A. Gandini

THE PHYSICS OF SUBCRITICAL MULTIPLYING SYSTEMS

In recent years an increasing interest is observed with respect to subcritical, accelerator driven systems (ADS), for their possible role in future nuclear energy scenarios, as actinide (Pu and MA) incinerators, and/or as claimed energy plants with potential enhanced safety characteristics. Important research programs are devoted to the various related fields of research. Extensive studies on the ADS behavior under incidental conditions are in particular made, for verifying their claimed advantage, under the safety point of view, with respect to the corresponding critical reactors. Related medium and long range scenarios are being considered to cope with a number of concerns associated with safety (power excursions, residual heat risk), as well as with fuel flow (criticality accidents, fuel diversion, radiological risk, proliferation). In the present work we shall comment on some issues relevant to these new reactor concepts, with the intent of giving a general view of the present state-of-the-art in the reactor physics domain. Specific calculation methods to be adopted for intercomparison analysis of these systems and experimental data interpretation are also discussed, as well as ongoing and perspective relevant experimental campaigns on experimental facilities.

HGPT based sensitivity time-dependent methods for the analysis of subcritical systems

Abstract In recent years an increasing interest is observed with respect to subcritical, accelerator driven systems (ADS). Considering the attention being given to these systems for their supposed ability to play a major role as actinides incinerators, as well as power production plants, the application of the heuristically-based generalized perturbation theory (HGPT) methodology for the cycle life analysis of these systems is reviewed and commented. It is discussed in particular the role of the importance function associated with the power control, and the definition of the concept of “generalized reactivity”, merging into the standard concept of reactivity with the system approaching criticality. Basing on these results, a formulation is also described of a point kinetic equation, with physically significant coefficients, similar to those presented by Usachev in 1955 (Usachev, L.N., 1955. Atomnay a Energiya, 15, 4726), using the standard adjoint flux as weighting function.

ON THE MULTIPLICATION FACTOR AND REACTIVITY DEFINITIONS FOR SUBCRITICAL REACTOR SYSTEMS

Abstract An analysis is made on the merit of different functions adopted for weighting neutron processes in subcritical nuclear reactor systems, as it appears in expressions of relevant integral quantities, such as reactivity worths, prompt neutron lifetimes, etc. All weight functions may be shown to depend on some sort of explicit or implicit, real or fictitious, system control. Associated with the importance function relevant to the reactor power control, the multiplication factor k sub and generalized reactivity values ρ gen are defined. The difference (1- k sub )/ k sub is shown to be the more appropriate index for generally representing the ADS subcriticality. However, in certain circumstances, when an accidental event is studied in which the criticality condition may be surpassed during the transient, it appears more appropriate to take into account the standard multiplication factor ( k eff ) and the reactivity values to which the transient is associated and for the definition of which the standard adjoint flux is adopted.

On the evaluation of ADS subcriticality

Abstract Based on a recent derivation of perturbation methods generally applicable for the analysis of subcritical reactors, at steady state or transient conditions, a simple procedure is described for the evaluation of their subcriticality level during normal operation. The precision of the method is shown to increase with the system approaching criticality conditions.

EVOLUTIONARY MOBILE FUEL REACTORS

In the present paper issues relevant to an extended, long term use of nuclear energy are commented. In particular, the role of ADS and of the so called evolutionary mobile fuel reactors are considered. A note is presented on specific calculation methods to be adopted for intercomparison and survey analysis of these systems.