“To provide [electricity] in today’s world, an ‘advanced reactor’ must improve over existing reactors in the following 4-core objectives. It must produce significantly less costly, cost-competitive clean electricity, be safer, produce significantly less waste and reduce proliferation risk. It is not sufficient to excel at one without regard to the others.” Dr. Christina Back, Vice President, Nuclear Technologies and Materials for General Atomics, May 2016 testimony before the US Senate Energy and Natural Resources Committee hearing on the status of advanced nuclear technologies.
There are eight new small modular reactors on a fast track to generic approval in the US – and more than 45 designs under development globally. A version of the high temperature, fast neutron technology was first built in Germany in the early 1960’s, a prototype was run at the Los Alamos National Laboratory in the late 1960’s, a demonstration plant is being constructed in China after a decade of running a prototype there. In February 2010, General Atomics (GA) announced a modified version of its gas turbine modular helium reactor (GT-MHR) as a fast neutron reactor – the Energy Multiplier Module (EM2). The project team is led by Dr. Christina Back. The EM2 concept is for a modular, 240-265 MWe helium cooled, fast neutron, high temperature reactor operating at 850°C. The company anticipates a 12 year development and licensing period. GA has teamed up with Chicago Bridge & Iron, Mitsubishi Heavy Industries, and the Idaho National Laboratory to develop the EM2.
The EM2 is a technology evolution designed to a performance specification that included cost, safety, fabrication, installation and operations that give it outstanding potential to change the energy landscape. The fast-neutron reaction first converts fertile material – including nuclear waste, uranium, plutonium and thorium – to fissile material which then splits under neutron bombardment to produce heat and lighter elements. Inert helium moving through and around the core is heated and drives a high efficiency Brayton cycle gas turbine. Helium is then cycled back through the reactor. Helium – rather than water – cooling enables siting flexibility in a footprint that is ten times less than conventional nuclear plants. The small modular design allows network grids to be developed in place of large and expensive grids shuffling energy across whole continents. A huge cost advantage in regions without existing electricity grids. Or indeed in regions with high penetration of wind and solar electricity generation – where grid augmentation is one solution to the wind and solar intermittency problem.
High temperature operation enables efficient conversion of heat to electricity. High efficiency – 50% greater than conventional nuclear – cuts costs of power by 40%. Sufficient to make it cost competitive against natural gas generation in the USA at a gas price of $6-$7/MMBtu. Natural gas prices are about half that at the moment – suggesting that a natural gas to advanced nuclear strategy is a feasible energy future for the US. The capital cost is estimated at less than $1.5 billion per modular unit – a far lower investment risk than conventional nuclear plants.
Source: General Atomics
Additionally, high temperatures enables efficient hydrogen production with what would otherwise be waste power and heat in low load periods. Liquid fuels compatible with existing infrastructure can be produced from hydrogen and captured carbon dioxide. It is not technically difficult. Audi estimated a cost of “e-diesel” at Au$2-$3/l. Liquid fuels are likely to be part of the energy mix for a considerable time to come.
The core design uses uranium carbide particles that are sintered into porous fuel plates and isolated from the main helium coolant flow with a silicon carbide coating. Silicon carbide is stable up to 20000C and won’t meltdown under any conditions. In normal operation heat is diverted to the turbine or passively circulated through 100% redundant heat sinks. The porous fuel plates provides for gases generated during fission to be vented and scrubbed through filters. The backup to the backups is full containment in an underground, reinforced concrete bunker.
The reactor units would be factory built, delivered on trucks and dropped into place – anywhere as they don’t need water cooling – for decades of hands free operation powering 350,000 houses per unit. The promise of EM2 is to deliver low cost power for centuries to come from spent nuclear fuel currently stockpiled – burning almost one hundred times more of the energy content of nuclear fuel than conventional reactors. Globally there are hundreds of thousands of tonnes of conventional nuclear waste sitting in stockpiles. The EM2 can burn conventional waste, plutonium, uranium or thorium providing essentially limitless energy.
In conventional plants high-level wastes are produced in the fission process from enriched uranium. The ‘spent fuel’ is easily radioactive enough to kill with a short contact exposure many years after removal from reactors. Isotopes from long lived waste can enter the environment and the food chain where the dose is far less but the exposure is longer and more widely spread. There is an invisible and insidious risk from storage of nuclear waste over periods that exceed by many millennia the lifespan of any engineered structure ever built.
In nuclear fission uranium atoms split converting mass to heat in accordance with Albert Einstein’s famous equation – E = MC2. The fission process creates radioactive isotopes of lighter elements such as cesium-137 and strontium-90. These are “fission products” and account for most of the heat and radioactivity in high-level waste. Some uranium atoms capture neutrons and form heavier elements – actinides – such as plutonium. Over time radioactive isotopes decay to harmless materials. Some decay in hours, but others over many thousands of years. Strontium-90 and cesium-137 have half-lives of 30 years – half the radioactivity will decay in 30 years. Plutonium-239 has a half-life of 24,000 years. Conventional nuclear waste contains 95.6% uranium oxide, 3.4% fission products and 1% long lived actinides.
The EM2 design provides for recycling fuel by passing it through a number of burn cycles in EM2 reactors. Fission products – a small proportion of the waste – can be removed using a plasma mass filter after each burn cycle. Waste fission products cannot be used in nuclear weapons and can be held safely in a repository where the radioactivity decays to levels in the original ore over 300 years. The remainder of the material – with added fertile material – is returned as fuel in the next burn cycle where it is transmuted into fission products. The fast neutron process converts fertile material to fissile materials within the reactor core. There is no enrichment of fuel outside of the reactor – and thus no risk of weapons proliferation. The reactor core design provides for factory sealing and a 30 year burn without refuelling. The reactor is designed to never be opened on site and thus there is little opportunity for diversion of radioactive material from operating plants.
Modern materials science and nuclear fuel cycles brings decades old nuclear technology into the 21st century. Several of these new designs are likely to be cost competitive. Due to the high fuel utilisation, heat conversion efficiency, small modular design and factory fabrication. By all means let’s avoid the folly of subsidising energy of any sort. The future of nuclear fuels seems quite likely to be focussed on fuel recycling and re-fabrication rather than mining and geological storage. The new designs are cheap, safe, reduce both the volume and half-life of waste and considerably reduce the potential for weapons proliferation.
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