Fast Reactors and Accelerator Driven Systems Knowledge Base

Conference Article: ADS Neutronic benchmark(stage 1), A new approach to the design of accelertor driven systems

Abstract

The main parameter characterizing the neutron economy of an accelerator driven subcritical fission device, like the Energy Amplifier (EA), is the factor M by which the “source” spallation neutrons are multiplied by the fission dominated cascade. A related quantity is the multiplication coefficient ksrc = (M-1)/M, that is the average ratio of the neutron population in two subsequent generations of the source-initiated cascade. Such a factor ksrc , depending on both the properties of the source and of the medium, is in general conceptually and numerically different from the effective criticality factor keff, commonly used in reactor theory, which is in fact only relevant to the fundamental mode of the neutron flux distribution, and is independent on the source. The effective criticality factor kef is however a meaningful measure of the actual safety characteristics of the device, that is 1-keff is a proper gauge of the distance from criticality. In this paper the difference between ksrc and keff is adressed numerically in the case of an externally driven Thorium fuelled and Lead cooled subcritical device representing a simplified version of the Energy Amplifier. It is found that codes or calculations implementing the “critical reactor” formalism (neutrons are distributed according to a cos-type imposed distribution together with a fission spectrum energy distribution and non-fission multiplication, i.e. n,Xn reactions, is not considered explicitly) in order to describe a subcritical device, systematically underestimate the reactivity of the system by about 0.028 in k (- 2800 pcm) which implies an error in the estimation of the necessary concentration of 233U close to 5% which in turn induces an adverse effect on the stability of k during burnup. Finally, the discrepancies arising from the use of different nuclear data libraries are as significant as the effects of using different neutron source approximations and hence also deserve attention. We think that a reevaluation of the nuclear libraries related to the Thorium cycle is necessary, particularly as far as the JEF-2.2 data are concerned

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Reference:

Technical Committee Meeting, Madrid, Spain 17-19 September 1997

International Atomic Energy Agency, Vienna (Austria)

IAEA-TC--903.3, pp:491-526