Feasibility of subcriticality monitoring with a digital reactivity meter based on inverse kinetics method has been investigated at the lead-based zero power reactor VENUS-II. A series of control rod drop experiments have been carried out in this work. The subcriticality before and after the rod drop were estimated considering the effect of external neutron source. The control rod worth was also extracted from the rod drop experiments. The measurement results exhibited obvious spatial dependence induced by the control rod insertion, and they could be corrected by correction factors calculated with MCNP code. The corrected values of control rod worth were found to be consistent with the reference ones predetermined at critical condition. Moreover, this method could also provide real time subcriticality without any correction when the detectors were placed at appropriate positions at slightly subcritical state. Based on these measurements, it has been proved that the digital reactivity meter could be a useful tool for continuous reactivity monitoring in the operation of ADS in future.

The transmutation reaction rate of the 9 kinds of MA and LLFP nuclides were calculated based on ADS subcritical facility Venus 1#.The transmutation reaction rate of 137Cs in the fast-thermal coupled zone was measured,and the transmutation speed of 137Cs was 10 times to natural decay.The results that Venus 1# has a certain transmutation ability,and the calculated result of the transmutation reaction rate for 137Cs agrees with the measuring result.

Based on LabVIEW development platform,the Quasi Macroscopic Neutron Noise Measurement and Analysis System was designed,which accepts neutron pulses from 3He detectors located near a subcritical reactor,and performs spectral analysis on the data,then obtains the prompt neutron decay constant of the reactor.The system has been tested by some experiments.

Relative neutron count rate change was studied on Venus 1# sub-critical assembly for the detectors at different positions of the core with external neutron source,Am-Be neutron source and 252Cf neutron source respectively.The experimental results show that neutron count rate changes for the detectors at different positions are different on Venus 1# sub-critical assembly,but it’s little effect on the results of sub-critical extrapolate experiment.

In the paper,the Venus 1# sub-critical assembly,configuration of reactor core,the external neutron source system and the neutronics research experiment system about accelerator driven sub-critical system (ADS) were introduced.The sub-criticality of reactor was studied using Source-Jerk method at different loadings.The results of measurement were compared with calculated results,and they are in agreement well.

The theory of interfering effects of neutron absorber and the criticality measurement experiments at the uranyl nitrate solution experiment facility was described.In the criticality experiments,the absorbing efficiencies and its interfering effects for multi-group solid neutron absorbers under different locations and arrangements with different concentrations of the uranyl nitrate solutions were measured.The measured results indicate that for eccentric symmetrical arrangement the interfering effect is positive,and for eccentric non-symmetrical arrangement the interfering effect is negative.Meanwhile,Monte-Carlo calculation code was used for calculations in these different cases.The calculated results show the consistence with the experimental data on the values and its variance trend of interfering effects.It confirms that the Monte-Carlo calculation code is suitable for analyses of interfering effects on the uranyl nitrate solution facility.

The paper presents the design objectives and structure of China's accelerator-driven subcritical system subcritical assembly Venus-1. The core of Venus-1 is a coupled core with afast neutron zone and a thermal neutron zone. The fast neutron zone is at the center of the core and is formed by natural uranium fuel. A fast neutron spectrum field can be produced in the fast neutron zone and used for transmutation. The thermal neutron zone surrounds the fast neutron zone and is formed by low-enriched uranium fuel; it is a fission zone. Venus-1 is driven by an Am-Be or other steady neutron source (252 Cf, D-D reaction or D-T reaction) to research the effect of an external neutron source with different energies on it or is driven by a D-T pulsed neutron source through the China Institute of Atomic Energy pulsed neutron generator to research the dynamic characteristic. On July 18, 2005, the first fuel element was loaded into the Venus-1 subcritical assembly, and some preliminary experiments on subcritical and neutron distribution have been performed. The final load has been determined by preliminary experiments on Venus-1. The relative neutron importance of the external source, the relative distribution of the thermal neutron flux in the fast neutron zone, and the variation of the neutron count with the addition of natural uranium fuel also have been determined by preliminary experiments on Venus-1.

China’s accelerator-driven sub-critical system (ADS) sub-critical experimental assembly—Venus-1 and the preliminary experiment is presented. The core of Venus-1 is a coupled one of a fast neutron zone and a thermal neutron zone. The fast neutron zone is at the centre of the core and formed by natural uranium fuel. A fast neutron spectrum field can be produced in the fast neutron zone and used for the transmutation of minor actinides (MAs). The thermal neutron zone surrounds the fast neutron zone and is formed by low-enriched uranium fuel. It is a fission zone. An epithermal neutron zone between the fast neutron zone and the thermal neutron zone can be established for the transmutation of longlived fission products (LLFP). On July 18, 2005, the first fuel element was loaded into the Venus-1 sub-critical assembly and some preliminary experiments about the sub-critical neutronics were performed. The Venus-1 can be driven by an Am-Be source or other steady neutron source (Cf-252, D-D reaction and D-T reaction) to study the effect of the external neutron source with different energies or a D-T pulsed neutron source on the dynamic characteristics.