Direct conversion of the electrical energy of charged fission fragments was first proposed in 1957 by Safanov. Over the next decade, considerable effort was put into understanding the nature and control of the charged fission fragments. This gave us an understanding of the fundamental physics, but it also gave us a clear idea of the many technical challenges that limited the viability of direct energy conversion (DEC).

By the late 1960s, it was clear that DEC was not a practical system, and most research into the area came to an end. However, the US Congress funded the Nuclear Energy Research Initiative in 1999, and interest in DEC has resurfaced.

The science

In a nuclear fission, over 80% of the total energy release is the kinetic energy of the two, positively charged, major fission fragments. These fission fragments move randomly in all directions, and possess a very high positive charge. Two major challenges limit the direct capture of the energy of these fission fragments.

First, the fission fragments have a relatively short range in solid materials, so the fission reaction must occur in a solid layer that is thin enough to ensure a high probability that the fission fragment will be released from the solid layer. Significant material science and reactor engineering issues face such systems.

Second, to capture the energy of this charged particle directly, it must be decelerated through a potential of 2-4 MV. The challenges associated with maintaining such charge differentials in an intense radiation field and capturing the charged particles efficiently are significant.

The range of gross fission fragments in U3O8 is 10mg/cm2, which corresponds to about 12 microns. The yields in Figure 1 are valid for uranium or uranium oxide layers much less than 12 microns. Deposition of uniform layers less than 2 microns has been achieved previously, and increased knowledge of the properties of fission fragments (energy, atomic number and mass distributions, energy-to-charge ratios, and stopping powers) will allow computational tools to investigate DEC concepts and their application to future reactors.

Reactor concepts

In previous efforts to develop fission fragment direct energy conversion, a number of reactor concepts were proposed. Most of these concepts were based on some form of a fission electric cell called a triode. The central cathode is coated with a thin layer of fuel to permit the fission fragments to escape. The anode is held at a potential of several million volts. The space between the electrodes is evacuated to serve as an electrical insulator. When a fission fragment leaves the cathode, it has a high kinetic energy and a large positive charge. It is decelerated by the charge differential and arrives at the anode with its kinetic energy exhausted, and it deposits its charge. Large numbers of electrons (100-400) leave the cathode with each fission fragment. Most of these electrons are at low energies (below 100eV), so they can be returned to the cathode by maintaining a relatively small negative bias (20-30kV) on a series of grid wires surrounding the cathode.

Hundreds of experiments were carried out to test the performance of these early triode fission cells. Most of the work was devoted to understanding the nature of the charged particles and their control in the triode. These experiments verified the fundamental physics of the process of fission fragment DEC, but these cells did not perform well enough for them to be considered for practical applications. A number of specific problem areas were identified, including:

• The understanding of electron and ion behaviour in complex electric and magnetic fields.

• The development of insulators for high radiation environments.

• The stability of high voltage differentials in radiation environments.

• The fabrication and performance of thin film reactor fuels.

These early fission cells performed poorly. Early attempts to model the performance of the fission cells were limited to simple analysis of only a few systems parameters. A more comprehensive systems model was developed towards the end of the research effort and, even though it was limited to simple geometries, it provided new insights into the design of fission cells.

Recent developments

There have been many advances in various fields that could find application in advancing DEC technology. Specific advances include:

Maintaining high voltage differentials in radiation environments

Research in pulse power inertial confinement fusion and accelerator development programmes have dramatically improved our capabilities in maintaining high voltage differentials in radiation environments. One obvious opportunity to improve the design of the triode cell is by eliminating the grid wires and employing a magnetic field to suppress electron flow. This concept was understood at the time of previous experiments in DEC, but never tested. The magnetically insulated diode has been extensively studied since its invention in the mid 1970s in the ion beam fusion programme. A magnetically insulated fission cell is essentially the reverse of a magnetically insulated diode. The entire design of the fission cell must be re-examined based on this improved understanding.

