In October 2014 a new technology was put to the test by the European Commission’s Joint Research Centre (JRC) in the search for new systems capable of tackling the illicit trafficking and development of nuclear materials. Designed around a liquid scintillator, the system uses advanced digital electronics to detect radioactive sources and nuclear material for nuclear security purposes, and to quantify fissile and special nuclear material in samples and fuel assemblies as part of safeguards against the proliferation of nuclear materials.
Safeguards play an important role in the control and monitoring of nuclear materials around the globe. They allow organisations such as the International Atomic Energy Agency (IAEA) to verify the amount of nuclear materials at civil nuclear facilities in order to ensure they are not being diverted into undeclared weapons programmes. By being able to uncover the presence of the materials used in nuclear technologies, the illegal trafficking of these materials can be targeted and prevented.
The organisation’s work is complicated by the quickly expanding use of nuclear science and technologies in applications such as the power, medicine, and agricultural industries; which is greatly increasing the number of nuclear facilities that must be monitored under the IAEA’s safeguards.
Member states use a number of international political and legal methods to commit to these safeguards – including political commitments, multilateral treaties and legally binding agreements – and the majority of these rely on self-reporting on the use of the nuclear materials. These materials include special fissionable material from which nuclear weapons or other explosive devices can be readily made – plutonium-239, uranium-233, and uranium enriched in 235 or 233 isotopes; and source materials – natural uranium, depleted uranium and thorium.
With the majority of states possessing and using these materials in a variety of non-weapons programmes, the IAEA requires the ability to independently verify that these materials are being used for these peaceful purposes only – and that they are not being transferred – in accordance with the 1970 Treaty on Non-Proliferation of Nuclear Weapons (NPT).
Each state’s provision of information on its nuclear research activities is supported by the provision of IAEA access to buildings on nuclear sites and open source information such as satellite imagery. The most important tool is the collection of environmental samples that can be analysed to determine whether or not undeclared nuclear activities are taking place, such the presence of separated plutonium or highly enriched uranium.
The last decade has seen significant development of potential tools with application here, particularly the move away from neutron detectors that use 3He gas (helium), which, being a finite resource of increasing scarcity, is giving way to the use of liquid scintillators.
A passive antineutrino liquid scintillator detector system that does not require the input of the facility operator has been demonstrated by a team from Lawrence Livermore National Laboratory and Sandia National Laboratories at the San Onofre Nuclear Generating Station (SONGS), to show how a cubic-metre-scale antineutrino detector can help monitor the operational status and thermal power of working nuclear reactors over hour-to-month timescales.
The system works by monitoring antineutrinos – the neutral particles produced in nuclear decay – to determine the operational amount of plutonium or uranium necessary to run the reactor in accordance with its declared operations, and keeps an inventory of both materials to ensure plutonium is not being diverted into other – potentially non-civil – activities.
However, liquid scintillator detectors have their challenges due to the associated hazard of having a low flash point, and this is prompting industry to develop alternative liquid scintillators with higher flashpoints and lower toxicity levels.
To address these issues, the team behind SONGS has gone on to develop prototypes that use a water-gadolinium mixture instead of liquid. Although these systems – which measure the light produced when antineutrinos collide with protons – give a fainter reading, they are much safer to operate and transport.
A safer alternative to traditional liquid scintillators has also been developed by Eljen Technology, called the EJ-309.
Pulse shape discrimination (PSD)
The system was developed as an alternative to commonly used pulse shape discrimination (PSD) liquid scintillators such as the well-known NW-231 scintillator, which is based on the solvent xylene which exhibits a high degree of solvent action and has a flash point of 77°C, making it a flammable liquid. The EJ-309, meanwhile, provides slightly poorer PSD characteristics, but has chemical properties that make it safer to use in a wider range of environmental conditions, including having a flash point of 144°C, a lower vapour pressure, and low chemical toxicity.
In the October 2014 testing conducted by the JRC together with the National Metrological Institute of Germany (Physikalisch-Technische Bundesanstalt) and the IAEA, the EJ-309 showed that it was capable of distinguishing between neutron and gamma radiation and could therefore be used for security purposes to detect nuclear materials being developed and trafficked.
The Lawrence Livermore National Laboratory has also developed a system designed to tackle the illegal transport of nuclear materials, with its ‘nuclear car wash’ solution. This system uses a high-energy neutron probe to scan sea-going cargo containers to detect the signature of fissionable material.
The need to restrict the production and transport of nuclear materials is a critical aspect of ensuring that the WMD threat posed by state-sponsored actors and terrorist organisations can be controlled. With the development of improved technologies such as these to aid nuclear non-proliferation, the IAEA will have greater capacity to monitor nuclear development activities worldwide, and ensure compliance with international standards.