Capture gamma-ray Assessment in Nuclear Energy (C-GANE)
Lead Research Organisation:
Lancaster University
Department Name: Engineering
Abstract
Nuclear energy is made available via two principles: 1) fission, in which energy is released by inducing heavy atoms to split into lighter elements, and 2) fusion, where energy is released by fusing light atoms together forming heavier ones. Fission is mature and is used throughout much of the world; fusion is the subject of significant research and investment, due to its potential to yield low-carbon, uninterrupted energy production without the yield of high-active radioactive waste produced in fission.
When the fuel used in fission reactors reaches the end of its useful life it is deemed spent, and is either stored or dissolved and separated (the latter known as being reprocessed). The widespread expectation is that spent fuel from fission reactors that is not reprocessed will be disposed of in the form of intact fuel assemblies. However, thus far in the UK much of it has been stored under water to ensure that it is cooled satisfactorily and that the radiation from it is shielded, and this has resulted in some of the assemblies having water inside them. Similarly, where fuel material exists in disordered form associated with, for example, miscellaneous wastes from processing operations and accidents (known as fuel containing materials - FCM), often it has been stored in silos and again the abundance of water present needs to be assessed. It is important to understand the extent of the situation concerning water abundance in spent fuel and FCM prior to it being disposed of permanently (for example in an underground repository) because the water constitutes a significant influence on the stability of the fuel against an inadvertent nuclear reaction, and this could influence how it is stored and the safety case concerning the design of the repository it is stored in.
A relevant recent example, and perhaps the highest-profile illustration of late, concerns the FCM at Chernobyl. This received widespread media coverage in 2021 when it was observed that the level of neutron radiation emitted by it was increasing. The debris in question had been shrouded by a new cover erected over the site to protect it from the elements and the suspicion arose that this was causing the fission rate in the material to escalate. Neutrons arise in materials containing fuel predominantly from fission in uranium-235, with the concern being that a fall in the water content in the debris was causing this to increase with the ultimate potential for uncontrolled energy release. However, the emission might also increase due to reduced shielding and absorption of neutrons by a reducing quantity of water, enabling more neutrons to get out, or by an increase in neutron-emitting reactions by alpha particles or due to the neutron detectors being used responding more efficiently at higher energies, none of which have implications as serious as an escalation in induced fission on uranium-235.
Rather than measuring the neutron flux, as was the source of concern for the FCM at Chernobyl, greater insight might be gained concerning this complex problem by detecting the gamma rays that are emitted when neutrons are captured by isotopes in the surrounding materials. This has the advantage that the gamma rays have energies that are characteristic of the isotope producing them and that they are measured relatively easily: this is the focus of this proposal. For example, hydrogen emits gamma rays with an easily-identifiable energy of 2.223 MeV which could be characteristic of changes in water content and which might be separable from changes in the neutron environment. Interestingly, one of the few ways to measure fusion power aside from the neutron emission is also to study these emissions, by for example considering the 16.7 MeV emission from the deuterium-tritium reaction. In this project, we intend to bring together these opportunities to determine whether fission and fusion energy might benefit from high-energy capture gamma spectroscopy.
When the fuel used in fission reactors reaches the end of its useful life it is deemed spent, and is either stored or dissolved and separated (the latter known as being reprocessed). The widespread expectation is that spent fuel from fission reactors that is not reprocessed will be disposed of in the form of intact fuel assemblies. However, thus far in the UK much of it has been stored under water to ensure that it is cooled satisfactorily and that the radiation from it is shielded, and this has resulted in some of the assemblies having water inside them. Similarly, where fuel material exists in disordered form associated with, for example, miscellaneous wastes from processing operations and accidents (known as fuel containing materials - FCM), often it has been stored in silos and again the abundance of water present needs to be assessed. It is important to understand the extent of the situation concerning water abundance in spent fuel and FCM prior to it being disposed of permanently (for example in an underground repository) because the water constitutes a significant influence on the stability of the fuel against an inadvertent nuclear reaction, and this could influence how it is stored and the safety case concerning the design of the repository it is stored in.
A relevant recent example, and perhaps the highest-profile illustration of late, concerns the FCM at Chernobyl. This received widespread media coverage in 2021 when it was observed that the level of neutron radiation emitted by it was increasing. The debris in question had been shrouded by a new cover erected over the site to protect it from the elements and the suspicion arose that this was causing the fission rate in the material to escalate. Neutrons arise in materials containing fuel predominantly from fission in uranium-235, with the concern being that a fall in the water content in the debris was causing this to increase with the ultimate potential for uncontrolled energy release. However, the emission might also increase due to reduced shielding and absorption of neutrons by a reducing quantity of water, enabling more neutrons to get out, or by an increase in neutron-emitting reactions by alpha particles or due to the neutron detectors being used responding more efficiently at higher energies, none of which have implications as serious as an escalation in induced fission on uranium-235.
Rather than measuring the neutron flux, as was the source of concern for the FCM at Chernobyl, greater insight might be gained concerning this complex problem by detecting the gamma rays that are emitted when neutrons are captured by isotopes in the surrounding materials. This has the advantage that the gamma rays have energies that are characteristic of the isotope producing them and that they are measured relatively easily: this is the focus of this proposal. For example, hydrogen emits gamma rays with an easily-identifiable energy of 2.223 MeV which could be characteristic of changes in water content and which might be separable from changes in the neutron environment. Interestingly, one of the few ways to measure fusion power aside from the neutron emission is also to study these emissions, by for example considering the 16.7 MeV emission from the deuterium-tritium reaction. In this project, we intend to bring together these opportunities to determine whether fission and fusion energy might benefit from high-energy capture gamma spectroscopy.
