The objective of this work is to test the functionality of an Optical Fiber Based Gamma Thermometer (OFBGT) in the HFIR spent fuel pool, in one of the spent HFIR cores. The DOE has shown interest in the advancement of new sensors and instrumentation for data generation to improve nuclear power plant control and operations. Sensing technology could be improved, by the development of an OFBGT. An OFBGT is designed to relate temperature measurements made with an optical fiber to an energy deposition rate in a thermal mass, as a consequence of irradiation. An OFBGT could be used to calibrate local power range monitors (LPRMs) in BWRs, and would provide significant advantage over currently employed sensors. The current LPRM calibration method uses traversing in-core probes (TIPs), which must be inserted upon calibration, and removed after calibration, which is a time-consuming, cumbersome and potentially hazardous process. The use of OFBGTs would result in a more rapid calibration procedure, because OFBGTs are not susceptible to sensitivity loss due to burnup, and could be left in-core. Additionally, OFBGTs offer less risk, higher axial spatial resolution, and better scalability for insertion at many locations in the reactor’s transverse plane, due to the small size of OFBGTs in comparison to TIPS and conventional thermocouple–based GTs. As such, the development and use of OFBGTs is consistent with the initiative to include big data in the operation of reactors. This feature of OFBGTs makes them applicable to advanced reactors. Partly for this reason, the proposers were awarded a NEET project to develop OFBGTs and test them in University research reactors. The proposed research is complimentary in that it would expose the OFBGTs to a high gamma-ray flux such as might be encountered in an operating power reactor, without the complications that are introduced by neutrons, such as activation of materials and neutron-induced displacement damage. A prototypical OFBGT has been fabricated already at ORNL, as a part of the NEET project. The OFBGT will be monitored in-situ with an optical backscatter reflectometer (OBR), which will allow for distributed temperature measurements (and therefore distributed dose rate measurements) to be taken by the proposers in the OFBGT. The OFBGT will be placed in 3 spent fuel cores initially, with different ages, for 2 hours each, and then it will be placed in the most recently utilized core for 3 days. It will not require active monitoring by HFIR staff, but will be monitored, with an OBR, continuously by the proposers. The OBR output will allow the us to analyze the functioning of the OFBGT and to compare the OFGBT’s output with the results of computer models. The evolution of the OFBGT output with irradiation time will reveal the functional impact of fiber degradation on the OFBGT; including the effects of radiation induced attenuation and signal drift. The fiber in the OFBGT will be inscribed with fiber Bragg gratings (FBGs). HFIR staff has confirmed that the timeline associated with the project is sufficient for obtaining necessary approvals.

The development of an OFBGT will directly address the DOE’s mission to advance nuclear power. Specifically, the development of an OFBGT advances the specific DOE-NE topics that are related to the development of advanced sensors and instrumentation for data generation to improve nuclear power plant control and operations. The DOE has specifically called for such research via the Nuclear Energy Enabling Technologies (NEET) program, under NEET 2.3 in the 2018 FOA. Dr. Thomas Blue, PI for this proposed work, is currently funded under this workscope, serving as PI also on project DE-NE0008810; this funded project can serve as leverage for the proposed RTE. Project DE-NE0008810 funding will ensure that all extraneous costs associated with the proposed RTE, aside from NSUF facility usage, will be covered. In addition, a DOE-NE NEUP graduate fellow will perform the proposed research in partial fulfillment of the goals of his Ph.D. The functionality testing of an OFBGT in HFIR will help to enable the utilization of the technology in BWRs. The OFBGTs could be used for the calibration of local power range monitors (LPRMs) in BWRs. OFBGTs could also be applied to next generation reactors for calibration of their neutron detectors. The currently employed sensors for calibration of the LPRMs, traversing in-core probes (TIPs) are disadvantageous because they are impermanent (they must be inserted and withdrawn from the core many times many times for each calibration), and the calibration procedure risks the release of radioactive material. OFBGTs could be permanently installed in a BWR such that the calibration procedure could be done more safely, and more efficiently. It is also worth noting that one OFBGT could be used to calibrate several LPRMs simultaneously, provided that the LPRMs are all in a line in a given instrument tube in the BWR. This is because distributed temperature measurements can be taken along an optical fiber; therefore, distributed measurements of linear power deposition can be taken along an OFBGT. An array of OFBGTs in a BWR core would be able to acquire a significant amount of data due to sub-cm spatial resolution sensing capabilities with optical fiber. The large amount of dose rate information from OFBGTs would conceivably allow for a more accurate calibration of LPRMs in a BWR, and addresses the call for “Big Data” generation in the NEET 2.3. We propose testing FBG inscribed fibers within the OFBGT. FBG inscribed fiber has been shown to provide a more stable response in radiation fields with regard to optical frequency domain reflectometry based sensing.