RAPID-L

Lore & Background

The RAPID-L study was commissioned by JAERI in 1999, inspired by the SP-100, Alkali Metal Thermoelectric Converter, JAERI's own high-temperature gas-cooled reactor, and CRIEPI's earlier RAPID and RAPID-A projects. The three-year plan included basic concept and material research in 1999, Fast Critical Assembly tests in 2000, and lithium injection module tests and plant dynamics analysis in 2001. The reactor was designed for natural circulation, using lithium-7 coolant chosen for its high boiling point (1342 °C) and its ability to generate helium gas via neutron absorption, which required helium removal systems.

Reader's Guide

The RAPID-L concept represents a significant attempt to create an inherently safe, long-life nuclear reactor for extraterrestrial use. Its reliance on passive reactivity control systems—Lithium Expansion Modules, Lithium Injection Modules, and Lithium Release Modules—eliminated control rods and enabled fully automated operation. The integrated fuel assembly design allowed quick refueling and a 10-year operational life without refueling. Although never built, the study advanced understanding of lithium-6 as a neutron absorber and demonstrated the feasibility of ultra-compact, low-mass reactors for space applications, with a reactor container 2 m in diameter, 6.5 m deep, and weighing 7.6 tons.

Did You Know?

• The RAPID-L reactor uses lithium-7 as the primary coolant, while lithium-6 serves as a neutron absorber in the control system.
• The reactor was designed to be installed below grade on the Moon or Mars so that the ground provides shielding.
• The RAPID-L concept achieved a 60% mass savings over comparable liquid metal cooled fast reactors by eliminating conventional fuel handling systems.
• The reactor startup is automated by melting a frozen seal in the Lithium Release Module at approximately 780 °C.