Chapter 16: Nuclear Applications – Nuclear Accelerators and Accelerator-Driven Systems

According to the World Nuclear Association (Updated August 2018):

• Powerful accelerators can produce neutrons by spallation;
• This process may be linked to conventional nuclear reactor technology in an accelerator-driven system (ADS) to transmute long-lived radioisotopes in used nuclear fuel into shorter-lived fission products; and
• There is also increasing interest in the application of ADSs to running subcritical nuclear reactors powered by thorium.

Used fuel from a conventional nuclear power reactor contains a number of radionuclides, most of which (notably fission products) decay rapidly, so that their collective radioactivity is reduced to less than 0.1 percent of the original level 50 years after being removed from the reactor. However, a significant proportion of the wastes contained in used nuclear fuel is long-lived actinides (particularly neptunium, americium and curium). In recent years, interest has grown in the possibility of separating (or partitioning) the long-lived radioactive waste from the used fuel and transmuting it into shorter-lived radionuclides so that the management and eventual disposal of this waste is easier and less expensive.

The transmutation of long-lived radioactive waste can be carried out in an accelerator-driven system (ADS), where neutrons produced by an accelerator are directed at a blanket assembly containing the waste along with fissionable fuel. Following neutron capture, the heavy isotopes in the blanket assembly subsequently fission, producing energy in doing so. ADSs could also be used to generate power from the abundant element thorium.

1        NUCLEAR ACCELERATORS:

An Electrostatic Nuclear Accelerator is one of the two main types of particle accelerators, where charged particles can be accelerated by subjection to a static high voltage potential. The static high voltage method is contrasted with the dynamic fields used in oscillating field particle accelerators. Owing to their simpler design, historically these accelerators were developed earlier. These machines are operated at lower energy than some larger oscillating field accelerators, and to the extent that the energy regime scales with the cost of these machines, in broad terms, these machines are less expensive than higher energy machines, and as such, they are much more common. Many universities worldwide have electrostatic accelerators for research purposes.

 

Historically, the first accelerators used simple technology of a single static high voltage to accelerate charged particles. While this method is still extremely popular today, with the electrostatic accelerators greatly out-numbering any other type, they are more suited to lower energy studies owing to the practical voltage limit of about 30 MV (when the accelerator is placed in a gas with high dielectric strength, such as sulfur hexafluoride, allowing the high voltage). The same high voltage can be used twice in a tandem accelerator if the charge of the particles can be reversed while they are inside the terminal. This is possible with the acceleration of atomic nuclei by first adding an extra electron or forming an anionic (negatively charged) chemical compound, and then putting the beam through a thin foil to strip off electrons inside the high voltage conducting terminal, making a beam of positive charge.

High current, high-energy accelerators or cyclotrons are able produce neutrons from heavy elements by spallation. A number of research facilities exist which explore this phenomenon, and there are plans for much larger ones. In this process, a beam of high-energy protons (usually >500 MeV) is directed at a high-atomic number target (e.g. tungsten, tantalum, depleted uranium, thorium, zirconium, lead, lead-bismuth, mercury) and up to one neutron can be produced per 25 MeV of the incident proton beam (These numbers compare with 200-210 MeV released by the fission of one uranium-235 or plutonium-239 atom.) An average fission event of U-235 releases 200 MeV of energy and is accompanied by the release of an average of 2.43 neutrons.

More than 15,000 accelerators are in use around the world. More than 97 percent of accelerators are used in commercial applications, such as the creation of ceramics, insulators, and plastics. Accelerators of many different designs have been developed. Some common accelerator applications include:

• Diagnosing and Treating Cancer;
• Locating Oil and Minerals in the earth;
• Processing Semiconductor Chips for Computers;
• Sterilizing Medical Equipment and Food Products; and
• Determining the age of Materials through Radiocarbon Dating

Both the radiation produced during operation and the radioactive waste created from operation, pose worker safety issues. During operations, accelerators produce x-rays. X-rays can be an external radiation hazard to those who work in close proximity to an accelerator. One of the major benefits of accelerators is that, unlike radioactive sources, they only produce radiation when they are operated. However, radioactive waste is produced during their operation. This waste is generally short-lived; decaying in less than one year and may be stored at laboratories or production facilities until it is no longer radioactive. An extremely small fraction of the waste can remain radioactive for more than one year.

