Systems and methods for assaying nuclear fuel

A system for assaying nuclear fuel, comprising:

• a nuclear fuel assembly comprising nuclear-fuel-loaded structures each independently comprising:
• a cladding structure;
• and
• a nuclear fuel material within the cladding structure and comprising one or more of uranium and plutonium;
• and
• a neutron collar laterally surrounding sides of the nuclear fuel assembly and comprising pressurized 4He scintillation detectors configured and positioned to receive fast neutrons from the nuclear fuel assembly, each of the pressurized 4He scintillation detectors independently comprising:
• a housing structure filled with 4He gas having a pressure greater than or equal to about 150 bar;
• and
• at least one sensor configured and positioned to detect radiation resulting from elastic scattering of the fast neutrons within the 4He gas.

A method of quantifying nuclear material, comprising:

• surrounding an assembly comprising structures loaded with nuclear fuel with a neutron collar comprising an array of pressurized 4He scintillation detectors;
• detecting fast neutrons emitted from the assembly using the array of pressurized 4He scintillation detectors;
• and
• determining a mass of the nuclear fuel at least partially based on the quantity of fast neutrons detected by the array of pressurized 4He scintillation detectors over a predetermined period of time.