, dew point temperature and relative humidity are indicators of atmospheric moisture. Wet bulb temperature refers to the lowest temperature that can be obtained by evaporating water into the air. The dew point is the temperature to which air must be cooled in order to reach saturation, assuming air pressure and moisture content are constant. Relative humidity is the ratio of the amount of atmospheric moisture present relative to the amount that would be present if the air were saturated.

12Note that for short averaging periods very intense precipitation can occasionally be observed from certain cloud cell systems, which would be smoothed out if a 24 h averaging period were used. This may be the case in particular in areas where there is extreme rainfall because of the orographic conditions.

13Some States have issued their own guidance and criteria for on-site meteorological monitoring programmes at nuclear power plant sites.

14A hydrological model can be used to determine hydrological data for a site using available data for another site.

15Hourly wet bulb temperatures can be calculated from concurrent measurements of dry bulb temperatures, dew point temperature (or relative humidity) and air pressure.

16For example, 1.0% and 2.0% values that are exceeded on average for 88 and 175 hours per year for the period of record analysed are typical design conditions. Likewise, 98% and 99% values are cold weather parameters for which the corresponding weather element is lower than the design condition for 175 and 88 hours.

17Depending on sources and on national practice or convention, extended pressure systems may also be designated as ‘extra-tropical’ storms, ‘extra-tropical’ depressions or ‘extra-tropical’ cyclones.

18In some States, extreme precipitation values are defined through the use of existing probable maximum precipitation characteristics that have been generated by the national meteorological service by means of a deterministic approach.

19Dust storms from the Sahara desert in North Africa are periodically observed in the European, North American and Caribbean regions.

20In relation to wind generated waves, the wind fetch is the maximum unobstructed distance that wind can travel over a water body in a constant direction.

21Use of the term ‘still water’ does not imply that the water is quiescent. Rather, the term is used to define the results of a hazard assessment before wind–wave or other hazard effects have been combined to produce the design basis parameter for the site (see Section 6).

22‘Deep water’ is water of a depth greater than L/2, where L is the wavelength of the surface wave under consideration.

23The ‘wave setup’ is the temporary buildup of water level at a beach due to breaking waves, which is to be added to the surge height.

24Use of the term ‘still water’ does not imply that the water is quiescent. Rather, the term is used to define the results of a hazard assessment before wind–wave or other hazard effects have been combined to produce the design basis parameter for the site (see Section 6).

25A ‘water body’ is a lake, river, estuary, sea or canal.

26‘Deep water’ is water of a depth greater than L/2, where L is the wavelength of the surface wave under consideration.

27‘Transition water’ is water of a depth less than L/2 but greater than L/25, where L is the wavelength of the surface wave under consideration.

28‘Shallow water’ is water of depth less than L/25, where L is the wavelength of the surface wave under consideration.

29The significant wave height Hs is the average height of the upper third of the wave heights in a wave record; the 1% wave height H1 is the average height of the upper 1% of the wave heights in a wave record. The approximation H₁ = 1.67 H_s is used in some States.

30The ‘wave setup’ is the temporary buildup of water level at a beach due to breaking waves, which is to be added to the surge height.

31‘Tsunami’ is a term in Japanese meaning a wave (‘nami’) in a harbour (‘tsu’).

32For meteorite induced tsunamis, assessments conducted to date do not demonstrate that the frequency of occurrence exceeds the screening level usually adopted.

33The current practice in some States is included in Annex II.

34In some States, formal elicitations are conducted to evaluate the significance of uncertainties in modelling and data uncertainties.

35In some States, probabilistic methods are used for hazard assessment for landslide induced tsunamis.

36A unit hydrograph is the runoff hydrograph that would result from a unit of rainfall uniformly distributed over the basin in a unit of time.

37For example, typical losses might be an initial loss of 10 mm, followed by a continuing loss of 2 mm per hour. It is often not worthwhile making detailed studies of losses as long as conservatively low estimates are selected. If, for example, the maximum hourly increment in the design basis precipitation is 150 mm, the effect of a loss of 2 mm per hour with such rainfall is insignificant compared with the errors inherent in the other parameters.

38In accordance with the practices of States, failures of these structures are considered either as internal events or as external events.

39Bulletin 111, published in 1998 by the International Commission on Large Dams, presents a review and recommendations on the analysis of dam break floods.

40Some parts of the installation (e.g. the pumping station for a nuclear power plant) could be more exposed to flooding and this would necessitate additional protective features.

41In most States method (a) is preferred to method (b) which includes the construction of permanent external barriers.

42The ‘swash zone’ is the zone of wave action on the beach, which moves as water levels vary, extending from the limit of rundown to the limit of runup.

43A cliff edge effect in a nuclear installation is an instance of severely abnormal system behaviour caused by an abrupt transition from one system status to another following a small deviation in a system parameter, and thus a sudden large variation in system conditions in response to a small variation in an input.

44For sites at which nuclear installations of different types are located, special consideration should be given to the use of a graded approach.

1The practice of the UN Secretariat is to use, in the absence of an internationally agreed standard, the most widespread and generally recognized denomination. This practice is without any prejudice to the position of any Member State of the United Nations on a particular appellation and does not imply the expression of any opinion whatsoever on the part of the Secretariat of the United Nations. The use of an appellation by the Secretariat based on the practice is without prejudice to any negotiations or agreements between the interested parties and should not be interpreted as advocating or endorsing any party’s position, and can in no way be invoked by any party in support of a particular position in the matter.

1References [II–4]–[II–15] are available either through the NRC ADAMS system using the ML ascension number (if shown), or through the NRC on-line reading room. Both systems can be accessed through the NRC web site: http://www.nrc.gov.