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.