Modeling Porous Corrosion Deposits (CRUD) in PWRs in CASL

In large-scale science and technology initiatives at the national or international level, often a goal is to seek innovations that are explicitly relevant to industry and at the same time also have significant scientific implications. An example is the Consortium for Advanced Simulation of Light Water Reactors (CASL), whose current efforts aim to impact nuclear power generation by exploiting the capabilities of multiscale modeling and simulation coupled to high-performance computing. One of the challenges to be addressed in achieving higher burnup and power uprates is a problem of fouling of reactor fuel rods. The phenomenon, known as Chalk River Unidentified Deposits (CRUD) after the location of their discovery, is a major concern in fuel reliability and reactor operation, with further consequences concerning safety under accident conditions. In existing CRUD models, simplifications such as infinitely soluble species, one-dimensional analysis, constant material properties, liquid saturation, and decoupled physical phenomena had been introduced. Although useful at the time, it is believed these assumptions can be removed by a formulation that is not only more science based in the physical description, but also scalable and adaptable to massively parallel, high performance computation.

MIT is developing a new framework and code, known as MAMBA-BDM (for Boron Deposition Model), to tackle the challenge of modeling CRUD in a coupled, multiscale manner. MAMBA-BDM is formulated and implemented in a highly parallel object-oriented physics simulation framework from the Idaho National Laboratory, known as MOOSE, which is dimension and geometry agnostic. The multiphysics nature of CRUD formation and growth is treated explicitly by allowing heat transport to have both conductive and convective contributions. The calculation of temperature, fluid pressure, fluid velocity, spatially-dependent material properties, soluble boron concentration, and regions of boron oxide precipitation in the CRUD is performed simultaneously. A control volume formulation consisting of one boiling chimney and the 2D, symmetric slice of CRUD surrounding it allows for the treatment of CRUD with a regular spacing of boiling chimneys.

MAMBA-BDM analyzes the spatial distributions of temperature, pressure, and boron concentration in 3D real or simulated CRUD, which reveal the interdependence of heat and mass flow, along with temperature-dependent solubility. A notable result is the non-negligible effect of heat convection, which has been overlooked previously. Another is the assumption of liquid saturation, which we show to be questionable in the process of estimating an effective thermal conductivity for CRUD. Overall, MAMBA-BDM represents a major development responding to the challenge of seeking technology progress coupled with scientific advances, because of further opportunities in this work for multiphysics modeling and multiscale simulations.