Protection against external events

External events are extensively addressed in several specific IAEA Safety Series pubhcations that also provide guidance for the safety assessment. Some key issues are, however, summarized in the following. 

The set of events which should be addressed in the safety assessment depends on the site chosen for the plant but would typically include  

The design basis should be adequate for the selected site and based on historical and physical data, and expressed by a set of values selected on the general probability distribution of each event according to specified thresholds.  

When such a probabihstic evaluation is not possible because of lack of confidence in the quality of data, deterministic approaches are apphed, relying upon enveloping criteria and engineering judgement. 

The SSCs which are required to perform the fundamental safety functions should be designed to withstand the loads induced by the design basis events and able to perform their functions during and after such events. This should be achieved through adequate stmctural design, redundancy and separation. 

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Protection against external events (e.g. in case the reactor pit area above ground level, the confinement should be bunkered)... 

The reactor building serves as a secondary confinement. The main function of this structure is the protection against external events. It also provides a subatmospheric enclosure to collect the leakage from the compact containment during LOG A. 

In a world of natural and human initiated vicissitudes, nuclear power plants are to be configured to protect against external events such as high winds/missiles, earthquakes, floods, fires, aircraft crash [7] and also purposeful attacks such as airplane attack or rocket or tank... 

Arrangement of the water space and coastal infrastructures needed for normal operation of the VBER-150 based floating NPP and its protection against external events, are similar to those for floating NPPs with the VBER-300 reactor installations [IV-1]. 

Provisions for safety under seismic conditions and protection against external events... 

These features and small strong building structures also provide an intrinsic protection against external events, such as aircraft crash and missiles. 

Section 2, concerning the general approach and the design philosophy for protection against external events, has been considerably expanded, with new information added on structures, systems and components (SSCs) to be protected against external events and on load combinations and acceptance criteria. 

APPLICATION OF SAFETY CRITERIA TO THE DESIGN FOR PROTECTION AGAINST EXTERNAL EVENTS... 

The evaluation of a nuclear power plant for protection against external events should be documented in a manner suitable for a detailed technical review of conceptual assumptions and of detailed calculation procedures. As a minimum, the documentation should identify the events considered, then-primary and secondary effects (if any) and the basis for determining the adequacy of protection for each case. The technical documentation should allow for a complete record of the data flow among the different design tasks for the purpose of accuracy assessment. 

The power supply system of the plant distinguishes 4 different levels according to the different tasks of the connected loads. It is arranged in so-called sections (supplying operational loads) and divisions (supplying safety equipment). For the Nuclear Island there are 3 sections and 3 divisions protected against external events. For the Conventional BOP two sections are provided. 

In general, design qualification is an accepted practice for protection against external events once siting questions have been resolved (i.e. if the site does not present hazards for which there is no adequate protection). The method for establishing the design bases for particular external phenomena can be summarized as follows ... 

Protection against external events [4, 5] and internal hazards [6]. The system should be so designed and laid out that no external event or internal hazard considered in the design (such as a pipe break or a flood) has the potential to prevent it from performing its intended safety functions. In particular the capability of the system or its components should be maintained under the most severe seismic conditions considered in the design. 

The safety function fuel cooling during transients and accidents is ensured by provision of sufficient coolant inventory, by coolant injection, by sufficient heat transfer, by circulation of the coolant, and by provision of an ultimate heat sink. Depending on the type of transient or accident, a subset of these functions or all of them may be required. Various passive systems/components are proposed for future reactor concepts to fulfil these functions. It is a feature of many new concepts that the water for replenishment of primary coolant inventory is entirely stored inside the containment. This ensures protection against external events and reduces the risk of loss of coolant accidents with containment bypass. Additional features implemented in some new designs to improve the replenishment of primary coolant inventory function include ... 

Line rupture HF release in area possible injurie s/fatal it ie s. None 2 No Action Unlikely event piping protected against external impact. 

The cylindrical containment building is made of prestressed concrete with a stainless steel liner of 67.8 m height. Its internal diameter is 37.6 m, and the wall thickness is 1.6 m. The containment is designed for internal pressure of 0.35 MPa, and a temperature of 150 C. It can protect the reactor against external events including aircraft impact. 

The CHTR would be located inside a pit with sealed barrier of reinforced concrete and steel covers, which would protect the reactor against external events. 

Regarding protection against internal events, the entire plant is designed to protect personnel and the external environment against radiological hazard. To summarize, the main technical features to protect the plant are ... 

I-l 16. General methods used to evaluate particular external and internal events, such as earthquakes, tornadoes or sudden catastrophic rupture of reactor pressure retaining components or reactor internals, should be presented in the appropriate chapter of the SAR. It may be difficult to model the effects of such events, or analyses may be highly speculative. Further guidance regarding protection against these events is given in Chapters A.2 and A.3 of the Appendix. 

It is necessary, therefore, to conduct specific studies to ensure that systems are adequately protected against all foreseeable external events, and the FAA (FAR25.1309), EASA (CS25.1309) and SAE (ARP4761) refer to this study as a Particular Risk Analysis (PRA). 

The containment serves to protect the reactor plant against external impacts, as well as for confining radioactive products in the event of incidents during reactor refuelling and following accidents with postulated loss of the guard vessel integrity. 

In the framework of the defence in depth approach, protection against all external events is part of level 1 of defence in depth. 

Examples of external events which are not expected, but which could occur and impair tiie fecility are flooding, earthquake and water intrusion. The access to an underground facility must evidently be protected against surface floods. It is sensible to design the surface and underground constructions against an earthquake with a expected frequency of 10 or even per year. 

Nuclear islands of the FUJI power plants could also be located underground, as shown in Fig. XXX-17. Such location may provide an additional degree of protection against certain external events, including those of human-induced malevolent origin [XXX-38]. The location depth should not exceed 10 m in order to keep the plant economic characteristics competitive. 

This information is drawn upon in the PCSR to show that the DBA has considered all potential initiating events that could result from external hazards, and that all claims that demonstrate the AP1000 to be adequately protected against the effects of external hazards have been identified and substantiated. 

Detailed discussions are provided in the External Hazards Topic Report relating to fire, turbine disintegration and pipe mpture arising as a consequence of a seismic event, to show that the withstand of the plant systems and stmctures, particularly those that are safety significant, is sufficient that loss of the KSFs is protected against. 

The auxiliary building provides protection against the consequences of either a postulated internal or external event. The auxiliary building also provides shielding for the radioactive equipment and piping that is housed within the building. 

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