Biological Shield 4 : A mass of absorbing material (e.g., thick concrete walls) placed around a reactor or radioactive material to reduce the radiation (especially neutrons and gamma rays respectively) to a level safe for humans
US Department of Labor, Electric Power eTool: Glossary of Terms, Occupational Safety & Health Administration OSHA, Washington, D.C., USA 2010. https://www.osha.gov/SLTC/etools/electric_power/gl ossary.html, (OSHA/ Electr Pwr - Amer) {618}