Over a period in 1961-1990 496 underground nuclear tests and explosions of different purpose and in different rocks were conducted in the Soviet Union at Semipalatinsk and anovaya Zemlya Test Sites. A total of 340 underground nuclear tests were conducted at the Semipalatinsk Test Site. One hundred seventy-nine explosions (52.6%) among them were classified as these of complete containment, 145 explosions (42.6%) as explosions with weak release of radioactive noble gases (RNG), 12 explosions (3.5%) as explosions with nonstandard radiation situation, and four excavation explosions with ground ejection (1.1%). Thirty-nine nuclear tests had been conducted at the Novaya Zemlya Test Site; six of them -in shafts. In 14 tests (36%) there were no RNG release. Twenty-three tests have been accompanied by RNG release into the atmosphere without sedimental contamination. Nonstandard radiation situation occurred in two tests. In incomplete containment explosions both early-time RNG release (up to *1 h) and late-time release from 1 to 28 h after the explosion were observed. Sometimes gas release took place for several days, and it occurred either through tunnel portal or epicentral zone, depending on atmospheric air temperature.

The value of the 133 Xe/ 133m Xe isometric activity ratio for the stationary regime of reactor work is about 35, and that for an instant fission (explosion) is about 11, which allowed estimation of the nuclear component of the instant (explosion) energy release during the NPP accident. Atmospheric xenon samples were taken at the trajectory of accident product transfers (in the Cherepovetz area); these samples were measured by a gamma spectrometer, and the 133 Xe/ 133m Xe ratio was determined as an average value of 22.4. For estimations a mathematic model was elaborated considering both the value of instant released energy and the schedule of reactor power change before the accident, as well as different fractionation conditions on the isobaric chain. Comparison of estimated results with the experimental data showed the value of the instant specific energy release in the Chernobyl NPP accident to be 2Á10 5 -2Á10 6 J/Wt or 6Á10 14 -6Á10 15 J (100-1,000 kt). This result is matched up to a total reactor power of 3,200 MWt. However this estimate is not comparable with the actual explosion scale estimated as 10t TNT. This suggests a local character of the instant nuclear energy release and makes it possible to estimate the mass of fuel involved in this explosion process to be from 0.01 to 0.1% of total quantity.

Monitoring of Xe and Kr radionuclides was conducted from August 2006 to 30 July 2008 within the framework of ISTC Project #2133. Cherepovets City in Vologda Province and St. Petersburg were chosen as monitoring locations. Kr-Xe concentrate samples were obtained as a result of processing of several thousand m 3 of atmospheric air. New results of 85 Kr monitoring show, that for last 15 years, the 85 Kr volumetric activity in the atmospheric air of the northwest region of Russia has increased approximately 50% and has achieved a level of 1.5 Bq/m 3 . This value correlates well with similar data for Western Europe and Japan. The xenon fraction (80-160 cm 3 under STP) is adsorbed on charcoal in the ampoule, which is measured in the well of HPGe gamma detector. Minimum detectable concentration (MDC) of 133 Xe for this technique is 0.008 mBq/m 3 , and it is the most sensitive method used today. The 133 Xe concentration in the atmospheric air of Cherepovets City varied in the monitoring period ranging from 0.09 to 2.5 mBq/m 3 . During the period of March 2007-30 July 2008, 133 Xe activity concentration in the atmospheric air of St. Petersburg changed from background values (0.2-0.3 mBq/m 3 ) to 185 mBq/m 3 and for approximately 20% of the samples 135 Xe was also measured with the 135 Xe/ 133 Xe activity ratio varied within the range of 0.03-3.5. -nuclides background in the air of St.-Petersburg, noble gas radionuclides release from nuclear power plants, high sensitivity methods of measuring Xe radionuclides.