Contamination levels observed on the Belgian territory subsequent to the Chernobyl accident

Contaminated air masses reached the Belgian territory from the South during the night of the first to the second of May. At this stage however the origin of this contamination was already identified through earlier observations over the Scandinavian area and the subsequent message about the reactor accident at the Chernobyl site. Later on radioactive clouds were also detected over the central part of Europe, demonstrating the persistent nature of the emissions from the damaged reactor. Consequently the influence on the Belgian territory was not unexpected. The authorities called on the SCK/CEN at Mol, and the IRE at Fleurus to assist the IHE at Brussels in collecting the necessary data for judging the radiological situation in our country. The KMI/IRM at Brussels was involved for the follow-up of meteorological conditions and analysis of the trajectories of contaminated air masses. Early detection possibilities for the arrival of contaminated air were provided by the continuous environmental monitoring apparatus for ambient γ-dose rate or for Β activity of airborne dust, available at nuclear institutions and nuclear power plants. On detection of enhanced air radioactivity, the sampling period of routine air dust samplers was significantly shortened to allow for the hour to hour renewal of data for gross Β activity as a general indication of the evolution of the air contamination. γ-spectrometric analysis of those filters provided the necessary data for the estimation of the dose equivalent due to inhalation. Ground deposition data at the location of the participating institutions were obtained by daily analysis of the radioactivity contents of a water container collecting both dust and rainwater. Field gamma spectrometry was used later on at a number of other locations, to estimate the integrated ground deposition of radioactivity and its distribution over the country. As the grazing season was just started or was about to be started in the following days for regions of higher altitude (Ardennes), the food chain grass-milk was intensively surveyed in first instance in respect of iodine-131 contamination. Extensive grass sample measurements were carried out during the first week of May, until further significant deposition ceased. Milk sampling covered the most important dairies of the country, while some individual farms and even individual cows were followed continuously over the whole period of significant milk contamination. High priority was also given to the follow-up of contamination on leafy vegetables by γ-spectrometry. The radioactivity in surface waters, especially those used for drinking water preparation, and in finished drinking water was monitored at high frequency for about two weeks mostly by gross B measurements. γ-spectrometry on meat samples, essentially to determine the long lived isotopes of Cesium, started after the critical period for iodine contamination during the second half of May. The build up of cesium in meat is indeed a much slower process