The Boiling Nuclear Superheater (BONUS) reactor, located northwest of Rincón, Puerto Rico, was one of only two boiling-water superheater reactors ever developed by the United States. Construction began in 1960 as a joint effort between the U.S. Atomic Energy Commission and the Puerto Rico Water Resources Authority.1 Approximately 6 kg of spent fuel was sent from the BONUS reactor to the commercial reprocessing plant in West Valley, New York before being sent to Hanford.

The BONUS reactor experienced numerous problems during its short operation, including 106 intentional reactor shutdowns. The BONUS reactor was operational for a total of six years (1962-1968) before being decommissioned, a process completed in 1970.

During the decommissioning process all special nuclear materials such as spent fuel and highly activated components such as control rods and shims were sent to the continental United States. All piping systems were flushed, the reactor vessel and associated internal components within the biological shield were entombed in concrete and grout, and systems external to the entombment were decontaminated. Many contaminated and activated materials were placed in the main circulation pump room beneath the pressure vessel and entombed in concrete. During subsequent years, more radioactive contamination was identified in portions of the building, and additional cleanup and shielding activities were conducted in the 1990s and early 2000s.

In 1993 the basement of the entombed reactor was flooded by rainwater as a result of a failed exhaust fan. The local government proposes developing a museum at the site of the BONUS facility.

References

http://www.lm.doe.gov/BONUS/bonus-factsheet.pdf https://www.osti.gov/opennet/document/purecov/nfsrepo.html#ZZ4 http://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=01639904