C. Le Brun

The thorium molten salt reactor : Moving on from the MSBR

A re-evaluation of the Molten Salt Breeder Reactor concept has revealed problems related to its safety and to the complexity of the reprocessing considered. A reflection is carried out anew in view of finding innovative solutions leading to the Thorium Molten Salt Reactor concept. Several main constraints are established and serve as guides to parametric evaluations. These then give an understanding of the influence of important core parameters on the reactors operation. The aim of this paper is to discuss this vast research domain and to single out the Molten Salt Reactor configurations that deserve further evaluation.

Neutronic Studies in Support of Accelerator-Driven Systems: The MUSE Experiments in the MASURCA Facility

Abstract The MUSE program (multiplication with an external source) is in progress at the MASURCA critical facility at the Cadarache Research Center of the Commissariat à l’Energie Atomique in France. The program is dedicated to the physics studies of accelerator-driven systems in support of transmutation studies of minor actinides and long-lived fission products. It began in 1995 with the coupling of a Cf source in MASURCA and was followed by a commercial (d,T) source. In 2001, a specially constructed (d,D)/(d,T) neutron generator (GENEPI) was placed in MASURCA and the MUSE-4 program commenced. We describe the first phases of the MUSE-4 program, with data presented that were obtained up to about the summer of 2002. We present some results from the “reference” configuration, which can operate at critical. We present traverses of measured fission reaction rates, with comparison to calculations. Also in the reference configuration, we performed activation foil measurements and present these results compared to calculations. Because a major objective of the MUSE program is to test and qualify methods of subcritical reactivity measurement, we have devoted a major portion of our studies to this area. We have used classical methods (rod drop, source multiplication) to attempt to measure the subcritical level. In these early phases we studied core configurations of around keff = 0.995. Deeper subcriticality (keff = 0.96) was achieved by inserting a safety rod. In addition to the methods mentioned above, we have devoted a lot of effort to pulse neutron source, fluctuation (Rossi-α and Feynman-α), and transfer function methods (e.g., cross-power spectral density). We present our preliminary results of all the methods, with some discussion regarding cross comparison.

Possible Configurations for the Thorium Molten Salt Reactor and Advantages of the Fast Nonmoderated Version

Abstract Molten Salt Reactors based on the thorium cycle were studied in the 1950 to 1960s to lead to the Molten Salt Breeder Reactor concept, which was finally discontinued prior to any industrial development. In the past few years, this concept has once again been studied in order to generalize it and seek configurations ensuring a high intrinsic safety level, an initial inventory compatible with intensive deployment on a worldwide scale, and a not-too-demanding salt chemical reprocessing scheme. The Thorium Molten Salt Reactor (TMSR) thus defined is studied in the Th-233U cycle in various configurations obtained by modulating the amount of graphite in core to obtain a thermal, an epithermal, or a fast spectrum. In particular, configurations of a fast spectrum TMSR have been identified with outstanding safety characteristics and minimal fuel-reprocessing requirements.

The decay of primary products in binary highly damped 208Pb + 197Au collisions at 29 MeV/u☆

Abstract Events with fragment multiplicities up to eight have been detected with a large detector array in Pb + Au reactions at 29 MeV/u. All collisions show a binary character irrespective of a possible further disassembly of the two highly excited primary partners. For the most violent collisions, dissipative orbiting is observed as well as full energy damping corresponding to excitation energies as high as 6 MeV/u.

Global Results from Deterministic and Stochastic Analysis of the MUSE-4 Experiments on the Neutronics of Accelerator-Driven Systems

Abstract The MUSE-4 program is a series of zero-power experiments carried out at the Commissariat à l’Energie Atomique Cadarache MASURCA nuclear facility from 2001 to 2004 to study the neutronics of accelerator-driven systems (ADSs). The program has investigated the coupling of a multiplying medium to neutron sources of 2.6 or 14 MeV provided by an accelerator (GENEPI) via D(d,n)3He or T(d,n)4He nuclear fusion reactions, respectively. The fuel was UO2-PuO2, the simulated coolant was sodium or lead, and the multiplication factor keff ranged from 1 to 0.95. The aim of the experiment was to develop new measurement techniques specific to ADSs and to test the performances of neutronic calculations codes for such systems. The interpretation of the MUSE-4 experiment has shown that the physical parameters of the system are globally well reproduced by calculations performed with the ERANOS code system, which proves good agreement with both the measurements and the reference Monte Carlo calculations; this concerns the critical mass, the delayed neutron fraction, the fission rate shapes, and the spectral indices. This is a particularly remarkable issue for ERANOS and its associated libraries, which had never been tested for such situations. Concerning the nuclear data, JEF-based cross sections provide a better agreement on critical mass than other libraries. A sensitivity of several measured parameters to the elastic and inelastic cross section of lead have been demonstrated, and possible biases on these cross sections have been indicated. We have shown that several methods based on deterministic or stochastic calculations allow us to relate the experimental neutron population decay after a source pulse with the reactivity of the system; these reactivity determination techniques are in good agreement with standard reactivity measurement techniques.

