Date: Dec 28, 2012 Source: Green Car Congress (
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Muons, Inc., a private-sector high-energy accelerator physics firm, and ADNA (Accelerator-Driven Neutron Applications) Corp., are proposing using spent nuclear fuel (SNF), natural uranium, or excess weapons-grade plutonium (W-Pu) in a proposed GEM*STAR accelerator-driven subcritical reactor (ADSR) to provide the process heat and steam for the Fischer-Tropsch production of synthetic diesel from natural gas and other carbonaceous feedstocks (e.g., biomass or coal).
An initial proposed plant using GEM*STAR's with the Fischer-Tropsch process would produce 70 million gallons of diesel fuel per year at an estimated cost of production of less than $2.00 per gallon, according to the company, while also dealing with the issue of waste nuclear materials.
Current nuclear power relies on fission—a reaction when the nucleus of an atom, having captured a neutron, splits into two or more nuclei, and in so doing, releases a significant amount of energy as well as more neutrons. These neutrons then go on to split more nuclei and a chain reaction takes place. The only materials in significant quantity capable of a sustained nuclear reaction are U235 and Pu239, which has been created in U235-based reactors. A critical mass of either of these isotopes will create enough neutrons to sustain a chain reaction. However, other materials including nuclear waste from conventional reactors, or natural uranium can be used to produce energy if additional neutrons can be added.
The two known methods to add neutrons use 1) fast breeders reactors or 2) particle accelerators similar to those developed for basic physics research. While the development of fast breeder reactors has been on hold in the USA because of nuclear weapon proliferation concerns, particle accelerator technology has reached the point where spallation neutrons can be produced in sufficient quantity for practical ADSR uses.
—Bowman and Johnson (2011)
Sub-critical reactors run by design with fewer neutrons than required to sustain the chain reaction; the external source (the accelerator) supplies the extra neutrons required to maintain the reaction, and thus enable the use of other nuclear materials such as SNF and W-Pu. In theory, the reactor is inherently safe; when then external neutron source is turned off, the reaction inside the reactor stops.
The external neutrons are provided by the interaction of accelerated charged particles with matter. The most widely proposed systems use high energy protons; the nuclear reaction of high energy protons with nuclei is called the spallation process. The accelerator, says Roger Barlow, one of the pioneers of work on ADSRs and president of ADNA, requires about 5-10% of the power generated by the reactor to run.
Challenges with ADSRs include accelerator performance; spallation target performance; and the subcritical reactor itself. Design decisions for ADSRs include:
The type of neutron spectrum: fast or thermal.
The type of fuel: solid (metallic, oxides, nitrides, carbides, etc.) or liquid (fluorides, chlorides).
The type of spallation target: lead, lead-- bismuth, tungsten, molten salt, etc.
The nature of the cooling agent: gas, molten metal, molten salt.
The accelerator system: cyclotrons or LINACs.
Adna
Conceptual design of a GEM*STAR reactor in underground placement. The vertical dimension is about 30 ft. The gray box is the graphite reflector for the core. Horizontal beams from two accelerators are shown at the top being bent by magnets about 45° into the core where both strike a target shown schematically in the center of the core. The tank below shows an accumulation of salt that overflowed into it from the free salt surface. Space is provided around the outside of the reactor for convective flow of air down to the bottom and then up by the core for passive removal of decay heat if required. Source: ADNA. Click to enlarge.
The GEM*STAR (Green Energy Multiplier*Subcritical Technology for Alternative Reactors) being developed by ADNA Corp. and Muons, is a molten-salt based ADSR that uses a linear accelerator to produce spallation neutrons.
Molten salt fuel can include natural uranium, depleted uranium, natural thorium, excess W-Pu, and SNF from conventional Light Water Reactors. The Molten Salt Fuel technology was successfully demonstrated at the ORNL Molten Salt Reactor Experiment (MSRE) conducted from 1965 to 1969.
A molten-salt fuel mixture is held in a graphite-reflector, Hastelloy-N container which also contains the heat exchanger (non-radioactive) liquid salt. Beams of energetic protons hit targets to cause spallation neutrons to enter the fuel mixture. Volatile radioactive by-products are constantly purged by a flow of helium gas.
The helium purging greatly reduces the possibility of accidental releases of radioactivity—a well-known problem of technologies that employ fuel rods that must contain years of built-up radioactive volatile elements.
ADSR using molten-salt fuel has impressive advantages: 1) ability to burn any number of materials including conventional reactor waste, excess plutonium from weapons, and very abundant thorium; 2) exceptional safety advantages including subcriticality to eliminate Chernobyl disasters, 3) no build-up of volatile radioactive elements to eliminate 3-Mile Island problems; 4) no storage of solid nuclear waste that can catch fire.
—Bowman and Johnson (2011)
In a recent talk at Oak Ridge National Laboratory, Dr. Rolland P. Johnson of Muons, suggested that the molten salt fuel and the relaxed availability requirements of process heat applications—such as the production of synthetic fuel—imply that the required accelerator technology is available now.
Gemstar2
Conceptual F-T production using process heat from GEM*STAR and coal as a feedstock, from a 2010 ADNA presentation. Click to enlarge.
One 10 MW proton accelerator feeding four GEM*STAR units could burn and destroy the 34 metric tons of excess weapons-grade plutonium slated to be destroyed by the US-Russian Plutonium Management and Disposition Agreement (PMDA) to provide the US DOD with green diesel fuel to satisfy most of its needs for the next 30 years, Johnson suggested.
The current cost estimates for the first pilot GEM*STAR power project is $25 million in design costs and $500 million in construction costs ($200M for the accelerator, $150M for the Reactor and $150M for the Fischer Tropsch diesel producing plant). To burn 34 Tons of W-Pu in 30 years for the National Nuclear Security Administration would require the 10 MW proton accelerator ($600M) and four GEM*STAR units ($300M/each).
Recently, a Chinese roadmap for long-term development of accelerator-driven subcritical (ADS) technology was proposed by the Chinese Academy of Sciences and a start-up budget of $260 million has been approved for ADS test setup construction. The Chinese ADS program was started in the first half of 2011.
