Risk Studies in Nuclear Power Stations (csni86-123)

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As it has been mentioned above, for ensuring the confinement of radioactive substances, the nuclear power plants are designed per principle of defense in depth. Failure of some structures, systems, and components SSCs can trigger a sequence of events at the plant deviating from normal operational conditions.

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If a sequence was considered in the design basis of the plant, the safe stable condition of the plant should be ensured by safety systems. The safety systems shall ensure the control of reactivity, that is, the chain reaction in the reactor shall be stopped, the heat generated by decay of radioactive fission elements shall be removed from the reactor core to the ultimate heat sink to the environment , and the radioactive substances shall be confined in the fuel elements.

The annual probability of the core damage, P CD , is limited by the nuclear regulations. Thus, the safety systems should withstand the effects of natural hazards and fulfill their intended functions for avoiding the core damage. In very improbable cases when the safety features fail, and the conditions are more severe than those accounted for in the design, the radioactive releases shall be kept as low as practicable.

The most important objective of this level is the protection of the confinement function. It is as dangerous as earlier it happens in the course of the severe accident. The annual probability of the early large releases, P LR , is also limited by the nuclear regulations. It means the acceptable value for a singular sequence should be less approximate by an order of a magnitude. Since the consequences of nuclear accidents caused by natural hazards can be enormous, the risk should be reduced by selecting effects for the basis of design with very low annual probability.

Some exception is the regulation regarding the tornado hazard in the USA, where the tornado hazard is a reality due to meteorological and topographical conditions. The Nuclear Regulatory Commission has determined the best-estimate design-basis tornado wind speeds for new reactors, which correspond to the exceedance frequency of 10 —7 per year [ 19 ].

Probably, the reason for this conservative approach is the complexity of post-event conditions. A target performance, P T , should be set to each category. Care should be taken to the convolved frequency, where there are multiple parameters used to define an event.

Nuclear Plant Risk Studies: Failing the Grade (2000)

Obviously, there is a strong correlation between the phenomena having the same physical origin. Contrary to this, for example, the combination of a big storm with a high tide could lead to the external flooding of a power plant. Specific considerations are made in case of causal-related events, like earthquake and liquefaction, earthquake and tsunami, or earthquake and failure of structures protecting the sites. In case of liquefaction, based on the soil date and the design-basis earthquake magnitude, the conditional probability of liquefaction can be calculated.

The total probability—earthquake and liquefaction—should be less than the probabilistic screening criterion for neglecting the liquefaction hazard see, e. This condition can also be formulated in terms of safety factor with respect to liquefaction. There are multiple causally correlated hazards. For example, possibility of multiple causally linked hazards has been recognized at Tricastin site in France that initiated a focused safety justification in [ 21 ]. The level of the Tricastin site is 6 meters below the nearby channel level. The nuclear site is protected by embarkment. Although the embarkment would resist the maximum historically credible earthquake, it could not be excluded that it would fail if the design-basis earthquake of the plant hits the site.

If the site would be flooded, loss of off-site and on-site electrical power supply and failure of the cooling systems of the reactors could be expected. Limited access to the site would hinder the emergency response. In this case the seismic resistance of the embarkment is the key question, since the plant remains safe in case of design-basis earthquake. The probability of loss of safety function in this case is defined by the probability of design-basis earthquake, since the embarkment failure and the consequent flooding are highly probable if a design-basis earthquake happens.

In case of causally linked hazards, the damaging effects of root cause event and the consequential event would not be necessarily simultaneous. The timing of effects should be considered in the design. The above considerations with the small probabilities may seem like the usual reasoning and magic of the nuclear industry.

As a matter of fact, that is the state of the art. However, this is recognized to be not sufficient.

Two fundamental questions have to be answered here: Whether the characterization of rare natural hazards can be performed with high enough assurance? The question is related to the possibility of definition of the hazard curve, which is the annual probability of an event that will occur at the NPP site with a damaging effect exceeding a given threshold. Whether there are proven engineering solutions available for ensuring enough capability of NPPs to withstand safely the effects of hazards?

Presentation of the state-of-the-art methodologies for hazard evaluation is out of scope of the recent chapter. The nuclear industry is adapting the most novel scientific achievements for the site characterization and investigation see, e. The hazards accounted for in the design are subject to regular review and update in countries where the regime of periodic safety review is established.

Book Risk Studies In Nuclear Power Stations (Csni86 )

Most extensive programs for natural hazard evaluation and upgrading and justification of operating plant safety have been implemented in the USA and several Eastern-European countries, where the operators should deal with the issues of underestimation of the seismic hazard for the design basis. Summary description of these programs is given in [ 25 , 26 , 27 , 28 ].