Given the human resources challenges in nuclear science and technology, small accelerators are also increasingly incorporated in nuclear science and technology academic curricula to help develop students’ general and subject specific skills. In 2009, Ghana established a National Accelerator Facility to further strengthen institutional capacity to support research and human resource development. Small accelerators, in particular, provide opportunities to acquire hands-on knowledge and experience, opportunities usually not available at larger facilities.

2.       ACCELERATOR-DRIVEN SYSTEMS (ADS):

Powerful accelerators can produce neutrons by spallation. Spallation is the process where neutrons are ejected from a heavy nucleus being hit by a high-energy particle. In this case, a high-energy proton beam directed at a heavy target expels a number of spallation particles, including neutrons.

The spallations have only a very small probability of causing additional fission events in the target. However, the target still needs to be cooled due to heating caused by the accelerator beam. If the spallation target is surrounded by a blanket assembly of nuclear fuel, such as fissile isotopes of uranium or plutonium (or thorium-232 which can breed to U-233), there is a possibility of sustaining a fission reaction. This is described as an accelerator-driven system (ADS). Accelerator-driven systems are also referred to as energy amplifiers since more energy is released from the fission reactions in the blanket assembly than is needed to power the particle accelerator. Professor Carlo Rubbia, a former director of the international CERN laboratory, is credited with proposing the concept of the energy amplifier, using natural thorium fuel.

Typically, in ADS, the neutrons produced by spallation would cause fission in the fuel, assisted by further neutrons arising from that fission. Up to 10 percent of the neutrons could come from the spallation, though it would normally be less, with the rest of the neutrons arising from fission events in the blanket assembly. ADS can only run when neutrons are supplied to it because it burns material, which does not have a high enough fission-to-capture ratio for neutrons to maintain a fission chain reaction. One then has a nuclear reactor, which could be turned off simply by stopping the proton beam, rather than needing to insert control rods to absorb neutrons and make the fuel assembly sub critical. Because they stop when the input current is switched off, ADS are seen as safer than normal fission reactors.

Used fuel from a conventional nuclear power reactor contains a number of radionuclides, most of which (notably fission products) decay rapidly, so that their collective radioactivity is reduced to less than 0.1 percent of the original level 50 years after being removed from the reactor. However, a significant proportion of the wastes contained in used nuclear fuel are long-lived actinides (particularly neptunium, americium and curium). In recent years, interest has grown in the possibility of separating (or partitioning) the long-lived radioactive waste from the used fuel and transmuting it into shorter-lived radionuclides so that the management and eventual disposal of this waste is easier and less expensive.

The transmutation of long-lived radioactive waste can be carried out in an accelerator-driven system (ADS), where neutrons produced by an accelerator are directed at a blanket assembly containing the waste along with fissionable fuel. Following neutron capture, the heavy isotopes in the blanket assembly subsequently fission, producing energy in doing so. ADS could also be used to generate power from the abundant element thorium. 

The technology available to accelerator designers and builders of today is substantially different from, and superior to, that which was utilized in early ADS studies, in particular in the accelerator design that was considered in the 1996 National Research Council report.  Since the publication of that report, there have been several key advances in accelerator and target technology that are summarized here, and explained in more detail in the sections that follow:

• The construction, commissioning and operation of a high-power continuous wave front-end system that meets the beam current performance required for up to 100 mA ADS accelerator system (the Low-Energy Demonstration Accelerator (LEDA) at Los Alamos);
• The construction, commissioning and MW-level operation with acceptable beam loss rates of a modern linear accelerator based on independently-phased superconducting accelerating structures (the Spallation Neutron Source at ORNL);
• The construction and deployment of a wide variety of pulsed and continuous-wave superconducting accelerating structures for proton/ion acceleration over a wide range in particle velocities, which is a key ingredient to achieving high reliability operation; and
• The high-power beam test of a liquid Pb-Bi eutectic spallation target loop at the Paul Scherrer Institute in Switzerland (the MEGAPIE project).

Perhaps more importantly, recent analyses of sub critical reactor response to beam interruptions reveal greater tolerance to and therefore more relaxed requirements for beam trips, which had been a key criticism of ADS concepts, as explained below.

