Lower Core Plate Research Articles

A universal formula for high-order added mass of reactor core barrel with lower core plate immersed in fluid

The high-order mode has non negligible influence on the flow induced vibration response of the core barrel, which requires accurately capturing its high-order added mass. In this regard, a theoretical model of high-order added mass for core barrel with lower core plate was developed. To fulfill this, the governing equations of this intrinsic fluid-structure interaction were first derived based on Galerkin method and potential flow theory. Then, some hypotheses were introduced to simplify the mathematical expressions and solutions. Based on these assumptions and governing equations, the hydraulic pressure exerted on the façade of core barrel was given. The calculation of the added mass of a certain high-order mode was proposed through the integration of hydraulic pressure. Compared with the finite element results, it indicates that the added masses calculated by our model are applicable. Consequently, the parameters such as thickness of lower core plate, core barrel thickness and radius ratio of core barrel to reactor pressure vessel were analyzed, so as to explore the variation rule of added mass. This contribution offered a concise formula to facilitate relevant engineering applications.

Research on debris in-vessel cooling and retention behavior for the small modular reactor ACP100

In this paper, the debris in-vessel cooling and retention behavior of the small modular reactor ACP100 design in severe accident is studied using MELCOR 2.1 code. A complete MELCOR input deck of ACP100 has been built. In ACP100, in-vessel retention strategy is essential for safety enhancement, and is realized by passive water injection into the flow channel between reactor pressure vessel outer surface and reactor cavity, namely cavity injection system, when the core exit temperature is higher than 650 °C. The severe accident is conservatively assumed to be triggered by a double-ended rupture of one direct vessel injection line. The collapse behavior of the fuel assembly is estimated by MELCOR fuel rod degradation model, and the failure behavior of the lower core plate is predicted by the ANSYS software. The results show that the core center partially collapse and melt to form a core molten pool, while most fuel assemblies surrounding the core molten pool remain intact. The lower core plate is still able to support the core molten pool and uncollapsed fuel assemblies throughout the simulation. The heat transfer processes associated with the core barrel, reactor pressure vessel, and lower core plate are also discussed. The decay heat can be effectively removed by the cavity injection system, so that debris retention in the reactor pressure vessel is feasible and even the appearance of molten pool in the lower head can be avoidable.

Hydraulic lift-off issues for application of high performance annular fuels in pressurized water reactors

In the PWR core, the fuel assembly is firmly seated on the lower core plate during operation. However, if the hydraulic force exerted on the fuel assembly by coolant flow is too large and the fuel assembly is lifted-off from the lower core plate, the excessive vibration will cause fuel failure. Therefore, the hydraulic lift-off issue needs to be addressed when the advanced fuel assembly is developed. It has been shown that the advanced annular fuel design with internal cooling allows power uprating up to 50% while the peak temperature of the fuel can be reduced and the MDNBR can be maintained. However, if the coolant condition in the core is kept unchanged, increasing the core power by 50% requires the core flow rate also increase proportionally, which will give rise to the hydraulic lift-off, an important issue to be addressed. In this paper, taking the 17×17 solid fuel design as the reference, the hydraulic lift-off issue is investigated for proposed 12×12 and 13×13 annular fuel designs. Both the steady-state and start-up operating conditions are evaluated. It is found that the hydraulic lift-off indeed is an issue for annular fuel design which requires careful analysis. By comparison, the lift-off forces and hold-down forces required for the externally and internally cooled annular fuels (13×13 and 12×12 arrays) are several times larger than that of the referenced solid fuel (17×17 array). Therefore, the hold-down mechanism for annular fuel needs to be carefully designed.

Observation of in-core instrument tube vibrations in a boiling water reactor by evaluating reactor noise data

Using reactor noise techniques, a vibrating in-core instrument tube in the Mühleberg Boiling Water Reactor could be detected. The normalized auto power spectral density (NPSD) of the current fluctuations of a mini in-core chamber, fixed within the instrument tube, has been analyzed before and after repair. There was a strong evidence of a peak at the resonance frequency of the vibration at 2.6 Hz in the NPSD. It vanished promptly after the bypass-streaming at the lower core plate, being the physical origin of the vibrating instrument tube, was re-arranged.