Active-Passive Drum Monitors

In conventional active totals mode, pulses of neutrons are used to interrogate a waste drum placed in the measurement chamber. A neutron generator is pulsed at 100 Hz producing 14-MeV neutrons. The neutrons are slowed down in graphite providing a source of thermal neutrons that cause induced fission in the  ²³⁵U and ²³⁹Pu in the waste. Each pulse generates approximately 1 x 106 neutrons. Cadmium shielded detector packages are used to measure the prompt fission neutrons (from induced fission) in a time period (after a delay of 500 µs) when neutrons from the initial generator burst have been cleared from the fast detector packages. At a generator frequency of 100 Hz, each measurement period is ~10ms. Neutrons are counted in an early gate (0.8 ms to 3.0 ms) and a late gate (6 ms to 9.5 ms).

The measurement result is the separate sum of many early and late gates as the overall measurement time is typically 5 minutes or more. Additional ³He flux monitors are placed in the measurement chamber and provide a measure of the interrogating neutron intensity and its die-away characteristics. A later time period is used to determine the background due to delayed neutron production from fission products and from any spontaneous fission nuclides present. The fissile mass is proportional to the net fissile signal normalised to the interrogating flux. The system is calibrated for uranium and plutonium mass using representative matrices.

Passive mode operation uses conventional neutron counting of the correlated neutrons arising from spontaneous fission of the even plutonium nuclides, principally ²⁴⁰Pu. Plant measured isotopic ratios (from high resolution gamma-ray spectrometry) can be used by software in order to convert ²⁴⁰Pu effective mass to total plutonium mass.