Posted on: 10th December 2012
At the heart of deuterium-tritium fusion is the neutron. Each fusion event produces neutrons with an enormous amount of energy – 14.1 megaelectron volts. This is a million times more energy than is produced by burning coal, and manifests itself in the speed of the neutrons, which leave the reaction at about 50 000 kilometres per second.
These neutrons have more energy than those produced by fission, which typically have only a few megaelectron volts. This enormous amount of energy seems to be a goldmine for our future energy needs, however neutrons are a bit tricky to catch. Because they are uncharged they pass through the electron shells of atoms unhindered; it is only when they get close to nuclei that they interact – the charge of the quarks in the neutron interacts with the charge of protons and neutrons in the nuclei. At this range neutrons often trigger nuclear reaction with their interaction, thereby changing the nature of the material – the large numbers of neutrons of this energy produced by a fusion reactor would damage most conventional structural materials beyond repair within weeks. Furthermore the blanket around a fusion reactor needs to be able to use these neutrons to breed tritium from lithium, and convert the energy of the neutrons to heat that can generate steam to drive turbines.
These multiple material science challenges are being met by a number of research groups across Europe, who are examining the effects of neutrons, both in simulations and with small experiments that produce fast neutrons. These groups have come up with a number of possible designs for the lithium blanket, for example in ceramic pebbles, or as a layer of liquid lithium-lead, which will be tested in ITER. “Neutronics simulations suggest producing tritium in sufficient amounts in a fusion power plant is feasible,” says Dr Lee Packer of CCFE.
However research into the activation of the fusion-facing components is not as advanced. Neutronics experts such as Dr Packer are developing detailed understanding of the defects that neutrons cause in material structures. They have already whittled down the possible 66 000 reactions that neutrons could cause to a ‘mere’ 5096 important reactions, but more remains to be done, says Dr Packer. “We need a large scale neutron source, such as the proposed IFMIF or CTF, to qualify materials. We have simulations, but there is no substitute for experimental data. Of those 5096 reactions, only 470 have complete experimental data, so there is a lot to do!”