The radiological characterization of activated components of nuclear reactor – as a result of neutron activation – which always appeared with surface contamination, is essential for the decision-making process on their management during decommissioning. The need of decontamination, the appropriate method selected before the dismantling / cutting process, as well as the cutting techniques to be followed in order to reduce the production of secondary radioactive waste and limit the doses to personnel, must be based on accurate radiological characterization of the radionuclides inside the material and also those which are settled as surface contamination.

A major issue most countries are facing is how to manage effectively the large volume of metallic radioactive waste from decommissioning. A prerequisite for their management is the accurate and precise radiological characterization.

In metallic radioactive wastes from nuclear reactors, there are probably radionuclides inside the materials as result of the neutron activation. At the same time radionuclides are deposited on the surface of the materials which are products of neutron activation or fission. It is important to decide in which cases the decontamination will work as well as to select the appropriate decontamination technique (s).

This thesis concerns the development of a technique for radiological characterization and segregation of metallic Radioactive Waste in different management routes, in order to decide which ones is worth to be decontaminated as well as to select the appropriate decontamination techniques (e.g. sandblasting techniques, chemical decontamination, melting).

The technique will be a combination of a non-destructive gamma spectrometry method and Monte Carlo simulations using the MCNPX code as well as sampling. The radionuclides in radioactive waste can be: either (a) gamma emitters which can easily be detected by a gamma spectrometry system; or (b) alpha or beta emitters without emitting gamma rays (difficult to detect radionuclides) and therefore can only be determined by sampling and costly radiochemical analyses. The samples are taken for determination of scaling factor (the ratio of difficult to detect radionuclides to key radionuclides which are gamma emitters). The total activity is determined by non-destructive measurement, based on gamma spectrometry as well as by the use of the scaling factors.

The presence of NORM (Naturally Occurring Radioactive Materials) has been recognized since early 1930s in petroleum reservoirs, in oil and gas production and in processing facilities. The origin of NORM waste in the oil industry is due to the formation of both sulphate and carbonate precipitates inside tubulars and other production equipment during oil production.

NORM waste is usually very low-level radioactive waste, which may be sometimes exempted from the radiation regulations, although they present an increased radiation dose rate. There is a widespread need for NORM waste to be characterized and treated properly because of their impact to the human health and environment. The successful characterization of NORM waste will significantly reduce their management costs.

The main objective of this PhD thesis is the development of a fast and cost-effective method for segregation and in situ characterization of NORM waste originating from Oil Industry by using a 1.5×1.5 in. scintillation detector LaBr3(Ce).

The last decade LaBr3(Ce) scintillation detectors have become commercially available and are very promising due to their high light yield (> 65000 photons/MeV) that results in a better energy resolution compared to NaI(Tl) detectors (< 3% FWHM at 137Cs), their decay time of 35 ns and their material density (5.29 g/cm3). Also, there is no need for cooling comparing to HPGe detectors. Thus, LaBr3(Ce) detectors could be a suitable choice for in-situ measurements of NORM.

The method development is based on a combination of experimental gamma spectrometry and computational Monte Carlo techniques for laboratory and in situ measurements.

LaBr3(Ce) scintillator was fully characterized in the laboratory environment (FWHM, energy and efficiency calibration) by using reference point sources and multi-nuclide volume sources made of epoxy material of different densities. Simulated and experimentally calculated efficiencies were compared to determine the accurate dimensions of the LaBr3(Ce) detector crystal.

Optimal NORM radionuclides and the corresponding emitting peak energies for the gamma spectrometry measurements were selected from uranium and thorium radioactive series. The activities of other radionuclides in the series, in radioactive equilibrium with the determined ones can be estimated. Also, the radionuclides not possible to be determined by gamma spectrometry or radioactive equilibrium, can be identified and sent for sampling and radiochemical analyses, if needed.

The method is being developed for Drilling the following NORM waste storage packages: (1) Metallic/plastic drum, (2) Plastic Big Bags FIBC – Flexible Intermediate Bulk Container and (3) IBC – Intermediate Bulk Container. The NORM waste characterization material is Oil Based Drilling Mud – OBM, which is a fluid used to drill boreholes into earth and used while drilling oil and natural gas wells and on exploration rigs.

Uncertainty analysis regarding the density inhomogeneities because of the material moisture will be carried out and a sampling procedure to segregate the NORM packages in groups of different origin will be developed.