of zirconium carbide (instead of niobium carbide) but Pewee also increased the power density of the system. A water-cooled system is known as NF-1 (for Nuclear Furnace) used Pewee 2's fuel elements for future materials testing, showing a factor of 3 reductions in fuel corrosion still further. Pewee 2 was never tested on the stand and became the basis for current NTR designs being researched at NASA's Glenn Research Center and Marshall Space flight Center. The NERVA/Rover project was eventually canceled in 1972 with the general wind-down of NASA in the post-Apollo era. Without a human mission to Mars, the need for a nuclear thermal rocket is unclear. Another problem would be public concerns about safety and radioactive contamination. Kiwi-TNT destructive test In January 1965, the U.S. Rover program intentionally modified a Kiwi reactor (KIWI-TNT) to go prompt critical, resulting in immediate destruction of the reactor pressure vessel, nozzle, and fuel assemblies. Intended to simulate a worst-case scenario of a fall from altitude into the ocean, such as might occur in a booster failure after launch, the resulting release of radiation would have caused fatalities out to and injuries out to . The reactor was positioned on a railroad car in the Jackass Flats area of the Nevada Test Site. United Kingdom As of January 2012, the propulsion group for Project Icarus was studying an NTR propulsion system. Israel In 1987, Ronen & Leibson published a study on applications of 242mAm (one of the isotopes of americium) as nuclear fuel to space nuclear reactors, noting its extremely high thermal cross section and energy density. Nuclear systems powered by 242mAm require less fuel by a factor of 2 to 100 compared to conventional nuclear fuels. Fission-fragment rocket using 242mAm was proposed by George Chapline at Lawrence Livermore National Laboratory (LLNL) in 1988, who suggested propulsion based on the direct heating of a propellant gas by fission fragments generated by a fissile material. Ronen et al. demonstrate that 242mAm can maintain sustained nuclear fission as an extremely thin metallic film, less than 1/1000 of a millimeter thick. 242mAm requires only 1% of the mass of 235U or 239Pu to reach its critical state. Ronen's group at the Ben-Gurion University of the Negev further showed that nuclear fuel based on 242mAm could speed space vehicles from Earth to Mars in as little as two weeks. The 242mAm as a nuclear fuel is derived