Tarja K. Ikäheimonen

Characterisation of projectiles composed of depleted uranium.

Projectiles suspected to be composed of depleted uranium (DU) were found in Kosovo. Their properties were analysed using alpha and gamma ray spectrometry, mass spectrometry and electron microscopy. They were found to be composed of DU with small amounts of other elements such as Ti. 236U was detected in the penetrators, reflecting the use of reprocessed fuel. No transuranium elements were detected. The typical external dose rate meter is not the best option for mapping the location of penetrators from the ground. Monte Carlo calculations were performed in estimating possible skin doses. Penetrators in long-lasting contact with skin may cause a notable equivalent dose to skin.

Uranium and plutonium containing particles in a sea sediment sample from Thule, Greenland

Particles composed of radioactive materials and probably originating from US nuclear weapons were identified in sea sediment samples collected from Thule, Greenland, in 1997. The weapons were destroyed close to the Thule Air Base in 1968 in an aeroplane crash, which dispersed radioactive materials in the environment. The presence of particulate radioactive materials in the sediment samples was revealed by combining gamma-spectrometry and autoradiography. Isolation and separation of a radioactive particle from a bulk sample were performed using autoradiography, phosphor plate imaging and scanning electron microscopy. Using X-ray microanalysis as well as alpha and beta activity analysis, U and weapons-grade Pu were detected in the granular, brittle particle.

Measurement of 241Pu in Environmental Samples

Concentrations of 241Pu in the air and deposition samples in Finland after the 1986 Chernobyl accident were estimated with the aid of ingrowth of 241Am. Plutonium-241 was clearly shown to be present in the Chernobyl fallout in Finland. Pu was unevenly distributed and the mean radioactivity ratio of 241/239,240Pu was 70 for air and 71 for deposition samples. Here, a radiochemical method for determination of 241Pu in the environmental samples was also developed. The method was based on the liquid scintillation measurement of 241Pu with a pulse shape analyser after Pu separation. Efficiency calibration was performed with the aid of 3H standard instead of the 241Pu standard. The method was tested by comparing the results with the method of 241Am ingrowth and some reference samples. The detection limit for 241Pu was 0.007 Bq per sample. The 241Pu concentrations in the sediment samples taken from the Baltic Sea varied between <0.5 and 27 Bq·kg-1. The effect of the Chernobyl accident could be seen both from the 241Pu concentrations and from the Pu isotope ratios in vertical distributions of sliced samples.

Regional distribution of Chernobyl-derived plutonium deposition in Finland

The Chernobyl nuclear power plant accident in April 1986 caused a widely spread plume of radionuclides containing, amongst other materials, plutonium isotopes. The regional deposition of these nuclides in Finland has been assessed, based on samples of lichen, peat, precipitation, surface soil and grass. Unlike the deposition of transuranium elements from the weapons tests in the 1950s and the 1960s, the deposition in Finland from the Chernobyl accident was very unevenly distributed. Even then, the Chernobyl-derived deposition of 239,240Pu in the most contaminated regions of Finland was only around 10% of the global fallout from weapons tests. The total amount of 239,240Pu deposited in Finland was 1.1011 Bq (»25 g), i.e., approximately half of a percent of the activity deposited in the 1950s and the 1960s. In addition to the alpha-emitting Pu isotopes, the Chernobyl plume also contained a significant amount of the beta-emitting 241Pu, which is the precursor of the long-lived alpha-emitter 241Am. The highest plutonium deposition values were found in a relatively narrow swath from the southwestern coast of Finland northeastwards across the country. This is related to the calculated route of the air parcel trajectory associated with the initial explosion of the Chernobyl reactor. The high deposition values found in the northeastern part of the plume route over Finland can be attributed to the simultaneous occurrence of precipitation. The relatively high plutonium deposition in the southwestern part of Finland occurred, however, without concurrent precipitation. This indicates that the plutonium was at least partly associated with relatively large particles having a substantial deposition velocity due to gravitational setting

Identification and analysis of a radioactive particle in a marine sediment sample

Abstract A radioactive particle was identified in a marine sediment sample. Characteristics of the particle were determined using various methods. Particle isolation and preliminary activity estimation were performed using gamma-ray spectrometry and autoradiography. Long-lived radionuclides were detected in the particle using alpha-, beta- and gamma-ray analyses. Particle size and elemental composition were determined using SEM. Elemental composition, nuclide composition and structure suggest that the particle may have originated from the Chernobyl accident. The present study shows that routinely used laboratory methods/procedures in environmental radiation detection are not necessarily appropriate for analyses of individual particles as they are (too much) focused on radiation monitoring and not on characterization of radioactive materials.

