Process of operating a nuclear reactor to minimize production of U-232

The method of operating a light-water cooled nuclear power reactor containing both fissile fuel elements and fertile fuel elements in predetermined arrangements, said arrangements comprising a central core having a plurality of first fuel elements therein, each said first fuel element comprising a central section of fissile uranium oxide pellets, outer sections of hollow, thin walled thorium oxide pellets, plenum sections interposed between said central and outer sections, spring and spacer means positioned within said plenum sections, a moderating material positioned on at least one side of each of said plenum sections, cladding enclosing said pellets, a core barrel spaced from and surrounding said central core, a second set of fuel elements spaced from and surrounding said central core and positioned between said central core and said core barrel in spaced relation thereto, a third set of fuel elements surrounding said core barrel in spaced relation thereto, the fuel in said second and third sets of fuel elements consisting of fertile thorium, each of the fuel elements in said second and third sets comprising a plurality of stacked thorium oxide pellets within a cladding wherein all of said thorium oxide pellets are hollow, said hollow thorium oxide pellets having inner diameters which vary axially from a maximum at the radial center of the fuel element to a minimum at each end of the fuel element, and characterized by minimum production of U-232 due to n,2n reactions on thorium comprising the steps:

• (A) initially charging said light-water nuclear power reactor with said first, second and third sets of fuel elements in said predetermined arrangements;
• (B) irradiating said first, second and third sets of fuel elements at an average driver fuel rod power output of approximately 7 kw thermal per foot, annually removing and replacing preselected ones of said first fuel elements such that the amount of U-232 mixed with the U-233 in the removed irradiated thorium initially averages 300 to 500 parts per million;
• (C) continuing said irradiation of said second and third sets of fuel elements for a period of two to three years at a core power density of 50 to 100 kw thermal per liter;
• (D) thereafter removing and replacing the fuel elements in said second and third sets such that the amount of U-232 mixed with the U-233 in the removed irradiated thorium initially averages 300 to 500 parts per million;
• (E) further continuing steps (B) through (D).