In-core detection/ analysis technique using laser/ fiber optics

Research purpose and contents

Currently, the measurement and analysis of the in-core situation in the Fukushima Daiichi Nuclear Power Station (hereinafter referred to as FDNPS) is being performed using remote operation robots. However, as the results that accurately understand the characteristics of fuel debris have not been obtained, what we are actually doing is perform estimations based on accident development analysis and acquired knowledge. With the actual analysis method, specimens sampled in the FDNPS building are taken and transported to research institutions, and nuclide analysis is performed using a variety of methods. However, if field measurement and analysis can be rapidly performed inside the building, by making assessments based on the actual situation, it is possible to construct plans and methods, determine work arrangement, understand the work situation, and confirm the situation subsequent to the work (checking of remnants) swiftly  on site. Although the accuracy of analysis results at specialist facilities is certainly higher, in situ swift measurement and analysis is actually realized using Laser-induced plasma emission spectrometry using radiation-resistant optical fibers (hereafter fiber optic LIBS analysis method).

Development based on the Fiber Optics LIBS analysis method

The Optical Fiber LIBS analysis method is a technique to analyze debris using the laser-induced plasma emitting analysis method by transmitting laser light using radiation-resistant optical fibers. With the previous technology, there were cases where higher energy was required depending on the materials being analyzed, and issues arose wherein the released laser decreased as a consequence of the lengthening of optical fibers. In response to those, we used the long pulse laser technology utilized by the Deep Seas Natural Resources Exploration Infrastructure Research at Kyoto University and were able to not only greatly reduce the damage caused by laser light to the optical fibers but also transmit energy with 10 times more intensity than before. In addition, using a technique to increase atomic emission intensity through microwave plasma generation technology (high efficiency combustion technology for practical use in the automobile industry) by i-Lab Inc., the analysis method was applicable to not only solid sintered bodies but also vitrified debris and corium, and we succeeded in achieving higher sensitivity than before and improving the substance analysis performance.

Combination of variety of methods that are applicable case-by-case

However, there are limits to the optical fiber LIBS analysis method. As the situation is unclear inside FDNPS, there is the issue that the environment and conditions in which it can be used cannot be specified. Thus, we have diversified the methods that use optical fiber LBS, so that they can be selected and used in combination, making a good use of features of each method, under any condition they are used.For technologies utilizing microchip laser, a pulse laser transmitter conventionally allocated outside the core (i.e., in the low radiation rate environment) was attached to the optical fiber tip and deployed directly inside the core. In this way, the reduction in laser released caused by the lengthening of the optical fiber is fundamentally resolved, and the devices such as spectrometer, etc. can be allocated even safer outside the core . By making it longer, more flexible, and more compact, we have achieved even higher levels of light-harvesting.