Patent Number: 041486870
Section: description

DETAILED DESCRIPTION OF EMBODIMENTS FIG. 1 shows the neutron multiplication factor versus lattice pitch as calculated by a Monte Carlo code, HWCOR-SAFE, for a typical CANDU 19-rods fuel element used in the Douglas Point Nuclear Power Station in Canada. In the original Douglas Point fuel element, all 19 rods are made of natural UO.sub.2 with Zircaloy-2 cladding and the square lattice pitch is 22.86 cm. In the proposed fuel element of the present invention including a beryllium containing rod, the central rod is replaced by a Zircaloy-2 sheathed beryllium rod of the same size. The calculation results as given in FIG. 1 show that the beryllium-embedded fuel with a pitch of 22 cm has about the same neutron multiplication value as that of the original Douglas Point fuel. A reduction of 0.86 cm in pitch here is equivalent to saving of about 9% of the D.sub.2 O moderator inventory. As a second illustration of the use of an (n, 2n) scatterer in the heavy water reactor fuel, the coolant of the above-mentioned fuel is changed from D.sub.2 O to H.sub.2 O and the central fuel rod is replaced by an unfuelled tubular central supporting rod with the lattice pitch enlarged to about 27 cm to simulate a CANDU-BLW fuel such as that used in the Gentilly Nuclear Power Station in Canada. The reactivities calculated by Monte Carlo method for this fuel with and without the central tubular tie-rod replaced by a beryllium rod of the same size are shown in FIG. 2. It is seen from this figure that by inserting a beryllium rod in the center of the fuel element the lattice pitch can be reduced by about 2 cm without a decrease in reactivity, thus giving a saving of approximately 16.5% of the D.sub.2 O moderator inventory for the present case. FIG. 3 is a drawing on the CANDU 28-pins fuel element as used in the Pickering Generating Station in Canada with twelve additional beryllium rods embedded at positions as shown. From the calculated results of reactivity as given in FIG. 4, it is seen that with the twelve beryllium rods embedded in the fuel the pitch can be reduced from the original 28.58 cm to 27.4 cm without diminishing the reactivity of the original no-beryllium-embedded fuel. This, in turn, yields a saving of about 9.78% of the D.sub.2 O moderator inventory. For light water reactors, in order to simplify the calculations by Monte Carlo method, a concentrically arranged fuel element may be utilized. FIG. 5 gives an example of the proposed LWR (light water reactor) fuel element with beryllium inserted therein. Each fuel element consists of bundle of 19 enriched UO.sub.2 fuel rods and 6 smaller beryllium rods. Here, the fuel rod dimension was set to be the same as that of a typical boiling water reactor (BWR). To illustrate what achievement can be obtained by embedding beryllium rods into the fuel element as shown in FIG. 5, the reactivity calculations were done for the fuel with various uranium enrichment values and the results were plotted in FIG. 6. The uranium enrichment chosen for the fuel element with the six beryllium rods removed was 1.95 weight percent of .sup.235 U which is typical for a boiling water reactor. A square lattice pitch of 9.5 cm was used in the Monte Carlo calculations here. It is seen from FIG. 6 that the uranium enrichment of a beryllium-embedded fuel can be lessened by about 0.1 weight percent and still yields the same neutron multiplication factor as compared to the fuel without beryllium embedded.