Nuclear. An assembly containing fissile material intended to sustain a controlled fission chain reaction at a low power level, used to investigate reactor core geometry and composition
International Atomic Energy Agency IAEA, Department of Nuclear Safety and Security, IAEA Safety Glossary (Terminology Used In Nuclear Safety and Radiation Protection), IAEA, Vienna. Austria 2007. ISBN 92-0-100707-8, http://www-pub.iaea.org/MTCD/publications/PDF/Pub1 290_web.pdf, (Nucl Sfty and Rad Prot - Brit) {459}