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The G 0 parity-violation experiment at Jefferson Lab (Newport News, VA) is designed to determine the contribution of strange/anti-strange quark pairs to the intrinsic properties of the proton. In the forward-angle part of the experiment, the asymmetry in the cross-section was measured for e ⇒ p elastic scattering by counting the recoil protons corresponding to the two beam-helicity states. Due to the high accuracy required to measure the few-part-per-million asymmetry, the G 0 experiment was based on a custom experimental setup with its own associated electronics and data acquisition (DAQ) system. Highly specialized time-encoding electronics provided time-of-flight spectra for each detector for each helicity state. More conventional electronics, processing only a small fraction of the events, was used for monitoring (mainly FastBus). The time-encoding electronics and the DAQ system have been designed to handle events from the 128 detector pairs at a mean rate of 2 MHz per detector pair with low deadtime and with minimal helicity-correlated systematic errors. In this paper, we outline the general architecture and the main features of the electronics and the DAQ system dedicated to G 0 forward-angle measurements.

In the framework of nuclear waste management, highly radiotoxic long-lived fission products and minor actinides are planned to be transmuted in a sub-critical reactor coupled with an intense external neutron source. The latter source would be created by a high-energy proton beam hitting a high atomic number target. Such a new system, termed an accelerator-driven system (ADS), requires on-line and robust reactivity monitoring. The ratio between the beam current delivered by the accelerator and the reactor power level, or core neutron flux, is the basis of one method which could give access to a core reactivity change. In order to test reactivity measurement technique, some experimental programs use 14-MeV neutrons originating from the interaction of a deuteron beam with a tritium target as an external neutron source. In this case, the target tritium consumption over time precludes use of the beam current for reactivity monitoring and the external neutron source intensity must be monitored directly. A range telescope has been developed for this purpose, consisting of the assembly of a hydrogenous neutron converter and three silicon stages where the recoiling protons are detected. In this article, the pergormances of such a telescope are presented and compared to Monte-Carlo simulations.

In the context of the world energy demand growing and of greenhouse effect concern, new nuclear power options are extensively studied, especially systems dedicated to high activity waste incineration. It is the case of fast accelerator driven subcritical systems which are the subject of several projects among which the recently completed MUSE programme. From the coupling of a fast subcritical mock-up to a pulsed neutron source, various methods leading to the measurement of the reactivity level and kinetic parameters, which are of prime interest for safety, were investigated. An analysis of pulsed neutron source time spectra taking into account the full distribution of the time intervals between two consecutive fission events – the kp method – was developed and is recalled in this paper. Complete results obtained during the MUSE experiment with this method are given and compared to reference measurements as well as to PNS measurement analysis like exponential fitting or area methods. The quality of the results led to the investigation of the applicability of the method in the case of a continuous source interruption and then in the case of a power ADS. In both cases, on the basis of simulations, no obstacles are found to get the prompt multiplication factor from this method. The aim of this paper is to give a complete overview of the kp method applications and limits.

We discuss here the concept of Thorium Molten Salt Reactor dedicated to future nuclear energy production. The fuel of such reactors being liquid, it can be easily reprocessed to overcome neutronic limits. In the late sixties, the MSBR project showed that breeding is possible with thorium in a thermal spectrum, provided that an efficient pyrochemical reprocessing is added. With tools developed around the Monte Carlo MCNP code, we first re-evaluate the performance of a MSBR-like reference system with 232Th/233U fuel. We find an important reduction of inventories and induced radiotoxicities at equilibrium compared to other fuel cycles, with a doubling time of about thirty years. We then study how to start this interesting reference system with theplutonium from PWR spent fuel. Such a transition appears slow and difficult, since it is very sensitive to the fissile quality of the plutonium used. Deployment scenarios of 232Th/233U MSBR-like systems from the existing French PWRs demonstrate the advantage of an upstream 233U production in other reactors, allowing a direct start of the MSBR-like systems with 233U. This finally leads us to explore alternatives to some MSBR features, for energy production with 232Th/233U fuel from the start. We thus test different options, especially in terms of core neutronics optimization and reprocessing unit adaptation.

