Of immobilising radioactive waste

Nuke power

The disposal of nuclear waste has been a point of debate for years now. With every reactor built, the need for safe disposal increases, writes S Ananthanarayanan 

Plutonium is a radioactive element that is a by-product of a nuclear reactor. It can also be used as nuclear fuel and to build an atom bomb. When it is used as nuclear fuel, it can be used to create more plutonium, which could make the process self-sustaining. If it isn’t used, it is still radioactive and needs to be sequestered and stored. But using it as fuel will produce further by-products which are radioactive and will need to be sequestered and stored. Frank von Hippel, Rodney Ewing, Richard Garwin and Allison Macfarlane, scientists and policy experts in the US have analysed the options in a paper carried in the journal, Nature.

Atoms consist mainly of a number of particles packed in their central nucleus. About half of the particles are electrically charged and the rest – the neutrons – are not. The force that holds the particles together, works to keep them that way, and the energy for this comes from the total energy of the nucleus. When the nucleus gets very large, the force holding the particles together begins to weaken and the nucleus may split into two, with or without some of the particles, like neutrons or groups of particles, which do not find a place in the ‘daughter’ nuclei, getting left out. But the main change is that the daughter nuclei are collectively more efficiently packed than the parent, and there is ‘saved’ binding energy to spare. This energy is expressed in violent separation of the daughters or the expulsion of the additional particles that are released. The appearance of these particles is what is usually called radioactivity.

The splitting of the nuclei of some atoms in this way can be induced by the impact of a stray neutron. The uranium 235 atom (U235) is one such. The number, 235, denotes the number of particles in the nucleus. The same element can also exist with a few more, or less, neutrons, because neutrons only affect the mass of the nucleus, not its electric charge. As the charge stays unchanged, the atoms with nuclei differing only in the number of neutrons behave as the same element. Now, when U235 atoms are struck by a neutron, they splits into a pair of daughters plus either two or three free neutrons.

These neutrons can then set off more nuclear fissions, which would release more free neutrons. As neutrons move fast and the distances are small, a mass of U235 atoms could undergo very rapid fission, releasing huge energy. This energy is used to boil water and generate power in the nuclear reactor.

But, this useful kind of uranium atom (U235) is only a small part of natural uranium. The large part is U238 atom, which has three more neutrons and cannot take part in the chain reaction. But the U238 atom is also affected by the neutrons zipping about and it  changes into a radioactive form, on being struck, which promptly breaks down into another radioactive element – plutonium. Plutonium decay also generates neutrons, which can set off more decay leading to a chain reaction, just like the one with the U235 atoms. If left-over U238 atoms are packed in a plutonium reactor, more plutonium gets generated, which promises economy of fuel material!

Dangerous remnants

Many daughter elements and by-products of nuclear reactions are also radioactive. Over years of operating nuclear reactors, there can be a build-up of such radioactive waste which poses great danger. In commercial reactors which use uranium ore there are advantages, if processed ore which concentrates the content of U235 atoms, is used.

But it is easier and quicker to use ordinary uranium, rich in U238 atoms. These U238 atoms, which do not take part in the chain reaction, still absorb neutrons and decay into plutonium. Plutonium is radioactive and is an important hazardous by-product of reactors that use natural uranium. As plutonium also undergoes fission during a chain reaction, like U235 atoms, it can also be used, not necessarily in reactors, to make atom bombs.

Simple facilities to generate power using natural uranium would provide a ready source of material for military use.

Apart from the danger of military application, even the plutonium build up needs to be stored, with arrangements to contain its radioactive emissions. The large number of natural uranium reactors world over are said to have generated 500 tonnes of plutonium, enough for 1,00,000 nuclear weapons and a dangerous treasure. This stockpile is largely the result of a separation of plutonium from spent fuel, for use in plutonium reactors which could then ‘breed’ more fuel. This fuel cycle is the grand plan of India’s nuclear programme, supplemented by generations of U233 atoms, another chain-reaction-worthy form of uranium, by exposing thorium, of which India has good resources, to neutrons in plutonium reactors. These proposals of ‘breeder’ reactors have not taken off and there has been a build-up of stockpiles of plutonium. 

Burying waste

The alternative is to ‘immobilise’ separated plutonium or to directly bury plutonium. The US and Russia had committed to dispose of 34 tonnes each of plutonium stocks. Russia objected to ‘immobilisation’ as this could be reversed and the fuel recovered. The US also considered converting plutonium into fuel pellets, rather than immobilise, to be economical, but the economy did not materialise. The UK will soon have over a 100 tonnes of separated plutonium and has plans to convert this into fresh ‘mixed’ fuel. The UK has walked this path earlier but not succeeded.

The Nature report thinks UK should abandon trying to make fresh fuel and resort disposing of the plutonium by ‘immobilisation’, which is to encase the waste in ceramic and bury it 500 metres deep in a geological repository. This can be done without the precise machining of pellets and if the plutonium is mixed with the waste that comes from the reprocessing plant, it would be so radioactive for a century that it would be safe from thieves or terrorists. The other method is to directly bury the waste out of reach, in boreholes that are 5000 metres deep.

This discussion is about disposal of the plutonium build up, thanks to the weapons programme, the pursuit of the plutonium reactor and the breeder reactor. But even if these programmes had succeeded with the plutonium getting consumed the spent ‘mixed’ fuel would have needed to be disposed of along with other spent fuel. There is no getting away from the need to dispose of nuclear waste. An evaluation of the geographical areas that would be barred for habitation or other use, over years of generating nuclear energy and burying waste, will place a limit on the power used. It may be more workable to find ways of reducing power consumption and for that, reducing power consumers i.e. population.