Crud-induced Power Shift Research Articles

A coupling method for CRUD mass calculation based on Eulerian-Eulerian two-fluid model

Abstract The corrosion products occur in the pressure boundary in the pressurized water reactor primary loop which is known as CRUD. CRUD could increase the risks of crud induced power shift and crud induced localized corrosion. The mechanism of CRUD production depends on nucleate boiling. Eulerian-Eulerian two-fluid model has been currently proved as effective means to understand flow boiling character. In this research, the input data of thermo-hydraulic parameter is obtained by CFD method. The CRUD mass has been calculated based on thermo-hydraulic and chemical coupled model. The main calculation code in this article is FLUENT and CAMPSIS. The calculation results show that the CRUD does most likely occur the 3rd grid mixing vanes from the top of the fuel rod. The maximum CRUD mass is closer to the position of onset of nucleate boiling.

Experimental investigation of local boron concentration in a 2 × 2 rod bundle channel under subcooled boiling flow

During the operation of pressured water reactors (PWRs), subcooled boiling occurring at the upper-span region of the fuel assembly causes migration of the boron concentration in the coolant. The distribution of boron concentration in the coolant is essential to predict the boron hideout within the Corrosion Related Unidentified Deposition (CRUD) and crud-induced power shift (CIPS). However, experimental data on local boron concentration of rod bundle geometries in subcooled boiling conditions are not available. In this paper, a vertical test channel is constructed to determine the distribution of the local boron concentration of a 2 × 2 rod bundle channel under subcooled boiling conditions. The bubble-boron concentration simultaneous measurement device is developed and calibrated to measure the local two-phase flow parameters and boron concentrations under the subcooled boiling conditions. The experimental results indicate that the boron concentration is highest near the narrowest gap and decreases toward the subchannel center or subchannel wall. The boron concentration increases linearly with void fraction in the central subchannel, while the boron concentration first increases rapidly and then slowly in the wall channel. The underlying mechanism for the boron concentration distribution is revealed by considering the boron enrichment on the bubble–liquid interface, diffusion of the boron concentration, boron mixing induced by bubble wake, and boron dilution caused by bubble condensation. On the basis of that, a boron equilibrium model considering the evaporation and condensation of bubbles is developed. The experimental data is used to support the development and validation of the boron equilibrium model. The comparison results show that the boron equilibrium model can predict the performance of the boron concentration in the rod bundle channel with an accuracy of 20 %.

Experimental investigation on distribution of boric acid in a vertical 1 × 2 rod array channel

During the reactor operation, the mixing mass flow of boron acid caused by diversion cross-flow, turbulent mixing and void drift changes the boron concentration distribution in the coolant and affects the amount of boron in the Chalk River Unidentified Deposit (CRUD). Accumulation of boron within the porous crud layer on the fuel rods in the cores of pressured water reactors (PWR) results in the crud-induced power shift (CIPS), threatening the operation safety of reactors. In order to obtain the mixing mass flow of boric acid between adjacent channels, a vertical test channel containing two subchannels simulating a PWR fuel rod bundle is constructed. Using this channel, flow distributions and mixing mass flow of boric acid are measured for single-phase and two-phase air–water flows. The measurements show that the boric acid distribution is affected by turbulent mixing for single-phase flow and the mixing mass flow of boric acid increases with the Reynolds number. For two-phase flow, the bubble-induced turbulence enhances the turbulence intensity of the liquid flow, the mixing mass flow of boric acid increases with the increase of void drift and reaches the maximum in the slug flow. The experimental data is used to support the development and validation of a new transverse source term model of boron mixing. The comparison result shows the significant improvement of the code prediction.

Research on the effects of corrosion resistant zinc injection in primary circuit of large-scale PWR plants on core crud amount

In recent years, the number of nuclear plants injecting zinc is increasing worldwide because zinc injection can reduce the corrosion rate of structural materials in the primary coolant circuit. However, at the initial stage of zinc injection, zinc can introduce a large amount of corrosion products entering into the coolant circuit by displacing other divalent species in oxide. These corrosion products will be carried into the core by the coolant and cause unevenness of the crud on fuel cladding surfaces, which can increase the probability of crud induced power shift (CIPS) and damage to fuel claddings. In this paper, the structural material corrosion model with or without zinc injection is built based on a mixed conduction model. The migration model of corrosion products in the primary circuit is established, and the relationship of crud amount with the time is obtained. The mechanism of interactions between zinc and formed oxide layers is analyzed. The extra amount of corrosion products due to zinc injection is calculated according to the different starting times. It is used to revise the crud mass. The influences of zinc injection on the crud amount are achieved, which can be the foundation of further research on the relationship between zinc injection and CIPS.

