12.58. Coolant Temperature Decrease. A decrease in the temperature of the coolant entering the reactor can cause an increase in reactivity, and hence in the neutron flux and thermal power, as a consequence of the negative temperature coefficient. In a PWR, this could occur as a result of malfunction in one of several systems. For example, a faulty valve might cause the feedwater rate to a steam generator to increase, or failure of a water preheater could result in a decrease in the feedwater temperature. However, such steam-generator effects in a PWR would tend to produce only a slow change in the primary coolant temperature. Since the feedwater (turbine condensate) in a BWR is pumped directly to the reactor vessel, the response from similar situations would tend to be faster. In each case damage to the fuel cladding would be prevented by a reactor trip actuated by the increase in the neutron flux.

12.59. Inadvertent startup of a pump in an inactive PWR coolant loop could also lead to an increase in reactivity and in the thermal power. Similarly, an accidental speedup in a BWR recirculation pump would result in a power increase. Such transients would be corrected automatically by the control system or the reactor would be tripped.

12.60. Control Material Removal. An inadvertent reduction in the con­trol (poison) material in the core is another cause of an unplanned increase in the thermal power. Although the reactor design limits the rate at which control elements can be withdrawn, the consequences of an uncontrolled withdrawal resulting from a malfunction must be considered in the safety analysis. The effects depend on the withdrawal rate, the reactivity worth of the control element (§5.168), and the operating state of the reactor, e. g., startup or full power. During startup, special precautions are taken to prevent a too rapid increase in the reactivity, since this could lead to a potentially unsafe condition (§5.219). Here again, reactor trip would be initiated by the protection system should design limits be exceeded, re­gardless of the operating state of the reactor.

12.61. In a chemically-shimmed PWR (§5.187), an accidental decrease in the boric acid concentration would lead to a power increase. However, the response of the reactor power to boron dilution is slow, and numerous alarms and other indications would alert the reactor operator to take cor­rective action should an error be made in the manual dilution operation.

12.62. System Pressure Increase. The system pressure in a BWR would increase if a valve in the main steam line should close automatically, e. g., as a result of a sudden decrease in the turbine load (see also §12.65). An increase in pressure would decrease the steam-void volume, thereby pro­ducing an increase in reactivity. The thermal power would consequently rise, thus tending to cause a further increase in the system pressure. The reactor pressure-relief valve would then release excess steam to the suppression pool, and the reactor would be tripped before any damage could occur. Radioactivity present in the steam would be removed by the water in the pool and by the air circulation system in the drywell.