Head, Reactor Vessel : The removable top section of a reactor pressure vessel. It is bolted in place during power operation and removed during refueling to permit access of fuel handling equipment to the core.
U.S. Nuclear Regulatory Commission (NRC), Full-Text Glossary, Washington, DC, USA 2014. http://www.nrc.gov/reading-rm/basic-ref/glossary/f ull-text.html, (Dzstr/ Disabl Mass Care - Amer) {826}