Traces of Pu isotopes originated from burnt-up fuel in Fukushima exclusion zone

The accident in Fukushima Dai-ichi Nuclear Power Plant (FDNPP) released high activity of fission and neutron activation products in the environment. Total amount of released Pu is still unknown. Estimations so far indicates that dispersion from FDNPP is about 0.01 – 0.1 % of total Pu isotopes released from Chernobyl NPP accident. This implies activity of distributed Pu (for FDNPP) at the level GBq. It is assumed that more amount might be deposited close to NPP in early April 2011. However, due to sequence of hydrogen explosion (Unit 3 contained MOX fuel rods) that caused emission of radioactive elements to the atmosphere and depending on meteorological conditions, a highly radioactive plume was moved on the north - western direction of FDNPP. As a consequence of coprecipitation on 15 March 2011, a large area was affected by wet deposition. The highly contaminated area, within 30 km radius of FDNPP was defined as an exclusion zone. This work focused on Pu isotopes as traces of non-volatile elements of burnt-up fuel which were dispersed and could be deposited on that area. Previous papers reported no significant increase of 239+240Pu activity and rather rarely provided information on 238Pu activity concentration in samples from exclusion zone. In the present studies, hot spots (places with high dose rate above 2 μSv h-1) in Namie district were selected. Samples of upper layer soil and plants were collected as well as mud from small “artificial collectors” of water like e.g. small dips below downspout of rain gutter, trench that was a part of road drainage system and dips in the road, etc. The activity level of caesium isotopes (134Cs, 137Cs) in collected samples reached value 5 MBq kg-1 (signature of high and relatively fresh contamination) while 239+240Pu and 238Pu reached 0.3 Bq kg-1 and 0.5 Bq kg-1, respectively. These values were significantly higher than average activity concentration of Pu in Japanese soil estimated as a 0.15 Bq kg-1. The activity ratio, 238Pu and 239+240Pu used as a sources marker, was determined as a 0.5 for Chernobyl accident. Our study reveals higher level of Pu activity ratio in some samples in exclusion zone. That indicates very well the nuclear burnt-up fuel non-volatile elements impact on sampling area. More results in details and their interpretation will be presented at the conference.Plutonium Futures-The Science 201