Energy deposition analysis for VVER‑1000 core internals

Calculation of energy deposition in metal structures under the influence of reactor irradiation is an important task in analyzing the state of reactor pressure vessel (RPV). Correct determination of energy deposition is important for obtaining initial data for assessing: temperature fields in the RPV, radiation swelling of the RPV material, and changes in the geometry of the RPV. The article presents the results of numerical modeling in the MCNP code of the process of energy deposition formation in the VVER‑1000 reactor pressure vessel. The main components of energy deposition in the core baffle were considered. The energy deposition is caused by the interaction of neutrons and gammas with the core baffle material. Since there are prompt and delayed neutrons and gammas, it is necessary to take them all into account. In the model of reactor core, the prompt neutron sources are fission neutrons. Prompt gammas are produced directly during fuel fission, as well as during the interaction of neutrons with fuel (without fission), fission products, structural materials, and coolant (moderator). Delayed neutrons and gammas are emitted during the radioactive decay of fuel fission products. The amount of delayed neutrons is insignificant (0.68% for ²³⁵U), the amount of delayed gammas is already significant and commensurate with the amount of prompt fission gammas. For the production of delayed particles (neutrons, gammas, beta particles, alpha particles and positrons) in the MCNP6 code, the ACT card can be used. The ACT card of the MCNP6 code is promising for modeling the delayed gammas from the decay of radioactive fission products, but it is not suitable for this calculation because it does not work together with the NONU card. Therefore, the determination of the delayed gammas was performed using the SCALE program package. The calculation results showed that the energy deposition is almost entirely due to the interaction of gammas with the core baffle material, with the contribution of delayed fission gammas to the total energy deposition amounting to 1.4−21.1%, depending on the core baffle region. Since the contribution from delayed gammas is maximum in the area where the most energy deposition is observed – on the inner surface of the core baffle – its accounting is mandatory.