A. D'Angelo

Some Features of Spatial Neutron Kinetics for Multiplying Systems

Abstract The paper considers some physical aspects of the neutron space kinetics of critical and source-driven subcritical systems. The possibility of introducing some indicators to qualify the spatial nature of neutronic transients is investigated. It is shown theoretically and then proved by numerical examples that the separation of the eigenvalues of the mathematical operator defining the problem can be taken as a good indicator of the importance of space effects in time-dependent conditions. To obtain good physical insight into the phenomena, paradigmatically simple configurations are considered, and whenever possible, a fully analytical approach is used. The presented results evidence the limits of applicability of classic simplified models for transient analyses, such as point kinetics. In a second part, the paper considers the open problem of the choice of the weighting function to be used either for the generation of the kinetic parameters of pointlike models or for the exploitation of quasi-static procedures, analyzing comparatively the effect of different options on the results of transient calculations.

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.

Application of the multipoint method to the kinetics of accelerator-driven systems

Abstract The mathematical foundations of the multipoint method are illustrated and the method is developed for the neutron kinetics of multiplying systems to treat physical situations in which spatial and spectral effects can play an important role in transient conditions, and hence the classical point-kinetic model can become inadequate. In the present paper the method is specifically developed for source-driven systems, through a proper adaptation of the factorization-projection technique used to derive other classic kinetic models. The results presented for some test cases show the advantages that can be attained with respect to the standard point model, even when treating relevant spatial and spectral transients. It is then shown how the technique can be inserted into a quasi-static framework.

Neutronic Analyses of the Trade Demonstration Facility

Abstract The TRiga Accelerator-Driven Experiment (TRADE), to be performed in the TRIGA reactor of the ENEA-Casaccia Centre in Italy, consists of the coupling of an external proton accelerator to a target to be installed in the central channel of the reactor scrammed to subcriticality. This pilot experiment, aimed at a global demonstration of the accelerator-driven system concept, is based on an original idea of C. Rubbia. The present paper reports the results of some neutronic analyses focused on the feasibility of TRADE. Results show that all relevant experiments (at different power levels in a wide range of subcriticalities) can be carried out with relatively limited modifications to the present TRIGA reactor.

The effective beta sensitivity to the incident neutron energy dependence of the absolute delayed neutron yields

The uncertainty of the [sup 235]U, [sup 239]Pu, and [sup 238]U absolute delayed neutron yields v[sub d] is one of the principal sources of uncertainty in predicting the fission reactor reactivity scale [beta][sub eff]. The current uncertainties in the dependence of v[sub d] on incident neutron energy is investigated for significance in the evaluation of [beta][sub eff]. The uncertainty effects on the GODIVA, JEZEBEL, Zero Power Reactor, SNEAK, and Masurca benchmark facility calculations are analyzed using ENDF/B and JEF basic data. Different assumptions about the energy dependence result in variations of up to 5% in the reactor spectrum averaged values of v[sub d], and these would result in variations of up to [approximately] 2% in the value of [beta][sub eff] for a typical liquid-metal fast breeder reactor.