Hybrid nuclear reactor grey rod to obtain required reactivity worth

A grey rod for use in controlling reactivity of a fuel assembly in a nuclear reactor, comprising:

• (a) tubular, elongated, cladding selected from the weak neutron absorbing material group consisting of stainless steel, zirconium and INCONEL;
• (b) a first cladding insert including a first plurality of pellets selected from the strong, neutron absorbing material group consisting of hafnium, silver-indium-cadmium, and boron carbide;
• and
• (c) a second cladding insert including a second plurality of pellets selected from the weak neutron absorbing material group consisting of stainless steel, INCONEL, zirconium and ZIRCALOY,
• wherein said first and second inserts extend substantially along the length of the cladding, and
• wherein each of the pellets of the first cladding insert has a same height shorter than a same height of each of the pellets of the second cladding insert.

A control arrangement for use with a nuclear fuel assembly having a plurality of reactivity control rods and a plurality of guide thimbles, said arrangement providing supplemental reactivity control and allowing zero boron change load follow mode, comprising:

• (a) a spider assembly;
• and
• (b) a plurality of grey rods supported at their upper ends from said spider assembly and extending generally parallel to one another downwardly therefrom for insertion into the guide thimbles of the fuel assembly,
• wherein each grey rod includes
• (i) a tubular, elongated, cladding selected from the weak neutron absorbing material group consisting of stainless steel, zirconium and INCONEL,
• (ii) a first cladding insert including a first plurality of pellets selected from the strong neutron absorbing material group consisting of hafnium, silver-indium-cadmium, and boron carbide, and
• (iii) a second cladding insert including a plurality of pellets selected from the weak neutron absorbing material group consisting of stainless steel, INCONEL, zirconium and ZIRCALOY,
• wherein said first and second inserts extend substantially along the length of the cladding, and
• wherein each of the pellets of the first cladding insert has a same height shorter than a same height of each of the pellets of the second cladding insert.

A control arrangement for use with a nuclear fuel assembly having a plurality of reactivity control rods and a plurality of guide thimbles, said arrangement providing supplemental reactivity control and allowing zero boron change load follow mode, comprising:

• (a) a spider assembly;
• and
• (b) a plurality of grey rods supported at their upper ends from said spider assembly and extending generally parallel to one another downwardly therefrom for insertion into the guide thimbles of the fuel assembly,
• wherein each grey rod includes
• (i) a tubular, elongated, cladding selected from the weak neutron absorbing material group consisting of stainless steel, zirconium and INCONEL,
• (ii) a first cladding insert including a first plurality of pellets selected from the strong neutron absorbing material group consisting of hafnium, silver-indium-cadmium, and boron carbide, and
• (iii) a second cladding insert including a plurality of pellets selected from the weak neutron absorbing material group consisting of stainless steel, INCONEL, zirconium and ZIRCALOY,
• wherein said first and second inserts extend substantially along the length of the cladding, and
• wherein pellets from the respective pluralities alternate.