137Cs and 90Sr depositions, and 137Cs/90Sr ratios of samples from the Kraton-3 underground nuclear explosion site, Russia

Strontium-90 activity concentrations in surface soils and areal deposition densities have been studied at a site contaminated by an accidental release to atmosphere from the underground nuclear explosion 'Kraton-3' conducted near the Polar Circle (65.9°N, 112.3°E) within the territory of the former USSR in 1978. In 2001-2002, the ground surface contamination at 14 plots studied ranged from 20 to 15000 kBq/m**2, which significantly exceeds the value of 0.44 kBq/m**2 deduced for three background plots. The zone with substantial radiostrontium contamination extends, at least, 2.5 km in a north-easterly direction from the borehole. The average 137Cs/90Sr ratio in the ground contamination originated from the 'Kraton-3' fallout was estimated to be 0.55, which is significantly different from the ratio of 2.05 evaluated for background plots contaminated mostly from global fallout. Although vertical migration of 90Sr in all undisturbed soil profiles studied is more rapid than that for 137Cs, the depth of percolation of both radionuclides into the ground is mostly limited to the top 10-20 cm, which may be explained, primarily, by permafrost conditions. The horizontal migration rate of radiostrontium in the aqueous phase exceeds the radiocaesium migration rate by many times. This phenomenon seems to be a reason for the significant enrichment of the soil surface layers by radiostrontium at some sites, with variations occurring in accordance with small-scale irregularities of landscape.

本研究针对1978年前苏联境内北极圈附近（北纬65.9°、东经112.3°）开展的地下核试验‘克拉通-3号（Kraton-3）’意外向大气释放放射性物质所污染的场地，对其表层土壤中锶-90（Strontium-90）的活度浓度与面沉积密度展开了调查。2001至2002年间，对14个研究样地的地表污染水平进行测定，其范围为20~15000 kBq/m²，显著高于3个背景样地推算得到的0.44 kBq/m²的基准值。具有显著放射性锶污染的区域，从钻孔位置向东北方向至少延伸2.5千米。由‘克拉通-3号’沉降物引发的地表污染中，铯-137（137Cs）与锶-90的活度比平均值经估算为0.55，与主要受全球沉降污染的背景样地所测得的2.05这一比值存在显著差异。尽管在所研究的全部未扰动土壤剖面中，锶-90的垂向迁移速率快于铯-137，但两种放射性核素在土壤中的渗透深度大多局限于表层10~20厘米，这一现象主要可由永久冻土条件解释。水相中放射性锶的水平迁移速率远高于放射性铯，该现象或是部分场地表层土壤出现放射性锶显著富集的原因，且富集程度随景观的小规模不规则性发生相应变化。