Much closer to the configuration of what an ADSR could be, the MUSE (Multiplication Source Externe) project is an experimental programme dedicated to the study of neutronic multiplication in a subcritical reactor. The first experiment was carried out in 1995 at CEA/Cadarache.

The principle of the MUSE experiment consists of coupling a fast neutron subcritical reactor (MASURCA) to an external neutron source. MASURCA is a very low power (maximum 5 kW) experimental reactor. The core is rather small (60 cm in height and about 100 cm in diameter). The fuel is a MOx fuel (UO2-PuO2) enriched with 25% plutonium. Sodium or lead rodlets are put in the fuel elements to simulate the coolant. It is also possible to remove these rodlets to simulate a gas cooled reactor.

The first tests have been done with a strong californium source, placed in the centre of the core. Today, a pulsed neutron generator is used (GENEPI: Generateur de Neutrons Pulse; Intense), in order to study the time response of the subcritical core to a burst of source neutrons. The neutrons are produced by a deuteron beam, impinging on a deuterium target (yielding around

Figure 13.1. Geometry of the MUSE experiment with the deuteron accelerator GENEPI feeding into the (sub)critical fast-neutron pile MASURCA.

2 MeV neutrons) or a tritium target (14 MeV neutrons) placed at the centre of the reactor. A schematic view of the coupling is shown in figure 13.1. A lead buffer zone is placed around the source to simulate a spallation target. Different subcriticality levels can be investigated (k = 0.95 up to criticality) by playing with the configuration of the fuel elements. The pulse width is less than 1 ms, with a shape very close to that of a square gate. These charac­teristics allow very precise measurements of the evolution of the neutron flux in the core over a few tens of milliseconds. The main characteristics of the neutron generator are listed in table 13.1.

Different types of measurement are performed. On one hand, the spectrum shape is studied, at different positions in the core, target and reflector, using different types of detector (fission chambers, helium detector

for example). On the other hand, the time response of the system to a neutron pulse is measured, in order to devise a method able to determine the subcriticality level without starting the reactor in a critical configuration, as is done at present. The subcriticality level is determined by following the calculation described in chapter 7. The results of such an experimental pro­gramme are very important in the context of the definition of an experimental ADSR at significant power. In particular, the MUSE project has to provide a precise method to control the subcriticality level of the system in operating conditions.