Analysis on the dynamic behavior of corrosion products in the nuclear fuel of a PWR and itsrelationship with the Axial Offset Anomaly (AOA)risk using BOA calculation code

This master’s thesis can be characterized as a research project about nuclear engineering.
The object of this thesis is the primary system of Vandellos II nuclear power plant.
Specifically, it has been focused on the dynamic behavior of corrosion products in the
nuclear fuel of a nuclear core Vandellos II PWR. The report describes the operation of the Reactor Coolant System (RCS), highlighting and making a detailed study of an existing and common problem in different nuclear plants around the world, the so-called Axial Offset Anomaly (AOA). The occurrence of PWR Axial Offset Anomaly (AOA) is a limiting operational condition preventing many PWRs from operating with efficient core designs. AOA occurs when boron incorporates in corrosion products deposited in the steaming regions of
high-duty fuel assemblies causing the reactor neutron flux to be skewed. It is for these reasons that in industry placed a high priority on developing a capability to assess fuel behavior and AOA risk for any particular core design. An expectation emerged that this capability would be versatile enough to model any PWR geometry and fuel design and would include the known physical parameters related to AOA.
Therefore, the first objective of this project consists to study the consequences that produce this anomaly: The AOA phenomenon is believed to result from three interrelated aspects: 1) Sub cooled boiling on high-duty assemblies cause, 2) enhanced corrosion product deposition on those rods, and as the deposits thicken (CRUD formation), 3) boron is incorporated in
those deposits causing a depression in neutron flux.
After that, the second objective was to provide the different solutions to prevent mitigate or avoid this phenomenon. Among these solutions, there is the use of Boron-induced Offset Anomaly Risk Assessment Tool or “BOA” code. So, using BOA code methodology an assessment of AOA risk and strategies of Zinc injection (as a proposed solution to this problem) is done and accurate risk assessment tool (AOA risk) for projected cycle core designs of Vandellos II are performed. In this section, the BOA code development incorporated the above processes (1),2),3)), first requiring a thermal-hydraulic code input to establish the requisite heat flux and assembly steaming. Then, a system-wide mass balance
approach to corrosion product release and deposition was applied. A boiling (porous) deposit model was added to address solute concentration processes as the deposits thicken. Finally, boron deposition processes have been incorporate based on both precipitation and physicsorption
phenomena. By this process a Conservative and Best Estimate study are done. In
addition, different sensitivity studies to analyze the effect of certain parameters on the behavior of the core in order to assess the AOA and to work with an optimal chemical
strategy.
On the other hand, an important aspect to reduce AOA risk is the ultrasonic fuel cleaning equipment. So, the third objective is the investigation of the Ultrasonic Fuel Cleaning (UFC) equipment. Ultrasonic Fuel Cleaning is a mechanical cleaning process developed by EPRI for reduction of CRUD inventory from irradiated fuel assemblies. UFC reduces susceptibility of PWRs to Axial Offset Anomaly or CRUD Induced Power Shift (AOA/CIPS), and is expected to become increasingly important in CRUD inventory reduction for CRUD-induced fuel failure mitigation, and radioactive material source term reduction initiatives.
However, in order to take proper credit for UFC application when using core design tools
such as BOA, a quantitative measure of CRUD removal is required. It is for this reason that the last objective is to propose a new method for quantifying UFC crud removal, and
therefore, the effective of the UFC equipment.
Keywords: Axial offset anomaly (AOA); BOA code; Boron; Core Boron; Core steaming rate;
CREL; CRUD; Corrosion products; Lithium; Primary water chemistry; PWR core design;
Reactor Coolant System (RCS); Steam Generator (SG); Subcooled nucleate boiling.