Withdrawal, controlled

Underflow is usually withdrawn continuously on the bases of bed level, rake torque, or underflow solids concentration in a feed-back mode. Most installations incorporate at least two of these parameters in their underflow withdrawal control philosophy. For example, the continuous withdrawal may be based on underflow solids density with an override to increase the withdrawal rate if either the rake torque or the bed level reaches a preset value. In some cases, underflow withdrawal has been regulated in a feed-forward mode on the basis of thickener feed sohds mass flow rate. Any automated underflow pumping scheme should incorporate a lower limit on volumetric flow rate as a safeguard against line pluggage. 

Underflow is usually withdrawn continuously on the bases of bed level, rake torque, or underflow solids concentration in a feedback mode. Some installations incorporate two or more of these parameters in their underflow withdrawal control philosophy. For example, the... 

Nuclear Reactor with a Hole in the Head On March 6,2002, personnel repairing one of the five cracked control rod drive mechanism (CRDM) nozzles at Davis-Besse Nuclear Plant, Oak Harbor, Ohio, discovered extensive damage to the reactor vessel head. The reactor vessel head is a dome-shaped structure made from carbon steel housing the reactor core. The reactor vessel head is placed such that it can be removed when the reactor is shut down to allow spent nuclear fuel to be replaced with fresh fuel. The CRDM nozzles connect motors mounted on a platform above the reactor vessel head to control rods inside the reactor vessel. Reactor operators withdraw control rods from the reactor core to start the operation of the plant and insert the control rods to shut down the operation of the reactor. 

There were no stops to prevent the operators withdrawing control rods to their full extent, but there were station rules which were intended to prevent this. 

Reactivity and power distribution anomalies uncontrolled control rod withdrawal control rod ejection boron dilution due to the startup of an inactive loop (for a PWR). 

Log Count Rate High or Interlock Defeated" alarm occurs at 10 cpm, or if Interlock switch is placed in "Defeat" position Cannot withdraw control elements when fission counter is moving. 

While withdrawing control rods during a reactor startup, the count rate doubles. If the same amount of reactivity that caused the first doubling is added again, the ... 

During startup an operator withdraws control rods until the reactor is shutdown by 6X AK/K, at which point the startup channel reads 5 cps. Rod withdrawal is continued until the reactor is shutdown by 0.5X AK/K. What would you expect the startup channel to read at this point ... 

While subcritical and withdrawing control rods, the operator observes a significant change in count rate while withdrawing a peripheral rod, The next rod he moves in the center of core, shows little change in multiplication. Is this an abnormal condition Explain your answer,... 

The following example apphcable to pressurized water reactors (PWRs) may further illustrate the approach described. One of the SFs relevant for Levels 1-3 of defence in depth is prevention of unacceptable reactivity transients. This SF can be challenged by insertion of positive reactivity. Several mechanisms lead to such a challenge, including control rod ejection, control rod withdrawal, control rod drop or misalignment, erroneous startup of a circulation loop, release of absorber deposits in the reactor core, incorrect refuelling operations or inadvertent boron dilution. For each of these mechanisms there are a number of provisions to prevent its occurrence. For example, control rod withdrawal can be prevented or its consequences mitigated by ... 

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