[en] Two transmutation methods, the spallation neutron and the muon-catalyzed fusion methods, both which use an accelerator, are employed for the transmutation of long-lived nuclides in high-level radioactive wastes. The transmutation energies and the effective half-lives of ⁹⁹ T_c for both transmutation methods are calculated by the Monte Carlo simulation codes for particle transport, the NMTC/JAERI code and the MCNP code. Both methods could obtain short effective half-lives, which are 17 times smaller than those of a fission reactor. The transmutation energies are calculated to be 25 to 55 MeV for both methods. These calculated transmutation energies reveal that it is possible for the foregoing two methods for transmutation of ⁹⁹ T_c to meet the energy balance criterion