Estimation of tritium production in VVER-440 reactor core during normal operation

Abstract: Slovakia as one of the world leading countries in the share of nuclear power in electricity production and currently operates 2 nuclear power plants, each with 2 VVER-440 units. In addition to these reactors there are 2 VVER-440 units under construction and 2 units in decommissioning. The VVER-440 technology features thermal neutron spectrum, low enriched uranium dioxide fuel and light-water coolant with diluted boric acid. The reactivity control during normal operation is ensured by the system of emergency reactivity control assemblies (ERC), symmetrically arranged in 37 positions in the reactor core, and by boric acid, homogenously diluted in the moderator. Due to the presence of 10B in the coolant/moderator which has high thermal neutron capture cross-section, the absorption of neutron on these atoms may lead to tritium production. Tritium as the radioactive isotope of hydrogen with half-life of 12.3 years, strongly contributes to the level of radioactivity of the primary coolant, therefore the NPP staff must have appropriate knowledge of its production during operation. The production rate of tritium may be assessed either by experimental measurement or by simulation using state-of-the-art calculation tools. This paper focuses on the estimation of the tritium production for a specific scenario of the operation of the 3rd unit of Mochovce NPP. For simulations the SCALE6 [1] system is used with the detailed calculation model [2] developed at the B&J NUCLEAR ltd. company. The whole-core 3D model of VVER-440 consists of the reactor in-vessel components such as fuel assemblies (including fuel rods, upper spacer grid, intermediate spacer grids, supporting grid, mixing grid, central tube and fuel endings), emergency reactor control assemblies (absorber and fuel part), core basket, barrel and the reactor pressure vessel. The boundaries of the created VVER-440 whole-core model are given by the outer surface of the dry shielding, the level of hot-leg piping and the basement of filtration mechanism. The calculations presented in the paper are performed using self-shielded multi-group cross-section libraries, taking into account the operation conditions of Mochovce unit 3 NPP [3] in the first fuel campaign.
Keyword: Tritium production, VVER-440, activity, criticality, SCALE system, Mochovce NPP