FUELLING SYSTEM FOR NATURAL URANIUM REACTOR WITH LONG RODS FOR HIGH BURN-UP WITHOUT RECYCLING

摘要

Since natural uranium as a reactor fuel becomes progress¬ively poisoned and the effect of the poison is approximately pro¬portional to (neutron flux)2, it is advantageous to irradiate fuel first in a region of high flux and then move it to a position of lower flux. Schedules based on this principle are examined in de¬tail with application to NPD, the Canadian Nuclear Power Demonstra¬tion reactor. This is a heavy water moderated reactor, fuelled by a lattice of 109 rod assemblies of natural uranium in the form of U02.nIt is shown that on the basis of the best data available if it is possible to operate at a mean reactivity level about 12.5 milli-k below that of a fresh fuel charge in equilibrium with full xenon and samarium poisons, it should be possible to attain an average energy yield of 4,000 MWD/tonne of uranium. An improved fuelling schedule described should increase this further by 300 MWD/ tonne. Moreover, if more reactivity proves to be available, the irradiation can be extended by about 85 MWD/tonne per milli-k.