Having now seen that the reactor system consists of a closed loop, in­volving the reactivity, power generation, temperatures, and back to the reactivity, and having seen that there are other feedbacks which operate with different time scales and from different initiators (temperature, flux, fuel composition, and control), it is necessary to see how the whole picture fits together for particular reactor systems.

The safety engineer is interested in setting up a complete model of the system in order to assess the various interactions within the model for their safety in the face of expected and unexpected external and internal disturb­ances. By modeling a fast reactor system and disturbing that model with a flow change which might arise, such as a pump coast-down, he can calcu­late resulting temperatures to determine whether the behavior is acceptable or not.

This section deals primarily with the principles of interacting variables. The next chapter treats specific disturbances to fast reactors cooled by gas, steam, and sodium but here we are concerned with seeing how the safety engineer deduces or calculates what the results of disturbances might be.