The neutron multidetector DeMoN has been used to investigate the symmetric splitting dynamics in the reactions 58,64 Ni + 208 Pb with excitation energies ranging from 65 to 186 MeV for the composite system. An analysis based on the new backtracing technique has been applied on the neutron data to determine the two-dimensional correlations between the parent composite system initial thermal energy (ECNth) and the total neutron multiplicity (νtot), and between pre- and post-scission neutron multiplicities (νpre and νpost, respectively). The νpre distribution shape indicates the possible coexistence of fast-fission and fusion-fission for the system 58 Ni + 208 Pb (Ebeam = 8.86 A MeV). The analysis of the neutron multiplicities in the framework of the combined dynamical statistical model (CDSM) gives a reduced friction coefficient β = 23 ± 1225 × 1021 s−1, above the one-body dissipation limit. The corresponding fission time is τf = 40 ± 2046 × 10−21s.

A simple model is proposed for the diffusion-controlled release of radioactive nuclei implanted into a metal. Experiments using a tungsten target bombarded by 25 keV 135,137–141 Xe 1+ ions have been conducted in order to make a comparison with this model. Experimental results agree with the expected behaviour and the Arrhenius relation is obeyed over the temperature range with an activation energy E A = 110 kJ/mole and a pre-exponential factor D 0 = 1.4 × 10 −11 cm 2 /s.

Particle-particle correlations are studied for large- and small-angle separations for the reactions 312 and 680 MeV $^{40}\mathrm{Ar}$${+}^{\mathrm{nat}}$Ag. The spectral shapes and angular ansiotropies allow a characterization of the effective temperatures and spin zones of the hot nuclear emission sources. Small-angle correlations are compared to reaction simulations that employ various parametrizations for the emission time scales. We conclude that the initial lifetime scale for $^{1,2,3}\mathrm{H}$ particle evaporation is of the order of ${10}^{\mathrm{\ensuremath{-}}22}$ s. These times are so short as to suggest a breakdown of the concept of ``sequential evaporation,'' for which a new ansatz is explored.

In the G0 experiment, performed at Jefferson Lab, the parity-violating elastic scattering of electrons from protons and quasi-elastic scattering from deuterons is measured in order to determine the neutral weak currents of the nucleon. Asymmetries as small as 1 part per million in the scattering of a polarized electron beam are determined using a dedicated apparatus. It consists of specialized beam-monitoring and control systems, a cryogenic hydrogen (or deuterium) target, and a superconducting, toroidal magnetic spectrometer equipped with plastic scintillation and aerogel Cerenkov detectors, as well as fast readout electronics for the measurement of individual events. The overall design and performance of this experimental system is discussed., Comment: Submitted to Nuclear Instruments and Methods

Large and small angle correlation measurements are reported for [sup 1]H-[sup 1]H, [sup 4]He-[sup 4]He, and [sup 2]H-[sup 4]He pairs from 7.8[ital A] and 17[ital A] MeV [sup 40]Ar+[sup nat]Ag. These measurements are interpreted with the aid of trajectory calculations designed to simulate evaporationlike emission of particles and fragments. The most important physical parameter needed to fit the correlation function peaks is the fractional yield of unstable fragments ([sup 8]Be or [sup 6]Li) compared to independently evaporated particles ([sup 4]He-[sup 4]He) or ([sup 2]H-[sup 4]He). In this approach, the peaks in the simulated correlation functions contain essentially no information concerning the space-time extent of the emitter source. Nevertheless, these simulations can account for the data very well.

