Digital Twins for Improving Accuracy of Thermal Fatigue and Environmental Qualification Time Limited Aging Analyses

Authors:

1. Galbally (Innomerics S.L)
2. Calatayud (Iberdrola Nuclear Generation)

Motivation

The design bases of nuclear power plants include assumptions regarding the service conditions to which their critical components may be subjected during plant operation. This paper describes how digital twins can help validate or refine those assumptions and ensure safe operation in the context of Long-Term Operation.

Abstract

Time-Limited Aging Analyses (TLAAs) is a term directly associated with the Long-Term Operation of Nuclear Power Plants. The revalidation of TLAAs is a necessary task to undertake before the plant enters its extended operational period beyond its original design life. In this regard, the international regulatory framework suggests reducing the conservatism applied in the original design calculations as an accepted method to demonstrate whether components within the scope of TLAAs are capable of performing their Safety Functions during the extended period of operation. The current state of technology provides us with an opportunity to achieve this through the improvement of accuracy compared to the original calculations.

Two use cases are presented,

Thermal Fatigue: The fatigue design of a component is based on a set of design basis transients that are anticipated or postulated to occur during the intended service life of the component. However, after the component is put into service, the actual frequency of occurrence of the different design transients typically differs from the frequency that was originally postulated. Additionally, the characteristics of those transients may also differ from those that were originally postulated during the design phase. Furthermore, additional requirements must be satisfied for renewal of operating licenses for nuclear power plants (NPPs) beyond their original design service life. The extrapolation of the original design bases assumptions to the end of the extended period of operation, combined with the additional new requirements applicable to license renewal, is often too conservative, resulting in the inability to meet the acceptance criteria of the applicable design code. This paper presents a digital twin framework that combines data obtained from custom sensors with high-fidelity multiphysics models to improve the accuracy of fatigue monitoring based in NPPs.

Environmental Qualification: The revalidation of the EQ program requires an accurate characterization of the ambient temperature and radiation that equipment are expected to be subjected to during plant operation. In some cases, it is challenging to define the representative temperature applicable to calculate the qualified life of equipment, especially in large rooms with harsh environments that contain multiple ECCS. The operational room temperature in a nuclear power plant undergoes complex dynamic behavior influenced by factors such as leakages, degradation of thermal insulation, operational conditions, HVAC reliability, among others. Historically, difficulties have been encountered in characterizing equipment temperature and hot spots, particularly when the room is not accessible during normal operation.

This paper introduces a digital twin of the environmental temperature inside the drywell of BWR. This model provides an opportunity to enhance accuracy in defining the ambient temperature affecting equipment within the EQ scope, as opposed to relying on the typically used design or bounding estimated room temperature.

These two examples are implemented in a BWR NPP with the following benefits:

• Online monitoring of the fatigue damage accumulation in the feedwater nozzles of the reactor pressure vessel.
  • Identify leakages through the thermal sleeve seals.
  • Characterize thermal stratification during transients.
  • Improve accuracy to determine fatigue damage due to thermal transients.

• Drywell environmental digital twin.
  • Dynamic identification of hot spots.
  • Determination of equipment specific operating temperature profiles to calculate qualified life.
  • Environmental 3D model to manage degradation mechanisms affected by humidity as a stressor.
  • Dose reduction based on precise identification of leakages that affect the temperature field inside the room.