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Astronuc
Staff Emeritus
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2023 Award
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theCandyman said:Isn't flux a given measurement?
What is the definition of flux? The number of neutrons passing through a unit area per unit time (n/cm2s), and it can vary between 0 (never negative) and whatever value can be achieved in a transient before the fuel and reactor geometry become disrupted.
At zero power, the neutron flux is due to whatever spontaeous fissions of certain transuranics and the few fissions they induce, which could be on the order of 10,000 - 100,000 n/cm2s, as compared to full power where the flux is on the order of 1013 n/cm2s.
The objective in having a startup source is to provide the neutron detectors with a strong enough signal to monitor the criticality of the core. In subcritical configuration, the neutron activity is a function of the source strength. When a reactor has gone through some cycles, and some of the fuel has achieved some reasonable level of burnup to where there is sufficient transuranics, a sourceless startup might be possible. If a reactor has incore detectors (IIRC all commercial plants were built initally with ex-core detectors), it is more likely that one could do a sourceless startup.
As for targeting particular eigenvalues, one can only expect the same eigenvalue in a subsequent startup if all initial conditions are the same, i.e. the core configuration is more or less in equilibrium, which means the same core average burnup (efpd), same power history, same burnup distribution, same feed batch size and enrichment, same burnable absorber content and distribution, same temperature spatial distribution. In reality, this never never happens.
As for reactor design, in a sense doing a core design is much like reactor design, but with many parameters already determined, e.g. the number of assemblies and their geometry (lattice), coolant temperature, control element configuration. Some of the key variables in a core design include energy, batch size, enrichment and distribution, and burnable absorber (either discrete or integral) and distribution. A core designer however does not go back and re-write the physics of the lattice code (CASMO, Phoenix, WIMS, TGBLA) or core simulator (SIMULATE, ANC, Panther, POLCA, MIRCOBURN, PANACEA).
Even today, the nuclear physics codes are still tweaked, particularly with respect to handling the presence of burnable absorber adjacent to guide tubes and water rods where the moderation is more significant.
Presently, ANL is conducting a program - National Numerical Reactor - which uses multigroup transport theory to do a core simulation. This has been coupled with a sophisticated CFD code which calculates the coolant properties, which then provides the input for the moderation of the core.