Special Nuclear Materials detection

Nuclear Weapons and Materials

Nuclear weapons require fissile material, atoms of which can fission (split) when struck by fast or slow neutrons; pieces of this material can support a nuclear chain reaction. The fissile materials that used in nuclear weapons are uranium and plutonium.

The Atomic Energy Act of 1954 designates those materials as “Special Nuclear Material” (SNM). Uranium enriched to 20% U-235 isotope is termed highly enriched uranium, or HEU. Weapons-grade plutonium (WGPu) contains at least 93% Pu-239 isotope. 

The small amount of SNM is required to fuel a bomb – about 26 kg of HEU or 5 kg of WGPu that would fit into cubes with sides 11 cm or 6 cm, respectively.

Problem of SNM detection

International smuggling of weapons grade nuclear material presents a significant security challenge. Between 1993 and 2004, the International Atomic Energy Agency has reported 18 incidents that related to smuggling of weapons grade nuclear materials. The proliferation of nuclear weapons can take place through the borders at ports, airports, road passengers and through the postal system. A crude nuclear device constructed with highly enriched uranium (REV) poses the greatest risk of mass destruction by terrorists.

Currently Radiation Portal Monitors and Radiography systems are widely applied for security purpose at the borders and checkpoints but they can not eliminate the problem of Special Nuclear Materials detection.

Radiation portal monitors are used to detect radioactive materials passively, by sensing radiation coming out of an investigated object. But radiation signature of HEU is very small and relatively low energy so it quickly attenuates. In additional the high density of uranium makes it strongly self-shielding. This means an HEU device will be very difficult to detect at the distance for example from outside a container, even if it is otherwise unshielded, using the most sensitive of passive detection systems.

Radiography inspection system used for imaging internal content of investigation object based on its opacity to the x-ray beams. Key limitations of such systems are that they cannot differentiate between different types of material and cannot automatically detect threats. They fully depend from operator decision so time of screening is uncontrolled parameter. While a complete nuclear weapon might or might not be noticed, detection probability decreases as the threat object becomes smaller, so it would be difficult for current radiography systems to detect a small piece of HEU.

Active methods of SNM detection

The purpose of active interrogation methods – is to provide a fully automated technique for threats detection that can be performed quickly and eliminate necessity of hand inspections.

The ability of neutrons and high energy photons to induce fission in fissile materials (233U, 235U, 239Pu) is utilized in active interrogation systems for SNM detection. Fission results in the emission of neutrons and gamma rays, which are detected and analyzed. 

Photonuclear reaction

High-energy x-rays systems can be exploit for U-235 and Pu-239 isotope detection: they fission when struck by photons with energy above approximately 5.6 MeV. The resulting fission products decay over many seconds, producing prompt and delayed neutrons and gamma rays which can be easy detected. The delayed neutrons and gamma are obvious evidence of SNM presents. High energy photon interrogation has one limitation – shielding the SNM with high atomic number materials that caused strongly attenuation of x-ray beam.

Neutron interrogation

Neutrons of any energy level induced fission in U-235 and Pu-239 with very high probability, so neutron interrogation can be very efficient tool for SNM detection. But there is concern of possible shielding of SNM by hydrogenous material as neutrons are highly absorbed by hydrogen.

Ratec technology of SNM detection

Ratec technology of SNM detection exploits several mechanisms combined in one system that make possible to provide fully automated solution and eliminate intrusive hand inspection procedures. It utilized passive and actives techniques includes neutrons and photonuclear activation providing comprehensive complementary set of mechanisms for false alarm clarification. Irradiation source of special design provides small dose – small focus beam with comparatively alleviated requirements for shielding.