Samarium compensation for nuclear reactor fuel

Claim 2

A fuel assembly for use with other fuel assemblies in a thermal nuclear reactor core, said core including control rods which are selectively movable into and out of said core to control reactivity thereof, said fuel assembly comprising an array of a plurality of spaced fuel elements containing new nuclear fuel initially lacking an equilibrium amount of samarium, said new fuel providing an excess of reactivity to compensate for said equilibrium amount of samarium which said new fuel will contain after a period of operation in said core, and an initial amount of dilute fixed gadolinium burnable poison in said fuel assembly to absorb neutrons during the period of samarium buildup in said new fuel to equilibrium to compensate for said initial lack of samarium and the said initial excess of reactivity whereby said burnable poison substantially depletes out of said fuel as the samarium builds up to the equilibrium amount, said dilute gadolinium poison having a density and distribution in the fuel assembly such that the poison will not be substantially self-shielding, said gadolinium being particularly useful in said fuel assembly as compared to other burnable poison in that it has both a neutron capture cross section which decreases more rapidly than l/v and than the cross section of samarium-149 over thermal neutron energy range of from 0.005-0.1 electron volts, and also provides either an increased ''''cold'''' shutdown margin or an increased ''''hot'''' operating reactivity margin the initial amount of said burnable poison being substantially defined by: burnable poison atoms/fuel atoms Y sigma ff/ sigma cbp where Y is the fractional yield of Sm-149 atoms per fission of fuel, sigma ff is the microscopic neutron fission cross section of the fuel, and sigma cbp is the microscopic neutron capture cross section of the burnable poison.

Claim 3

The fuel assembly of claim 1 wherein said initial amount of said burnable poison is between 1 and 3 times an amount defined by: burnable poison atoms/new fuel atoms Y sigma ff/ sigma cbp

Claim 4

The fuel assembly of claim 1 wherein said burnable poison is mixed with the fuel of said fuel elements.

Claim 5

A fuel assembly for use with other fuel assemblies in a nuclear reactor core, said core including control rods which are selectively movable into and out of said core to control reactivity thereof, said fuel assembly comprising an array of a plurality of spaced fuel elements containing new nuclear fuel initially lacking an equilibrium amount of samarium, said new fuel providing a first excess of reactivity in said core to compensate for fuel depletion during operation said core, said fuel providing a second excess of reactivity in said core to compensate for said equilibrim amount of samarium in said new fuel which said new fuel will contain after a period of operation in said core; an initial amount of fixed self-shielded burnable poison in said fuel assembly to control at least a portion of said first excess of reactivity; and an initial amount of fixed dilute gadolinium burnable poison in said fuel assembly to control said second excess of reactivity during the period of samarium build-up in said new fuel to equilibrium to compensate for said intial lack of samarium whereby said dilute burnable poison substantially depletes out of the fuel as the samarium builds up to the equilibrium amount, said dilute gadolinium poison having a density and distribution in the fuel assembly such that the poison will not be substantially self-shielding, said gadolinium being particularly useful in said fuel assembly as compared to other burnable poisons in that it has both a neutron capture cross section which decreases more rapidly than l/v and than that of Sm-149 over thermal neutron energy range of from 0.005-0.1 electron volts and also provides either an increased ''''cold'''' shutdown margin or an incReased ''''hot''''operating reactivity margin, said amount of said dilute burnable poison being about 1 to 3 times an amount defined by: burnable poison atoms/fuel atoms Y sigma ff/ sigma cbp where Y is the fractional yield of Sm-149 atoms per fission of fuel, sigma ff is the microscopic neutron fission cross section of the fuel, and sigma cbp is the microscopic neutron capture cross section of the burnable poison.

