Year
2007
Abstract
Delayed neutron activation analysis (DNAA) provides a powerful tool for rapid, cost-effective determination of the fissile isotope content of environmental samples. For more than 30 years, the Oak Ridge National Laboratory (ORNL) has operated a DNAA facility and has excelled in nondestructive analysis of fissile materials. DNAA offers significant advantages for nuclear nonproliferation determinations, including the ability to address a myriad of complex matrices such as soil, vegetation, plastic, rock, cloth, and water. The DNAA method is free from matrix effects and yields results very rapidly at less than 10 min per sample. The determinations are nondestructive and require minimal sample preparation. Materials are analyzed by comparison with laboratory-prepared standards. The DNAA facility at ORNL’s High Flux Isotope Reactor (HFIR) previously utilized BF3 detectors that yielded a detection limit of 15 pg of 235U. Well over 10,000 samples have been analyzed in this configuration. The facility has recently been upgraded with more-sensitive 3He detectors that provide an even lower detection limit. Simultaneous determination of multiple fissile isotopes in mixtures (for instance, 235U and 239Pu) may also be possible through decay curve fitting. After the installation of a cold neutron source at HFIR is complete in spring 2007, the upgraded DNAA facility will resume operation and its performance will be characterized. Because of its high throughput (less than 10 min per sample, 50–100 samples per day) and low detection limit (less than 15 pg of 235U), DNAA is useful for screening and analysis of environmental samples. Applications include wide-area monitoring and quality control. The ability to differentiate between 235U and 239Pu in environmental samples is especially valuable for safeguards purposes such as detecting undeclared fuel cycle activities.