Studies of the Criticality of 20% Enriched Uranium Fast Critical Assemblies (FCA-I)

Hirota, Jitsuya; Nomoto, Shoji; Hirakawa, Naohiro; Nakano, Masafumi

doi:10.3327/jnst.6.35

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“…Critical assemblies had different names, different design approaches were applied, but, at the same time, all the test facilities embraced the same general idea -constructing universal instruments allowing simulating fast nuclear reactor cores with different geometries, nuclear fuel compositions, compositions and volume fractions of coolants, with internal and external nuclear fuel breeding blankets at minimum financial and time expenditures. Such test facilities were constructed in the USA (ZPR, ZPPR) (Ishikawa and McKnight 2013;Lell et al 2013), in the USSR (BFS, KOBRA) (Leypunsky et al 1974;Rozhikhin and Semenov 2013), in France (MA-ZURKA) (Bouget et al 1980), in Germany (SNEAK) (Bickel et al 1965), in Japan (FCA) (Hirota et al 1969), and in the Great Britain (ZEBRA) (Rowlands and Zukeran 2013). The main purpose of such test facilities was to allow creating the model of the reactor under design and obtaining, using the model, experimental values, comparison of which with calculated values allowed drawing certain conclusions with regard to the reliability and uncertainties of the design values of neutronics parameters of the modeled reactor design and entering appropriate corrections of these values in timely manner if the necessity emerged.…”

Section: The Formulation Of the Problemmentioning

confidence: 99%

Heterogeneous effects in simulating a fast nuclear reactor on the BFS test facility

Kazansky¹,

Karpovich

2019

NUCET

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Simulating fast neutron reactor cores for comparing experimental and calculated data on the reactor neutronics characteristics is performed using zero power test stands. The BFS test facilities in operation in Russia (Obninsk) are discussed in the present paper. The geometrical arrangement of materials in the cores of the simulated reactors (fuel pins, fuel assemblies, coolant geometry) differs from the simulation assembly on the BFS. This can cause differences between the experimental results obtained at the BFS and theoretical calculations even in the case when homogenized concentrations of all materials of the reactor are thoroughly observed. The resulting differences in neutronics parameters due to the geometry of arrangement of materials with the same homogeneous concentrations are referred to as the heterogeneous effect. Heterogeneous effects tend to increase with increasing reactor power and its size, mainly due to changes in the neutron spectra. Calculations of a number of functional values were carried out for assessing the heterogeneous effects for different spatial arrangements of the reactor materials. The calculations were performed for the following cases: a) heterogeneous distribution of materials in accordance with the design of a fast reactor; b) heterogeneous arrangement of materials in accordance with the capabilities and design features of the BFS test facility; c) homogeneous representation of materials in the reactor core and breeding blankets. The configuration of materials in accordance with the design data for fast reactors of the BN-1200 type was accepted as the basic calculation option, relative to which the effect called the heterogeneous shift of the functional value (HSF) was calculated. The effect of neutron leakage on the HSF obtained as the result of calculations using different boundary conditions was estimated. All calculations were carried out for the same homogeneous concentrations of all materials for all the above three configurations. Calculations were carried out as well for the case when plutonium metal fuel was used in the BFS. The values of the following functionals were calculated for different cases of arrangement of materials: the effective multiplication factor (reactivity), the sodium void reactivity effect, the average energy of fission-inducing neutrons, and the ratios of radioactive capture cross-sections to fission cross-sections for 239Pu. The calculations were performed using the Serpent 2.1.30 (VTT, Finland) Monte Carlo software package for neutronics simulations and ENDF/B-VII.0 and JEFF-3.1.1 evaluated nuclear data libraries. The effects of various options of material arrangement on the values of keff were found to be the greatest (about 1.6%) for the case when fissile material in the form of dioxide is replaced with metal fissile material. Homogenization of the composition reduces the keff value by about 0.4%. The average energy of fission-inducing neutrons depends to a significant extent on the leakage of neutrons and the presence of sodium (the average energy of neutrons increases and reaches in the presence of sodium about 100 keV, that is, it increases by about 11–13%). Replacing fissile material metal with its dioxide in the BFS test facility (while maintaining homogeneous concentrations, including that of oxygen) allows reducing the average energy of fission-inducing neutrons by about 60 keV. The highest values of HSF, reaching 65%, are observed when calculation of sodium void reactivity effect is performed with materials distributed homogeneously; however, HSF is equal to 1.5% when calculation of the reactor mock-up assembled on the BFS is performed. In the absence of neutron leakage (infinitely extended medium), the sodium void reactivity effect becomes positive and the HSF is equal to 4–7%. The heterogeneous effect of α for 239Pu noticeably (6–8%) depends only on the replacement of metallic plutonium with its dioxide (maintaining, of course, the homogeneous concentrations).

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Section: The Formulation Of the Problemmentioning

confidence: 99%