A new integral measurement of the 7 Li neutron induced elastic scattering cross section was determined in a wide neutron energy range. The measurement was performed on the LPSC-PEREN experimental facility using a heterogeneous graphite-LiF slowing-down time spectrometer coupled with an intense pulsed neutron generator (GENEPI-2). This method allows the measurement of the integral elastic scattering cross section in a slowing-down neutron spectrum. A Bayesian approach coupled to Monte Carlo calculations was applied to extract natural C, 19 F and 7 Li elastic scattering cross sections.

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.

International audience; 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.

Particle-particle coincidence measurements have been made for the reactions 312 and 680 MeV {sup 40}Ar+{sup nat}Ag. The large-angle correlation data indicate essentially complete energy thermalization even for 680 Mev {sup 40}Ar+{sup nat}Ag. They also allow for an assignment of {approx}64{h bar} for the rms spin and {approx}4.3 MeV for the average initial temperature of the composite nuclei formed by incomplete fusion. The small-angle correlation data for {sup 1}H-{sup 1}H and {sup 2}H-{sup 2}H are consistent with phase-space, statistical-model predictions of the evaporation lifetimes. This result implies that energy mixing and evaporation particle emission steps occur with intervals of {approx}10{sup {minus}22} s, a time period comparable to the traversal time for a projectile velocity of 17 MeV/nucleon.

International audience; 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.

We have observed Doppler-shifted x rays characteristic of Sb and Sn atoms in coincidence with {sup 108}Xn evaporation residues resulting from the emission of four protons by the compound nucleus (CN) formed in the reaction {sup 58}Ni+{sup 54}Fe. We have attributed the Sb x rays to atomic inner-shell vacancies which are created on the way into the nuclear reaction and filled before the emission of the last proton in the deexcitation cascade. The mean proton partial width of the nuclear states, formed at a mean excitation energy of 25{plus minus}4 MeV by the evaporation of three protons by the CN, has been deduced.

We have measured the beam-normal single-spin asymmetry in elastic scattering of transversely-polarized 3 GeV electrons from unpolarized protons at Q^2 = 0.15, 0.25 (GeV/c)^2. The results are inconsistent with calculations solely using the elastic nucleon intermediate state, and generally agree with calculations with significant inelastic hadronic intermediate state contributions. A_n provides a direct probe of the imaginary component of the 2-gamma exchange amplitude, the complete description of which is important in the interpretation of data from precision electron-scattering experiments., 5 pages, 3 figures, submitted to Physical Review Letters; shortened to meet PRL length limit, clarified some text after referee's comments

Long-lived fission products and minor actinides produced in nuclear power plants are the most radiotoxic nuclear wastes. They can be transmuted into stable nuclei or into nuclei with shorter lifetime thanks to the so-called Accelerator Driven Systems (ADS), consisting of the coupling of an intense high energy proton beam, a spallation target and a sub-critical reactor core. For safety reasons, an on-line and robust measurement of the reactivity during loading and power operation is mandatory. The investigation of the relationship between the current of the accelerator and the power level (or neutron flux) of the reactor appears to be powerful, any change in reactivity being accessible through the measurement of the current and the flux. Such a relationship will be studied in an experiment to be performed at the YALINA facility (JIPNR Sosny - Belarus) in the framework of the EUROTRANS IP (6 th FP). At this installation, 14 MeV neutrons are produced in T(d,n) 4 He reactions by a deuteron beam impinging on a TiT target. Due to the tritium consumption over time, the intensity of the deuteron beam cannot be used for the monitoring of the neutron beam. The source neutron yield itself has to be accessed. This contribution describes the performance of a three-element silicon telescope dedicated to the detection of recoil protons produced in reactions induced by 14 MeV neutrons.

