12.56. Transients caused by operational (and other) occurrences can cause an imbalance between the rate of heat generation in the fuel (or thermal power) and the rate of heat removal by the coolant. If the former exceeds the latter, there is danger of overheating and hence of damage to the fuel-rod cladding. In many cases, the imbalance can be rectified au­tomatically by the reactor control system without interruption of normal operation. The reactor protection system is designed to trip the reactor if the imbalance is too severe to be handled by the control system. The redundancy and diversity available in the control and protection systems make their total failure highly improbable.

12.57. The relationship between heat generation and heat removal rates may be considered in terms of the maximum heat flux (or heat generation rate) in a coolant channel and the critical heat flux (§9.98) at which some fuel cladding damage may be expected to occur. In order to prevent such damage, the critical flux must be well above the maximum (actual) flux, as depicted in Fig. 9.22, for example. An increase in the thermal power of a reactor will increase the (actual) flux, whereas a decrease in the coolant effectiveness will decrease the critical heat flux. Thus, transients capable of causing either (or both) of these effects will increase the possibility of cladding damage.