Design basis tsunami

So, TEPCO s application of the 2002 JSCE guidelines apparently got this issue very wrong indeed. The methodology of the 2002 JSCE guidelines was to use a database of historical tsunami in order to calculate a design basis tsunami height, which... 

Press reports have actually suggested that TEPCO knew by the time of the accident that the design basis tsunami was insufficient. The Japan Tunes for July 3, 2011 reported that TEPCO was aware as early as 2008 that a 10-m-plus tsunami could hit the Fukushima Daiichi nuclear power station, and that the results of new analysis work had been reported for the first time to the safety regulator (the Nuclear and Industrial Safety Agency or NISA) on March 7 - only four days before the tsunami occurred. 

The role of the safety regulator NISA should have been central in this issue. It should not have been up to TEPCO to interpret the JSCE guidelines. Either (i) NISA should have given detailed prescriptive requirements to TEPCO, or otherwise (ii) TEPCO should have had to prepare a detailed justification, for NISA approval, why their chosen height for design basis tsunami was justified. (The first is the US regulatory model and the second is the UK regulatory model.)... 

Very soon after the accident, the role of NISA began to be called into question. TEPCO had reported their concerns about the size of the design-basis tsunami to NISA on March 7, 2011, only four days before the tsunami struck. NISA officials had replied to TEPCO that countermeasures are urgently needed , calling for modifications, but it was too late. 

Ocean impacts may also be the cause of a tsunami. The impact of an asteroid or comet on an ocean may be a significant cause of major tsunamis that affect populations quite different from the populations affected by tsunamis caused by geological events. Such an event may be considered in the analysis of beyond design basis events. 

It should be borne in mind that, in spite of the accepted terminology for the probable maximum storm surge, probable maximum tsunami, probable maximum seiche and probable maximum dam break, such events cannot always be characterized in a purely probabilistic framework. However, the terminology emphasizes that an estimate should always be made of the probability of exceedance associated with the design basis scenarios, even when they are investigated by means of deterministic approaches. 

When sites are located along semi-enclosed bodies of water, such as river estuaries, the reference water level may depend on astronomical tides in combination with the river flow. In regions where extreme floods arise mainly from oceanographic causes, it is necessary only to choose an appropriate value for the river flow (not to be exceeded in tens of years) this should be considered in conjunction with the appropriate combination of probable maximum surge, tsunami, wind wave and tide to derive the design basis flood. In other cases, where the river flood is more important, the solution adopted should be appropriate to the particular case . 

With hindsight, changes in height of only 20 cm or so seem ridiculous when the actual tsunami was some 14 m, i.e., 8.3 m above the design basis. 

This core meltdown is, and was caused by, a rare event a seismically induced tsunami of immense proportions that caused loss of almost all power and control. Thus the initiating event lay outside of the safety analysis envelope of what had been considered at the design stage, beyond the design basis of what had been considered for safety margin, and system and structural design and was more severe than considered in risk assessments for natural hazards. 

The risks of hydraulic hazards such as tsunamis should be analysed at the initial (feasibility) stage of a reclamation project. The frequency of occurrence and the expected impact of extreme events should be properly evaluated and related to the expected costs for protection against such events. Such a risk-cost assessment should be the basis for the design of reclamation areas in terms of being hazard-proof or not. 

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