Consideration of hazardous and especially hazardous hydrometeorological impacts in design of buildings and structures of nuclear power plants

External impacts of the hydrometeorological origin have a significant influence on the safety level of objects of use of atomic energy (OUAE), including nuclear power plants (NPP). Therefore, the existing NPP-related safety regulations demand to consider such impacts at all stages of the NPP life cycle. It is important to make decisions on considering or ignoring certain external impacts while designing NPP buildings and structures. The main criterion for such decisions is the probability of a non-project accident associated with the release of radionuclides into the environment when an extreme phenomena occurs. The aim of this study is to develop a concept for refinement regulatory requirements, considering hydrometeorological factors in organization of NPP engineering protection. Criteria for consideration of hazardous and especially hazardous hydrometeorological impacts for design of NPP buildings and structures were analyzed, and recommendations for refinement of regulatory requirements, considering hydrometeorological factors in organization of NPP engineering protection, were developed.


Introduction
External impacts of the hydrometeorological origin have a significant influence on the safety level of nuclear facilities, including NPP [1,2]. International and Russian OUAE safety codes and regulations state that such impacts shall be considered at all stages of their life cycle [3,4].
These codes and regulations generally require to consider a wide range of hydrological and meteorological phenomena and processes. However, they do not provide clear criteria determining the need for consideration of relevant hydrological and meteorological loads in calculations. So it is necessary to refine regulatory requirements for consideration of hydrometeorological factors in provision of NPP engineering protection.
The abovementioned normative technical documents [3,4] require consideration of external hydrometeorological factors listed in Table 1, which also indicates general impacts of such factors on NPP buildings and structures. According to the table, potential impacts of the listed factors are quite diverse. However, the mentioned extreme hydrometeorological phenomena and processes can cause a non-project NPP accident only in extremely rare occasions. Taking into account the importance of NPP radiation safety assurance, even such cases require detail analysis of possible consequences of a hypothetical accident.

Probabilistic safety criteria for NPP
Decision on consideration or ignoring of certain hydrometeorological factors according to [4] shall be based on the threshold probability P 0 (basically, recurrence), which is 10 -4 /year per reactor. However, taking into account high dangers of tornados and their impacts on NPPs, their threshold probability is assumed equal to 10 -7 /year per reactor [5,6]. Lightning impacts on NPP buildings, structures and infrastructure also have specific features. In many instances, parameters of their impacts considered by design shall be determined at the expert level.
Collection and analysis of input data for statistical processing and determination of design hydrometeorological characteristics are carried in the process of engineering hydrometeorological surveys [7]. Statistical data processing is performed using standard methods (e. g., refer to [8,9]).
In Russia and other IAEA member counties, NPP safety requirements are based on nonexceedance of the non-project accident probability with the accidental release limit of P G = 10 -7 /year [4]. This value is the main criterion of NPP safety in relation to external impacts of different origin.
The non-project accident probability caused by a specific hydrometeorological factor is where P Adimensionless probability of an accident resulting in maximum permissible release into the environment upon occurrence of the impact event for this factor. With P A = 1 for tornado impacts and P A > 10 -3 for impacts of other hydrometeorological factors, the non-project accident probability will reach or exceed the main safety criteria of P G = 10 -7 /year. In case of tornado impacts the non-project accident with radionuclides release is not always inevitable. This is due to the fact that according to the tornado zoning scheme of the ex-USSR territory the estimated characteristics of the maximum probable tornado with the NPP impact probability of 10 -7 /year can be relatively weak [5,6,10]. As for impacts of other hydrometeorological factors, their consideration shall be formally based not on the occurrence of the threshold probability of P 0 = 10 -4 /year, as it is provided for by federal regulations [4], but on the following condition:  (2) To ensure appropriate NPP protection against external impacts, including those of the hydrometeorological origin, engineering measures shall be implemented for NPP protection.
If the threshold probability of P 0 = 10 -4 /year is still considered, the NPP engineering protection is required, when the non-project accident probability resulting from the impact of a specific hydrometeorological factor P A exceeds 10 -3 . In its turn, the maximum estimated value of a relevant hydrological or meteorological factor, e.g. water level or wind speed, is the probability level quantile of 1 -10 -4 /year and is determined basing on the statistical distribution of this value. In addition, this greatly varying quantile will by no means always result in a non-project accident with a probability above 10 -3 . Otherwise stated, the potential hazard from the impact of such meteorological factor can be overestimated and cause excessive material costs for NPP engineering protection.
To avoid this, it is necessary to proceed not from the assumption of the threshold level of P 0 = 10 -4 /year, but from the following condition: In this formula x C is the estimated extreme value of the variable x (probability level quantile of 1 -P 0 ×year). Dependence of the non-project accident probability P A (x C ) on the impact of a given hydrological or meteorological factor x is derived from construction calculations. The cumulative distribution function 1 -P 0 (x)×year of the variable concerned is determined based on results of statistical processing of observation data. Therefore, it is generally not difficult to determine values of x C , P 0 and P A . It may turn out that the quotation (3) will have no solution, if the variation range of the hydrological or meteorological variable x does not result in a potential non-project accident with a probability above 10 -3 . This means that in some cases the danger of occurrence of a certain hydrological or meteorological event may be overestimated.
The engineering protection of NPPs from hydrometeorological factors provides for significant expenditures for new construction of each object as well as for its expansion or reconstruction. In some cases there will not be necessary to design expensive protection from some factors without reduction of the NPPs safety.
In this regard, it seems advisable to refine regulatory requirements to consideration of hydrometeorological factors, as well as other factors of the natural origin in designing a NPP.

Conclusions
• Based on the applicable NPP safety codes and regulations, analysis of consideration criteria for hazardous and especially hazardous hydrometeorological impacts in NPP design.
• Recommendations for refinement of regulatory requirements were developed to consider hydrometeorological factors in organization of NPP engineering protection against such factors.
• It is noted that there may be situations when there is no need to design the engineering protection from some factors without reduction of the NPP safety.