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Gérard Mourou
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Super-powered lasers to transform nuclear waste

Gérard Mourou, Nobel prize in Physics and member of the Haut Collège de l'École polytechnique

It is no secret that some prod­ucts of nuclear fis­sion are extreme­ly radioac­tive. Treat­ing this waste is there­fore one of the biggest chal­lenges in cre­at­ing a sus­tain­able nuclear indus­try. Radioac­tive prod­ucts of fis­sion include plu­to­ni­um (the most com­mon) and oth­er minor actinides such as nep­tu­ni­um, ameri­ci­um and curi­um, which we find in quan­ti­ties of around 800 grams per ton of spent fuel. Cur­rent­ly, waste prod­ucts from nuclear fis­sion must be secure­ly stored for extreme­ly long peri­ods of time because of their pro­tract­ed half-lives (the time for their radioac­tiv­i­ty to drop by half). Plu­to­ni­um-239, for exam­ple, has a half-life of around 24,000 years.

Along­side secure stor­age we are now look­ing at how to trans­form, or rather “trans­mute”, this waste with the help of pow­er­ful lasers so that it can be bet­ter man­aged. The­o­ret­i­cal­ly, these lasers are pow­er­ful enough to stim­u­late nuclear fis­sion in plu­to­ni­um and oth­er minor actinides (cur­rent­ly the most haz­ardous waste). In short, trans­mu­ta­tion treats waste from nuclear fis­sion in two ways. Either by con­vert­ing it into sub-prod­ucts with weak­er atom­ic mass­es and much short­er half-lives. Or by adding par­ti­cles to cre­ate less radioac­tive iso­topes, which also short­ens very long half-lives – some­times down to just one year.

Ampli­fied laser power

We hope to be able to achieve trans­mu­ta­tion of nuclear waste using chirped pulse ampli­fi­ca­tion (CPA). With this tech­nique, the extreme light from lasers can be boost­ed to lev­els long thought impos­si­ble, mea­sured in petawatts (PW) – the equiv­a­lent of 1015 watts. To give an idea of scale, one PW is around fifty times the pow­er of the world’s entire elec­tric grid, and the total ener­gy the earth receives from the sun is mea­sured at 174 PW.

CPA uses an ultra-short laser pulse, spread­ing out its spec­tral com­po­nents before ampli­fi­ca­tion. The pulse is then com­pressed again for a very short time, gen­er­at­ing enough pow­er to cre­ate par­ti­cles (elec­trons and pro­tons) by detach­ing them from mat­ter, before accel­er­at­ing these par­ti­cles using con­ven­tion­al tech­niques. These kinds of lasers are already oper­a­tional – no small feat since one of the chal­lenges was to cre­ate a mech­a­nism that would not be destroyed by the pow­er of the beam itself! And we also know how to detach par­ti­cles. How­ev­er, the chal­lenge is now around effi­cien­cy: how do we make these lasers small enough and at suf­fi­cient scales with­out con­sum­ing too much ener­gy in the process? It is there­fore a prob­lem around indus­tri­al pro­duc­tion – a prob­lem that the XCAN project is specif­i­cal­ly designed to meet.

An alter­na­tive to uranium

On top of that, a CPA-boost­ed laser could be used to gen­er­ate fis­sion, pro­vid­ing an alter­na­tive to the neu­tron bom­bard­ment method used in cur­rent reac­tors. This is an option we are explor­ing, and which could be piv­otal for devel­op­ing tho­ri­um-based pow­er. Tho­ri­um is a heavy met­al found in far greater quan­ti­ties around the world than ura­ni­um; in the­o­ry, it could pro­vide a replace­ment for ura­ni­um in nuclear pow­er. More­over, the by-prod­ucts of tho­ri­um fis­sion are also less haz­ardous than ura­ni­um and have short­er half-lives of just a few hun­dred years. Giv­en that this kind of fis­sion does not pro­duce plu­to­ni­um, it is also a good option for pure­ly civ­il nuclear appli­ca­tions, with­out the risks sur­round­ing pro­duc­tion of nuclear weapons.

But tho­ri­um fis­sion is dif­fi­cult, involv­ing an extra step as com­pared to ura­ni­um fis­sion. The tho­ri­um must first be brought to a sub­crit­i­cal lev­el; neu­trons from an exter­nal source are then used to pro­voke fis­sion. Lasers could play a key role in this process pri­or to fis­sion, in par­tic­u­lar because they allow for a high lev­el of con­trol. The sys­tem can be fed with a con­stant light source, sim­i­lar to oper­at­ing a tap.

Many physi­cists, includ­ing myself, are con­vinced of the poten­tial of tho­ri­um-based pow­er. But while it is sci­en­tif­i­cal­ly sound, there are still many tech­ni­cal prob­lems to be solved. In par­tic­u­lar, con­cern­ing the ener­gy required to gen­er­ate fis­sion. But lasers could be part of the solution.

Contributors

Gérard Mourou
Gérard Mourou
Nobel prize in Physics and member of the Haut Collège de l'École polytechnique

Gérard Mourou has spent a large part of his career in the United States, in particular at the University of Michigan, where he is now Professor Emeritus. Upon his return to France in 2005, he was head of the Applied optics laboratory (a UMR ENSTA ParisTech/CNRS/École polytechnique) until 2008. Gérard Mourou is a Knight of the legion of honour. His awards include the Frederic Ives medal (2016) from the Optical society of America and the Arthur L. Schawlow prize in laser science from the American Physical Society in 2018. Gérard Mourou received the Nobel Prize in Physics in 2018, which crowns a career dedicated entirely to lasers and physics.