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

Activity concentration of plutonium in atmospheric precipitation

Activity concentrations of plutonium isotopes (238Pu, 239+240Pu) were determined in atmospheric precipitation samples collected in Kraków from August 2005 to December 2007. The volume of 29 samples varied in the range 14–269 dm3. The method of samples collection, preparation and radiochemical analysis proper for the separation of plutonium are indicated in brief. Monthly plutonium deposition a showed seasonal variation for 239+240Pu the first maximum was observed in August 2005 and amounted to about 2.65±0.31 mBq/m2, the second one in July 2006 – 0.371±0.063 mBq/m2 and the third one in April 2007 – 0.859±0.075 mBq/m2 and for 238Pu in September 2005 – 0.090±0.038 mBq/m2, July 2006 – 0.177±0.074 mBq/m2 and the third maximum in May 2007 – 0.333±0.028 mBq/m2. Similar behaviour of activity concentration of plutonium was also observed in other parts of the earth. Anomalously high deposition of 239+240Pu (2.65±0.31 mBq/m2) was found in August 2005. The 238Pu/239+240Pu activity ratios higher than the global fallout value ~ 0.55 (in Poland) suggested participation from different sources of plutonium

Activity ratios of thorium isotopes in living species compared with other environmental samples

Since about ten years our laboratory is conducting studies on the determination of Pu in many environmental samples, among them also the biological ones. Plutonium results were already discussed elsewhere [1, 4, 5, 8–21, 27]. In the course of radiochemical procedure for plutonium separation a thorium fraction was separated for each sample. Since we had not used any thorium tracer in the past, thorium activities were not determined. However, always alpha sources were prepared and then measured. This produced a relatively large archive of thorium alpha spectra from which one can study activity ratios of thorium isotopes. Two ratios seems to be interesting. The first is the Th to Th activity ratio. Both these isotopes are the Th-series members and, therefore, one might expect equilibrium. Usually this is not the case. Another subject is the activity ratio between Th and Th. One can tell that there is no relation between these two isotopes and, therefore, this ratio should not bring any information. The first isotope belongs to the U series, it is a decay product of U. The second one is just a primordial parent of the Th series. So, one can expect, that this ratio will generally reflect the activity ratio between the U and Th series for a given environment. However, this ratio might be modified by weak effects and this possibility should be taken into consideration

238Pu/(239+240)Pu activity ratio as an indicator of Pu originating from the FDNPP accident in the terrestrial environment of Fukushima Prefecture

The Fukushima Dai-ichi Nuclear Power Plant (FDNPP) accident has caused significant radionuclide contamination. Pu isotopes at the level of GBq were released from the damaged reactors to terrestrial and marine ecosystems. In this work, 35 samples were collected at different locations of Fukushima. Samples consisted of three types, soil, forest litter and alluvial dust (road dust, sludges from drainage systems and below gutter pipe outflows). The obtained activity ratios of 238Pu/(239+240)Pu ranged from 0.030 to 1.86. 14 of our samples contained trace amounts of Pu originating from the damaged reactors (2SM verification). Our study identified a few previously unknown “hot spots” of 238Pu/(239+240)Pu activity ratio localized in an area between 15 and 30 km in the northwest direction from the FDNPP. Additionally, results obtained in this study combined with previously published data allowed us to prepare a map of spatial distribution of the Pu isotope fingerprints (238Pu/(239+240)Pu) in Fukushima Prefecture