After the Challenger accident, a study considered additional shielding. Additional shielding was not adopted, even though it would offer some protection near the launch area, because the great complexity of such a design significantly increased the risk of mission failure. If a failure on orbit occurred, additional shielding would significantly increase the consequences of a ground impact. The two close flybys of Earth had raised questions about the possible inadvertent reentry of Galileo into Earth's atmosphere.

Exhaustive studies were made by JPL and were independently reviewed regarding the safety of the Venus-Earth-Earth Gravity Assist (VEEGA) mission. The Department of Energy, as required by law, completed a mission risk analysis, with full disclosure of those results to State and local governments. The Interagency Nuclear Safety Review Panel completed an independent Safety Evaluation Report and the Office of Science and Technology Policy approved the mission. As a result of the Project's testing and rationale for the Galileo mission, the Court found in the Project's favor and the launch was a splendid success. The United States has an outstanding record of safety in using RTGs on 23 missions over the past three decades. While RTGs have never caused a spacecraft failure on any of these missions, they have been on-board three missions which experienced malfunctions for other reasons. In all cases, the RTGs performed as designed. More than 30 years have been invested in the engineering, safety analysis and testing of RTGs. Safety features are incorporated into the RTG's design, and extensive testing has demonstrated that they can withstand physical conditions more severe than those expected from most accidents. First, the fuel is in the heat-resistant, ceramic form of plutonium dioxide, which reduces its chance of vaporizing in fire or reentry environments. This ceramic-form fuel is also highly insoluble, has a low chemical reactivity, and primarily fractures into large, non-respirable particles and chunks. These characteristics help to mitigate the potential health effects from accidents involving the release of this fuel. Second, the fuel is divided among 18 small, independent modular units, each with its own heat shield and impact shell. This design reduces the chances of fuel release in an accident because all modules would not be equally impacted in an accident. Third, multiple layers of protective materials, including iridium capsules and high-strength graphite blocks, are used to protect the fuel and prevent its accidental release. Iridium is a metal that has a very high melting point and is strong, corrosion resistant and chemically compatible with plutonium dioxide. These characteristics make iridium useful for protecting and containing each fuel pellet. Graphite is used because it is lightweight and highly heat-resistant.

Potential RTG accidents are sometimes mistakenly equated with accidents at nuclear power plants. It is completely inaccurate to associate an RTG accident with Chernobyl or any other past radiation accident involving fission. RTGs do not use either a fusion or fission process and could never explode like a nuclear bomb under any accident scenario. Neither could an accident involving an RTG create the acute radiation sickness similar to that associated with nuclear explosions.

NASA places the highest priority on assuring the safe use of plutonium in space. Thorough and detailed safety analyses are conducted prior to launching NASA spacecraft with RTGs, and many prudent steps are taken to reduce the risks involved in NASA missions using RTGs. In addition to NASA's internal safety requirements and reviews, missions that carry nuclear material also undergo an extensive safety review involving detailed verification testing and analysis. Further, an independent safety evaluation of the Cassini mission will be performed as part of the nuclear launch safety approval process by an Interagency Nuclear Safety Review Panel (INSRP), which is supported by experts from government, industry and academia.

More information from the Dept of Energy (DOE)

Information about radioisotope thermoelectric generators can be located via the DOE Reports Bibliographic Database at
http://apollo.osti.gov/html/dra/dra.html

Select the link "Search The DOE Reports Bibliographic Database."

perform a search using the keywords radioisotope thermoelectri generators in the "Enter Search Terms Here" area. By selecting a report link you will reach information about that report. For each report, availability information can be found by selecting the link "Report Ordering and Retrieval

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