REDUCTION OF THE REACTIVITY OF WATER INGRESS IN MODULAR PEBBLE-BED HIGH-TEMPERATURE REACTORS

In high-temperature reactors (HTRs), ingress of water introduces posit ive reactivity. Normally, this is controlled by the reactor itself, bu t in hypothetical situations, there could be a need for an active supp ort by the control system. Calculational research identifies three rea sons for the reactivity change caused by the water: (a) a negative con tribution by the absorption of the hydrogen, (b) a positive contributi on by the softening of the neutron energy spectrum, and (c) a reductio n of the neutron leakage losses due to a shift in the neutron flux loc al distribution. By increasing the carbon/heavy metal ratio, the react ivity effect can be reduced to almost zero or even to negative values. In the modular pebble-bed HTR, this effect can be accomplished in a s imple manner. By adding 25% of graphite spheres to the regular batches of feed fuel elements, the neutron spectrum effect is reduced, and th e fractional absorption of hydrogen is increased; thus, the maximum ex cess reactivity is limited to 0.3%. The effect on economy and safety i s negligible.