Method and apparatus for measuring the reactivity of a spent fuel assembly

The combination comprising:

• a nuclear fuel assembly containing therein an array of longitudinally extending, spaced apart, hollow tubes at least two of which are adapted to telescopingly receive individual elements of a control element assembly;
• a first rod having outer dimensions substantially equal to those of a single control element;
• a second rod having outer dimensions substantially equal to those of a single control element;
• a neutron source fixedly attached to the first rod;
• a neutron flux detector fixedly attached to the second rod;
• means for positioning the first and second rods into selected first and second tubes such that the source and detector are at selected relative elevations within the respective tubes;
• and
• means for receiving the output signal from the detector and for converting the output signal to a remote flux measurement.

A method for comparing the subcritical multiplication factor between two movable nuclear fuel assemblies containing therein an identical array of at least two longitudinally extending, spaced apart hollow tubes, comprising the steps of:

• (a) supporting a first assembly such that the assembly is in a unique location and orientation with respect to the surrounding environment;
• (b) inserting a neutron source to a predetermined position within a first tube of the assembly;
• (c) inserting a neutron flux detector to a predetermined position within a selected second tube of the assembly;
• (d) remotely measuring the output signal from the detector;
• (e) removing the source and detector from the first assembly;
• (f) removing the first assembly from the unique location;
• (g) supporting a second assembly in the unique location and orientation of step (a);
• (h) repeating steps (b) through (d);
• (i) comparing the measurements from the first and second assemblies.

A method for loading nuclear fuel assemblies from a transfer area into a standard storage area where the effective reactivity of each stored assembly may not exceed that of a standard assembly having a known effective reactivity and subcritical multiplication, and wherein each assembly and the standard assembly contain therein an identical array of longitudinally extending, spaced apart, hollow tubes, comprising the steps of:

• (a) supporting the standard assembly in a unique location and orientation in the transfer area;
• (b) inserting a neutron source to a predetermined position within a first tube of the standard assembly;
• (c) inserting a neutron flux detector to a predetermined position within a selected second tube of the standard assembly;
• (d) obtaining a standard output measurement from the detector commensurate to the subcritical multiplication of the standard assembly;
• (e) removing the source and detector from the standard assembly;
• (f) removing the standard assembly from the unique location;
• (g) supporting in said unique location and orientation one of the assemblies to be loaded into the storage area;
• (h) inserting the neutron source to the predetermined position within said one assembly;
• (i) inserting the neutron flux detector to the predetermined position within said selected second tube of said one assembly;
• (j) obtaining an output measurement from the detector commensurate to the subcritical multiplication of said one assembly;
• (k) removing the source and detector from said one assembly;
• (l) loading said one assembly into the standard storage area only if the measurement of step (j) does not exceed the measurement of step (d);
• (m) repeating steps (g) through (l) for the remainder of the assemblies.