Reduction of the External Dose Rate of a Research Nuclear Reactor Shielding Utilizing Concrete with Higher Incorporated Water Quantity

摘要

The ordinary concrete used in constructions hold about 4 wt% to 6 wt% of water content in itsmixtures under two distinct fractions: free water fraction and chemically constituted water fraction. Studies havebeen showing that is possible and convenient to increase 3 to 4 wt% the water content in the special concretemixtures used in nuclear reactor shielding, increasing the free water fraction and yet considering an adequateconcrete composition of the components in order not to compromise the structural resistance with the increasedwater to cement ratio. This is technically viable, because these structures have dimensions much superior thanthose imposed by structural calculations since they are determined as a function of the shielding design itself.The use of a concrete with larger quantity of incorporated water assures a more efficient neutron capture processin the foremost concrete shielding layers leaving a great portion of material thickness to attenuate the highenergygamma radiation resulting from these neutron captures. This in turn reduces the dose rate in the externalshielding wall satisfying the ALARA principle, and assures a less radioactive environment in the reactorsurroundings, which is important for the procedures of radiological protection of the installation. This workevaluated the reduction of the dose rate in the outer wall of the shielding of a reactor type pool of 10 MWth dueto variations of the water fraction content in concrete, in a range from 4 to 7 wt%. This variation range waschosen conservatively and took into account the free water fraction incorporated into an ordinary concrete andexperimentally estimated through neutron radiography techniques carried using the irradiation channel of theArgonauta reactor installed at the Instituto de Engenharia Nuclear. The discrete ordinate code ANISN, theEURLIB4 library of neutrons and gammas, and heavy concrete of different densities were used in thecalculations.