VENUS-1# 装置脉冲源实验分析

1. VENUS-1#装置脉冲源实验分析 谢金森（xiejinsen@139.com） 南华大学

2. Contents Background and Purpose Core Composition & PNS Experiment of VENUS-1# Preliminary Results from PNS Experiment Simulation of PNS Experiment on Certain Condition Comparison of Simulated and Experimental Results Summary & Future Suggestions

3. Background and Purpose • Background • R&D on Accelerator Driven System for MA and LLFP Transmutation • Supported by two “973” Projects • The World-first Fast-Thermal Coupled ADS core- VENUS-1# has been built in 2005; • Abundant research works on Sub-critical neutronics have been carried out on VENUS-1#. • Supported by Chinese Academy of Science • 5MWt~10MWt Pb-Bi cooled ADS core will be built (First-step, Critically Operation; Second-step, Accelerator coupling with sub-critical core ); • Although massive meaningful results has been obtained, some issues on Sub-criticality measuring still exist.

4. Background and Purpose • Background • To support the development of sub-criticality measuring technique, Pulsed Neutron Source experiments on VENUS-1# has been performed • 2005, VENUS-1# was coupled with CPNG (CIAE Pulsed Neutron Generator), five different Sub-critical levels; • 2007, the first PNS experiment analysis work was published (Thesis of Shanghai Jiaotong Univ.) ; • 2011, PNS experiment simulation work was performed, three different Sub-critical levels, D-T, 252Cf, Am-Be neutron sources(Thesis of CIAE). • All the above works indicate: Prompt neutron attenuation constants depend on detectors’ locations & driven source energy.

5. Background and Purpose

6. Background and Purpose • Purpose • Confirm the results from former researches; • Feasibility study of PNS method on Fast-thermal coupled core; • Investigate the potentially special phenomenon of Fast-thermal coupled core under pulsed neutron condition.

7. Core Composition & PNS Experiment of VENUS-1# • Main parameters of VENUS-1# • External source region: coupled with accelerator, neutron tube, or isotopic neutron source; • Fast spectrum blanket: natural uranium pins + aluminum block, 10 layers; • Thermal spectrum blanket : 3wt% uranium pins+ CH2 block, 15 layers maximum; • Reflector: CH2 block, 200mm thick; • Shield: CH2+B, 200mm thick; Core Composition

8. Core Composition & PNS Experiment of VENUS-1# CPNG neutron generator (CIAE Pulse Neutron Generator ) • Main parameters of CPNG • Type: Cockcroft-Walton • High voltage: 200~600kV • Pulse beam frequency: 50~200Hz • Pulse width:1~5us

9. Core Composition & PNS Experiment of VENUS-1# Experimental conditions & instrument system Detectors Sub-criticalities: adjusted by thermal fuel pin loading; 1926, 1962, 1998, 2022, 2046; Detectors: 3 3He tubes (6th, 10th layer of fast blanket & reflector); 1 BF3 tubes (Shield) Electronic system: 60 multi-channels; 50us/channel; Data collection: 3 groups data; 105 pulses; statistical errors for first several channels < 1%;

10. Preliminary Results from PNS Experiment

11. Preliminary Results from PNS Experiment • Discussion on the experimental results • Prompt neutron attenuation constants show strong spatial dependence; • The attenuation constants obtained by detectors show large discrepancies; • The experimental results conflict with theory of experimental reactor physics (lumped parameter Point Reactor Dynamics) ; • Which attenuation constant should be used as a sub-critical indicator, or how to make spatial corrections.

12. Simulation of PNS Experiment on Certain Condition • Theoretical considerations • Harmonics neutron flux seriously affects detector responses in deep sub-critical condition (both static & transient ) ; • For Fast-thermal coupled system, typical prompt neutron life time is 10-5 second, width of multi-channel should be correspond to the condition for better inferring of harmonics (can be easily realized in simulation); • To obtain the prompt neutron attenuation constants (fundamental mode), harmonics should be filtered.

13. Simulation of PNS Experiment on Certain Condition Theoretical considerations • In transient conditions, neutron flux can be expressed as: • Consider Alpha Eigen-value problem, neutron density in PNS: • Contribution of external source • Contribution of fundamental Alpha mode • Contributions of the ith harmonic Alpha modes

14. Simulation of PNS Experiment on Certain Condition Theoretical considerations • Relationship of each attenuation constant: • When harmonics and source contribution disappeared:

15. Simulation of PNS Experiment on Certain Condition Methodology of simulation • Simulation conditions: • MCNP4CTM selected as PNS simulation code; • 2046 thermal fuel pins loading for comparison with experiment; • Multi-channel width set as 5us for better inferring harmonics effect, totally 600 channels (0~3000us, the same as experiment); • Data processing technique: • Detector response of R selected as benchmark; • Relative responses to R used for fundamental attenuation time region search;

16. Simulation of PNS Experiment on Certain Condition Simulation results Simulated detector responses Simulated relative detector responses

17. Simulation of PNS Experiment on Certain Condition • Simulation results • Detector responses in time interval 500~3000us selected as fundamental attenuation constant fitting

18. Comparison of Simulated and Experimental Results • Comparison of detector responses • The simulated data use the same channel width of experiment; • Quite different attenuation trends observed. Simulated data Experimental data

19. Comparison of Simulated and Experimental Results • Comparison of Alpha constants • Original experiment data; • harmonics filtered simulated data; • harmonics filtered experimental data. Original Exp. data Harmonics filtered Exp. Data(500~3000us)

20. Comparison of Simulated and Experimental Results • Comparison of Alpha constants • original experimental data; • harmonics filtered simulated data; • harmonics filtered experimental data. • The Alpha indicated by detector 10th has been improved; • The Alpha value obtained by detector S is closest to simulated one; • The Alpha value of detector R still show big discrepancy.

21. Comparison of Simulated and Experimental Results Verification of the simulated Alpha • In one neutron generation time, the prompt neutron population: • For sub-critical system: • Relationship between kp and Alpha:

22. Summary & Future Suggestions Summary • Using relative neutron count rates as an indicator, time region for fundamental Alpha mode decay can be easily obtained (a harmonics filtering technique); • The simulated data with harmonics filtering technique give a spatially independent prompt neutron attenuation constants; • Some Alpha values from experiment is improved by harmonics filtering, however some other values are worsen.

23. Summary & Future Suggestions Suggestions • Since the big discrepancy between experiment and simulation, as well as the short neutron life time and harmonics influence in the Fast-thermal couple ADS core, more rapid electronics data collection system should be used in future PNS experiment; • Although the harmonics filtering technique show the capability for Alpha measuring, the accuracy of calculated prompt neutron generation time plays a key role in verification work (consider the coupling between external neutron source region, fast blanket, thermal blanket and reflector, the effective generation time should be introduced, as well as its calculation theory)

24. Thanks for your attention!