From binary fission to multifragmentation in the decay of heavy excited nuclei

Abstract The decay modes of heavy excited nuclei formed in dissipative collisions for the systems Ar + Au at 60 MeV/u and Kr + Au at 43 MeV/u are studied. A transition from binary fission towards multi-fragment emission is observed around 3 MeV/u excitation energy, independently of the considered system. The charge distributions of the three target-emitted fragments suggest a smooth change from “fission” events (as a “natural” continuity of binary fission) to “residue production” then to “symmetric ternary fragmentation” events when the excitation energy increases from 3 MeV/u to 5 MeV/u.

Energy damping and intermediate-velocity fragment emission in peripheral Kr+Au collisions at 43 MeV/u☆

Abstract Triple and four-fold coincidences among fragments have been measured in the reaction 84 Kr+Au at 43 MeV/u. All events showing the projectile-like nucleus and fission fragments of the target-like nucleus, and all events with one additional intermediate-velocity fragment, were analysed in the frame of a dissipative collision and a participant-spectator model. The mechanism is basically that of a dissipative collision but the emission of the intermediate velocity fragment by the target differs from an equilibrated evaporation.

Dynamical effects in nuclear collisions in the fermi energy range: aligned breakup of heavy projectiles

Abstract Recent experimental results concerning heavy systems (Pb + Au, Pb + Ag, Pb + Al, Gd + C, Gd + U, Xe + Sn, …) obtained at GANIL with the INDRA and NAUTILUS 4 π arrays will be presented. The study of reaction mechanisms has shown the dominant binary and highly dissipative character of the process. The two heavy and excited fragments produced after the first stage of the interaction can decay into various decay modes from evaporation to multifragmentation including fission. However, deviations from this simple picture have been found by analyzing angular and velocity distributions of light charged particles, and fragments. Indeed, there is a certain amount of matter in excess emitted between the two primary sources suggesting either the existence of a mid-rapidity source similar to the one observed in the relativistic regime (participants) or a strong deformation induced by the dynamics of the collision (neck instability). This last possibility has been suggested by analyzing in detail the angular distributions of the fragments. More precisely, we observe an isotropic component which is compatible with the prediction of statistical models and a second one corresponding to breakup aligned with the recoil direction of the projectile like source which should be compared with the predictions of dynamical calculations based on microscopic transport models.

Evidence for fast and simultaneous multi-fragment emission in central Kr + Au collisions at 60 MeV/u

Abstract Decay products emitted in Kr+Au collisions at 60 MeV/u have been detected with a large experimental array. Events with fragment multiplicities up to 5 have been recorded. After a careful selection of central collisions leading to equilibrited events, the data are compared to two fragmentation models in which fragments are emitted either sequentially or simultaneously. It is shown that the data can be explained very well by the second model, thus suggesting a prompt multifragmentation mechanism. Possible expansion effects in the disassembly process are also investigated.

Introduction to the Physics of Molten Salt Reactors

In the frame of a major re-evaluation of the molten salt reactor (MSR), we have developed a new concept called Thorium Molten Salt Reactor (TMSR), particularly well suited to fulfill the criteria chosen by the Generation IV forum. This reactor may be operated in simplified and safe conditions in the Th/U fuel cycle with fluoride salts. Amongst all TMSR configurations, many studies have highlighted the configurations with no moderator in the core as simple and very promising. Since U does not exist on earth and is not being produced today, we aim at designing a critical MSR able to burn the Plutonium and the Minor Actinides produced in today’s reactors, and consequently to convert this Plutonium into U. Thus, the current fuel cycle can be closed thanks to TMSRs started with transuranic elements on a Thorium base, i.e. started in the Th/Pu fuel cycle, similarly to fast neutron reactors operated in the U/Pu fuel cycle. We analyze the characteristics of these reactor configurations, in terms of fissile matter inventory, salt reprocessing, waste production and burning, and finally deployment capabilities. Using a simple kinetic-point model, we analyze the reactor’s behaviour as the total reactivity margins are introduced in the core. We thus confirm, beyond the classical advantages of molten salt reactors, the satisfactory behaviour of the TMSR and the excellent level of deterministic safety which can be achieved in such reactors. We then illustrate how the reactor can be driven with no control rod, either by controlling the extracted power or by monitoring the operating temperature. Finally we stress the hardiness and the flexibility of this TMSR concept, allowing it to be adjustable without loosing its advantages in the event of any technological stumbling block.