Technology to measure the flow of subatomic particles known as antineutrinos from nuclear reactors could allow continuous remote monitoring designed to detect fueling changes that might indicate the diversion of nuclear materials. The monitoring could be done from outside the reactor vessel, and the technology may be sensitive enough to detect substitution of a single fuel assembly.
source https://www.lifetechnology.com/blogs/life-technology-news-blog/antineutrino-detection-could-help-remotely-monitor-nuclear-reactors