Having now constructed a mathematical model of the system and knowing the interaction between its relevant variables, we can calculate the system behavior resulting from certain disturbances.

These disturbances may be operational (the closure of a valve restricting steam flow or the insertion of rods to obtain a reactivity reduction), or they may be faults (pump failure, a burst steam main, or an inadvertent withdrawal of control). The model may be suitable for investigating each of them, or it may have to be modified for each set of circumstances. One might have to add a representation of the pumps in more detail if their behavior off their normal characteristic is required.

The calculations are performed either on an analog or a digital computer. The analog is especially convenient for time-dependent differential equa­tions but the sheer size and nonlinearity of the calculation may demand the use of a digital machine. Presently it is customary for a reactor designer to have several standard codes that he may use as tools in the representation of certain types of transient—using one with a detailed fuel model, for example, to calculate the response to a reactivity transient and one with a detailed pump and circuit representation to calculate flow transient be­havior. These are usually digital codes (see the Appendix).

Analog machines are in use for stability investigations and for control simulations, their range having been extended by the addition of some digital facilities, making them into hybrid computers with a combination of digital and analog functions.