Fission reactors are experiencing a kind of renaissance while the world awaits fusion reactors to take over as the primary carbon-free power source. Even in this well-established technology—still under close scrutiny—precise and reliable power monitoring is a cornerstone of nuclear safety. Conventional instrumentation relies primarily on expensive neutron-sensitive detectors, such as ionization chambers. These systems are not only costly but also exhibit limitations when measuring fast power transients. And no matter how mature some systems are, new technologies can always improve safety, accuracy, and, if possible, reduce costs.
In their paper Cherenkov reactor power meter: Development of an innovative and cost-effective technique for reactor power monitoring—published in Nuclear Engineering and Technology—Julijan Peric et al., from the Reactor Physics Department at the “Jožef Stefan” Institute in Ljubljana, Slovenia, explain how Cherenkov radiation can be used to measure nuclear reactor power output, both in steady-state and pulsed operation.
Anyone who has visited a nuclear power plant and its spent fuel pools is likely familiar with the light-blue glow emitted by submerged fuel rods. In operating reactors, this Cherenkov effect is also present, primarily generated by high-energy electrons produced via Compton scattering of prompt gamma rays. When these electrons travel through a dielectric medium at speeds exceeding the speed of light in that medium—such as light or heavy water—they emit the characteristic blue light visible to the naked eye.
Knowing that the prompt fraction of this light inside water-cooled reactors is produced with an intensity proportional to the reactor power output, the authors developed a seemingly simple setup to measure this intensity and subsequently cross-calibrate the system against existing instrumentation.
A Cherenkov power meter was installed in the JSI TRIGA Mark II research reactor, a light-water-cooled reactor designed for steady-state operation up to 250 kW and high-power pulses reaching the gigawatt range. The power meter is mechanically simple, minimally intrusive, and fully compatible with the existing reactor infrastructure. The measurement system consists of the following main components (see Figure 1):
Figure 1: Schematic of the Cherenkov power meter experimental setup
In steady-state operation, the contribution of prompt gamma rays dominates the total field intensity and is instantaneously visible. Delayed gamma radiation from fission and activation products generates a gradually increasing signal during operation, as shown in Figure 2. This signal drift must be compensated to obtain a reliable reactor power readout.
Figure 2: Processing of power measurements from the TRIGA reactor using the Cherenkov power meter—non-compensated (up) and compensated (down) signals
During pulsed reactor operation, the Cherenkov power meter excels at tracking fast transients, since the pulses are too short to generate significant delayed gamma radiation, eliminating the need for compensation. Cross-calibration with standard TRIGA instrumentation shows excellent agreement for key pulse parameters: peak power, full width at half maximum (FWHM), and released energy.
Signals from the detector are sampled, digitized, and processed by a Red Pitaya STEMlab board (illustrated on the right-hand side of Figure 1). In steady-state mode, high-speed acquisition windows are averaged to monitor power trends and rapid fluctuations. In pulse mode, FPGA-level triggering captures fast power transients with precise timing. With adequate ADC resolution, flexible FPGA-based triggering, and software-defined processing, STEMlabs provide accurate time stamping and efficient handling of rapidly changing signals, making them an ideal DAQ solution for both operation modes.
Reliable instrumentation in nuclear environments usually comes with a high price tag. Considering the relatively simple setup, a total cost of approximately €2,300, and performance that rivals—or even surpasses traditional instruments for fast transients, it is fair to say that with Red Pitaya boards, more often than not, you can achieve more with less.