Development of Crud Chemistry Model using MOOSE Amit Agarwal, Jim Henshaw & John McGurk
Introduction: MOOSE MOOSE software tool developed by Idaho National Labs MOOSE used for solving partial differential equations Uses finite element methods Suitable for coupled chemistry/flow/heat transport problems Use requires some C++ knowledge, understanding of MOOSE language and manipulation of differential equations and boundary conditions Implemented on NNL computer cluster Applied here to investigate nuclear fuel clad behaviour in reactor systems
Problem: Background Pressurised Water Reactors and Boiling Water Reactors circulate high-temperature water that slowly corrodes Inconel and stainless steel system surfaces. Nickel/iron based corrosion products deposit on fuel clad in regions of the core where subcooled/saturated boiling occurs (deposit called crud ). Fuel Crud - porous material (Fe/Ni oxides) containing relatively large channels, chimneys, penetrating down to the clad surface. Heat transfer across the crud by wick-boiling, water permeates the porous deposit, boils as it approaches the clad and exits as steam via the chimneys. Very high concentrations of dissolved species occur within the crud increasing the local saturation temperatures. Accumulation of boron in the crud causes a drop in the neutron flux (CIPS crud induced power shift or AOA, axial offset anomaly). Crud hinders heat transport. In some cases fuel-clad integrity can be compromised because of crud-induced localized corrosion.
Background Fuel Clad Failures - Increased personnel doses (Iodine isotope releases + tramped uranium) - Increased reactor outage times ( 5M/day) - Increased cost of cause analysis - Fuel retrieval and storage problems - Problems persist for many years after events Current plants uprating/extending fuel cycles, also UK-EPR and AP1000 large amount of subcooled boiling Problem likely to get worse in future!!
Moose Chemistry Crud Model It is important therefore to understand this phenomena theoretically, to know how thick a crud can be tolerated before problems start to occur, how this is related to the power output from the fuel pin and how this is affected by the concentration levels of the various dissolved chemicals in the coolant, which can be controlled by the operator. Moose Chemistry Crud Model (MCCM) has been developed which solves the relevant partial differential equations for this problem. The MCCM model is one dimensional through the depth of the deposit and consists of one steam chimney and its surrounding porous shell. Model contains differential equations + boundary conditions for the following components: 1.) A thermal hydraulics model of heat transfer within and fluid flow of water and steam through the porous deposit. 2.) Transport (flow, diffusion, ion migration) of chemical species in solution through the porous shell. 3.) Partitioning of volatile species into the steam phase. 4.) Chemical reactions and precipitation of solids from solution.
Concentration (mol/l) Deposit Temp ( o C) Results / Further work/ Comments 10 1 B(OH) 3 375 Extending the MCCM includes: 0.1 0.01 0.001 0.0001 B 3 O 3 (OH) 3 Li + 370 365 360 1. Coupling the heat-transfer equation through the crud to the heat transfer equation of bulk coolant. Accurate clad temperatures 1E-05 1E-06 1E-07 1E-08 1E-09 1E-10 1E-11 340 0 5 10 15 20 25 30 35 40 45 50 Water/Crud Interface Temperature Deposit Depth (mm) LiBO 2 (s) Precipitation MCCM compared to existing Facsimile model, good comparison. - Crud thickness at which LiBO 2 precipitates - Temperature at crud/clad interface - Species concentrations within the crud 355 350 345 Crud/Clad Interface 2. Blocking of the deposit pores when lithium metaborate precipitate - Accurate clad temperatures/boron uptake amounts. 3. Addition of other salts such at Li 2 B 4 O 7, ZnO, Zn 2 SiO 4. Define plant operating limits on Li, B, Zn, SiO 2 4. Extend model to 2D and 3D More realistic representation
Conclusion NNL have developed a MOOSE based model (MCCM) to study the effect of fuel crud Model predicts conditions (temperature, ph) at the fuel clad interface which control clad corrosion rates Currently improving the MCCM model by choosing more realistic boundary conditions for the problem, adding other relevant chemistry and extending from 1D to 2 and 3D A number of simplified models form part of the Electric Power Research Institute (EPRI) BOA code, used by PWR plants worldwide to maximise core design efficiency. MCCM will help to validate the BOA code. The final aim will be to have an accurate 3D model to give realistic predictions, but more importantly can be used to validate simpler models that are used in complete core design codes where modelling on the μm size scale is not feasible.