2.4.1 System Modeling

Section 1.3 showed that thermal effects formed a link between the power input to the system and the feedback reactivity effects which, in turn, affect power. Thermal modeling was included in the section.

Thermal changes in the system can be system-induced but they can also be induced by external means: (a) inactive loop start-up, (b) heat exchanger rupture, (c) secondary pump failures, (d) feed supply failure, (e) turbine stop-valve closure, or a large number of minor steam cycle malfunctions.

These disturbances will be witnessed by the core as a change in the inlet temperature. This has, in turn, three effects: (a) an immediate coolant re­activity temperature feedback; (b) a delayed Doppler feedback; and (c) a further delayed (by the loop time constant) effect on the inlet temperature.

All these effects may be taken into account in a comprehensive system model such as would describe Fig. 1.20. Some models which do not repre­sent the steam cycle may have to be supplemented by additional calculations performed to produce a perturbation for the model in the form of the inlet temperature as a function of time.

Trips available during such transients are the following: (a) primary signals such as pump control or valve control monitors or steam generator pressure relief signals; (b) thermocouples in secondary and tertiary loops;

(c) heat exchanger primary outlet temperature; and (d) core outlet tempe­rature (eventually). There is considerable time available in steam cycle incidents due to the insulation of the core from the steam cycle by the intermediate or secondary loop. The trips are in order of occurrence. It can be seen that the core is very well protected from this kind of disturbance.