Asymmetric faults

These faults give a large power increase in very few channels in a small area of the core. Temperatures do rise to some extent over the whole core, but the rise of temperature away from the fault rods is usu­ally insufficient to activate trip thermocouples. Hence only those few thermocouples in the fault region are effective. Furthermore, flux measurements are made at three discrete points on the periphery of the core set at 120° from each other. It is necessary for two of these instruments to detect the excursion before a trip will occur and, for most asymmetric faults, at best only one will be effective. The study of these faults is very extensive since large numbers of possi­ble combinations of rods being withdrawn need to be considered. In addition, the behaviour of the regulat­ing rods is most important in limiting the numbers — of thermocouples which provide effective protection. This occurs because the regulatory rods act to main­tain the temperature in their area to a predetermined value. Hence, if a single rod runs out, the regulatory rods in compensation for this may trip thermocouples and a very sharp peak in the temperature distribution across the core results.

A special case of the asymmetric faults is the problem of local criticality. If with a reactor in the shutdown condition, a number of control rods are withdrawn from a local area of the core, criticality can result. Although the reactivity of the whole core in­creases, and ultimately the core kerf becomes greater than unity, the flux distribution is very peaked in the area from where the rods have been withdrawn. Further, since the reactor is shut down, the mass flow is very low or even zero and channel gas outlet thermocouples are ineffective. Flux protection is also largely ineffective because of the distribution of the sensors. In this case reliance is placed on adminis­trative control to prevent sufficient rods being removed to allow criticality to occur. A fault study is carried out to determine the minimum number of rods needed to be withdrawn to produce local criticality; the number of adjacent rods allowed to be removed for maintenance and repair is then limited to a figure considerably below this. Typically seven or eight would be required in the most sensitive area of the core, and the allowable number would be two or three.