Long Term Effects of Gamma Irradiation on In-Service Concrete Structures

Concrete is used extensively in the nuclear industry. The majority of studies examine neutron irradiation for nuclear power plants with studies into only gamma radiation effects, which is of relevance for waste stores, being relatively sparse. This study examines archive samples of fly ash blended concrete which are over 30 years old. Some samples were kept under in-service operating conditions in a higher activity nuclear waste store whilst control samples were kept in normal atmospheric conditions on the nuclear site. Electron microscopy, XRD and elemental analysis of leachate showed little difference between the control and in-service samples. Applying a higher radiation dose rate to a sub-set of the control samples led to a decrease in porosity, as determined by N2 adsorption, with a more pronounced decrease evident in the in-service samples. This observed effect of dose rate suggests that accelerated radiation testing can be used to indicate a general change of in-service concrete, but should not be used to determine quantitative changes. The compressive strength of in-service samples was similar to control samples of the same age and historic data sets also showed no loss of compressive strength. Consequently, the trend identified in the literature where compressive strength decreases with increasing gamma radiation doses may not be applicable to concrete waste stores currently in use within the nuclear industry.