Carbon Nanotube Based Dosimetry of Neutron and Gamma Radiation

Abstract

As the world's nuclear reactors approach the end of their originally planned lifetimes and seek license extensions, which would allow them to operate for another 20 years, accurate information regarding neutron radiation exposure is more important than ever. Structural components such as the reactor pressure vessel (RPV) become embrittled by neutron irradiation, reducing their capability to resist crack growth and increasing the risk of catastrophic failure. The current dosimetry approaches used in these high flux environments do not provide real-time information. Instead, radiation dose is calculated using computer simulations, which are checked against dose readings that are only available during refueling once every 1.5-2 years. These dose readings are also very expensive, requiring highly trained technicians to handle radioactive material and operate specialized characterization equipment. This dissertation describes the development of a novel neutron radiation dosimeter based on carbon nanotubes (CNTs) that not only provides accurate real-time dosimetry, but also does so at very low cost, without the need for complex instrumentation, highly trained operators, or handling of radioactive material. Furthermore, since this device is based on radiation damage rather than radioactivation, its readings are time-independent, which is beneficial for nuclear forensics. In addition to development of a novel dosimeter, this work also provides insight into the particularly under-investigated topic of the effects of neutron irradiation of carbon nanotubes.

This work details the fabrication and characterization of carbon nanotube based neutron and gamma radiation dosimeters. They consist of a random network of CNTs, sealed under a layer of silicon dioxide, spanning the gap between two electrodes to form a conductive path. They were fabricated using conventional wafer processing techniques, making them intrinsically scalable and ready for mass production. Electrical properties were measured before and after irradiation at several doses, demonstrating a consistent repeatable trend that can be effectively used to measure dose. Changes to the microstructure were investigated using Raman spectroscopy, which confirmed that the changes to electrical properties are due to increasing defect concentration.

The results outlined in this dissertation will have significant impacts on both the commercial nuclear industry and on the nanomaterials scientific community. The dosimeter design has been refined to the point where it is nearly ready to be deployed commercially. This device will significantly improve accuracy of RPV lifetime assessment while at the same time reducing costs. The insights into the behavior of CNTs in neutron and gamma radiation environments is of great interest to scientists and engineers studying these nanomaterials.