Super-powered lasers to transform nuclear waste

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.

We are look­ing at how to trans­form, or rather “trans­mute”, nuclear waste with the help of pow­er­ful lasers.

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.

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.