An improved nuclear shielding material based on a resistant organic matrix that is flexible or resilient after room temperature polymerization, and sufficiently fluid before polymerization so as to effectively fill voids in radiation containment structures. The material can be formulated to undergo pyrolysis and transform into a strong ceramic material. Along with the organic matrix the material contains a primary radiation shielding component such as tungsten carbide powder. Additional optional components include: a neutron absorbing/gamma blocking compound such as boron carbide powder, a heat conducting material such as diamond powder, a high temperature resistant compound such as silicon dioxide powder, a second neutron blocking compound which also imparts electrical conductivity, namely barium sulfate powder, and a hydrogen gas surpassing component which readily absorbs hydrogen such as sponge palladium- Refractory materials and rare earth oxides can be included to favor effective ceramic transition.