The ANTECH Neutron Active Crate Counter (nACC) is an enhanced efficiency active/

passive neutron counter designed for the measurement of plutonium and uranium in nuclear

wastes contained in a variety of crates and waste boxes. Different configurations of the instru-

ment are available for different crate sizes, such as B25 and SWB. As in the design of the

LANL CTEN drum instrument, and the ANTECH model 4201 FFnC differential die away

drum counter the inner assay chamber walls contain a thick graphite liner. This graphite wall

design extends the neutron slowing-down time enhancing the thermal neutron interrogating

The nACC contains 1 inch Cd shielded 3He proportional counters in a 4π geometry. The de-

tectors in the four side walls are positioned vertically with the top and bottom detectors ori-

ented horizontally. Each detector package is connected to a high voltage junction box contain-

ing an Amptek charge sensitive amplifier and discriminator circuit and connections for high

voltage, low voltage and signal cables. Polyethylene back shielding is provided to reduce the

effects of ambient background. The outer surface is covered in stainless steel cladding and

the graphite liner is clad in aluminium. Additional detector packages mounted on the interior

walls of the chamber cavity that are used to acquire information on the degree of interrogating

neutron self-shielding in fissile material. Flux monitors are installed to correct for waste ma-

trix bias effects.

The D-T neutron generator (MF Physics, Zetatron accelerator) is mounted on a motorized

track to allow generator movement to three different positions on the crate during the assay.

This is performed to account for non-uniform interrogating flux profiles expected to occur in

the large assay chamber. In conventional active totals mode pulses of neutrons are used to in-

terrogate a waste drum. A neutron generator is pulsed at 100 Hz producing 14-MeV neu-

trons, that are slowed down in the graphite providing a source of thermal neutrons that cause

induced fission in the 235U and 239Pu in the waste. Cadmium shielded detector packages are

used to measure the prompt fission neutrons (from induced fission) in a time period (~500 µs)

when neutrons from the initial generator burst have been cleared from the fast detector pack-

ages. 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 neutrons present. The fissile mass is proportional

to the net fissile signal normalized to the interrogating flux. The system must be calibrated

for uranium and plutonium mass using representative matrices.

The passive mode operation uses conventional neutron counting of the correlated neutrons

arising from spontaneous fission of the even Pu nuclides, principally Pu-240. Plant measured

isotopic ratios (from high resolution gamma spectrometry) can be used by the software in

order to convert 240Pueffective mass to total Pu mass.

Die-away time fast detector packages: ~40ms

Die-away time (chamber): ~1 ­ 1.5ms

Cd shielded detection passive mode detection efficiency approximately 5% (empty

Optional HPGe gamma spectrometer for Pu isotopic ratio determination.

Active mode Lower Limit of Detection (LLD) for the measurement of 239Pu in an

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empty nACC chamber is of the order of 10mg. For measurement of waste in an SWB

containing a metal matrix up to ~2g/cc the LLD is estimated to be 100mg 239Pu. In

severely moderating matrices the LLD could be as high as 750mg ­ 1g 239Pu. The

LLDs for 235U measurement in similar waste matrices are likely to be higher by ap-

proximately 50% across the board.

Approximate Footprint: (L x W x H) 10' x 8' x 7'; Weight ~8000kg