Analysis of radioactive particles from the Kola Bay area

Two types of radioactive particle were identified in marine sediment and lichen samples collected from the Kola Bay, NW Russia. The particles were identified by means of gamma-ray spectrometry and autoradiography, separated and subjected to various analysis techniques. Several complementary techniques are needed to characterise particle properties thoroughly. 137Cs was present in the sediment matrix in large (approximately 100 microns) greenish particles that were most probably pieces of paint. Although their element composition was heterogeneous, 137Cs was found to be evenly distributed. 60Co in the lichen matrix was present in small (approximately 1 micron) particles. No U or transuranium elements were detected in either type of particle.

Plutonium and americium in the sediments off the Thule air base, Greenland

In 1968 a U.S. B-52 bomber from USA with four nuclear weapons crashed on the sea ice near the Thule air base and contaminated the surroundings. Most of contamination was recovered during the cleanup program. However, some of the plutonium still remains in the bottom sediments of the area. In 1997 an international expedition with a comprehensive sampling program was performed. A lot of sediment samples were taken with a Finnish Gemini corer. Sediment samples from 5 sampling stations, 80 samples altogether, were pretreated in Risř, Denmark, and analysed for transuranic elements at STUK, Finland. The samples were analysed primarily for Pu, but gamma-spectrometric measurements of 241Am were also made. Generally, the Pu concentrations in the area studied were from 1 to 3 magnitudes higher than the fallout level in the Arctic sea area. None of the vertical profiles of Pu at the various stations showed a typical Pu peak originating from the accident in 1968, but the sediments were mixed down and contaminated to the depths studied (20 cm). The presence of large quantities of hot particles distorted the estimations of mean concentrations and inventories. Samples with plutonium from the accident show significant variations of nuclide ratios, 238Pu/239,240Pu, 241Pu/239,240Pu and 241Pu/241Am. This indicates that Pu at Thule may be from more than one source of different quality.

Towards quality excellence in radioanalytical laboratories at STUK, Finland

Abstract The development of a uniform and modern quality system for the whole of the Finnish Radiation and Nuclear Safety Authority (STUK) began in 1997. The principle of continuous improvement is widely applied by performing annual self-assessments as well as internal and external audits. One of the most important steps in a quality assurance programme is the proficiency of the personnel. Good initiation, non-stop training during work and knowledge of the physical and chemical properties that affect analytical procedures are the requirements for first-rate and motivated working. Working procedures have been planned and followed through in order to prevent any contamination in laboratories. Considerable emphasis has also been placed on the standardisation of uncertainty estimations. This demands good knowledge of the sources of errors in sample preparation, analysis and measurement. Participating in intercomparison exercises and proficiency tests is one way to demonstrate the validity and quality of the methods used. However, a lack of intercomparison exercises and reference material on certain natural radionuclides occurring in waters and certain artificial radionuclides has complicated the validation and proficiency testing of these measurements. Moreover, the arrangement of intercomparisons has not always been optimal.

Joint Russian—Finnish study of radioactive contamination in the NW part of Lake Ladoga

In August 1992 a joint Russian-Finnish expedition was arranged to the NW part of Lake Ladoga to study radioactive contamination in the region. Special attention was paid to the area surrounding the Heinamaa Islands, where the wreck of the former mine carrier ship “Kit” had been lying about 30 years before it was moved to Novaya Zemlya in 1991. During this period the wreck had been used as a store for radioactive waste containing principally 90Sr, 137Cs and 239,240Pu. Lake water, bottom sediment and some biological samples were collected for strontium, plutonium and gammaspectrometric analyses. In all the samples the radioactivity concentrations were very low, indicating radioactive contamination of about the same level as caused by global fallout in the 1960’s and the Chernobyl fallout in the area. Only in two water samples taken close to the former site of the wreck slightly elevated 239,240Pu concentrations were detected. The great water volume of Lake Ladoga and effective water exchange at the wreck site may explain the very low levels of radioactive wastes detected in the aquatic environment.