A Study on the Crystalline Boron Analysis in CRUD in Spent Fuel Cladding Using EPMA X-ray Images

Chalk River Unidentified Deposits (CRUDs) were collected from the Korean pressurized water reactor (PWR) plant (A, B, and C) where the axial offset anomaly (AOA) occurred. AOA, also known as a CRUD-induced power shift, is one of the key issues in maintaining stable PWR plant operations. CRUDs were sampled from spent nuclear fuel rods and analyzed using an electron probe micro-analyzer (EPMA). This paper describes the characteristics of boron-deposits from the CRUDs sampled from twice-burnt assemblies from the Korean PWR. The primary coolant of a PWR contains boron and lithium. It is known that boron deposition occurs in a thick CRUD layer under substantial sub-cooled nucleate boiling (SNB). The results of this study are summarized as follows. Boron was not found at the locations where the existence was confirmed in simulated CRUDs, in other words, the cladding and CRUD boundaries. Nevertheless, we clearly observed the presence of boron and confirmed that boron existed as a lump in crystalline form. In addition, the study confirmed that CRUD existed in a crystal form with a unique size of about 10 μm.

Implementation of a Spacer Grid Rod Thermal-Hydraulic Reconstruction (ROTHCON) Capability into the Thermal-Hydraulic Subchannel Code CTF

The Consortium for Advanced Simulation of Light Water Reactors is developing a core simulator capability known as the Virtual Environment for Reactor Applications (VERA) to address nuclear industry challenge problems such as crud-induced power shift (CIPS). The CTF thermal-hydraulic (T/H) subchannel code provides thermal feedback in the coupled neutronics, T/H, crud chemistry simulation that VERA performs. It has been discovered that the coarse meshing approach used by CTF (in which fuel rods are discretized into four azimuthal segments) can be a source of error in predicting crud growth and boron distribution in VERA CIPS calculations. Spacer grid effects lead to complex rod-to-fluid heat transfer behavior that, when not resolved, can lead to error in the prediction of crud growth and boron deposition. A higher-fidelity computational fluid dynamics approach can be used instead of CTF, but this leads to excessive simulation times. This paper presents an approach for using high-fidelity computational fluid dynamics data to create shape functions that are used in CTF to reconstruct rod surface heat transfer behavior as a function of spacer grid geometry. The approach is demonstrated for a 5 × 5 rod bundle facility with five mixing vane grids under a range of operating conditions encountered in nominal pressurized water reactor conditions. It is demonstrated that the grid heat transfer maps are successful at introducing a higher-fidelity heat transfer modeling capability into CTF.

Modeling heat transfer through corrosion product deposits on fuel rods in pressurized water reactors

Abstract CRUD is the corrosion product deposit found on a fuel rod surface of pressurized water reactors (PWRs). Problematic phenomena caused by CRUD – CRUD Induced Power Shift (CIPS) and CRUD Induced Localized Corrosion (CILC) – are closely related to the heat transfer mechanism of CRUD. However, heat transfer regimes of CRUD are still not well clarified. Therefore, the heat transfer regimes of CRUD were investigated using existing database from the CRUD heat transfer experiment. As a result, it was found that there are three heat transfer regimes: (i) liquid-saturated regime, (ii) wick-boiling regime, and (iii) film-boiling regime. In addition, the boiling curves from the experiment were categorized according to their heat transfer regimes so that models for CRUD can be validated with appropriate database for the heat transfer regime where they are concerned. After obtaining the categorized databases, the model for the film boiling of CRUD was suggested. Unlike the conventional approaches that use capillary force as a unique source of the liquid supply in pores, the model we present in this paper adopted the disjoining force as an additional liquid supply term. This assumption was validated with the previous heat pipe experiment; the root-mean-square error (RMSE) for the prediction of the vapor film thickness was notably reduced from 790% to 18.7% by including the disjoining force to the supply term. The supply term considering the disjoining force was also applied to a model for the CRUD film boiling. The model estimates the wall superheat by balancing the supply term and the hydraulic resistance terms from fluid flows within CRUD. The model we present in this paper successfully predicted the wall superheat during the film boiling; the RMSE was 19% when the predicted wall superheats were compared with ones from the database categorized as film boiling. Finally, it was found from the sensitivity study using the model we present in this paper that the heat transfer performance of CRUD during the film boiling is highly sensitive to the pore radius.

Optimization method to determine mass transfer variables in a PWR crud deposition risk assessment tool

ABSTRACTOptimization numerical method was implemented to determine several mass transfer coefficients in a crud-induced power shift risk assessment code. The approach was to utilize a multilevel strategy that targets different model parameters that first changes the major order variables, mass transfer inputs, then calibrates the minor order variables, crud source terms, according to available plant data. In this manner, the mass transfer inputs are effectively simplified as “dependent” on the crud source terms. Two optimization studies were performed using DAKOTA, a design and analysis toolkit, with the difference between the runs, being the number of model runs using BOA, allowed for adjusting the crud source terms, therefore, reducing the uncertainty with calibration. The result of the first case showed that the current best estimated values for the mass transfer coefficients, which were derived from first principle analysis, can be considered an optimized set. When the run limit of BOA was increased for the second case, an improvement in the prediction was obtained with the results deviating slightly from the best estimated values.

