Neutrons characteristics

Hidaka, H. Holliger, P. 1998. Geochemical and neutronic characteristics of the natural fossil fission reactors at Oklo and Bamgombe, Gabon. Geochimica et Cosmochimica Acta, 62, 89-108. 

The analytical and experimental methods applicable for justification of neutronic characteristics of spent removable parts (SRP) of nuclear submarine liquid metal reactors (NS LMR) are briefly reviewed. 

Belyaev, S.T., Bondarenko, L.N., et a/. (1991) Methods and Techniques in Studying Neutronic Characteristics of Fuel-Containing Mass in The Fourth Unit of Chernobyl NPP, lAE preprint 5312/3, Moscow (in Russian). 

NAA Neutron Activation Analysis Bulk, >0.5 g Thermal neutrons Characteristic y-rays, (-1 MeV) Bulk Trace concentrations (ot isotopes) ot elements trans. metals, Pt-group detection limit 10 -10 atoms/cm 43... 

NAA Bulk, >0.5 g Thermal neutrons Characteristic Bulk — Trace concentrations (of isotopes) of 43... 

In the first half of 1994 there were measured characteristics of the fast reactor core in the center of which a representative insert of 400 1 volume was set up containing inert diluent (alximinium oxide) - based fuel (the BFS-58 - assembly). In the insert fuel U-238 is absent and average fuel density is noticeably lower than that in the traditional cores. These circumstances result in substantial variation of neutron characteristics for the core composition. 

The reactor that first succeeded in generating electricity was not the LWR, which has been commercialized, but the FR. Thereafter in the USA, many FRs such as the Enrico Fermi Atomic Power Plant, EBR-II and FFTF were constructed and operated. Test data have been accumulated from these reactors through R D for core fuel, structural materials, neutronic characteristics of core, safety, and so on. CRBR with 380 MWe power output passed the safety review in 1975, and the construction began. However, the nuclear policy changed during President Carter s administration, and the constmction was called off. 

The natural properties of lead coolant and mononitride fuel and the neutronic characteristics of the fast reactor combined with the design of the core and cooling circuits raise the BREST reactor to a radically different level of safety and provide for its stable behavior without involving active safety features in the severe accidents unmanageable in any one of the existing reactors, such as ... 

Design and neutronic characteristics of the reactor core ensure negative power, temperature, and void reactivity coefficients. As a result, self-limitation of the reactor power at the reactivity accidents and transients without scram takes place. Reactor self-control properties enable to change reactor power in the range of 20 - 100% of rated power (No) at a rate up to 0.5%No/s without control rods displacement and just for automatic control of feed water flow rate. 

An important factor in the criticality of the actinides is that those containing even number of neutrons characteristically have a fission threshold and experience little or no subthresbold fission. Consequently, the inelastic scattering process, which degrades the neutron spectrum, is particularly Important in determining the criticality of these actinides. Also, the presence of small amounts of moderator has the same effect on the neutron spectrum and can easily reduce k of these nuclides to less than unity. This is Illustrated in Fig. 1, where calculated k values are shown as a function of metal atom concentration in water. For the even nuclides, Pu, Pu, em, and Am, k rapidly becomes less than one upon the addition of water to the theoretical metal system and criticality is no longer possible for these nuclides. 

Flattop is at the opposite extreme from our unreflected critical assemblies, inasmuch as it provides a thick natural uranium reflector around spherical cores of uranium (93.2% and 98.1% U) and plutoniiun (94.9% Pu). A complete set of data has been obtained for the neutronic characteristics of each of the three core materials. Critical masses are 6.0 kg for the Pu (94.9%) ball, 5.7 kg for the (98.1%), and 17.8 kg for the (93.2%) ball after correction to a completely spherical geometry. Flattop is used for replacement measurements and service irradiations. 

The conformity of the neutronic characteristics to the design data has been established, which may serve as a validation of the adopted calculation models and codes ... 

