Book excerpt: This investigation report examines a vessel explosion that occurred on April 11, 2003, at D. D. Williamson & Co., Inc., in Louisville, Kentucky. The explosion caused a massive release of aqua ammonia. Twenty-six residents were evacuated, and 1,500 people were sheltered in place. This report identifies the root and contributing causes of the incident and makes recom- mendations on overpressure protection, hazard evaluation sys- tems, layers of protection, and operating procedures and training. The U.S. Chemical Safety and Hazard Investigation Board (CSB) is an independent Federal agency whose mission is to ensure the safety of workers, the public, and the environment by investigat- ing and preventing chemical incidents. CSB is a scientific investi- gative organization; it is not an enforcement or regulatory body. Established by the Clean Air Act Amendments of 1990, CSB is responsible for determining the root and contributing causes of accidents, issuing safety recommendations, studying chemical safety issues, and evaluating the effectiveness of other govern- ment agencies involved in chemical safety. No part of the conclusions, findings, or recommendations of CSB relating to any chemical incident may be admitted as evidence or used in any action or suit for damages arising out of any matter mentioned in an investigation report (see 42 U.S.C. § 7412 [r][6][G]). CSB makes public its actions and decisions through investigation reports, summary reports, safety bulletins, safety recommendations, case studies, incident digests, special technical publications, and statistical reviews. More information about CSB may be found at www.csb.gov