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Nuclear energy is currently one of the world’s main sources of electricity. Closely connected to the use of nuclear energy are important issues such as the nonproliferation of fissile material that may potentially used in nuclear weapons (safeguards), and the management of the highly radioactive nuclear waste. This thesis addresses both these issues by contributing to the development of new experimental methods for ensuring safe and secure handling of the waste, with focus on methods to be used prior to encapsulation and final storage.

The methods rely on high resolution gamma ray spectroscopy (HRGS), involving the measurement and analysis of emitted gamma radiation from the fission products ¹³⁷Cs, ¹³⁴Cs and ¹⁵⁴Eu. This technique is nondestructive, making it relatively nonintrusive with respect to the normal operation of the nuclear facilities.

For the safeguards issue, it is important to experimentally verify the presence and identity of nuclear fuel assemblies and also that the fuel has experienced normal, civilian reactor operation. It has been shown in this thesis that the HRGS method may be used for verifying operator declared fuel parameters such as burnup, cooling time and irradiation history. In the experimental part of the work, the burnup and the cooling time has been determined with an accuracy of 1.6% and 1.5%, respectively (1 σ).

A technique has also been demonstrated, utilizing the ratio ¹³⁴Cs/¹⁵⁴Eu, with which it is possible to determine whether a fuel assembly is of MOX or LEU type. This is of interest for safeguards as well as for the safe operation of a final storage facility.

As an improvement to the HRGS technique, measuring a part of the fuel assembly length in order to reduce measurement time has been suggested and investigated. A theoretical case for partial defect verification has also been studied as an extension of the HRGS technique.

Finally, HRGS has been used for determining the decay heat in spent nuclear fuel assemblies, which is of importance for the safe operation of a final storage facility. This application is based on the radiation from ¹³⁷Cs, and the accuracy demonstrated was within 3% (1 σ).