Gamma rays reveal power levels with precision in nuclear fusion reactors

What could be called a major breakthrough, researchers have developed a groundbreaking new method for measuring power in nuclear fusion reactors.

They have discovered that gamma rays, produced during the deuterium-tritium nuclear reaction, can serve as a precise and alternative means of measuring the power output of new fusion reactors.

Currently, magnetic confinement fusion devices rely solely on absolute neutron counting as a direct way of measuring fusion power.

“However, this technique presents several difficulties: the emission and the transport of neutrons from an extended source like the tokamak, and their interaction with reactor materials, require the use of complicated simulation codes, as well as long and costly calibration campaigns to validate the codes,” said researchers in the study .

The research team has now identified an alternative approach. The new method utilizes the gamma-ray-to-neutron branching ratio in the deuterium-tritium reaction, a measurement that was previously unidentified.

In simpler terms, the researchers have found a way to count the rare gamma rays emitted during this fusion reaction.

Gamma-ray measurement

By counting the rare gamma rays emitted during this reaction, researchers can now obtain valuable information about fusion power, independent of traditional neutron counting techniques.

“Absolute counting of deuterium-tritium gamma rays could provide the secondary neutron-independent technique required for the validation of scientific results and as a licensing tool for future power plants,” explained the research team.

This novel method involves the precise measurement of two specific gamma rays with energies around 13 MeV and 17 MeV.

“From this measurement, never before carried out with sufficient accuracy, it was possible to determine the energies and relative intensities with which the two gamma rays are emitted,” asserted Marica Rebai, a researcher at Consiglio Nazionale delle Ricerche (CNR-ISTP) and an author of the study.

“This gamma ray emission process has a relative probability (called branching ratio) much lower than that of 14 MeV neutron emission.”

Implications for future fusion reactors

Andrea Dal Molin and Davide Rigamonti, who led another study on the same subject, said that this result enabled them to find that one gamma ray is emitted for every 42,000 14 MeV neutrons produced.

“It paves the way for the use of absolute gamma-ray measurement as a new alternative and complementary method to neutron measurements for determining the power achieved in new fusion reactors based on the deuterium-tritium reaction, such as ITER and SPARC,” added Dal Molin and Rigomonti.

For context, the International Thermonuclear Experimental Reactor (ITER) is a global collaborative effort focused on proving the viability of fusion power. It requires two independent methods for accurately measuring the power it generates.

“Until now, the absence of a direct and alternative method to absolute neutron counting has been an obstacle to the independent validation of results obtained from ongoing experiments and the authorization of future commercial plants,” underlined project coordinator Marco Tardocchi, a research director at CNR-ISTP.

Broader applications and future outlook

This newly developed gamma-ray-based technique has the potential to function as the essential second method, thereby enhancing the precision and dependability of ITER’s power measurements.

This discovery extends beyond ITER and has significant implications for fusion energy research. It also enables the study of alternative fusion reactions, like proton-boron or deuterium-helium-3, which do not produce neutrons. These reactions have the potential to offer cleaner and more efficient energy production in the future.

The research team is optimistic about the future and plans to implement their gamma-ray measurement technique in the next generation of magnetic confinement reactors.