The range of applications of ADS technology discussed within the worldwide community span four missions. These missions are ordered in increasing complexity, increasingly stringent beam requirements, greater development time and cost. Presumably, each successive mission would build upon the technological developments of the preceding mission.

ADS may be employed for the following applications:

2.1       Transmutation Demonstration:

The demonstration of ADS and transmutation technologies in a flexible research facility in which a sub-critical core is coupled to a MW-scale proton accelerator. This requires building a prototypic accelerator, target and fuel blanket to operate with low power density, an order of magnitude lower than an industrial scale facility.

A facility for Transmutation Demonstration, being primarily a test-bed and research facility, requires a beam power of 1-2 MW to deliver a thermal power of 50-100 MW, depending on the neutron multiplication factor of the sub critical assembly. This can be realized within a broader range of available beam energies, from approximately 0.5 GeV to as much as 3 GeV, with continuous-wave (CW) beams, and perhaps even with a pulsed beam.

What was claimed to be the world’s first ADS experiment was begun in March 2009 at the Kyoto University Research Reactor Institute (KURRI), utilizing the Kyoto University Critical Assembly (KUCA). Japan’s Ministry of Education, Culture, Sports, Science and Technology (MEXT) commissioned the research project. The experiment irradiates a high-energy proton beam (100 MeV) from the accelerator on to a heavy metal target set within the critical assembly, after which the neutrons produced by spallation are bombarded into a subcritical fuel core.

The Indian Atomic Energy Commission is proceeding with design studies for a 200 MWe PHWR accelerator-driven system (ADS) fuelled by natural uranium and thorium. Uranium fuel bundles would be changed after about 7 GWd/t burn-up, but thorium bundles would stay longer, with the U-233 formed adding reactivity. This would be compensated for by progressively replacing some uranium with thorium, so that ultimately there is a fully-thorium core with in situ breeding and burning of thorium. This is expected to mean that the reactor needs only 140 tU through its life and achieves a high burnup of thorium – about 100 GWd/t. A 30 MW accelerator would be required to run it.

The Belgian Nuclear Research Centre (SCK.CEN) is planning to begin construction on the MYRRHA (Multipurpose Hybrid Research Reactor for High-tech Applications) research reactor at Mol in 2015. Initially it will be a 57 MWt ADS, consisting of a proton accelerator delivering a 600 MeV, 2.5 mA (or 350 MeV, 5 mA) proton beam to a liquid lead-bismuth (Pb-Bi) spallation target that in turn couples to a Pb-Bi cooled, sub critical fast nuclear core.

2.2       Industrial Scale Transmutation:

A facility for transmutation of nuclear waste on an industrial scale.  Such a facility would require a beam power of at least 10 MW and as high as 75 MW, depending on the specific design. The produced heat may be utilized without direct connection to the power grid.

Industrial scale systems, being fully optimized with regard to system cost and technology, require beam energies nearer the peak in neutron yield, in the range of ~1-2 GeV. Tens of MW of continuous wave beam power are required, yielding thermal power in the GW range. The range in beam power evident in the existing worldwide designs reflects the range in neutron multiplication factors, thermal power, burn-up compensation requirements and accelerator system redundancy.

It was also reported that the US nuclear industry operates on a “once through” nuclear fuel cycle. All of the used fuel (currently about 60,000 metric tons) is stored on the sites of operating nuclear plants, with about 2,000 metric tons added each year. The President has convened a Blue Ribbon Commission on America’s Nuclear Future to conduct a comprehensive review of policies for managing the back end of the nuclear fuel cycle, including all alternatives for the storage, processing, and disposal of used nuclear fuel.

One option offering several potential benefits is to recycle the used LWR fuel. The benefits of recycling are i) better resource utilization and a reduction in the amount of uranium that must be mined, and ii) a substantial reduction in the volume, heat load, and radiotoxicity of the high-level waste that must ultimately be emplaced in a geologic repository.

France, Great Britain, Russia and recently Japan have major recycling facilities that are capable of reprocessing several hundred tons or more of used nuclear fuel annually. These are large, expensive (~$10B) government-owned facilities that use the PUREX process to extract the U and Pu from the used fuel, which are then recycled as mixed oxide (MOX) fuel in LWRs. The remaining portion of the used LWR fuel is vitrified, and the intent is to place this waste in geologic repositories.

