Study of particle transport in a high power spallation target for an accelerator driven transmutation system

Transmutation of highly radioactive nuclear waste can be performed using an accelerator driven system (ADS), where high energy protons impinge on a spallation target to produce neutrons. These neutrons are multiplied in a subcritical core, while simultaneously fissioning the minor actinides into short lived or stable nuclides. AGATE is a project envisaged to demonstrate the feasibility of transmutation in a gas (helium) cooled ADS using solid spallation target. Development of the spallation target module and assessing its safety aspects are studied in this work. According to the AGATE concept parameters, 600 MeV protons are delivered on to the segmented tungsten spallation target. Tungsten is an ideal solid spallation target material because of its high melting point, other than the many desirable properties. Spallation is by far the most attractive means of neutron production when it comes to energy deposition per neutron. The spallation mechanism initiates with intranuclear cascade (INC) reactions, followed by deexcitation of nuclei through evaporation, multifragmentation and fission. Both the INC and deexcitation processes lead to the production of neutrons and spallation products. Spallation neutron energy spectrum is relatively harder compared to fission because of the higher energetics involved. The monte carlo toolkit Geant4 has been used in the simulation of particle transport. From the systematics study of incident projectile types (proton, deuteron and alpha), neutron yield due to proton and deuteron are generally higher than that for alpha. At higher energies, deuteron fares better than proton. Given the lower kinetic energy of proton (600 MeV) and owing to the fact that acceleration cost increases with increasing mass, proton turns out to be the ideal projectile for the current system. Energy cost of neutron production is the most efficient for protons of energy between 800 and 1000 MeV. Nuclear collision probability increases with increasing proton energy, reaching a saturation value at about 1 GeV for tungsten. To ensure maximum interaction, the target length needs to be as long as the range of protons in the material. For 600 MeV protons, the range is about 15 cm in tungsten. There exists an optimum radius of the target determining the neutron yield. While lower radii means the leakage of energetic secondaries without producing further neutrons, larger radii results in the parasitic absorption. For tungsten, target radius of about 10 cm turns out to be a good option. In a monolith tungsten target, neutron buildup near the target head is not very suitable to illuminate the subcritical core coupled to the target. Hence splitting the target using fluxtraps into segments of varying thicknesses is necessary to produce a homogenized neutron field. This also leads to the hardening of the emitted neutron energy spectrum required for transmutation. Further, the fluxtraps allow efficient cooling and reduced parasitic absorption in the target. Power density in the optimized target is still very high to be cooled. Fluidizing the target with pebbles instead of solid material is a feasible option. Radiation damage has been calculated using the NRT theory for one full power year operation and per mA proton current. Damage is significant in the first few segments of the target, decreasing gradually with increasing tungsten length. Maximum damage in the target is inflicted on the first segment, about 4.5 dpa and the total damage is about 53 dpa. Total specific activity in the target at shutdown after one full power year operation is about 2.4E14 MBq/g per mA. Unlike the actinides, which need to be transmuted, radionuclides produced in the target are less radiotoxic and have shorter life time. Spallation produces high energy neutrons and gammas which need to be shielded. An additional dimension in the shielding calculation is introduced by the high energy forward peaked neutrons. Shields composed of boronated steel and boronated concrete are used, which greatly reduces the shielding dimensions while exhibiting good shielding performance. This also reduces the amount of activated materials. An inner thick layer of iron is required to attenuate the high energy of neutrons. The concrete block following this is efficient in shielding against the low energy neutrons