Abstract
Understanding the effect on neutronics of the changing physical parameters of a pile is a key element in safety analysis. These effects are known as “neutron feedback” and have various and complex origins. In industrial reactors, a high moderator temperature is required either to vaporize water or to increase the thermal efficiency of the reactor. This high temperature implies significant fluctuations in the fuel temperature, which in turn affects cross sections via the Doppler effect. Furthermore, the moderator density also varies significantly, thereby influencing neutron slowing-down.
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Notes
- 1.
If the balance is carried out at equilibrium, the number of neutrons lost within an energy interval is given by: ∫Σ t (E) Φ(E) dE = ∫ dE ∫ dE′ Σ s (E′ → E) Φ(E′) + ∫ vΣ f (E) Φ(E) dE. Given that the resonance width is narrow compared to the mean energy loss per collision (NR hypothesis), the second term in the balance is almost constant. Thus, Φ(E) = cst/Σ t (E).
- 2.
Lahoussine Erradi : Etude des effets de température dans les réseaux caractéristiques des réacteurs nucléaires de la filière à eau ordinaire [Study of the effects of temperature in characteristic networks of light-water nuclear reactors], PhD, Université d’Orsay (1982).
- 3.
238 Pu = 2.10, 239 Pu = 54.50 %, 240 Pu = 25.00 %, 241 Pu = 9.30 %, 242 Pu = 6.40 %, 241 Am = 2.70 %, depleted uranium support 0.25%
- 4.
Gérald Rimpault : Etude de l’effet en réactivité de vidange de sodium dans les expériences critiques à neutrons rapides, transposition aux réacteurs de puissance [Study of the effect on reactivity of sodium voiding in critical fast-neutron experiments, transposition to power reactors], PhD thesis, Université d’Aix-Marseille (1979).
- 5.
Loïck Martin-Deidier : Mesure intégrale de la capture des produits de fission dans les réacteurs à neutrons rapides [Integral measurement of the capture cross-section of fission products in fast-neutron reactors], PhD thesis, Université d’Orsay (1979).
- 6.
Philippe Coppe : Etude de la représentation des produits de fission dans les réacteurs de la filière à neutrons rapides [Study of the representation of fission products in fast-neutron reactors], PhD thesis, Université d’Orsay (1978). The weak absorption of fission products (the highest capture cross section in the SuperPhenix spectrum is for samarium 151 and is barely 4 barns!) must be compared to the high burn-up (100,000 MWd/ton) reached in fast reactors. In SuperPhenix, for a 300-day cycle, the weight of fission products is estimated at 2300 pcm, or 75% of the total loss in reactivity.
- 7.
J.M. Fabre, Code Jason note de présentation [Presentation note for Jason code], Note EDF E-SE-PN-86-146-A (1986)
- 8.
J.C. Lefebvre, R. Seban: Nouveau modèle de contre-réactions neutroniques appliqué au calcul des cœurs de réacteurs [A new model of neutronic feedback applied to reactor core calculations], Note EDF E-SE –TB 81-117 A, 1982.
- 9.
Some authors, like R. D. Mosteller in Impact of moderator history on physics parameters in pressurized water reactors, Nuclear Science and Engineering, 98, pp. 149–153 (1988), use the term historical water temperature, defined in the same way as the historical density. Similarly, others use a historical boron concentration or a historical fuel temperature. The moderator density effect is predominant over the others.
Bibliography
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Marguet, S. (2017). Neutronic Feedback. In: The Physics of Nuclear Reactors. Springer, Cham. https://doi.org/10.1007/978-3-319-59560-3_16
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DOI: https://doi.org/10.1007/978-3-319-59560-3_16
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