Tag Archives: particle accelerator

Transmuting Waste and Worries Away

The philosopher stone yearned for by Alchemists, was actually not a stone.

Alchemists of old yearned for the philosopher stone, a substance of magical quality that would allow them to transmute other elements into gold.  Nowadays, it would be even more valuable to have this mythical device, since its transmuting power could be harnessed to transform long lasting nuclear waste, if not into gold, then at least into less dangerous isotopes.

Scientists at the Belgium nuclear research center SCK CEN in Mol are working to accomplish exactly that. Lacking a philosopher stone, they are deploying the Swiss army knife of contemporary physics: A particle accelerator to create fast neutrons for the treatment of problematic nuclear waste.

A modern version of the philosopher stone: The MYRRHA (Multi-purpose hYbrid Research Reactor for High-tech Applications) concept allows for industrial scale treatment of nuclear waste. Fast neutrons are produced via nuclear spallation.

By capturing these neutrons, the waste can be transmuted into fission products with much shorter half-lives. The beauty of this concept is that the nuclear waste simultaneously serves as fuel in a sub-critical reactor.  A recent paper on this concept can be found here, and there is also an excellent presentation online (which contains the MYRRHA diagram and radiotoxicity graph).

Not only does this technology allow us to get rid of extremely worrisome long lasting radioactive material, but a nice side effect is that it will also alleviate our energy security worries, as this design is well suited to also use Thorium as fuel.

The following graph illustrates how dramatically this could alter our nuclear waste problem (note: the x-axis is logarithmic).

The requirement for save storage of nuclear waste could be drastically shortened from about 200,000 years to a mere 200, if processed in a suitable spallation reactor.

This flies in the face of conventional wisdom, as well as a world increasingly turned off by conventional nuclear energy in the wake of its latest catastrophe in Fukushima. After all, this new waste treatment still requires a nuclear reactor, and given the track record of this industry is it a risk worth taking?

To answer this, one must take into consideration that the idea of using a particle accelerator as a neutron source for a nuclear reactor is actually quite old, and significantly predates the recent research in Mol.  I first encountered the concept when preparing a presentation for nuclear physics 101 almost twenty years ago.  My subject was differing approaches to inherently safe reactor designs, "safe" in this context defined as the inability of the reactor to engage in a run-away nuclear chain reaction. (The treatment of nuclear waste was not even on the horizon at this point because the necessary reprocessing to separate the waste material from the depleted fuel rods did not exist).

The ratio of surface to volume is key in determining if a a neutron triggers enough follow up reactions to sustain a critical cascading chain reaction.

The idea is simple, design the reactor geometry in such a way that neutrons produced by the reaction don't have the opportunity to spawn more in follow-up reactions by having them escape the reactor vessel. Then, make up the balance by providing enough neutrons from an accelerator-driven reaction to sustain the nuclear fission process.  Once you pull the plug on the accelerator, the fission reaction cannot sustain itself.  Compare this with the situation in Fukushima or Chernobyl.  The latter was a classic run-away fission chain-reaction, and the biggest problem at Fukushima was that melted down fuel can become critical again (there is some indication that this may actually have happened to some degree).

Will an inherently safe fission reactor design, one that can melt away the stockpiles of most long lasting nuclear waste into something that will only be kept in safe storage for some hundreds of years, sway the environmentally motivated opposition to nuclear technology? Doubtful.  Many will argue that this is too good to be true and point to the fly in the ointment: The fact that reprocessing is essential to make this happen, and that doing this on an increased industrial scale will make accidental releases more likely.  Then there will be the talking point that the nuclear industry will simply use this as an opportunity to establish a Plutonium fuel cycle (one of the purposes that reprocessing technology was originally developed for).  Not to mention the strongly entrenched ideological notion, especially in some European countries, with Germany topping the list, that humanity is simply too immature to handle this kind of technology. In a way, this is the temporal analog to the Not In My Back-Yard (NIMBY) attitude, maybe it should be called the NIMLT principle, as in Not In My LifeTime, let future generations deal with it.

Do you think a core catcher would have helped in Fukushima?

Of course, the nuclear industry did little to earn any trust, when considering what transpired in the wake of the Chernobyl disaster.  In a pathetic display of a "lesson learned" they added "core catchers" to existing blueprints, rather than invest R&D dollars into an inherently safe design. Instead of fixing the underlying problem, they simply tried to make the worst imaginable accident more manageable.  It was as if a car manufacturer whose vehicles rarely, but occasionally, explode was improving the situation in the next model line by adding a bigger fire-extinguisher.
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In 2006 France changed its laws and regulations in anticipation of this new technology, and now requires that nuclear waste storage sites remain accessible for at least a hundred years so that the waste can be reclaimed. To me this seems eminently reasonable and pragmatic. The notion that we could safely compartmentalize dangerous nuclear waste for many thousands of years and guarantee that it would stay out of the biosphere always struck me as hubris. This technology offers a glimpse at a possible future where humanity may finally gain the capability to clean up its nuclear messes.