In a reactor dismantling technology development project being conducted by the Japan Atomic Energy Research Institute under contract from the Science and Technology Agency, Kajima Corporation has been performing research and development on an abrasive water jet cutting system. This system is to be applied to the cutting of biological shields, which cannot be performed by humans due to the high level of radioactivity of the shields. A mock-up test of dismantling the biological shield of the Japan Power Demonstration Reactor (JPDR) has been performed to demonstrate that the abrasive water jet cutting system is fast, safe, and reliable under actual working conditions. The mock-up test provided valuable experience and various reference data useful for the dismantlement of the JPDR, which started in 1991. The abrasive water jet cutting system is expected to prove very useful in the dismantlement of atomic power plants.