Study on the Curved Channel Model in the Initial Core Loading of Pebble Bed High Temperature Reactors

Continuous on-line fuel cycling is the essential feature of the pebble bed high temperature reactor (PB-HTR). The flow speed of the fuel pebbles in a PB-HTR presents a radial distribution in the reactor core, mainly due to the friction between the pebbles and the wall and the conical structure at the core bottom. In the VSOP fuel shuffling model, the simulation of unequal pebble flow speed is achieved by dividing the reactor core into some vertical flow channels with different numbers of the equal-volume regions in each channel. However, the fuel shuffling with equal-volume batches bring complexity when dealing with the change of fuel composition, such as the fuel fraction of fuel-graphite pebble mixture, during the initial core loading and early running-in phase. In this work, a curved channel model with unequal flow speed and the bottom cone is established based on the DEM simulation of pebble flow in the HTR-PM. The batch-wise fuel shuffling strategy is adapted to fit the complex situation during mixing and re-assigning the discharged fuels by employing a rounding strategy for the actual volume of fuels with similar irradiation history. The key of the adapted strategy is to divide the total number of the mixed batches with similar irradiation history by the number of flow channels, and round the quotient as the number of reloaded batches in each top region. The fuel loading process to build up the initial core, accompanied by the low-power reactor running to compensate the reactivity provided by the fresh fuels, is simulated by using the fuel shuffling model mentioned above. On the other hand, the simulation on the same process with an effective cylindrical core mesh and straight flow channels is carried out, in which dividing and rounding the batch numbers are unneeded. The results of both models are compared, indicating that the curved channel model presents less core reactivity and shorter fuel loading period than those of the cylindrical model. From the point of view of fidelity, the former is more suitable for the simulation of initial core loading process. The results in this work are important for enhancing the economy of fuel cycling of PB-HTRs.

Experimental Measurement on Pebble Flow Discharge in a Hopper Silo Based on a Drainage Method

The dropping of absorption sphere from the storage vessel under accident conditions and the transportation of spent fuel elements in the reactor will both lead to the pebble flow discharging process driven by gravity in a hopper silo. Therefore, the research on the gravity-driven discharging rate of pebbles in a hopper silo has significant engineering guidance for reactor safety. In general, the idea of falling pebbles weighing to obtain the discharging rate becomes the most common experimental measurement method. However, due to the limitation of response frequency and the disturbance of pebbles falling, the resolution of experimental results is limited, and the uncertainty is introduced into the data error, which is difficult to eliminate. In this experiment, a volume measurement based on drainage method is adopted. This is a new experimental method to measure the discharge process of hopper silo. The magnetostrictive liquid level sensor is applied to measure the rise of liquid level caused by the volume of falling pebbles. Compared with the weighing method, this method has two advantages. First, the resolution of this method has a higher controllability. On the one hand, the disturbance caused by the momentum of falling pebbles will not be introduced into this method, on the other hand, the measurement accuracy is determined by the multiple controllable factors. Second, this method can obtain higher measurement frequency. the sampling frequency of liquid level sensor is 1–2 orders of magnitude higher than that of electronictong balance. Based on this new experimental method, the reliability of the method is validated by comparing the experimental results of discharge flow rate with the Beverloo’s and Nedderman’s empirical formula. Furthermore, the effect of silo outlet size on pebble discharge flow rate fluctuation have been also analyzed in this study. By use of fast Fourier transform, the fluctuation of particle discharge flow rate is separated from the discharging sampling results of liquid level sensor.

Correction to: A review of pebble flow study for pebble bed high temperature gas-cooled reactor

The article “A review of pebble flow study for pebble bed high temperature gas-cooled reactor” written by Shengyao Jiang, Jiyuan Tu, Xingtuan Yang, and Nan Gui, was originally published electronically on the publisher’s internet portal (currently SpringerLink) on 11 June 2019 without open access. After publication in Volume 1, Issue 3, page 159–176, the author(s) decided to opt for Open Choice and to make the article an open access publication. Therefore, the copyright of the article has been changed to © The Author(s) 2020 and the article is forthwith distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits use, duplication, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made.