E. Liatard

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.

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.

Prompt reactivity determination in a subcritical assembly through the response to a dirac pulse

The full understanding of the kinetics of a subcritical assembly is a key issue for its online reactivity control. Point kinetics is not sufficient to determine the prompt reactivity of a subcritical assembly through the response to a dirac pulse, in particular in the cases of a large reflector, a small reactor, or a large subcriticality. Taking into account the distribution of intergeneration times, which appears as a robust characteristic of each type of reactor, helps to understand this behaviour. Eventually, a method is proposed for the determination of the prompt reactivity. It provides a decrease rate function depending on the prompt multiplication coefficient Keffp. Fitting a measured decrease rate with this function, calculated once for the reactor, gives the true value of keffp. The robustness of the method is tested.

INFLUENCE OF THE PROCESSING AND SALT COMPOSITION ON THE THORIUM MOLTEN SALT REACTOR

Abstract Molten salt reactors (MSRs) are one of the six systems retained by Generation IV as a candidate for the next generation of nuclear reactors. The MSR is a very attractive concept especially for the thorium fuel cycle, which allows nuclear energy production with a very low production of radiotoxic minor actinides, so it has been selected by the Generation-IV International Forum. Its main characteristic is a strong coupling between neutronics and salt processing. Such nuclear reactors use a liquid fuel that is also the coolant. Elements produced during the reactor’s operation, like fission products or transuranic elements, modify the neutronic balance of the reactor by capturing neutrons. As the fuel is liquid, partial processing of a limited amount taken from circulating salt is possible, in order to remove the poisoning elements, without stopping reactor operation. In this paper, we present a configuration that we consider to be a reference one for a thorium molten salt reactor (TMSR), and we study the influence of efficacy of different types of processing on the neutronic behavior of this reactor. By considering both the possibilities in chemistry and the neutronic effects, our aim is to work out an efficient, reliable, and realistic processing scheme. The processing includes in fact two components: an in-line bubbling system within the reactor that extracts the gaseous and metallic fission products quickly and a slower external processing unit that extracts the other fission products. A salt volume equal to the core volume is thus cleaned in several months. We have studied the influence of different processing rates on the reactor’s behavior. This mainly affects the breeding ratio. Properties of the salt are also crucial. We choose in our simulations of the TMSR a 78 mol% LiF–22 mol% [heavy nuclei (HN)] F4 salt for the fuel, but lower HN proportions in the fuel salt are also examined in order to minimize the 233U inventory in the reactor. The neutron spectrum is largely modified by the HN proportion and has a deep impact on the reactor behavior. Our simulations evaluate the degradation of the breeding ratio from >1 for the reference configuration down to 0.86 due to a decrease of the HN proportion in the fuel salt. We conclude that the simplification of the salt processing that is addressed in this work improves the feasibility of the TMSR system.

Monitoring fast neutron sources for accelerator driven subcritical reactor experiments

In the framework of nuclear waste management, minor actinides could be incinerated in subcritical reactors driven by an accelerator coupled to a spallation target. For safety reason, this so-called Accelerator-Driven System (ADS) requires on-line and robust core reactivity monitoring. In such a system, a simple proportional relationship exists between the reactivity and the ratio of the beam current to the thermal power of the reactor core. This relationship is planned to be exploited as a relative reactivity indicator by the measurement of both the beam current delivered by the accelerator and the core neutron flux. The GUINEVERE experiment facility, which is being built at SCK-CEN in Mol (Belgium), will be devoted to the study of such reactivity measurement techniques. This zero power experiment will consist of the coupling of a subcritical fast core to an external source of 14-MeV neutrons originating from fusion reactions between a deuteron beam and a tritium target. In this case, the target evolution over time and target inhomogeneities preclude from using the beam current for reliable reactivity monitoring and therefore the external neutron source intensity must be monitored directly. This paper presents the systems envisioned to achieve such a monitoring and the results of test experiments.