Neutronic feedback

Natural circulation driven main coolant system. A passive system for normal core heat removal. A 1/3-scale integral system test loop has been constructed and operated at Oregon State University in conjunction with INEEL. It includes all of the primary side and secondary side passive safety systems. Initial testing completed. Current status Primary side stability tests being developed Secondary side control logic under development Neutronic feedback tests being developed. 

Because it is based on well-established LWR technology and implements benchmarked thermal hydraulic safety analysis codes, and because each module is a relatively small reactor, it is possible that all of the information required for design certification may be obtained using a full-scale demonstration of a single module. Of significant interest for design certification would be the impact of neutronic feedback on flow stability, particularly during plant transients. 

The neutronic feedback involves the neutron kinetics, the fuel dynamics, the core thermal-hydraulics, and the reactivity feedback dynamics. The neutron kinetics affects and is affected by the power generation in the fuel, and is directly responsible for the power perturbations. The fuel dynamics affects and is affected by the fuel surface heat flux, and is responsible for the time delays between power production and the response of coolant flow heating. The core thermal-hydraulics affects the power production and the response of the water density perturbations to fuel surface heat flux perturbations. Finally, the reactivity feedback dynamics is responsible for the feedback reactivity due to water density perturbations and fuel temperature perturbations, and is affected by neutron kinetics. 

Since the coolant temperature and hence the density ratio decrease, the density feedback effect is reduced and the stability is improved (neutronic feedback). 

The net effect on the stabihty depends on the balance between the above two effects. In the present analysis, the neutronic feedback effect is found to be dominant over the hydraulic feedback effect. Here, the core power and flow rate are kept constant and the decay ratios are calculated with various feedwater temperatures. The decay ratio is found to decrease when the core inlet temperature decreases as shown in Fig. 5.61. 

Later, after von Neumann died, I saw his memoirs, and he had written a long series of discussions about putting a semiconductor near an atomic pile, producing neutrons which would excite the electrons in the semiconductor. He hoped to get very intense light out of the excited electrons, and he talked about stimulated emission. Apparently, he had sent this proposal to Edward Teller saying, Don t you think we should try this Edward Teller never bothered to answer. And so von Neumann dropped it. But he did not mention a resonant cavity for feedback, and he didn t mention coherence. He was just getting an intense light. 

The nuclear reactor kinetics was modelled using simple point kinetics. The point kinetics model utilised in the calculation was developed as an analogue to the point kinetics module of the RELAP5 code. The number of delayed neutron groups considered was six. A Doppler feedback coefficient of -0.0095 was used. Xenon feedback was also modelled, although due to the time scales considered in this document the xenon feedback is not relevant and has almost no impact on the results. 

Once within the GPT neutronics solver during optimization, an estimate of the CMFD operator of equation (2) for a given perturbed LP is readily obtained by performing a single feedback update using the first-order, reconstructed flux results of equation (5). An estimate of the NEM operator, required for use in the GPT functional, is then obtained as follows ... 

The temperature feedback mechanisms provide a link between the reactor s neutronics and its coolant systems independent of any action of the control system. The size and relative importance of the temperature effects will vary from reactor to reactor, but designers work hard to ensure that there is an overall negative coefficient of reactivity with temperature, which provides for automatic limiting or mitigation of temperature excursions. Some important temperature feedback mechanisms are listed below ... 

Fourth, for comparable reactor systems, the one using a thorium-base fuel will have a larger negative feedback on neutron multiplication with increased fuel temperature (Doppler coefficient) than will a U-fueled reactor. 

To examine its viability, a variety of parametric scans were performed using MCNP-5 [LANL, 2003] on a model of the proposed core. The core was designed with sufficient reactivity to ensure 10 years of full-power operation even when including such negative feedback coefficients as temperature, temperature based expansion, and bumup. The neutronics runs done include ... 

An organisation has been set out with means for guaranteeing satisfactory processing of experience feedback from the plant, other French and Overseas fast neutron reactors and the PWR plants... 

The role of Group R is to organise expenence feedback from French and foreign fast neutron reactors together with PWR plants... 

The RCSS and NCSS must provide the capability to control heat generation with moveable poisons and to control heat generation with inherent feedback. The moveable poison control function is accomplished both with a primary and a diverse secondary moveable poison control, while control with inherent feedback requires a negative temperature coefficient of reactivity. The NCSS and the RISS within the RS, also perform the function of heat generation control by maintaining the geometry for insertion of moveable poisons into the core. The NCSS monitors the neutron flux. 

Preserving acquired knowledge not only includes feedback obtained from the Rapsodie, Phenix, and SuperPhenix reactors, but also knowledge acquired at the time of the EFR Project (1988-1998) that allowed considerable improvements to be made after careful observation of the SuperPhenix reactor in terms of technology, in-service inspection, safety, steam generator design, and neutronics. 

The start-up and operation of the Na-cooled fast reactors Phenix and Superphenix has provided a large amount of valuable experience, as the decommissioning of Superphenix is doing right now. We shall focus here on the feedback of the neutron physics experiments performed in both reactors on the neutron physics calculation tools and methods. 

The validation phase will consist in optimising the oxide core thanks to a large experimental feedback on fuel element behaviour under irradiation (tfaermics, tiiermomecanics, thermodynamics...), physics and neutronics (new adjusted formulaire, code optimization,...) and safety field. As regards the irmovative core, it will consist in proposing a core design at tiie same level of feasability than the reference one. 

Accident studies, or hazard analyses, often assume that reactivity is inserted so as to make the reactor supercritical on the prompt neutrons alone. In such events, sufficient power may be generated to cause feedback effects due not only to temperature changes, but also to shock motions or inertial accelerations of reactor parts. A discussion of some of these effects in a detailed accident study is presented by Brooks in his report to the symposium. 

It is of interest that two additional papers on the subject of Reactor Dynamics in this volume include problems involving expansions in space dependent modes, first, the paper on Tem perature coefficients and stability by Harvey Brooks, and, second, the paper on System kinetics by T. A. Welton. Brooks is also interested in representations of the neutron density with the aid of a multi-mode analysis, but his problem is more complicated than that of this section because of feedback considerations. However, he confines his detailed analysis to a case in which the fundamental mode is dominant and where the effect of higher modes can be treated by a perturbation method. Welton s multi-mode analysis is peculiar to the aqueous homogeneous reactor and bears little resemblance to the corresponding problems treated by Brooks and this writer. Neither Brooks nor Welton appear to be interested in graphical representations of their results. 

In these equations n(r) is the neutron density, c (r) the density of precursors, d the fractional production of precursors per neutron produced in the core. The symbols G and P are linear integral operators, the operator G denoting absorption and transport processes while P denotes production by fission. We will assume that all the reactivity feedback takes place through the operator G, although this assumption is not essential. This assumption may be expressed in the form ... 

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