Calculation of energy deposition in the pore water of the cements

In the civil nuclear industry, long-term storage and deep geological disposal of waste require safety assessments to demonstrate the durability of the materials used. Among these materials, concretes are complex and heterogeneous materials that trap large quantities of free interstitial water. The irradiation of these materials by the confined radionuclides can lead to the accumulation of oxidizing and explosive molecular species. The molecular hydrogen generated by the radiolysis of water presents risks if its concentration in the air exceeds 4% by volume. The air exchange must be continuous and sufficient in a waste containment storage, so that the atmosphere does not become explosive. In the framework of studies of the radiolysis in cement-based waste containers, CP2C laboratory of CEA Saclay makes calculation of the deposited energy in the water contained in the cement. This deposited energy is then used by chemists from CEA to calculate a chemical simulation code the quantity of hydrogen relased. Furthermore, in a sealed vessel, the continuous production of hydrogen will result in an increase in the internal pressure of the cement pores. If the mechanical strength limit is exceeded, an instantaneous failure of the vessel is possible. Concrete is a composite material consisting of a granular skeleton covered with a cement paste (mixture of water and mineral binder). Cements belong to a category of hydraulic binders composed of calcium silicates and aluminates, called CSH (silica and calcium hydrates). Since water is introduced in excess to facilitate the processing of the material, the cement matrix contains residual alkaline water with a pH between 12 and 14. The most common cement is Portland cement with pore sizes ranging from a few tens of micrometers to a few hundred of micrometers. In the CP2C laboratory, the radiation shielding unit at the CEA Safety Critical Competence Pole, I studied the deposited energy in the water pores of cement. The calculation of the deposited energy in the water contained in cement is usually performed by homogeneising the cement with the water. The aim of this work is to compare the classical homogeneous model to heterogeneous models of water in cement and to compare results obtained with TRIPOLI-4® and with PENELOPE. This led us to use two computer codes of the Monte Carlo type (called PENELOPE and TRIPOLI-4®) which deals with the transport of particles (neutrons, photons, electrons and positrons) in matter. First the calculations are carried out by homogeneising the materials, without taking into account the difference of density. We then studied different configurations taking into account the heterogeneous model. Lastly it was interesting to study the effects of energy depositions in 1D and compare it to a more detailed calculation that takes into account the shape and the position of the pores (stochastic geometry). From the reasoning developed and the simulations, we will give some ideas for reflection on the energy deposited in the water and therefore which kind of configurations are better to used.