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Chapter 1 – INTRODUCTION
One of the major concerns in nuclear safety issues regarding NPP operation is the maintenance of the integrity of the core fuel rods, as a way of preventing radiological hazards to the environment or individuals [1]. This is closely associated with the avoidance of fuel rod overheating, which can occur during a series of postulated accidents. The evaluation of prevailing margins against this, mostly on a licensing basis, is obtained by means of detailed thermal-hydraulic transient analyses of specified accident sequences, mainly based on conservative assumptions to minimize fuel refrigeration and/or maximize fuel power excursions [2].
The recent incorporation of full three-dimensional modelling of the reactor core into system transient codes allows “best-estimate” simulations of interactions between reactor core behaviour and plant dynamics. Traditionally, thermal-hydraulic and neutron kinetics codes were developed to pursue different objectives and with little or no common connections. However, with recent computer developments resulting in the availability of powerful computation capabilities at reasonable costs, the interconnection between the two disciplines has become feasible. It is now possible to perform detailed dynamic T-H reactor system analysis together with coupled detailed dynamic 3-D core kinetics simulation even on a relative low-cost readily-available PC system.
This type of detailed 3D-NK-TH overall capability for transient simulations in LWR NPP provide a basis to undertake a more in-depth evaluation of the safety margins found in previous (licensing) TH simulations for which a point kinetics model or a 1D model was used. Thus provisions are now at hand to accurately reveal the actual safety margins, which could provide incentives to more efficiently utilise the fuel and obtain cost benefits in the operation of the nuclear power plants while still preserving – and possibly even improving – safety.
On the other hand, coupled analyses require detailed and elaborated plant and core specific inputs to reflect important plant and core design features and operational states. The latter also includes appropriate consideration of the core state changes during past operation periods (power history and burn-up effects).
The development of the present work will focus on the application of 3D-NK-TH coupled codes methodology in the study of reactivity insertion accidents, in particular a control rod ejection accident. This event is characterised by significant space-time effects in the core caused by asymmetric power excursion. Simulation of the transient requires evaluation of core response from a multi-dimensional perspective (3D) supplemented by a one-dimensional (1D) simulation of the remainder of the reactor coolant system.
2 For this study, it has been chosen the Three Mile Island Unit 1 reactor (TMI-1), which was object of a OECD benchmark [5] in 1999, and, for this reason, there is enough public information available.
The main objectives of the present work are the following:
• Study of the coupled codes methodology, identification of its potentialities and fields of application;
• Appraisal of the computational tools (TH and 3D-NK codes) involved;
• Verification of the capability of these system codes to analyse complex transients with coupled core-plant interactions;
• Acquisition of capability to perform transient analysis using 3D-NK-TH coupled codes; • Study the TMI-1 reactor behaviour during a control rod ejection accident and analysis of the
main important parameters regarding nuclear safety; and
• Comparison between predictions of different codes in best-estimate transient simulations.
As a way of pursuing these main objectives, the work was divided in five main parts. Chapter 2 describes the coupled codes methodology and applications; Chapter 3 gives a detailed description of the reference plant and of the rod ejection accident studied; Chapter 4 outlines the plant modelling and computer codes adopted; Chapter 5 presents performed calculations and the achieved results; finally, Chapter 6 focuses on the conclusions that can be drawn from the present work.