In recent years great interest has been displayed, worldwide, for Accelerator Driven Sub-critical reactors (ADS) to incinerate the minor actinides generated by existing energy-producing reactors. In sub-critical systems, the effective neutron multiplication factor is lower than 1.0 and the neutrons otherwise required to maintain the chain reaction can be put to other uses, in particular, the destruction of nuclear wastes such as minor actinides (MA). One of the major advantages of such ADS systems is that it can be operated with very high MA content without jeopardizing the overall safety due to a small effective delayed neutron fraction, a small Doppler temperature coefficient and possibly also a large void coefficient depending on the chosen coolant. This enhanced safety, however, requires at all time a sufficient sub-criticality margin. Reliable reactivity monitoring techniques are hence required to achieve this goal. The MUSE-4 program is a series of low power experiments carried out at the CEA-Cadarache MASURCA facility to investigate the various methods leading to the measurement of the reactivity level and associated kinetic parameters such as the effective delayed neutron fraction. The aim of this paper is to present the results obtained with a method which directly gives the ratio, for a sub-critical assembly, between the reactivity ρ and the effective delayed neutron fraction β eff . By combining these results to those obtained with the k p -method for the prompt neutron multiplication coefficient, we have access to the parameters which govern the prompt and the slow kinetics of a sub-critical assembly. These parameters can be obtained without reference to any calibration measurement in critical configuration. It opens the way to the control of larger sub-critical demonstrators that are operating with fuels which cannot be used in critical reactor, and, thanks to sub-criticality, which are characterized by a deterministic safety.

We have studied the quasielastic He-3(e,e(')p)H-2 reaction in perpendicular coplanar kinematics, with the energy and the momentum transferred by the electron fixed at 840 MeV and 1502 MeV/c, respectively. The He-3(e,e(')p)H-2 cross section was measured for missing momenta up to 1000 MeV/c, while the A(TL) asymmetry was extracted for missing momenta up to 660 MeV/c. For missing momenta up to 150 MeV/c, the cross section is described by variational calculations using modern He-3 wave functions. For missing momenta from 150 to 750 MeV/c, strong final-state interaction effects are observed. Near 1000 MeV/c, the experimental cross section is more than an order of magnitude larger than predicted by available theories. The A(TL) asymmetry displays characteristic features of broken factorization with a structure that is similar to that generated by available models.

Results of the Jefferson Lab Hall A quasielastic 3He(e,e'p)pn measurements are presented. These measurements were performed at fixed transferred momentum and energy, q=1502 MeV/c and omega=840 MeV, respectively, for missing momenta p(m) up to 1 GeV/c and missing energies in the continuum region, up to pion threshold; this kinematic coverage is much more extensive than that of any previous experiment. The cross section data are presented along with the effective momentum density distribution and compared to theoretical models.

We have studied the quasielastic 3He(e,e(')p)2H reaction in perpendicular coplanar kinematics, with the energy and the momentum transferred by the electron fixed at 840 MeV and 1502 MeV/c, respectively. The 3He(e,e(')p)2H cross section was measured for missing momenta up to 1000 MeV/c, while the A(TL) asymmetry was extracted for missing momenta up to 660 MeV/c. For missing momenta up to 150 MeV/c, the cross section is described by variational calculations using modern 3He wave functions. For missing momenta from 150 to 750 MeV/c, strong final-state interaction effects are observed. Near 1000 MeV/c, the experimental cross section is more than an order of magnitude larger than predicted by available theories. The A(TL) asymmetry displays characteristic features of broken factorization with a structure that is similar to that generated by available models.

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 k_eff = 0.995. Deeper subcriticality (k_eff = 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-alpha and Feynman-alpha), 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.

This paper deals with the neutronic Monte Carlo simulations in view of an eXperimental Accelerator Driven System (XADS) optimization and characterization. With this aim, the quite realistic concept of XADS chosen for our investigations, comes from the preliminary studies done by Framatome ANP. First, we simulate this basic concept with different fuels and characterize the time evolution of the conceivable fuels, especially with minor actinides. Secondly, we consider the neutronic parameters in view of an extrapolation of the XADS toward a large ADS. We also consider some alternative solutions to reduce the flux peak in the first assemblies around the spallation target of a large ADS and consequently to improve the shape factor and the efficiency of the core. Finally, we focus on the generation of 233 U based on a plutonium and minor actinide fuel associated to thorium oxide to start power Molten Salt Reactors (MSR) and to accelerate the transition toward a 232 Th/ 233 U cycle which offers an interesting option for nuclear waste reduction and nuclear energy production during many centuries.