Events, like the Great Tohoku Earthquake, triggered an overall review, correction, and justification of hazard evaluation at the plants see the stress test initiated by the European Union and the reviews and upgrading programs in several countries, e. The Fukushima accident is the worst-case example for improper characterization of tsunami hazard. The NPPs can be protected from the flooding due to tsunamis, assuming that the design-basis wave height is adequately defined and the uncertainties of the tsunami characterization are properly compensated by the conservative design.

Contrary to the Fukushima Dai-ichi plant, the m high seawall protected the Onagawa NPP from flooding due to tsunami [ 31 ]. The basic difficulties of the hazard characterization are the epistemic and aleatoric uncertainties that should be evaluated and accounted for. Considering the design-basis hazards, the uncertainty is compensated by conservative approach: in the definition of the demand and calculation of the resistance of the SSCs.

The generic design rules are fixed in the nuclear regulations and acceptable standards see, e. Therefore, the design should cope with this uncertainty not only within the design basis but also beyond. It is required that the NPPs should be prepared for the unexpected exceedance of E DB and the sudden loss of safety functions a cliff-edge phenomena shall be eliminated.

In the case of earthquakes exceeding the design basis, the design should provide an adequate margin to protect items ultimately necessary to prevent escalation of the event sequence to severe accident. According to the regulations, the best-estimate approach can be adopted for the evaluation of this margin [ 34 ].


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The high-confidence of low-probability of failure HCLPF could be the measure of the seismic margin [ 35 , 36 ]. For new plants, depending on the regulatory framework and design practice, a HCLPF capacity of at least 1. These values are based on the conservatism of the nuclear design standards and justified by extensive studies. The above concept can be adopted for other hazards as it is proposed, for example, in [ 37 ]. From our point of view, the most important are the real NPP experiences regarding natural hazard events and consequences. There are several sources archiving the experiences of extreme natural events.

The International Atomic Energy Agency International Seismic Safety Centre collected the information on the earthquake experiences reported by the operators. The World Nuclear Association has also collected the data of nuclear accidents that could be compared by other industrial activities [ 39 ]. According to this study, apart from earthquakes and tsunamis Fukushima case , the fouling events biological fouling of water intakes affecting also the ultimate heat sink and chemical fouling causing corrosion and extreme weather conditions, including lightning strikes and floods, are dominating.

A few events reported have safety significantly according to the International Nuclear Event Scale. The examples show that the nuclear plants can withstand and properly respond to extreme natural events, if the design basis defined is adequate that was not the case at the Fukushima site with respect to the tsunami.

The industry has the tools, the analytical and testing capabilities, and the consolidated standards to design and build safe plants. There are plenty of examples demonstrating that the codes and standards accepted in the nuclear praxis ensure sufficient capacity of SSCs to withstand the ground vibratory effects of earthquakes. Although the recorded ground motions exceeded those values for what the plants were designed, the safety consequences of the earthquakes were negligible. In case of the Great Tohoku earthquake, the behavior of 13 nuclear units in the impacted area on the East shore of the Honshu Island demonstrated high resistance against ground vibrations due to earthquake.

Even the Fukushima Dai-ichi plant survived the strong motion period of the earthquake.

In August the North Anna plant in Virginia, USA, also survived a beyond-design-basis earthquake thanks to the designed and built margins. The North Anna case demonstrated also the adequacy of definition of damage criteria formulated in terms of cumulative absolute velocity and justified the correctness of predefined measure of margin. Although the ground motion experienced at the site exceeded the design-basis level, the damaging effect of the earthquake was found below the margin evaluated, and the damages were really negligible [ 43 ].


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Sufficient capability of plants to withstand beyond-design-basis vibratory motion of earthquakes has been demonstrated by the stress tests performed in the European Union and by focused reviews implemented in other countries. The stress tests have been aimed to the review of seismic hazard assessments for sites of nuclear power plants and to the verification of the design bases, as well as to the evaluation of margins against external hazard mainly earthquakes and floods effects, whether the beyond-design-basis hazard effects can cause cliff-edge effect, that is, sudden loss of safety functions due to effects exceeding the design-basis one.

Food safety can be ensured by combination of technical and procedural measures, reducing the power generation or shutting down the reactors. The protection of plants against floods is feasible even at rather unfortunate sites like the Tricastin one [ 21 ]. In spite of this, floods at some sites caused safety issues. For example, at Fort Calhoun site in [ 41 ], the plant should be protected by extraordinary temporary measures.

The flood and fire resulted in a 3-year shutdown of the plant. At Blayais Nuclear Power Plant in [ 44 ], the high tide and storm flooded the plant and caused an event Level 2 according to the International Nuclear Event Scale. Safety upgrading measures and improved procedures have been developed and implemented to achieve the required safety level.

The case turned the attention to event combinations that are capable to cause extreme flood event. Both cases reveal the importance of design-basis definition, regular review of the hazard characterization, and checking the protection capabilities and upgrading if necessary.