D. Schubring

Modification of SCWR Assembly Designs with Coupled MCNP5/SCA Methods to Promote Safer Operation

Abstract This work concerns the comparison of supercritical water reactor (SCWR) assembly designs using coupled reactor physics and thermal-hydraulic methods. In the SCWR, large density gradients in the supercritical water (used as coolant and moderator) will require detailed multiphysics analysis. The Super Light Water Reactor (SLWR) was analyzed previously [Hughes et al., Nucl. Eng. Des., Vol. 270 (2014)], where MCNP5 was coupled with density and temperature results from a single-channel code. MCNP5 then provided the single-channel code with a linear heat profile. In the present work, that proposed assembly design is determined to have a negative density coefficient of reactivity. Two alternate designs with different geometries and water-to-fuel ratios are presently considered to address this issue. It is found that adding an additional row of pins is more effective at producing a positive density coefficient than is reducing the size of the moderator boxes.

Coupled Computational Heat Transfer and Reactor Physics for SCWR

To fully model the physics present within the proposed supercritical water reactor (SCWR), the thermal hydraulics calculations (yielding temperatures and densities in each material as a function of space) must be coupled to the neutronic calculations (yielding reactivity and neutron flux shapes). To enable this full coupling, a 3D model of a supercritical water reactor is being implemented in the CFD software OpenFOAM with the same geometry as a 3D MCNP (neutronics) model. Coupling will performed through result exchange between the two codes — densities and temperatures from OpenFOAM to MCNP, with heat generation returned from the neutronics calculations. Use of a reduced-geometry model is advisable due to the high computational cost of each OpenFOAM/MCNP coupling iteration. In the present work, a 1.5D thermal model of a single fuel pin was coupled with a 3D MCNP model. The thermal model includes single channel analysis (cladding/coolant heat transfer) as well as heat transfer within the cladding, helium gas gap, and uranium dioxide fuel itself. These heat transfer zones provide specific data points of the radial temperature profile. Because no radial mesh is considered, full radial dependence is not possible. The model provides limited radial dependence unlike what a 1D code could provide; thus, 1.5D is used to indicate the incomplete radial dependence that is included in the model. Iteration between the two codes is performed until heat generation is converged to within 10% between successful MCNP results. A discussion of these 3D/1.5D coupled results and the path forward to fully 3D coupling is provided.Copyright

CFD Modeling of Single Bubble Collapse in Subcooled Boiling

The open-source software OpenFOAM has been employed to better understand the transport, deformation, and collapse of single, isolated bubbles in subcooled boiling. High pressure steam-water was selected as the fluid system of interest due to its relevance to the power generation industry. Ranges of bubble size, subcooling, pressure/saturation temperature, and mesh refinement have been employed. Both quantitative (bubble rise velocity and collapse times) and qualitative (bubble shape) results are discussed across the parameter ranges studied.Copyright