Researchers at the University of Bristol have excited geeks around the world by announcing they had successfully demonstrated artificial diamonds that produce an electrical current when exposed to radiation. Their research has been inspired by a fact that is almost unique to the UK. There is a large stockpile of radioactive carbon-14 available as a result of operating a number of graphite-moderated, gas cooled reactors starting in 1956 and continuing today.
When faced with a large inventory of potentially hazardous material that no one seems to want, people have two primary choices. They can fret about ways to store or dispose of the waste or they can find ways to extract value from the material.
The University of Bristol researchers have produced a short video to explain how they have chosen the second path and to describe the destinations to which it might lead.
The development has the potential to address an issue has been one of many obstacles to building Adams Engines. I’ve been wondering how to convince others to accept C-14 as a potentially useful byproduct that needs to be handled instead of thinking of it as an insurmountable barrier.
What is an Adams Engine?
Adams Engines have been under conceptual development since 1991. The fundamental idea is to use proven nuclear fuel capable of heating a flow of moderate pressure gas to ~ 800 ℃ or higher. That flow of gas would be piped directly to a simple cycle gas turbine to produce electricity. A variant would eliminate the power turbine and instead produce a hot stream of gas useful for process heat applications.
This path breaks from precedent but seems to be a path toward a radically simplified power system using atomic fission as a heat source.
High temperature reactors
High temperature gas cooled reactors have been under development since the late 1940s. Several, including Dragon, AVR, THTR, Peach Bottom 1, Ft. St. Vrain and HTR-10 have operated with varying degrees of success. Technolgists in the field have almost universally decided to use a tristructural isotropic (TRISO) coated particle fuel that contains fission products even when raised to temperatures in excess of 1600 C.
Almost everyone in the field of high temperature gas reactors selects high pressure helium as the heat transfer medium to move thermal power out of the reactor.
Using helium locks out the direct use of commercially available compressors and turbines. That doesn’t seem like a very big obstacle, but developing turbo machinery that is well suited for high pressure helium has proven to be challenging enough to kill several promising development projects.
A few designers have considered using a helium to air heat exchanger and a piping loop with a helium circulator to enable the use of conventional, air breathing turbomachinery, but that idea locks in a higher number of components. The heat exchanger, which is easy to insert in a drawing, has material and manufacturing challenges that will fundamentally change the system cost structure.
The Adams Engines concept avoids the challenges imposed by helium and instead uses nitrogen as the reactor coolant. The same hot nitrogen that transfers heat from the reactor serves as the working fluid in the turbo machinery that converts fission-originating heat into motion. Using nitrogen at appropriate temperatures, pressures and flow rates gives Adams Engine designers access to a wide range of proven machinery that has been designed and manufactured for the combustion turbine industry.
Why does everyone else use helium?
From a reactor engineer perspective, helium is an almost ideal coolant. Though its ability to move heat is limited by its low natural density, helium has a high specific heat transfer coefficient per unit mass. That gives it a reasonable volumetric heat remove capacity, especially if the system is operated at 50 – 100 times atmospheric pressure. It’s inert, eliminating corrosion inside pipes and tanks as a concern.
Its real beauty as a nuclear reactor coolant is that it doesn’t absorb neutrons. Helium doesn’t affect reactor core reactivity and it doesn’t become radioactive.
From a heat transfer engineer’s perspective, there are other gases that can move heat from a reactor just as well. Two of those available gases, atmospheric air and nitrogen, are useful in commercially available gas turbine machinery because they have the gas properties (density, specific heat transfer coefficient, etc) for which those machines are designed.
Atmospheric air has three drawbacks.
- Air contains oxygen that has the possibility of reacting with materials in the reactor and in the piping systems when at system operating temperature.
- Air contains a small amount of Ar-40, which gets activated to Ar-41 when exposed to a neutron flux. That isotope decays with a penetrating gamma emission that can cause exposure hazards. It is a short lived isotope with a 1.8 hour half life, so it is not a long term pollutant.
- Air contains nitrogen.
Using just the nitrogen component of atmospheric air eliminates the first two issues. Challenges associated with the third issue are increased a bit due to the marginally higher concentration of N2.
The main reason that almost all other high temperature gas reactor projects have chosen helium over nitrogen is that N-14, the most abundant natural isotope of nitrogen, has a moderate cross-section for neutron absorption. It undergoes a neutron absorption/proton emission reaction that produces C-14 and hydrogen. Hydrogen is stable and not a challenge to eliminate in the tiny quantities that it is produced.
Neutron absorption has a small effect on core reactivity, but that issue can be mitigated.
The issue that other designers have chosen to avoid is that C-14 is mildly radioactive. It decays with a 5,730 year half life by emitting a single low energy beta particle that turns it back into N-14. C-14 isn’t a general area radiation issue because its weak beta is readily shielded and will only travel a few centimeters even in dry air. There are concerns, however, that C-14 may cause health problems if enough of it is ingested to give the host a dose large enough to cause harm.
If the University of Bristol researchers successfully create battery products that use C-14 as a raw material, a former barrier could turn into a potential revenue opportunity. That’s an outcome that would stimulate a small celebration.