A system for attenuating seismic forces in a reactor module, the system comprising: a reactor building comprising a reactor building floor, a reactor module, and a plurality of reactor building walls; and a support structure seismically isolated from the reactor building walls, the support structure comprising: a mounting structure configured to securely connect the support structure to the reactor building floor of the reactor building; a receiving area sized to receive a lower portion of the reactor module, wherein the support structure is configured to at least partially surround the lower portion of the reactor module within the receiving area, and a retention system located near a top surface of the support structure, wherein the retention system is configured to contact the lower portion of the reactor module during a seismic event; characterized in that an elevation of the top surface of the support structure above the reactor building floor coincides with the top level of the lower portion of the reactor module having less than or approximately equal to one-half an overall height of the reactor module from the bottom of the reactor module, wherein an upper portion of the reactor module extends above the retention system without contacting the support structure, and wherein the upper portion of the reactor module is the remainder of the reactor module above the lower portion. a reactor building comprising a reactor building floor, a reactor module, and a plurality of reactor building walls; and a support structure seismically isolated from the reactor building walls, the support structure comprising: a mounting structure configured to securely connect the support structure to the reactor building floor of the reactor building; a receiving area sized to receive a lower portion of the reactor module, wherein the support structure is configured to at least partially surround the lower portion of the reactor module within the receiving area, and a retention system located near a top surface of the support structure, wherein the retention system is configured to contact the lower portion of the reactor module during a seismic event; a mounting structure configured to securely connect the support structure to the reactor building floor of the reactor building; a receiving area sized to receive a lower portion of the reactor module, wherein the support structure is configured to at least partially surround the lower portion of the reactor module within the receiving area, and a retention system located near a top surface of the support structure, wherein the retention system is configured to contact the lower portion of the reactor module during a seismic event; characterized in that an elevation of the top surface of the support structure above the reactor building floor coincides with the top level of the lower portion of the reactor module having less than or approximately equal to one-half an overall height of the reactor module from the bottom of the reactor module, wherein an upper portion of the reactor module extends above the retention system without contacting the support structure, and wherein the upper portion of the reactor module is the remainder of the reactor module above the lower portion.