The WM3400 counter measures the plutonium content in 200 L (55 gal) drums by detecting coincidence neutrons from the spontaneous fission of the even numbered isotopes of plutonium. The counter is designed to make a go/no-go measurement. A JSR-12™ Neutron Coincidence Analyzer is required for coincidence counting, but not included with the WM3400 counter.

The WM3400 unit is a slab of high-density polyethylene attached to a cart. A drum positioning fixture is attached to the front of the polyethylene slab. The drum is rolled up to the counter and the measurement is made. A shelf on the back of the cart can be used to hold the JSR-12 analyzer, and the cart tabletop is available for a portable computer and printer (not included with WM3400 unit).

Six ³He tubes are placed in the slab of high-density polyethylene. The tubes are arranged in a single row, wired together and connected to one JAB-01 Amplifier/Discriminator circuit board. The JAB-01 is mounted inside a high voltage junction box. An LED indicator on the outside of the junction box indicates the proper operation of the JAB-01 channel. Electrical connections between the WM3400 counter and the JSR-12 analyzer include +5 V, HV, and a single "ORed" output signal.