Claim 6

A fuel assembly for use with other fuel assemblies in a nuclear reactor core, said core including control rods which are selectively movable into and out of said core to control reactivity thereof, said fuel assembly comprising an array of a plurality of spaced fuel elements containing new nuclear fuel initially lacking an equilibrium amount of samarium, said new fuel providing a first excess of reactivity in said core to compensate for fuel depletion during operation said core, said fuel providing a second excess of reactivity in said core to compensate for said equilibrim amount of samarium in said new fuel which said new fuel will contain after a period of operation in said core; an initial amount of fixed self-shielded burnable poison in said fuel assembly to control at least a portion of said first excess of reactivity; and an initial amount of fixed dilute gadolinium burnable poison in said fuel assembly to control said second excess of reactivity during the period of samarium build-up in said new fuel to equilibrium to compensate for said intial lack of samarium whereby said dilute burnable poison substantially depletes out of the fuel as the samarium builds up to the equilibrium amount, said dilute gadolinium poison having a density and distribution in the fuel assembly such that the poison will not be substantially self-shielding, said gadolinium being particularly useful in said fuel assembly as compared to other burnable poisons in that it has both a neutron capture cross section which decreases more rapidly than l/v and than that of Sm-149 over thermal neutron energy range of from 0.005-0.1 electron volts and also provides either an increased ''''cold'''' shutdown margin or an incReased ''''hot''''operating reactivity margin, said amount of said dilute burnable poison being about 1 to 3 times an amount defined by: burnable poison atoms/fuel atoms Y sigma ff/ sigma cbp where Y is the fractional yield of Sm-149 atoms per fission of fuel, sigma ff is the microscopic neutron fission cross section of the fuel, and sigma cbp is the microscopic neutron capture cross section of the burnable poison.

Claim 7

The fuel assembly of claim 4 wherein said self-shielded burnable poison is gadolinium.

Claim 8

The fuel assembly of claim 4 wherein said dilute burnable poison is mixed with the fuel of at least one of said fuel elements.

Claim 9

A fuel element for use in a thermal nuclear reactor core, said core including control rods which are selectively movable into and out of said core to control reactivity thereof, said fuel element containing new nuclear fuel initially lacking an equilibrium amount of samarium which said new fuel will contain after a period of operation in said core, said new fuel providing an excess of reactivity to compensate for said equilibrium amount of samarium, and an initial amount of dilute gadolinum burnable poison in said element to absorb neutrons during the period of samarium build up in said fuel element to equilibrium to compensate for said initial lack of samarium and said initial excess of reactivity whereby said burnable poison substantially depletes out of the fuel as the samarium builds up to the equilibrium amount said dilute gadolinium poison having a density and distribution in the fuel assembly such that the poison will not be substantially self-shielding, said gadolinium burnable poison being particularly useful in said fuel element as compared to other burnable poisons in that it has both a neutron capture cross section which decreases more rapidly than l/v and than that of samarium-149 over thermal energy range from 0.005-0.1 electron volts and also provides either an increased ''''cold'''' shutdown margin or an increased ''''hot''''operating reactivity margin, said initial amount of burnable poison being substantially defined by: burnable poison atoms/fuel atoms Y sigma ff/ sigma cbp where Y is the fractional yield of Sm-149 atoms per fission of fuel, sigma ff is the microscopic neutron fission cross section of the fuel, and sigma cbp is the microscopic neutron capture cross section of the burnable poison.

Claim 10

A fuel element for use in a thermal nuclear reactor core, said core including control rods which are selectively movable into and out of said core to control reactivity thereof, said fuel element containing new nuclear fuel initially lacking an equilibrium amount of samarium which said new fuel will contain after a period of operation in said core, said new fuel providing an excess of reactivity to compensate for said equilibrium amount of samarium, and an initial amount of dilute gadolinum burnable poison in said element to absorb neutrons during the period of samarium build up in said fuel element to equilibrium to compensate for said initial lack of samarium and said initial excess of reactivity whereby said burnable poison substantially depletes out of the fuel as the samarium builds up to the equilibrium amount said dilute gadolinium poison having a density and distribution in the fuel assembly such that the poison will not be substantially self-shielding, said gadolinium burnable poison being particularly useful in said fuel element as compared to other burnable poisons in that it has both a neutron capture cross section which decreases more rapidly than l/v and than that of samarium-149 over thermal energy range from 0.005-0.1 electron volts and also provides either an increased ''''cold'''' shutdown margin or an increased ''''hot''''operating reactivity margin, said initial amount of burnable poison being substantially defined by: burnable poison atoms/fuel atoms Y sigma ff/ sigma cbp where Y is the fractional yield of Sm-149 atoms per fission of fuel, sigma ff is the microscopic neutron fission cross section of the fuel, and sigma cbp is the microscopic neutron capture cross section of the burnable poison.

Claim 11

The fuel element of claim 7 wherein said initial amount of said burnable poison is between about 1 and 3 times the amount defined by: burnable poison atoms/new fuel atoms Y sigma ff/ sigma cbp

Claim 12

The fuel element of claim 7 where said gadolinium is in the form of an oxide mixed with at least a portion of said new fuel in said fuel element.