Strontium-90 in milk and mixed diet in the Czech Republic

Among various activities of National Radiation Protection Institute (SURO) monitoring of radionuclides in environment and food chain plays an important role. The monitoring takes place in frame of Czech Radiation Monitoring Network (RMN). Determinations of ⁹⁰Sr are performed according to the monitoring schedule of RMN. Strontium-90 was released into environment during the tests of nuclear weapons in the fifties and sixties of XX. century and to some extent in 1986 owing to the Chernobyl accident. Strontium is a chemical analogue of calcium and accordingly when entering a mammal body, it behaves very much like calcium. A large portion of the strontium will accumulate in bone and like calcium it transfers to milk. Since the Sr uptake by the human body from milk is an important pathway for radiostrontium incorporation, milk and milk powder are good indicators of ⁹⁰Sr content in human diet. Along with milk and milk powder, mixed diet samples composed of representative locally purchased food items according to food consumption statistics are also analysed for ⁹⁰Sr.The analyses are made using oxalate precipitation method, when interfering radionuclides are removed by precipitation scavenging and after its ingrowth ⁹⁰Y is separated and repeatedly measured by gas flow proportional counter. Strontium-90 activity is calculated as equal to ⁹⁰Y activity in equilibrium. Time evolution of ⁹⁰Sr activities in milk in Northern Moravia region (1988 present) and in Prague (1965 - present), in milk powder from the regions of the Czech Republic and in the mixed diet samples from the whole republic is presented on the poster. A very small increase due to the Chernobyl accident can be seen in milk. After the ban of atmospheric tests the level of ⁹⁰Sr has steadily been falling. The decrease is caused by radioactive decay and migration of the radionuclide in the environment. Due to this decrease activity concentrations in many samples have recently fallen under minimum significant activity (MSA) of the method. For example in cow's milk we can see decrease from nearly 0.7 Bq/L in 1965 to < MSA = 0.06 Bq/L in 2014. (authors)