Ultimate Shutdown Mechanism

It Is reassuring that a simultaneous failure of all automatic shutdown devices during a maximum startup excursion leaves about 1 seconds after the initial power turnaround by the temperature coefficients to manually scram the reactor before boiling commences. 

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Where feed lines have short pipe runs, where hot wells or FW tanks are of small volume, or when FW is too cold, there often is insufficient time for full DO scavenging to take place, even when using catalyzed scavengers. The inevitable result of this lack of contact time is the formation of oxygen-induced corrosion products, which by various secondary mechanisms may settle out to form permanent deposits within the boiler system. These deposits may develop in several forms (e.g., where DO removal is particularly poor, they often appear as reddish tubercles of hematite covering sites where pitting corrosion is active). Active pitting corrosion combined with the presence of waterside deposits ultimately may lead to tube failure in a boiler or other item of system equipment and result in a system shutdown. 

Fouling may be defined as the formation of deposits on heat transfer surfaces which impede the transfer of heat and increase the resistance to fluid flow. The growth of these deposits causes the thermai and hydrodynamic performance of heat transfer equipment to decline with time. Fouling affects the energy consumption of industrial processes and it can also decide the amount of material employed in the construction of heat transfer equipment. In addition, where the heat flux is high, fouling can lead to local hot spots and ultimately it may result in mechanical failure, and hence an unscheduled shutdown of the equipment. 

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