MOSCOW (AP) — Russia's military said Wednesday that an old Soviet-built nuclear-powered satellite has spewed fragments in orbit, but insisted they do not threaten the international space station or people on Earth.
The military's Space Forces said the decommissioned Cosmos-1818 satellite "partially fragmented" in July.
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http://www.latimes.com/news/nationworld/world/wire/sns-ap-eu-russia-nuclear-satellite,1,2439002.story
And from the NASA Orbital Debris Newsletter
A 21-year-old satellite containing a dormant nuclear reactor was the source of an unexpected debris cloud in early July 2008. Launched by the former Soviet Union in February 1987, Cosmos 1818 (International Designator 1987-011A, U.S. Satellite Number 17369) was the first of two vehicles designed to test a new, more advanced nuclear power supply in low Earth orbit. Dozens of small particles were released during the still unexplained debris generation event.
Cosmos 1818 and its sister spacecraft, Cosmos 1867 (Figure 1), carried a thermionic nuclear power supply, in contrast to the simpler, thermoelectric nuclear device which provided energy to the well-known RORSATs (Radar Ocean Reconnaissance Satellites) during the 1970s and 1980s. The most infamous RORSAT was Cosmos 954, which rained radioactive debris over Canada in 1978 after suffering a loss of control malfunction.
Unlike their RORSAT cousins, which operated in very low orbits near 250 km, Cosmos 1818 and Cosmos 1867 were directly inserted into orbits near 800 km, eliminating any threat of premature reentry. According to Russian reports, the nuclear reactors on Cosmos 1818 and Cosmos 1867 functioned for approximately 5 and 11 months, respectively. For the next two decades, the two inactive spacecraft circled the Earth without significant incident.
Following the fragmentation event on or about 4 July 2008, the U.S. Space Surveillance Network was able to produce orbital data on 30 small debris (Figure 2). The majority of these debris were ejected in a posigrade direction with velocities of less than 15 meters per second, suggesting a relatively low energy event. From radar detections, a larger number of very small debris appear to have also been released, but routine tracking of these debris has proven difficult.
Special observations of a few of the debris revealed characteristics generally indicative of metallic spheres. Cosmos 1818 employed sodiumpotassium (NaK) as a coolant for its reactor, as did the older RORSATs. Although the post-Cosmos 954 RORSATs were known for releasing significant amounts of NaK droplets after reaching
their disposal orbits (Kessler et al., 1997), Cosmos 1818 and Cosmos 1867 did not follow this precedent.
Simplified illustration of Cosmos 1818 and Cosmos 1867. The dimensional units are millimeters.Much of the NaK within Cosmos 1818 probably was in a solid state at the time of the debris generation event. However, some NaK present in the radiator coolant tubes might have reached a temporary liquid state, particularly when the spacecraft was exposed to direct solar illumination. A breach in a coolant tube (for example, due to long-term thermal stress) at such a time could have resulted in the release of NaK droplets. Alternatively, the hyper-velocity impact of a small particle might have generated sufficient heat to melt some of the NaK, which then would have formed spheres with metallic properties. Additional analysis of the debris is underway in hopes of providing new insights into the nature of the objects and the possible cause of their origin. To date, no similar debris generation by Cosmos 1867 has been observed.
Courtesy of the NASA Orbital Debris Quarterly News Volume 13, Issue 1 January 2009, Pages 1 and 2.