Two techniques have been developed for the underwater dismantlement of the steel components of the Japan Power Demonstration Reactor (JPDR): the plasma arc for dismantling the reactor internals and the arc saw for cutting the reactor pressure vessel. In parameter tests, the cutting capability for these techniques was improved, and the characteristics of the byproducts were evaluated to enable proper design of the water purification and dross collection systems. Mockup tests, conducted as final systems performance checks, proved that the cutting systems developed are practical for dismantling the JPDR steel structures.