Insulators and other material developments

Developments in space nuclear power for in-core conversion techniques such as thermionics have advanced the state of the art in insulators and related technologies, such as metal-to-ceramic seals, for high radiation environments. In many ways, the environment in a DEC fission cell is less hostile than that encountered in in-core thermionic power conversion. These technologies should address many of the issues that challenged previous researchers.

Reactor pumped laser technology

Research in reactor pumped laser technology has dramatically improved our understanding of fission fragment release from solids. Related developments in fuels technology has produced thin film fuels that have long lifetimes and are readily manufactured. Like reactor pumped lasers, direct energy conversion fission reactors will also have highly dilute reactor cores. Research in reactor pumped lasers has improved our understanding of the design issues in such systems.

Advanced simulation technology

The greatest opportunity to improve the performance of DEC fission reactors is through the use of advanced simulation. The ability to accurately predict the behaviour of the fission fragments in three-dimensional electric and magnetic fields is crucial to developing efficient concepts. Dramatic breakthroughs in this type of modelling, combined with the performance of modern supercomputers, provide tools to fully optimise the performance of such devices.

Utilisation of high voltage direct current power

The power form produced by fission fragment direct energy conversion, high voltage direct current was a major barrier to commercial utilisation 30 years ago, as it was incompatible with the power generation and transmission infrastructure. Today, the conversion between high voltage direct current and alternating current can be performed with losses of only about 0.6% of total power, and high voltage direct current power transmission is recognised as being more economical for long-distance power transmission (over 600-800km). This advantage exists even when the power conversion must be performed at both ends. High voltage direct current power generation would eliminate one of these conversion steps.

Recent research

Even though the technical challenges associated with DEC fission reactors remain formidable, the payoff for success is a revolutionary method of power production. A team consisting of researchers at Sandia National Laboratories, Los Alamos National Laboratory, General Atomics, the University of Florida, and Texas A&M University is now conducting research on these concepts, funded by a grant from the NERI programme. This three year research project will apply modern technologies to the development of DEC fission reactors. After a down-selection process, a preliminary design will be developed and assessed in terms of performance, technology development needs, ease of manufacture, and economics. At the end of this research programme, potential customers will be able to make an informed decision as to whether DEC fission reactors are now ready to enter engineering development.

To begin evaluation of alternate concepts for this reseach programme, a number of potential ‘direct’ conversion technologies were presented for consideration by members of the research teams. Although some of these concepts would not directly convert fission-fragment energy into electricity, they do take advantage of fission fragments or their ionised state, and they offer much higher conversion efficiency than thermal cycles. The concepts that are being examined are:

• Quasi-spherical magnetically insulated cell. This is an attempt to overcome deficiencies of previous work that were caused by poor performance of electric grid suppression, while simultaneously taking advantage of advanced understanding of magnetic field analysis and superconducting materials.

• Fission fragment magnetic collimater. This concept is similar to direct conversion for fusion reactors, with a different mass, energy and charge distribution than fusion fuel and ash.

• Knock-off electron collector. This attempts to collect electrical energy from electrons that are knocked free from materials as high energy fission fragments and electrons pass through them.

• Pulsed MHD generator.

• MHD generator.

• Reactor pumped laser or maser.

• Solid state converters.

• Hybrid converters.

• Radioactive isotope direct converter spin-offs.

During phase 1 of the NERI DEC project, these concepts are examined from several different perspectives. A decision will be made to screen out those that are less promising, and to select two or three concepts for concentrated research and analysis. During phase 2 of the project, NERI DEC teams will develop conceptual designs of components, systems, and reactors; will identify critical technologies; and will identify the most promising concept for further design work. In phase 3, the teams will develop a detailed design of the most promising technology, perform detailed design analyses focused on the key issues, and perform selected experiments.

Although evaluations of these concepts for DEC are not yet complete, the research team has completed an initial comparison. This comparison showed that the primary candidates for future research are the Quasi-spherical magnetically-insulated cell, the fission fragment magnetic collimater, the MHD generator, and thermophotovoltaic (TPV) hybrid converters.