The major isotopes of plutonium in the reactor fuel are plutonium-238, 239, 240, 241, and 242, which account for as high as 1% of the total weight of the heavy elements in high burn-up fuel, and is recognized to have high priority in radiological protection. Analysis of a-radioactivity of soil samples by MEXT showed non-negligible distribution of plutonium isotopes in the environment. Despite the low concentration detected in soil, plutonium was found at some locations in the region up to several tens of km radius from the NPP, especially in the northwest direction. Plutonium is generally observed in the environment as a result of weapons testing and the Naga­saki atomic bomb. The radioactivity ratio between plutonium-238 and the sum of 239 and 240 is a fingerprint for the origin of plutonium contamina­tion in the environment. The ratio observed for the weapons testing fall-out in Japan is approximately 0.026 on average, but after the Fukushima acci­dent it ranged from 0.33 to 2.2. This proves that the observed plutonium is from the accident.

However, according to MEXT, the sum of the radioactivity concentration of plutonium-239 and 240 was from 0.6 to 3.3 Bq/m2 , and this was within the range of the background contamination by weapons fall-out, which is 17.8 Bq/m2 on average over the period from 2001 to 2010. Therefore, the radiological effect of the plutonium by this accident is considered to be within the existing effect of weapons testing fall-out. Zheng et al. [9] ana­lysed soil samples in the area from 25 km to as far as 230 km from the NPP.

They found 1.4 Bq/kg at locations about 30 km from the NPP, which is several times higher than the background. Despite its low radiological effect, the fact that plutonium, being quite a non-volatile element, was found significant distances away, suggests the need for more careful follow­up for the environmental effects of this accident.