Radiological status of Rongelap Island in 1999

A radiological survey and whole body counting of 137Cs were carried out in Rongelap Island (main island) of Rongelap Atoll in July 1999. The maximum values of 137Cs contamination and of γ-ray dose rate were 39 kBq/m2 and 0.033 μSv/h, respectively. The maximum α and β gross counting rates on the surface of ground were 1 cpm and 182 cpm in active area of 72 cm2, respectively. Activity of 239,240Pu for soil was 80 Bq/kg in the top 5 cm and aerial deposition was 3.4 kBq/m2 in Rongelap Island in 1999. The body burden of 137Cs was observed to be 27±11 Bq/kg for 6 workers. Assessment of external and internal annual doses (0.1 and 0.07 mSv/y) indicates that as of 1999 there is no large risk to the inhabitants of Rongelap Island from a radiological point of view. The radiological status of Rongelap Island, which was severely contaminated by the radiological fallouts of nuclear testings carried out in 1954, has improved year by year as shown by the decrease of 137Cs. The effective halftime of 137Cs, which is estimated to be 6.6 y is much shorter than the physical half-life of 137Cs. Radioactive contamination in Kaballe Island, (a part of the northern islands used for farming) which is located 25 km northeast of Rongelap Island, was still high in 1999. One site nearby a beach was highly contaminated with 137Cs, where the maximum activity of 137Cs was 3.4 MBq/m2, α-ray of 2 cpm, β-ray of 1205 cpm and γ-ray of 0.73 μSv/h. Activity of 239,240Pu in soil (n = 1) was 294 Bq/kg (top 5 cm) and 16 kBq/m2