Instrument designed for the ITER fusion facility
The neutron spectrometer will measure the neutron flux in ITER's fusion reactor. Illustration: Uppsala University
The world's largest experimental fusion power facility, ITER, is currently being built in southern France. This facility will be used to create fossil-free energy the same way as in the sun, through the fusion of light atoms. Researchers at Uppsala University are participating in several different ways. Anders Hjalmarsson from the Department of Physics and Astronomy is leading the work to design a high-resolution neutron spectrometer.
Anders Hjalmarsson, Researcher at Department of Physics and Astronomy. Photo: Mikael Wallerstedt
The instrument is needed to monitor how the fusion fuel, which is made up of a mixture of the two hydrogen variants deuterium and tritium, behaves inside the reactor. In ITER’s reactor, the fusion fuel, i.e. the hydrogen, is enclosed by strong magnetic fields. During the fusion process, neutrons that are released can leave the reactor – and that is what needs to be measured.
“The neutron spectrometer not only registers neutrons, but can also measure their energies, which is a necessary instrument feature for determining the ratio between the number of deuterium and tritium ions in the reactor fuel in real time. There is an optimal mixing ratio that maximises fuel utilisation, and the aim is therefore to achieve a stable fuel ion ratio that is a 50:50 mixture of deuterium and tritium ions. With a neutron spectrometer, we can monitor how close we are,” explains Anders Hjalmarsson.
A preliminary design has been developed
When the instrument is ready, it will be able to measure the ratio between the different ion types every tenth of a second.
The work is proceeding as planned, and the researchers have now reached an important milestone.
“A preliminary design of the instrument has been developed, and work is underway to investigate the fusion conditions under which the instrument will meet ITER’s requirements specifications. In addition, there is extensive work being done on instrument integration at ITER”, says Anders Hjalmarsson.
Measuring the speed of neutrons
To be precise, Anders Hjalmarsson and his colleagues are involved in developing an entire system of different neutron spectrometers. One of the biggest challenges is that it must work for ITER’s highly variable fusion effects. When the reactor is put into operation, they will fluctuate between 0.5 megawatts and 500 megawatts. This means that the fusion effects, and thus the neutron fluxes, will vary by a factor of 1,000.
“It is very challenging. In order to determine the fuel-ion ratio for this large range of fusion effects, a system of several neutron spectrometers is needed, with Uppsala University responsible for the design of two time-of-flight spectrometers,” says Anders Hjalmarsson.
Simply put, a time-of-flight spectrometer measures the time it takes for a neutron to travel a certain distance.
The construction of ITER has taken longer than originally planned. Initially, the facility was supposed to be completed in 2016, but those plans had to be revised.
“According to the latest schedule for ITER, the neutron spectrometer system is to be installed in 2033–34, and ITER is to be commissioned in mid-2035. But these dates are, of course, somewhat uncertain and may be adjusted in the future,” says Anders Hjalmarsson.
Åsa Malmberg
International Thermonuclear Experimental Reactor (ITER)
Construction of ITER began in 2009 in Cadarche in southern France. The project is an international collaboration between the 27 EU countries (including Sweden), China, India, Japan, South Korea, Russia, the USA, Switzerland and the United Kingdom.
The heart of the facility will be the tokamak-type reactor. It is cylindrical in shape, 24 metres high and 30 metres wide.