fuel claddings as follows: the minimum critical heat flux ratio (minimum DNBR) is equal to or above the limit; the maximum fuel center temperature is equal to or lower than the melting point of fuel pellets; and the maximum fuel enthalpy does not exceeding the fuel allowable design limit of 712kJ/kg ■ UO₂ (which corresponds to 170 cal/g ■ UO₂ in the “Guide for Evaluation of Reactivity Initiated Events in Light Water Nuclear Power Reactor Facilities").One criterion is related to the integrity of the reactor coolant pressure boundary: the pressure of reactor coolant pressure boundary is equal to or lower than 1.1 times the maximum service pressure (17.16 MPa (gage) x 1.1 =18.88 MPa (gage)).

Three of the above 14 events of “abnormal transients during operation” are explained as examples of safety evaluations for a 3-loop plant

®-5 “Partial loss of reactor coolant flow” as a representative example from the viewpoint of DNBR

(D -1 “Uncontrolled control rod withdrawal at reactor startup” chosen from the viewpoint of fuel enthalpy.

(2)-8 “Loss of normal feedwater flow” chosen from the viewpoint of integrity of the reactor coolant pressure boundary.

2. Partial Loss of Reactor Coolant Flow

The transient postulated here is a reduction in reactor core coolant flow during power operation due to failure of a primary coolant pump or loss of a power supply or other causes. If nothing is done against the partial loss of coolant flow, heat removal from the fuel rods becomes insufficient due to the flow reduction in the core and fuel rods will be damaged. However, a number of measures are taken to prevent progression of the situation as follows.

(D Hie reactor coolant pumps are provided with a flywheel to increase the pump inertia so as to prevent rapid decrease in the coolant flow and hence rapid reduction in heat removal capability upon loss of the power supply to the pump motor.

® The reactor is automatically shut down by the following signals from the reactor protection system.

•Low reactor coolant flow

S

• Reactor coolant pump circuit breaker open Figure 8.2.1 shows the analysis results for reactor core flow, reactor power, heat flux, minimum DNBR, and reactor pressure in a failure of one reactor coolant pump. The event development with time is as follows.

CD After one reactor coolant pump fails, the core coolant flow decreases gradually due to the inertia effect of the flywheel (© in Figure 8.2.1).

(2) In response to the decrease in coolant flow, the "low reactor coolant flow” signal is generated, and about 2.4s after the occurrence of the event, control rod clusters start to drop and the reactor is shutdown automatically and the reactor power decreases rapidly ((2) in Figure 8.2.1). Accordingly, the heat flux at the fuel rod surface decreases slowly. On the other hand, the reactor pressure increases slightly with the decreasing coolant flow, and continues to gradually decrease for a long time after the

Time (s)

Figure 8.2.1 Partial loss of reactor coolant flow (• indicates the initial value)

Minimum DNBR

8-5

NSRA, Japan