Prediction of CRUD deposition on PWR fuel using a state-of-the-art CFD-based multi-physics computational tool

In the present paper we report about the first attempt to demonstrate and assess the ability of state-of-the-art high-fidelity computational tools to reproduce the complex patterns of CRUD deposits found on the surface of operating Pressurized Water Reactors (PWRs) fuel rods. A fuel assembly of the Seabrook Unit 1 PWR was selected as the test problem. During Seabrook Cycle 5, CRUD induced power shift (CIPS) and CRUD induced localized corrosion (CILC) failures were observed. Measurements of the clad oxide thickness on both failed and non-failed rods are available, together with visual observations and the results from CRUD scrapes of peripheral rods. Blind simulations were performed using the Computational Fluid Dynamics (CFD) code STAR-CCM+ coupled to an advanced chemistry code, MAMBA, developed at Los Alamos National Laboratory. The blind simulations were then compared to plant data, which were released after completion of the simulations.

Proof-of-principle of high-fidelity coupled CRUD deposition and cycle depletion simulation

A multiphysics framework for the high-fidelity simulation of CRUD deposition is developed to better understand the coupled physics and their respective feedback mechanisms. This framework includes the primary physics of lattice depletion, computational fluid dynamics, and CRUD chemistry. The three physics are coupled together via the operator-splitting technique, where predictor–corrector and fixed-point iteration schemes are utilized to converge the nonlinear solution. High-fidelity simulations may provide a means to predict and assess potential operating issues, including CRUD induced power shift and CRUD induced localized corrosion, known as CIPS and CILC, respectively. As a proof-of-principle, a coupled 500-day cycle depletion simulation of a pressurized water reactor fuel pin cell was performed using the coupled code suite; a burnup of 31MWd/kgHM was reached. The simulation recreated the classic striped CRUD pattern often seen on pulled fuel rods containing CRUD. It is concluded that the striping is caused by the flow swirl induced by spacer grid mixing vanes. Two anti-correlated effects contribute to the striping: (1) the flow swirl yields significant azimuthal temperature variations, which impact the locations where CRUD deposits, and (2) the flow swirl is correlated to increased shear stress along the cladding surface and subsequent erosion of the CRUD layer. The CIPS condition of the core is concluded to be primarily controlled by lithium tetraborate precipitation, referred to as boron hideout, which occurs in regions experiencing subcooled nucleate boiling as soluble boron and lithium species reach their solubility limit within the CRUD layer. Subsequently, a localized reduction in power occurs due to the high neutron absorption cross section of boron-10.

Physics-based multiscale coupling for full core nuclear reactor simulation

Numerical simulation of nuclear reactors is a key technology in the quest for improvements in efficiency, safety, and reliability of both existing and future reactor designs. Historically, simulation of an entire reactor was accomplished by linking together multiple existing codes that each simulated a subset of the relevant multiphysics phenomena. Recent advances in the MOOSE (Multiphysics Object Oriented Simulation Environment) framework have enabled a new approach: multiple domain-specific applications, all built on the same software framework, are efficiently linked to create a cohesive application. This is accomplished with a flexible coupling capability that allows for a variety of different data exchanges to occur simultaneously on high performance parallel computational hardware. Examples based on the KAIST-3A benchmark core, as well as a simplified Westinghouse AP-1000 configuration, demonstrate the power of this new framework for tackling—in a coupled, multiscale manner—crucial reactor phenomena such as CRUD-induced power shift and fuel shuffle.

Coupled Thermal-Hydraulic/Neutronics/Crud Framework in Prediction of Crud-Induced Power Shift Phenomenon

A thermal-hydraulics (TH)/neutronics/crud multiphysics coupling framework to simulate the crud deposits’ impact on crud-induced power shift (CIPS) phenomenon is proposed in this paper. The coupling among three essential physics (i.e., TH, crud, and neutronics) was implemented by coupling the computational fluid dynamics software STAR-CCM+, a newly developed crud module, and the neutronics code DeCART. A typical 3 × 3 pressurized water reactor fuel pin problem was analyzed with this framework and simulation results are presented. Time-dependent results are provided for a 12-month simulation. Simulation results provide the history of crud deposits inventory and their distributions on fuel rods, boron hideout amount inside crud deposits, and power shape changing over time. The obtained results clearly showed the power shape suppression in regions where crud deposits exist, a clear indication of CIPS phenomenon.