A design approach similar to that followed in the case of Savannah, has been adopted for the collision protection of the First Nuclear Ship of Japan (FNSJ) (Ando, 1969). An investigation to assess her performance in case she is involved in a ship-ship collision aimed in the calculation of the critical speeds of 15 striking vessels versus her navigation speeds. The 15 ships had displacements varying from 6,360 tons to 239,000 tons and navigation speeds varying from 15 knots to 20.8 knots. The analysis showed that in some cases the navigation speed of the striking ship was higher than the critical speed, but it was claimed that the most probable percentage of the world fleet capable of penetrating the reactor compartment of the FNSJ was 0.7%. It was further noted that as low speeds are used in harbours, a collision resulting to rupture in a harbour area is practically impossible.