The choice of make-up fuel is also crucial. If spent fuel from light water reactors (LWRs) is used within a short period after its discharge, then any delay in operation of the SPINNORA SPINNOR may produce an unfavourable effect on the neutronic characteristics of the core. Therefore, special validation, including nuclear data adjustment based on integral experiments, is considered necessary. 

Delayed Neutrons - Those neutrons with an eaisslon time after the fission process that is delayed by the half-life of certain fission products (precursors). An arbitrary time lini3.t exists (10 sec.) to distinguish between prompt and delayed neutrons. The delayed neutron fractions and half-lives are different for the various fissionable isotopes. This means that the delayed neutron characteristics of a pile are exposure-dependent. 

The thermal hydraulic design of CAREM reactor core was carried out using an improved version of the 3D two fluid model realized in the THERMIT code. In order to take into account strong coupling of the thermal-hydraulic and neutronic characteristics of the core, THERMIT was linked with the neutronic code CITVAP. This coupled model makes it possible to produce a 3D map of power and thermal-hydraulic parameters at any moment of the bum-up cycle. 

Moderation of neutrons, characteristics of moderators, variation of moderation (and cross section) with temperature. 

Bednyakov, S. et al. Investigation of Neutron Characteristics of Minor Actinides in Experiments at the BFS Facility and Fast Reactors. Proc. of Int. Conf. on Reactor Physics and Reactor Computations, Tel-Aviv, Israel (1994). 

Chain reaction a process that initiates its own repetition in a fission chain reaction a fissile nucleus absorbs a neutron and fissions spontaneously releasing additional neutrons Characteristic waste hazardous waste that exhibits one of four characteristics ignitability, reactivity, toxicity, or corrosivity... 

Attempts at materials diversion from spent fuel would encounter all the challenges discussed above, plus performing the required activities in a high radiation field. The enrichment step could be eliminated by chemically separating out plutonium. However, the isotopic content of the plutonium in the spent fuel is not attractive for weapons use due to the neutronic characteristics of the GT-MHR LEU cycle. The quantity of fissile material (plutonium and uranium) per GT-MHR spent fuel element is low (50 times more volume of spent GT-MHR fuel elements would have to be diverted than spent light water reactor fuel elements to obtain the same quantity of plutonium-239). 

Neutronic characteristics of MSRs have been explored in the literature. Flow effects were considered when calculating the effective multiplication factor and fast neutron, thermal neutron, and delayed neutron precursor distribution of the liquid-fuel MSR based on the multigroup neutron diffusion equation and delayed neutron precursor conversation equation (Zhang et ah, 2009b Cheng and Dai, 2014 Zhou et ah, 2014). Spatial kinetic models were developed for better neutronic analysis of the MSRs (Zhang et ah, 2015 Zhuang et ah, 2014). 

The general treatment of the time-dependent response of a reactor to a local change in reactivity is quite complicated and involves explicitly the geometry of the reactor. 12 if the reactivity change is distributed uniformly over the reactor volume, only the fxmdamental space mode of the reactor is involved (i.e., the initial space distribution of neutron flux is preserved, and the only reactor characteristics needed to describe the response are the neutron lifetime and the delayed neutron characteristics. The same is true for a localized reactivity variation if the time rate of change is low enough and if the response is measured at some distance from the location of the reactivity change. 

Neutronic studies indicate AI2O3, MgO and MgAl204 could be used but with mass penalty and differences in neutronic characteristics... 

The coupling of neutronic and thermal-hydraulic calculations is especially important for designing the Super LWR core. The density change of the coolant (and moderator) is large and sensitive to the enthalpy rise of the coolant as it flows from the core inlet to the outlet. On the other hand, the core neutronic characteristics strongly depend on the coolant and moderator density distributions. 

This section describes the basic design concepts of the Super LWR core including the fuel rod and fuel assembly designs. The core thermal-hydraulic characteristics are unique and strongly coupled with the neutronic characteristics of the core. 

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