The time scales for nuclear fission have been explored using both pre- and postfission neutrons and GDR gamma rays. Four systems were investigated: 133-MeV /sup 16/O+/sup 176/Yb and /sup 208/Pb and 104-MeV /sup 4/He+/sup 188/Os and /sup 209/Bi. Fission fragments were measured in coincidence with PPACs. The neutrons were detected using eight detectors from the DEMON array, while gamma rays were measured using the US BaF/sub 2/ array. The pre- and postfission gamma rays were determined using moving source fits parallel and perpendicular to the fission fragment emission directions. The time scales for fission for the neutrons were determined using the neutron clock technique. The gamma-ray data were fitted using a statistical model calculation based on the code CASCADE. The results of the fits from both data types were used to extract nuclear friction coefficients, gamma , and fission time scales. The gamma values ranged from 7 to 20, while the fission times were (31-105)*10/sup -21/s.

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. (Elsevier)

The γ-rays from the multimodal fission of the 226Th formed in 18O+208Pb was investigated at the near- and sub-barrier energies. The corresponding excitation energies at the saddle point, Esp*, ranged from 23 to 26 MeV. The average γ-ray multiplicities and relative γ-ray energies as a function of the mass of the fission fragments exhibits a complex structure and strong variations. Such strong variations have never been previously observed in heavy ion-induced fusion-fission reactions. Obtained results may be explained with the influence of shell effects on the properties of the fission fragments. Present work is the one in series of investigation of the multimodal fission phenomena in At-Th region.

The fission of the systems 220,224,226Th was investigated by measuring the mass-energy distributions of the fission fragments. The corresponding excitation energies at the saddle point, Esp*, ranged from 16 to 40 MeV. As Esp* decreases, an asymmetric mass component becomes visible on the predominately symmetric mass distribution. The contribution of the asymmetric mode is characterized by the total yield ratio Ys/Ya, which decreases rapidly for the heavier isotopes of thorium. This behavior of Ys/Ya is in qualitative agreement with theoretical calculations. For all isotopes studied, the subtracted asymmetric fission component, Ya=Y1−Ys, exhibits a complex structure, actually showing two components, Ya=Ya1+Ya0, which have average masses Ma1=132 and Ma0=140.

Mean square radii of light radioactive projectiles, including exotic isotopes close to the neutron drip line (11Li, 14Be, and 17B), have been deduced from total reaction cross-section measurements, using a simple microscopic model. The isospin dependence of nuclear radii is discussed and compared to other experimental results.

Total nuclear reaction cross-sections are determined by means of a 4π-γ method. The results cover a wide span of targets for various stable beams. The validity of the method is shown in a combined systematics including also the results of transmission-type experiments. The data are very well described by the formula developed by Kox et al. The same method is applied to secondary fragment beams produced from a 44 MeV/u22Ne beam on a 332mg/cm2 181Ta target. Using the LISE spectrometer the fragments4, 6He,6–9, 11Li,7, 9–12, 14Be,10–15, 17B11–19C,13–19N,15–21O,18– 21F and20,21 Ne are analyzed and transported to interact with a 199.4 mg/cm2 Cu target surrounded by a 4π-γ counter. The measured total reaction cross-sectionsσ R are discussed in terms of the reduced strong absorption radiusr 0 and compared with other experimental results.

The different paradigms on which traditional information retrieval systems are built are reused to design Web search engines. However, designers have to innovate in order to adapt to this particular documentary context, which has an important impact on the way tools operate. Through an analysis of French and English scientific literature, and observations made during Web search training sessions, we try to define how far users are taken into account in the design of such systems, by comparing paradigm implementation to recent researches on information needs and the informational process. An analysis of the different steps of an information retrieval process highlights the complexity and the many dimensions of the concept of relevance in document retrieval.

