Monju is a research prototype of a fast-breeder reactor, which burns plutonium extracted from spent nuclear fuel and generates even more plutonium. Its output is about 300,000 kilowatts. The so-called nuclear fuel cycle, which would allow the endless use of spent nuclear fuel, hinges on the very practicability of a fast-breeder reactor. It has constituted the centerpiece of Japan's efforts to develop advanced nuclear reactors, as the country is poorly endowed with energy resources. Unlike light-water reactors at existing power plants, which use water as a coolant, a fast-breeder reactor uses sodium that burns upon contact with the air, which makes the substance difficult to handle. Currently, no ways have been found to use Monju on a commercial basis because the necessary technology to ensure safety is too expensive.

(Sanskrit: Manjusri) Many Buddhists believe that this bodhisattva embodies the wisdom and insight of the Buddha. Monju is often depicted riding a lion and holding a sword and a sutra (a scroll of sacred text).

Monju is a framework that makes available the internal states of JBoss application server to Nagios remote monitoring system.