Development of the Neutron-Gamma-Neutron (NGN) approach for the fresh and spent fuel assay

Övrigt konferensbidrag, 2012

In connection with current safety and complexity limitations for installations which use keV neutrons for irradiation in the fresh and spent fuel assay, in medicine, geophysical and detection fields, there is a long felt need of effective, light, inexpensive systems with longer lifetimes and the possibility to ”switch off” the source during transportation and work break. The present paper discusses an opportunity of creating a novel technique based on using a pulsed neutron generator, hydrogen moderator and beryllium, which will enable creation of a compact and inexpensive facility capable to satisfy all requirements. The main idea of the proposed method consists in using photonuclear reaction in beryllium due to gamma irradiation originating from (n,gamma) reaction in a hydrogen containing moderator. After the neutron pulse of a modern compact DD neutron generator in hydrogen-containing moderator, high-energy neutrons (approximately 2.5 MeV) get slowed down mainly by elastic scattering to near thermal energies. At thermal energies, the neutrons diffuse through the material until they undergo thermal capture (capture is dominated by hydrogen neutron absorbers). When a hydrogen atom captures a thermal neutron, it turns into deuterium with the release of a large component of 2.23 MeV gamma-rays. Beryllium is one of the few elements in nature that undergoes a photonuclear reaction with this gamma energy range (e.g. deuterium has a low energy 2.225 MeV photonuclear threshold). Thus, as the beryllium has lower neutron-binding energy, 1.667 MeV (photonuclear reaction threshold), a photonuclear reaction will take place with emission of neutrons with energy defined by the kinematic equation. Thus, this method allows to obtain a high quality epithermal neutron beam without using a complex system with particle accelerator or radioisotope sources with limited decay time. The proposed method has been studied using Monte Carlo simulations, which made it possible to define the theoretical limits of the use of the NGN approach in industrial applications. The paper will presents the results of these investigations.