To date no country employs a fuel cycle that destroys the minor actinides (MA) present in used LWR fuel. Minor actinide destruction through transmutation is one mission that ADS are well suited to address. Unlike critical fast reactors, which generally incorporate uranium or thorium in the fuel for safe operation, ADS can potentially operate on a pure MA feed stream, meaning a smaller number of ADS can be deployed to burn a fixed amount of minor actinides.

ADS can recycle the MA multiple times until it is completely fissioned, such that the only actinide waste stream from these systems would derive from the recycling residuals, which could yield a significant reduction (by a factor of hundreds) in the amount of actinide waste per kW-hr of electricity generated, as compared to a once-through fuel cycle. Because accelerator driven systems do not require fuels containing uranium or thorium, they are more efficient at destroying MA waste – up to seven times more efficient according to one study – than critical reactors, based on grams of minor actinides fissioned per MW-hr of energy generated.

2.3       Industrial Scale Power Generation with Energy Storage:

A power generation facility that utilizes energy storage technology – developed for solar and wind energy – to mitigate lengthy beam interruptions. Such a system could burn minor actinide fuel to also fulfill a transmutation mission, or could burn thorium-based fuel for the purposes of power generation and 233U production.

A facility for transmutation of waste would also generate substantial power; the process heat could be utilized to produce another form of energy (e.g. biofuels) or could be used to generate electrical power.

Many proposed ADS concepts with the goal of power production utilize thorium-based fuel to take advantage of some of its benefits, including greater natural abundance (3-4 times greater than uranium), proliferation resistance, and significantly reduced production of transuranics which are a major source of radiotoxicity and decay heat relative to uranium-based fuel. Both liquid and solid fuel blankets have been proposed. An ADS system based on Th fuel would not require incorporation of fissile material into fresh fuel, and could operate almost indefinitely in a closed fuel cycle.

A limited number of critical reactor concepts based on thorium have been designed and operated (e.g., the Molten Salt Reactor at ORNL, and the Light Water Breeder Reactor at Shippingport). Expanded use of thorium-based fuels is actively pursued in some countries with large reserves of thorium, principally India, Norway and China. These programs are investigating whether ADS can speed up the deployment of the 233U-Th fuel cycle by breeding 233U, which does not exist in nature; and

2.4       Industrial Scale Power Generation:

A power generation facility that burns either transuranics or thorium-based fuel and is an integral part of the electric grid.

An accelerator driven system consists of a high-power proton accelerator, a heavy-metal spallation target that produces neutrons when bombarded by the high-power beam, and a sub-critical core that is neutronically coupled to the spallation target. To achieve good neutronic coupling the target is usually placed at the center of a cylindrical core. The core consists of nuclear fuel, which may be liquid (e.g., molten salt) or solid as in conventional nuclear reactors.

During the 1990’s, the Defense Programs Office within the US Department of Energy (DOE) funded the Accelerator Production of Tritium (APT) program to evaluate the efficacy of using high-power accelerators for producing tritium for the US stockpile. Many technical advances in accelerator technology resulting from this program are directly applicable to high-power accelerators for meeting the ADS missions. In addition, the accelerator and target technologies required for ADS applications have much in common with accelerators and targets that have been developed since then for scientific application at spallation neutron sources and nuclear and particle physics accelerator facilities.

Resources:

1. Wikipedia – Particle Accelerator;
2. Wikipedia – Oscillating Particle Accelerators;
3. Wikipedia – Dielectric Strength;
4. Wikipedia – Sulfur Hexafluoride;
5. Wikipedia – Atomic Nucleus;
6. World Nuclear Association – Accelerator-driven Nuclear Energy;
7. Rad Town USA – Industrial Particle Accelerators;
8. Rad Town USA – Radiation Therapy – External Beam; and
9. Science Energy – Accelerator and Target Technology for Accelerator-driven Transmutation and Energy Production.

• This chapter was published on “Inuitech – Intuitech Technologies for Sustainability”
  on February 10, 2012; and
• This chapter was updated on 20 June 2020.

Chapter 17 …