Patent Number: 041486870
Section: summary

FIELD OF INVENTION The present invention relates generally to method for saving in nuclear reactors and more particularly to methods by embedding or inserting beryllium (or its compounds) into the fuel bundle of nuclear reactors to reduce the inventory of D.sub.2 O in a heavy water reactor or to relax the requirement of uranium enrichment in a light water reactor and the resulting fuel elements without substantially detrimentally affecting the neutron multiplication value of the reactor. Having low (n, 2n) threshold and extremely small neutron capture cross sections, the beryllium can be used in a reactor as not only a good neutron moderator but also a fast neutron multiplier. With one or several beryllium rods properly embedded in a bundle-type fuel element at regions where the neutrons have energies at or above the (n, 2n) threshold of the beryllium; the (n, 2n) reactions in beryllium can be effectively utilized to take part of the neutron slowing down job and also to achieve some additional neutron multiplication. Because of these two fast neutron effects, the lattice pitch (and thus the D.sub.2 O inventory) of a heavy water reactor fuel cell can be reduced by properly embedding beryllium rods in the fuel without diminishing the neutron multiplication factor. In the case of light water reactors, a properly beryllium embedded fuel element of lower enriched uranium can be made to have the same neutron multiplication as that of a higher enriched one. BACKGROUND OF INVENTION Reactors, such as CANDU, employing heavy water moderation are attractive because of their ability to use natural uranium as the feed fuel. The lattice pitch needed for such D.sub.2 O reactors, however, is usually large as compared to other types of reactors using enriched fuels, resulting in a big core and large inventory of the heavy water moderator which is quite costly. It is recalled that a significant part of the capital cost of a heavy water reactor is for the D.sub.2 O inventory. As for light water reactors, it is well known that only enriched uranium fuels can make the reactor critical. However, if the uranium enrichment of the fuel can be reduced without decreasing the overall neutron multiplication effect, the fuel cost of this type of reactors will be more economic than that of the existing ones. SUMMARY OF INVENTION The invention disclosed here is to properly embed a light element such as beryllium (or its compounds), which has low (n, 2n) threshold and extremely small fast and thermal neutron capture cross sections with good neutron slowing down power, in nuclear fuel bundles positioned to obtain reactivity increases by utilizing the beryllium (n, 2n) reactions to partially slow down the fast neutrons and to gain one extra neutron in the meantime from each of these reactions. Because of these fast neutron effects of beryllium (n, 2n) reaction and the negligible capture of both fast and thermal neutrons by beryllium, the resonance absorption in the fuel, the fast leakage probability and the slowing down area are reduced and the neutron multiplication factor of a nuclear fuel element is increased if the beryllium rods are properly inserted into the fuel element. Since (n, 2n) reactions can be induced only by neutrons having energies not less than the threshold, the beryllium rods should be placed at positions inside the fuel bundle at positions where the neutrons have energies at or above the (n, 2n) threshold for the beryllium in order to effectively utilize the fast neutron effects.