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In a pebble bed nuclear reactor, each fuel pebble draining through the core experiences a different amount of burnup depending on the precise trajectory that it follows. Understanding the burnup profile of pebbles is essential for reactor safety, as well as for fuel economy. Here, we introduce a method for constructing the burnup profile based on performing a discrete element simulation of the pebble drainage, followed by a burnup calculation in each individual pebble. This method is more accurate than previous approaches, and in particular it captures the extremal cases of pebble burnup. We demonstrate the method using the geometry, neutron flux data, and thermal characteristics from the HTR-10 reactor being developed at Tsinghua University. We examine pebble burnup during a single drainage cycle, and over multiple drainage cycles characteristic of normal reactor operation. Our results show that the presence of slow-moving boundary layers of pebbles near the reactor wall strongly influences the burnup profile. We perform a systematic study where the pebble–pebble and pebble–wall friction coefficients are independently varied, and we show that the strength of the boundary layers is a complex interplay of these two parameters.

Widespread autotoxicity is an important obstacle to natural regeneration of many plants. The rare relict plant Metasequoia glyptostroboides is a difficult to natural regeneration and is affected by litter allelopathy. However, little is known about the potential influence of autotoxic substances on different regeneration stages of M. glyptostroboides. We identified multiple chemical compounds of aqueous extracts from fresh (recently accumulated) and natural litter (mixture of litter different phases of decomposition), to evaluate the autotoxic effects of the four most important detected compounds applied individually on seed germination and seedling growth of M. glyptostroboides. Results found that the 28 chemical compounds were identified in the aqueous extracts of M. glyptostroboides litter. The Jaccard similarity coefficient of chemical compounds in aqueous extracts of fresh and natural litter of M. glyptostroboides reached 50%. The number of chemical compounds in fresh litter was 5.56% more than that in natural litter. Catechol, trifluoroacetamide, benzoic acid and D-(+)-arabitol significantly affected seed germination rate, seed germination index, vigor index, shoot length and main root length of M. glyptostroboides. Specifically, benzoic acid had the strongest inhibitory effect, followed by catechol, trifluoroacetamide and D-(+)-arabitol. The autotoxic effect was concentration dependent, low concentrations were positive and neutral, high and extremely high concentrations were negative for all the chemical compounds. Moreover, catechol, trifluoroacetamide, benzoic acid and D-(+)-arabitol were autotoxic substances affecting the natural regeneration of M. glyptostroboides, as well as strongly inhibited at the shoot growth stage. This study confirms that natural regeneration of M. glyptostroboides is restricted by a large amount of litter coverage under the forest, highlighting how the chemical compounds responsible for the autotoxic characteristics of M. glyptostroboides affect the different regeneration stages of M. glyptostroboides. [ABSTRACT FROM AUTHOR]