Preliminary safety analysis for heavy water-moderated molten salt reactor

The heavy water-moderated molten salt reactor(HWMSR)is a newly proposed reactor concept,in which heavy water is adopted as the moderator and molten salt dissolved with fissile and fertile elements is used as the fuel.Issues arising from graphite in traditional molten salt reactors,including the positive temperature coefficient and management of highly radio-active spent graphite waste,can be addressed using the HWMSR.Until now,research on the HWMSR has been centered on the core design and nuclear fuel cycle to explore the viability of the HWMSR and its advantages in fuel utilization.However,the core safety of the HWMSR has not been extensively studied.Therefore,we evaluate typical accidents in a small modular HWMSR,including fuel salt inlet temperature overcooling and overheating accidents,fuel salt inlet flow rate decrease,heavy water inlet temperature overcooling accidents,and heavy water inlet mass flow rate decrease accidents,based on a neutronics and thermal-hydraulics coupled code.The results demonstrated that the core maintained safety during the investigated accidents.

Correction to:Assembly-level analysis on temperature coefficient of reactivity in a graphite-moderated fuel salt reactor fueled with low-enriched uranium

Following publication of the original article,the authors observed that both Fig.5 and Fig.4 depict the same image.Figure 5 was inaccurately referenced and displayed.The correct Fig.5 is copied below:The original article has been updated.

Assembly-level analysis on temperature coefficient of reactivity in a graphite-moderated fuel salt reactor fueled with low-enriched uranium

To provide a reliable and comprehensive data reference for core geometry design of graphite-moderated and low-enriched uranium fueled molten salt reactors,the influences of geometric parameters on the temperature coefficient of reactivity(TCR)at an assembly level were characterized.A four-factor formula was introduced to explain how different reactivity coefficients behave in terms of the fuel salt volume fraction and assembly size.The results show that the fuel salt temperature coefficient(FSTC)is always negative owing to a more negative fuel salt density coefficient in the over-moderated region or a more negative Doppler coefficient in the under-moderated region.Depending on the fuel salt channel spacing,the graphite moderator temperature coefficient(MTC)can be negative or positive.Furthermore,an assembly with a smaller fuel salt channel spacing is more likely to exhibit a negative MTC.As the fuel salt volume fraction increases,the negative FSTC first weakens and then increases,owing to the fuel salt density effect gradually weakening from negative to positive feedback and then decreasing.Meanwhile,the MTC weakens as the thermal utilization coefficient caused by the graphite temperature effect deteriorates.Thus,the negative TCR first weakens and then strengthens,mainly because of the change in the fuel salt density coefficient.As the assembly size increases,the magnitude of the FSTC decreases monotonously owing to a monotonously weakened fuel salt Doppler coefficient,whereas the MTC changes from gradually weakened negative feedback to gradually enhanced positive feedback.Then,the negative TCR weakens.Therefore,to achieve a proper negative TCR,particularly a negative MTC,an assembly with a smaller fuel salt channel spacing in the under-moderated region is strongly recommended.

Preliminary analysis of fuel cycle performance for a small modular heavy water-moderated thorium molten salt reactor

Heavy water-moderated molten salt reactors(HWMSRs)are novel molten salt reactors that adopt heavy water rather than graphite as the moderator while employing liquid fuel.Owing to the high moderating ratio of the heavy water moderator and the utilization of liquid fuel,HWMSRs can achieve a high neutron economy.In this study,a large-scale small modular HWMSR with a thermal power of 500 MWth was proposed and studied.The criticality of the core was evaluated using an in-house critical search calculation code(CSCC),which was developed based on Standardized Computer Analyses for Licensing Evaluation,version 6.1.The preliminary fuel cycle performances(initial conversion ratio(CR),initialfissile fuel loading mass,and temperature coefficient)were investigated by varying the lattice pitch(P)and the molten salt volume fraction(VF).The results demonstrate that the temperature coefficient can be negative over the range of investigated Ps and VFs for both 233U-Th and LEU-Th fuels.A core with a P of 20 cm and a VF of 20%is recommended for 233U-Th and LEU-Th fuels to achieve a high performance of initial CR and fuel loading.Regarding TRU-Th fuel,a core with a smaller P(~5 cm)and larger VF(~24%)is recommended to obtain a negative temperature coefficient.

Host plants and natural enemies of Bemisia tabaci （Hemiptera： Aleyrodidae）in China

The sweetpotato whitefly, Bemisia tabaci, has been a destructive pest in China for over the past two decades. It is an extremely polyphagous insect, being recorded feeding on hundreds of host plants around the world. Potential host plants and natural enemies of B. tabaci in the south, southeast, middle, north and northwest of China were investigated during the last decade. In total 361 plant species from 89 families were recorded in our surveys. Plants in the families Compositae, Cruciferae, Cucurbitaceae, Solanaceae and Leguminosae were the preferred host species for B. tabaci, which therefore suffered much damage from this devastating pest due to their high populations. In total, 56 species of parasitoids, 54 species of arthropod predators and seven species of entomopathogenic fungi were recorded in our surveys. Aphelinid parasitoids from Encarsia and Eretrnocerus genera, lady beetles and lacewings in Coleoptera and Neuroptera were found to be the dominant arthropod predators of B. tabaci in China. The varieties of host plant, their distribution and the dominant species of natural enemies of B. tabaci in different regions of China are discussed.

Thermoresponsive Nanogels from Dendronized Copolymers for Complexation,Protection and Release of Nucleic Acids

A series of thermoresponsive cationic dendronized copolymers and their corresponding nano gels containing den dritic oligoethylene glycol(OEG)units and guanidine groups were prepared,and their complexation,protect!on,and release of nucleic acids were investigated.The dendritic OEGs endow these copolymer materials with good biocompatibility and characteristic thermoresponsiveness,while cationic guanidine groups can efficiently bind with the nucleic acids.The dendritic topology also affords the copolymers specific shielding effect which plays an essential role in protecting the activity of nucleic acids.At room temperature,dendronized copolymers and the corresponding nanogels could efficiently capture and condense the nucleic acids,while above their cloud points(Tcps),more than 75%of siRNA could be released in 1 h triggered by ATP.More importantly,the copolymer showed protective capability to siRNA,while nano gels exhibit even better protection when compared to the copolymers due to the synergetic effect from the three-dimensional cross-linked network and high density of dendritic units in vicinity.This kind of smart dendr on ized copolymer nano gels form a no vel class of scaffolds as promisi ng materials for biomedical applicatio ns.