The response of photovoltaic cells to heavy ions and fission products have been tested in-beam. Their main advantages are their extremely low price, their low sensitivity to energetic light ions with respect to fission products, and the possibility to cut and fit them together to any shape without dead zone. The time output signals of a charge sensitive preamplifier connected to these cells allows fast coincidences. A resolution of 12 ns (FWHM) has been measured between two cells.

International audience; 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.

The thorium- 233U fuel cycle might provided safer and cleaner nuclear energy than the present Uranium/ Pu fuelled reactors. Over the last 10 years, a vast campaign of measurements has been initiated to bring the precision of neutron data for the key nuclei (232Th, 233Pa and 233U) at the level of those of the U- Pu cycle. This is the framework of these measurements, the energy dependent neutron induced fission cross section of 232Th and 233U has been measured from 1 to 7 MeV with a target accuracy lesser than 5 %. These measurements imply the accurate determination of the fission rate, the number of the target nuclei as well as the incident neutron flux impinging on the target, the latter has been obtained using the elastic scattering (n, p). The cross section of which is the very well known in a large neutron energy domain (~ 0,5 % from 1 eV to 50 MeV) compared to the 235U(n, f) reaction. This technique has been applied for the first time to the 232Th(n, f) and 233U(n, f) cases. A Hauser- Feshbach statistical model has been developed. It consists of describing the different decay channels of the compound nucleus 234U from 0,01 to 10 MeV neutron energy. The parameters of this model were adjusted in order to reproduce the measured fission cross section of 233U. From these parameters, the cross sections from the following reactions could be extracted: inelastic scattering 233U(n, n'), radiative capture 233U(n, γ) and 233U(n, 2n). These cross sections are still difficult to measure by direct neutron reactions. The calculated values allow to fill the lack of experimental data for the major fissile nucleus of the thorium cycle., Le cycle du thorium (232Th/233U) présente des caractéristiques moins polluantes que celui utilisé actuellement dans les combustibles des réacteurs nucléaires (U/Pu). Depuis une dizaine d'années, il fait l'objet d'un vaste programme de mesures dont l'objectif est d'atteindre une précision des données neutroniques des noyaux clés du cycle (232Th, 233Pa et 233U) comparable à celles du cycle U/Pu. Ce travail s'inscrit dans ce cadre d'études et consiste à mesurer les sections efficaces de fission induites par neutrons rapides (de 1 à 7 MeV) des noyaux 232Th et 233U avec une incertitude inférieure à 5 %. Ces mesures impliquent de déterminer le taux de fission émis par la cible, le nombre d'atomes composant la cible et le flux de neutrons arrivant sur celle-ci. Cette dernière quantité a été obtenue en utilisant comme réaction de référence, la diffusion élastique (n, p) dont la section efficace est la mieux connue sur un large domaine en énergie de neutron (~ 0,5 % de 1 eV à 50 MeV) comparée à celle de la réaction 235U(n, f). Cette technique a été appliquée pour la première fois à ces deux noyaux. Un modèle statistique de type Hauser- Feshbach a été également développé dans ce travail. Il a permis de décrire les différentes voies de désexcitation du noyau composé 234U pour des énergies de neutron de 0,01 à 10 MeV. Les paramètres de ce modèle ont été ajustés afin de reproduire la section efficace de fission mesurée de 233U. A partir de ces paramètres, les sections efficaces des réactions suivantes ont pu être extraites : diffusion inélastique 233U(n, n'), capture radiative 233U(n, γ) et 233U(n, 2n). Ces sections efficaces évaluées, dont les mesures sont encore difficilement réalisables, permettent de combler le manque de données expérimentales concernant 233U, le noyau fissile de la filière thorium.