The optimization of the high-temperature reactor fuel cycles leads always to rather undermoderated cores. As a consequence an introduction of water has a positive effect on reactivity. However, it is in most cases possible to design the system so that the water entering the core is limited to a very small amount.

In many cases it is not possible to have an ingress of liquid water into the hot core. The water will be vaporized in the high temperature ducts before reaching the core.

If the introduction of liquid water into the core cannot be absolutely excluded it is also possible to poison the water of the secondary system with a strong neutron absorber.

This is not necessary in big systems where only the introduction of steam is possible and the resulting Ak is limited to a few per mill. In this case the greatest damage is not caused by the power excursion, but by the corrosion due to chemical reaction of water with the core materials. An evaluation of the power excursion is made evaluating the maximum rate of insertion of water into the reactor and calculating its reactivity worth. The group constants for reactivity calculations in presence of water must be obtained from codes able to deal with neutron moderation in water (spectrum calculations based on multi-group diffusion theory, like in the MUPO code, cannot be used in this case).