Umasankari Kannan

Reactivity worth of liquid poison jets in the moderator

The safety requirement of two shutdown systems functioning on diverse principles has led to the introduction of a secondary shutdown system which functions by injecting liquid neutron poison jets at high pressure into the bulk moderator. The poison enters the moderator in the form of high-speed jets, in which the cross-section increases and the poison concentration falls as the jet develops. A formalism has been developed for estimating the reactivity worth of these jets. This formalism is different from currently used formalisms, in which the conical jet is approximated by equivalent cylinders whose radius changes from cell to cell. In the formalism presented in this paper, both the cross-section of the jet and the concentration of the poison are taken to be continuously varying from the origin of the jet to the end of the jet where it starts diffusing into the moderator. The formalism has been used to calculate the reactivity worth of the secondary shutdown system of the 500 MW(e) PHWR, making use of jet growth data that have been made available from experiments performed in our laboratory.

Studies on Reactivity Coefficients of Thorium-Based Fuel (Th-233U)O2 with Molten Salt (Flibe) Cooled Pebble

Abstract A combination of the neutronics features of gas-cooled high-temperature reactors by using the fuel in the form of ceramic-coated particles, called tristructural-isotropic, and the heat removal feature of molten salt reactors by using molten salt as a coolant is an attractive option in designing a reactor with a high-power density operation without compromising the safety aspects. Neutronics feasibility of such a combination of the molten salt (LiF-BeF2) as a coolant and thorium-based fuel, in particular (Th-233U)O2, in a graphite-moderated system is investigated. This technical note presents the influence of the heavy metal (HM) loading on neutronics features of a pebble lattice cell, that is, infinite multiplication factor (K-inf), temperature coefficients of reactivity (TCR), the void reactivity coefficient, etc. In addition, enriched uranium fuel has also been studied just to make a comparison with thorium-based fuel. Furthermore, the minimum HM loading of fuel per pebble that is needed to achieve negative coolant-temperature reactivity coefficients and void reactivity coefficients has been estimated for molten salt coolant. The analyses show that Th2/U3 fuel gives a less negative fuel temperature reactivity coefficient as compared with that of uranium-based fuel. This study also shows that all the TCR of both fuel types improve, becoming less positive or more negative, by increasing HM loading per pebble. Further, the burnup dependence of K-inf and the reactivity coefficients are studied for limiting HM loadings, e.g., 30 g per pebble. The change in the spectrum and the four-factor formula are used to explain the behavior of the reactivity coefficients as a function of HM loading and burnup.

Rotational Symmetry Boundary Condition in Current Coupled Whole Core Pin by Pin Transport Theory Code

The advances in computer processing power have made it possible to perform a detailed pin by pin calculation of the whole core. The methods based on response matrix are being used to perform whole core transport calculations. This includes the current coupled methods based on 2D collision probability (CP) and method of characteristic (MOC). The basic approach in the whole core transport theory methods is not to homogenize the lattice cells and subdivide each cell location in the fuel assembly (FA) into finer regions. The coupling of lattice cells within the assembly and assembly to assembly coupling can be achieved using interface currents. Due to very fine discretisation of the lattice structure and large core size, the physical memory requirements for the whole core simulations are huge. This requirement is compounded if ultra-fine discretisation of energy domain is also considered. When there is an inherent symmetry one can solve for the symmetric portion of the core, thereby save both memory and computational time. Rotational symmetry boundary condition in the whole core is normally considered. Application of this boundary condition gets very complicated when the whole core is modeled by a pin by pin approach. The present paper describes the methodology to apply the rotational symmetry boundary condition in the core discretized with complex microstructures of various heterogeneous cells of the problem.