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Reactor Physics: ADS (Accelerators Driven Systems)
The MUSE-4 experiment
Neutron multiplication kinetics
The MUSE-4 experiment took place at Cadarache in the MASURCA experimental reactor over the years 2000-2004. The aim was to understand the physics of sub-critical assemblies and, more specifically, how, within such systems, the kinetics of neutron multiplication can be mastered. To this end, the LPSC (reactor physics group and accelerator department) developed GENEPI, an intense pulsed neutron generator. GENEPI was coupled to the MASURCA reactor in early 2000. In the course of 2003-2004, the behavior of the generator-reactor system was studied for various sub-criticality levels of MASURCA. The analysis of these experiments has demonstrated that it is possible to:
- Measure the prompt multiplication factor in a sub-critical assembly with a fast neutron spectrum.
- Measure the percentage of delayed neutrons which, combined with the prompt multiplication factor, gives the overall sub-criticality level of the system.
An original method was developed for both of these measurements. The prompt multiplication factor is obtained thanks to the complete modelization of the time interval distribution between two fissions of the same chain. By taking into account all the details of the distribution, we avoid the insufficiencies of the usual reactor kinetics models which generally consider only the initial part of the distribution. The delayed neutron multiplication factor was evaluated using a temporal modulation of the frequency of neutron pulses produced by GENEPI. By alternating phases of high and low frequency pulse repetition, we were able to measure the delayed neutron fraction. The analysis of the experimental results validates these methods over a sub-criticality range that covers the likely domain of operation of a future power sub-critical system (0.995 > k eff < 0.96). We have also explored how this method, developed for pulsed excitation, could be extended to the case of an accelerator that would operate continuously, this being the expected mode in a power system. Such an extension implies temporary beam interruptions that can be short enough to avoid disrupting the system’s thermics.
Neutron Spectrometry
The methods developed for the control of the reaction kinetics rest largely on a stochastic modelization of the neutron multiplication process in the reactor which, in turn, rests on a good knowledge of the neutron energy spectrum. For verification purposes, we have developed a neutron spectroscopy technique, based on the use of mini proportional counters partly loaded with 3He.
For the first time, the energy distribution of the multiplication neutrons was measured within a reactor, and compared to the numerical simulation results. The excellent agreement of the two, as shown in the figure, represents an a posteriori validation of the nuclear data bases used for the modelization. |
Experimental and simulated neutron energy distributions in the MASURCA reactor. |
The results obtained in this program should naturally find their place in a power prototype to come, second step on the way to the development of a system dedicated to the incineration of minor actinides produced by the nuclear power industry.
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