Ultimate shut-down systems

Figure 15.21 Concept of ultimate shut-down system for taking care of re-criticality.

The reactor shut down system (RSS) is a constituent part of the CPS and is designed in such a way that a CPS failure does not affect operability of the RSS. An important feature of the reliabili of the RSS actuation is the fact that apart from 6 RSS clusters all 19 clusters of the CPS fall to their ultimate lowest position under gravity. 

Following an accident such as a loss of heat sink without scram in which the reactor power has passively decreased to a low level of afterheat typical of decay heat levels, it may be enough to simply return to power. Or it may only be required for an operator to ultimately insert the shutdown rod(s) to terminate possible fission power at low afterheat levels and render the core subcritical i.e. to ultimately shut down the reactor neutronically. Until this action is taken, the reactor would continue to generate power at a low level that is removed by the guard vessel natural convection air-cooling system and transported to the inexhaustible atmosphere heat sink. 

Disturbances In case of failed shut down cooling system, heat dissipation via reactor cavity coolmg system (S) In case of (partial or complete) cavity coolmg system failure, heat dissipation to ultimate heat sink by heating up concrete, steel structures and surrounding envuronment (L)... 

Apart from this automatic shutdown function which is an "ultimate safety" feature, there is a conventional protection system which initiates scram when operation limits are exceeded. Such scrams are performed by means of a scram valve, letting pool water in to the recirculation pump suction side. In addition, the reactor can be shut down by boron Injection via the normal control system. 

The KALIMER adopts the ultimate shutdown system (USS) as an alternative means of shutting down the reactor. The design concept is a self-actuated shutdown system (SASS) located in the centre of the core. 

To avoid accidents altogether, a complete elimination of human errors may be seen as the ultimate goal. This goal is not very practical, however, and will have severe side effects. Due to the intrinsic variability in human performance, errors will occur. Errors also provide the operators with task feedback and on-the-job learning about the systems that they operate. This experience is extremely valuable in situations when the operators have to handle unanticipated situations to avoid shut-down or accidents. This has often to be done under tight time constraints and psychological stress. 

Collective protection is not without drawbacks. Unless hardened, it is vulnerable to direct hits by conventional weapons, with the risk that those sheltered might thus be exposed to chemical contamination. Even near misses could downgrade the effectiveness of seals or other protective systems - for example, by causing blast damage. Ultimately, airmen must fly their aircraft and infantry must dismount from their vehicles. There is a limit to the time that a tank crew can stay shut down. In a CW environment personnel cannot re-enter collective protection without decontamination - which may not always be practical. Collective protection cannot therefore replace individual protection, but it has valuable applications, particularly where the requirement for protection is of short duration. It is an essential feature of modem static facilities, such as operational headquarters. In the field, it can provide opportunities for rest and... 

Operating period and duration affect selection of electrical control equipment and motor rating. Where possible, all of these parts of the system are specified for continuous operation. The operating period will further affect ultimate settlement during the shut-down period, and timing delays can be fitted to the primary section to ensure overloads do not occur on recommencement of operations. 

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