RIKEN develops trailer-mounted neutron system for on-site bridge deterioration inspection

A joint research team led by Senior Research Scientist Tomohiro Kobayashi, Senior Visiting Scientist Yujiro Ikeda, and Team Director Yoshie Otake from RIKEN Center for Advanced Photonics (RAP), together with Assistant Professor Shota Ikeda from the Laboratory for Zero-Carbon Energy, Institute of Integrated Research, Institute of Science Tokyo, has developed RANS-III, a transportable compact neutron source system capable of non-destructive measurement of internal deterioration in bridges at real-world sites. The team has successfully demonstrated neutron generation within this system. Compared with previous models, the accelerator section has been reduced in weight, decreasing from 5 tons to 600 kilograms. Furthermore, the shielding section weighs 2 tons, downsized from its prior weight at 20 tons. The total length is 4 meters, previously being 10 meters, making it significantly smaller and lighter so it can be mounted on a trailer. In addition, by suppressing proton beam energy while still generating the necessary neutrons, RANS-III complies with regulatory standards, enabling on-site non-destructive neutron inspection. Otake commented, "First, we will build up a track record of various experiments in a dedicated building that can accommodate the trailer. We want to conduct outdoor demonstration tests at a steel bridge at the Public Works Research Institute around June next year, and at the Fukushima Robot Test Field around autumn. We need to submit an amendment application (to the Nuclear Regulation Authority), so, depending on the procedures, we hope to bring it to an actual site by the end of the next fiscal year."

Aging infrastructure has become a serious societal challenge. Currently, infrastructure inspections are conducted through visual examination and hammering tests; however, these methods render it difficult to accurately assess the state of internal corrosion and deterioration.

The research group previously developed RANS, the first RIKEN compact neutron source system, and the smaller RANS-II. These systems have been in continuous operation, enabling the visualization of steel plate corrosion and measurement of austenite phase fraction in steel materials. The group has also developed a new non-destructive visualization method intended for outdoor use: fast neutron scattering imaging (pulsed neutron time-of-flight method). Using simulated samples, they successfully visualized internal bridge deterioration — such as water retention, soil accumulation, and unfilled PC grout - and developed a method for detecting salt concentration inside concrete.

In conventional neutron imaging, a detector must be installed on the opposite side of the sample. However, the joint research team attained a visualization of the internal state simply by irradiating neutrons from the bridge surface using fast neutron scattering imaging (ToF method). The ToF approach developed in this study irradiates materials with high-energy, deep-penetrating fast neutrons (maximum energy 700 keV) and detects scattered neutrons with a two-dimensional detector installed on the same side. The dispersed neutrons carry information about the interior of the material, enabling two-dimensional visualization of water retention conditions as well as the distribution and location of internal voids.

The researchers also developed a new dedicated compact accelerator system. It comprises a permanent magnet electron cyclotron resonance (ECR) ion source, a radio-frequency quadrupole (RFQ) linear accelerator, a 4-channel RF input system, and a beam transport system.

In the ion source, neodymium magnets were installed on the sides of the plasma chamber in place of the solenoid electromagnets previously used for plasma generation. This enabled the ion source body's miniaturization and power savings. The RFQ linear accelerator increased the resonance frequency to 500 MHz, approximately 2.5 times that of the previous RANS-II. As a result, the cross-sectional area was reduced to about one-quarter and the weight to about one-third, now approximately 700 kilograms. This greatly improves its installability within a trailer. Furthermore, by adopting a three-body structure that integrates the acceleration electrodes and vacuum vessel, high rigidity was ensured, enabling stable operation resistant to vibration.

For on-site inspections, the trailer is transported to the bridge, and a two-dimensional ToF neutron detector is installed on the road surface. A vertically adjustable suspension-type neutron generation target, including shielding, is then lowered to road level to cover the neutron detector, and pulsed neutrons are irradiated onto the bridge. The detector measures neutrons scattered within the slab beneath the road surface, allowing information on internal deterioration to be extracted and visualized.

Neutron-based measurements not only reveal invisible structural changes within infrastructure but also enable elemental analysis. As a result, even when similar cracks exist, this method can distinguish whether the surrounding area has deteriorated or whether water has infiltrated the structure. Given there are many aging infrastructure facilities throughout Japan, this technology provides information that helps to decide which repairs should be prioritized.

Although nationwide neutron-based infrastructure inspection is desirable, efficient infrastructure inspection is constrained under the current legal framework which assumes the use of "installed equipment." For example, when RANS-III is brought to a site, it would be more efficient to take measurements while moving, but the system must repeatedly stop, be installed, and then perform measurements. A review of the legal framework will likely become necessary to keep pace with the speed of technological development.