Most countries currently have radiological monitoring to monitor radioactivity levels in the environment and to detect the level of natural radioactivity. Also amongst its aims is to monitor the compliance with agreements such as the Comprehensive Nuclear Test Ban Treaty, as well as to detect and quantify a possible nuclear or radiological accident.
Depending on the frequency of the samples taken, these networks are classified into two groups: control networks, the aim of which is to measure radioactivity in the air, the water, the soil and in foods; and alert networks for informing of the anomalous presence of values in air and water.
To this end, alert networks take samples with frequencies of less than one hour and operate 24 hours a day. In Spain there exists a Radiological Alert Network and there are also four Autonomous Communities with their own networks, one of which the Basque Government launched in 2001.
Nevertheless, there has not existed to date a scientific methodology which enabled distinguishing between natural and anomalous values of radioactivity, to the point that certain radiological situations could go undetected. Ms Natalia Alegría Gutiérrez addressed this lacuna at the University of the Basque Country (UPV-EHU) with her PhD thesis, Drawing up alarm levels and analysis of transitory situations at radiological monitoring stations.
Dr. Alegría is an industrial engineer specialising in Energy Techniques and is currently working as lecturer-collaborator in the area of Fluid Mechanics at the Higher School of Engineering in Bilbao. Her PhD thesis was led by Dr. Fernando Legarda Ibáñez, of the Department of Nuclear Engineering and Fluid Mechanics and has been undertaken with the cooperation of the Nuclear Security Council (CSN).
The influence of precipitation
In order to detect a radiological incident, Dr. Alegría took the dose rate of gamma radiation, i.e. the amount of energy we receive per unit of mass over a specified unit of time. Her prime goal was to establish values for normal radiation situations, their origin being in natural sources of radiation: cosmic radiation from the Sun and interstellar spaces; and terrestrial radiation from radionucleids (originating in uranium or potassium, amongst others) present in the earth. Ms Alegría has chronologically ordered recorded dose rate values and shown that their evolution is constant (being represented on a horizontal line), although they do have an irregular component due to changes in meteorological values such as rainfall and lightning. That is, recorded values increase during periods of precipitation. The fact is that rainfall makes radioactivity present in the air fall to the earth’s surface (or to the rooftop where the probe or detector is located), and this causes the recorded dose rate to increase, without the cause being a radiological incident.
Effectively, the meteorological variables make it impossible for the evolution of radioactivity to be graphically represented using histograms. This is why Dr. Alegría decided to separate the behaviour of the dose rates over different periods, depending on the level of precipitation. Thus, she defined a dry period (0 litres per square metre), a wet period (whenever rainfall is greater than 0 litres per square metre) and a transition period, that encompasses the period from the end of the precipitations to the point where the gamma radiation dose rates return to those of the dry period.
Natural radioactivity and anomalous values
Taking as reference the magnitude that Currie called critical limit, Dr. Alegría calculated the critical limit of both the dry and rainy periods. In this way, every time the limits of the previous year are exceeded, a base alarm level is generated which will alert the experts to a possible radiological incident. To this end, she drew up a mathematical model in order to represent the increase undergone by the radioactivity during the rainy period with respect to the dry period.
Although the dose rate increases during the rainy period, it does not do so in a manner directly proportional to the amount of water fallen, but exponentially. According to Dr. Alegría, this is due to the rainfall transporting components derived from chains of radon (a radioactive gas) to the earth’s surface. In her PhD she also describes the systems of equations she used to distinguish between concentrations of radon due to natural causes and those caused by a radiological incident.
In conclusion, the PhD work presented by Ms Alegría at the UPV/EHU has made the radiological monitoring network in the Basque Country more sensitive, significantly reducing the number of alarms, but without there being radiological incidents that have gone undetected. The alarms of this network currently are triggered whenever there are external causes and not when a rise in natural radioactivity is caused by precipitations.
Alaitz Ochoa de Eribe | alfa
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