Features of the coolant flow in a fuel rod bundle of the fuel assembly for the RITM-200S reactor of a modernized floating power unit

Background: A modernized floating power unit with the RITM-200S reactor was developed to provide electricity to the areas of decentralized energy supply. An advanced design of the cassette core with increased energy resource, reliability, and safety indicators requires final comprehensive justification.

Aim: To experimentally study the coolant hydrodynamics in a fuel rod bundle of a fuel assembly for further justification of characteristics for an advanced cassette core of the RITM-200S reactor.

Materials and methods: Developed by order of the Rosatom State Corporation, a scale model of a fuel rod bundle fragment was tested on an aerodynamic facility of the Nizhniy Novgorod State Technical University named after R.E. Alekseev. The flow structure was studied according to a pneumometric method using a five-channel sensor.

Results: In regular cells behind the spacer grid, the formation of transverse flows is recorded at a distance of L/d_h ≈ 1 from the plates. Regular cells align the structure of the axial flow at a distance of L/d_h ≈ 10 from the grid; the dimensionless velocity is 1.0–1.1. The lowest axial flow velocity is observed in corner cells with a section covered by plates; the dimensionless value axial of the flow velocity ranges 0.3–0.6 at a distance of L/d_h ≈ 1 from the plates. The velocity of the axial flow at the central displacer is higher than at the periphery; its dimensionless value at a distance of L/d_h ≈ 10 from the grid is 0.75–0.9.

Discussion: Three structural zones of the flow are identified in the area of regular cells, central displacer, and on the periphery near the fuel assembly jacket; the flow velocity in them differs by 25–30%. The arrangement of the grid plates has a significant effect on the flow structure at a distance exceeding L/d_h ≈ 10.

Conclusion: The identified flow characteristics should be taken into account for justifying the thermal reliability of newly developed cassette cores using the KANAL thermal-hydraulic code. The calculation methodology of the code should be changed by increasing the number of calculation cell types and taking into account the non-uniformity of the flow rate over the areas of the fuel rode bundle and cell types.