Numerical method on the underground containment of fission products at a hypothetical accident in underground nuclear power plant

The safety of containing nuclides in an underground nuclear power plant at the time of a hypothetical accident was evaluated. As the special feature of underground nuclear plants, fission products are released and transmitted in strata very slowly and may partly appear on the ground after a hypothetical accident. The moving path of nuclides and the fundamental equation for this movement are shown. The numerical simulation process for the underground movement of radioactive nuclides is presented, utilizing the flow chart. The fundamental analytical equations concerning the nuclide movement in porous ground, the two-dimensional analytical process utilizing the finite element method and several boundary conditions are explained. As for the evaluation of the underground effect of containing long life nuclides such as Kr85 and I134 at the time of a hypothetical accident, several assumptions were made: 1) the electric output of a reactor is 1000 MWe, 2) the reactor type is PWR, 3) the kind of an accident is LOCA, 4) the ground thickness on an underground cave is 30 m and so on. Also, the analytical model of the ground around the containment cavity was set. The gas velocity distribution in the ground and the nuclides which are released inside the containment vessel at the time of a hypothetical accident were calculated and plotted. The leakage of radioactive nuclides to the ground surface at the time of a hypothetical accident was also analyzed and evaluated, utilizing the diffusion and adhesion coefficients in the ground. (Nakai, Y.)