Nuclear Reactor Basic Principles

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  • Topic: Neutron, Neutron moderator, Nuclear fission
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The Nuclear Reactor Core Basic Principles

Wang Ming ( 2012380022)
(Filippo Fiori)
1.1The Qualification Process6
1.2The Objective: the On-Transient Qualification by “CNA2 Scaled Calculations”9 1.3Structure of the Report10
2Facility description11
3Test description11
4Scaling approach12
5CNA2 scaled nodalization16
5.1Reference CNA2 Input16
5.2Common Features of CNA2 Scaled Nodalization22
5.2.1Atucha-2 Scaled Nodalization with Moderator System24
5.3Hardware differences between CNA2 and ITF24
5.4Geometry modifications24
5.5Boundary conditions25
6Reference CNA2 scaled calculation27
6.1Steady State Achievement27
6.2Transient results29
6.2.1List of resulting events29
6.2.2Visual comparison of time trends29
6.2.3RTA analysis31
6.2.4Summary judgment31
7Sensitivity analysis32
7.1Sensitivity 1: Calculation with Moderator system32
7.1.1Steady-state achievement33
7.1.2Transient results36
7.2Sensitivity 2: e.g. Kloss at the break37
7.3Sensitivity 3: e.g. Scaling with Kv37

List of Figures
Figure 11: Similarity Analysis (block k) in UMAE flow chart.9 Figure 51: REALP5-3D© “60ch” nodalization of CNA2: sketch of the RPV including the moderator tank.18 Figure 52: RELAP5-3D© nodalization of CNA2: sketch of RCS, SGs, pressurizer and spray system.19 Figure 53: REALP5-3D© “60ch” nodalization of CNA2: sketch of the moderator loops.20 Figure 54: REALP5-3D© “60ch” nodalization of CNA2: sketch of the FW and KAG systems.21 Figure 55: LPIS injection law.26

List of Tables
Table 4—1: Scaling factors for a general KV scaled calculation.12 Table 4—2: Scaling applied for initial and boundary conditions.14 Table 5—1: Model geometry modifications.24
Table 5—2: Reference CNA2 scaled boundary conditions.25
Table 5—3: MCP-1 coastdown.26
Table 5—4: MCP-2 coastdown.26
Table 6—2: Relevant Thermal-Hydraulic Aspects.31
Table 7—1: List of sensitivities32
Table 7—2: CNA2 scaled (with Moderator) steady state results.34

nuclear reactor core basic principles
Neutron balance
Nuclear fission is characterized by the fact that is originated by a neutron and produces neutron, the average number of the neutron that is produced in each fission event ϑ depends of the fissile isotope and form the energy of the incident neutron. The new produced neutron can induced further nucleons fission events in a process called chain reaction. The new produced neutrons in each fission event do not all induce a new fission event, (part of the neutrons can leak from the system or can be absorbed by other nuclei present in the system without originating a fission event), for this reason a chain reaction can be sustainable only if the number of new neutrons is higher than one. It is possible to express the neutron balance for each fission process as:

ϑ=Af+ Ac+FEquation 1
ϑ average number of neutron produced in each fission event.
Af number of neutron absorbed by fissile nuclei inducing fission.
Ac number of neutron absorbed by all the materials without inducing fission.
F number of neutron that leak from the system.
It is clear that a stable chain reaction can be achieved only when Af is equal to one. In the design of a nuclear core is of outmost importance to consider the material to be used, limiting the choice to the material with a low absorption cross section, in particular if the fissile material to be used is natural uranium in which the number of neutron produced by fission of U235 available so sustain the chain reaction is lower than the in case of enriched uranium. The number of neutrons that leak from the system is inversely proportional to the dimension of the multiplying system ( the surface/volume ration decrease), and to the fuel mass present (the production of neutron decrease). It can be now understand that for a set of different material combinations and...
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