98
Chapter 6 – CONCLUSIONS
The 3D-NK-TH coupled codes methodology was applied successfully to a control rod ejection accident. It was possible to perform a detailed analysis of the reactor core and the plant itself, although the transient involved sharp parameters variation (e.g. the reactor power and the fuel temperatures).
The TMI-1 Unit was modelled to provide transient analysis and three reference cases were conceived, in which the ejection of the CR with maximum worth value was performed.
With regard to the REA at HZP condition, the reactivity inserted (0.8$) caused a power increase of approximately four times the initial reactor power. This limited power excursion is particularly true for REA with reactivity insertion below prompt critical condition. This behaviour confirms that the reactor in object, with respect to core history and operational condition evaluated (set of cross sections used at EOC, HZP) reacts very well to the ejection of the CR with maximum worth.
A REA with the reactor operating at Full Power (HFP) was also studied. The ejection of a CR partially inserted (40% withdrawn) was idealized. Maximum power of 115% of PN was reached
in the case of delayed reactor trip, and no event of safety concern was identified.
In order to assess core behaviour in a more extreme condition, an accident with reactivity insertion beyond prompt criticality was idealized, considering an ejection of two adjacent CR as if they were collapsed and inserted in the same thermal-hydraulic channel. It should be noted that this reference case was motivated on academic basis, as it does not seem very realistic to consider the ejection of two adjacent rods at the same time or even a transient likely to insert such magnitude of positive reactivity (1.44 $), especially at HZP condition.
The 3D-NK-TH coupling approach used in this study should be completed by an evaluation of the results’ uncertainties induced by code modelling and/or input parameters uncertainties.
After analyzing the results for all reference cases studied, the following conclusions can be drawn regarding safety parameters of the NPP:
• The fuel centerline temperatures calculated remained below the fuel melting point as well as the clad temperature did not reach the safety limit of 1204 °C;
• No degradation of heat transfer (DNB) that could have caused damage to fuel rods was observed during transients; and
99 Concerning the modelling used and the capabilities of the codes, it was also noted that:
• Thermal-hydraulic and neutron kinetics modelling demonstrated to provide adequate resolution level required for the accident analysis;
• Adopted codes predicted correct design values for the steady-state conditions;
• Although PARCS yielded very good results, the version used (NRC v.2.43) demonstrated to be limited as it crashed during some transient calculations (e.g. because array bound was exceeded when reduced time step was used in Relap5/3.3 during coupling process). Moreover, limited code documentation is available and program is not user-friendly, as it most often does not give clues to user about errors encountered. Evaluation of a newer version is therefore recommended for future studies;
• On the other hand, Relap5/3D was found to be very well-documented and the integration of the NK code Nestle into the THCS required simplified procedures for transient execution, reducing possible errors during calculations and simplifying output data handling;
• Both coupling codes adopted, regarding the results obtained, sort of “complement” each other, as they possess different modelling features and generate distinct output information, providing a better understanding of the problem being analysed; and
• All coupled codes runs were adequately performed on a standard commercial notebook (1.6 GHz processor, Windows XP), with no special features.
Finally, the general conclusions of the activity performed for this thesis are synthesized below:
• Capability to perform 3D neutron kinetic and thermal-hydraulic analysis has been acquired; • Familiarization with an analytical procedure to perform calculation of the energy released
during a REA has been developed;
• In-depth study of the state-of-the-art of the research activities and of the physical phenomena involved in a REA has been accomplished;
• Concerning core behaviour during REA, no power spike was observed during ejection of the CR with highest worth value, at HZP and EOC condition, as the positive reactivity inserted remained below prompt critical;
• TMI-1 NPP demonstrated a safe response during all transients performed;
• Very good agreement was observed between the coupled codes adopted – RELAP5/3.3-PARCS and RELAP5/3D-NESTLE – for transient simulation; and
• 3D-NK-TH methodology demonstrated its strong capability to perform multi-dimensional analysis of a complex transient such as a REA.
