The solution of coupled neutronic/thermal hydraulic nuclear reactor calculations
requires the treatment of the nonlinear feedback induced by the thermal hydraulic
dependence of the neutron cross sections. As a result of these nonlinearities, current
solution techniques often diverge during the iteration process. These instabilities arise
due to the low level of coupling achieved by these methods between the neutronic and
thermal hydraulic components. In this work, this solution method is labeled the
Decoupled Iteration (DI) method, and this technique is examined in an effort to
improve its efficiency and stability. An examination of the DI method also serves to
provide insight into the development of more highly coupled iteration methods. After
the examination of several possible iteration procedures, two techniques are developed
which achieve both a higher degree of coupling and stability.
One such procedure is the Outer Iteration Coupling (OIC) method, which
combines the outer iteration of the multigroup diffusion calculation with the controlling
iteration of the thermal hydraulic calculations. The OIC method appears to be stable for
all cases, while maintaining a high level of efficiency. Another iteration procedure
developed is the Modified Axial Coupling (MAC) procedure, which couples the
neutronic and thermal hydraulic components at the level of the axial position within the
coolant channel. While the MAC method does achieve the highest level of coupling
and stability, the efficiency of this technique is less than that of the other methods
examined.
Several characteristics of these coupled calculation methods are examined during
the investigation. All methods are shown to be relatively insensitive to thermal
hydraulic operating conditions, while the dependence upon convergence criteria is quite
significant. It is demonstrated that the DI method does not converge for arbitrarily
small convergence criteria, which is a result of a non-asymptotic solution
approximation by the DI method. This asymptotic quality is achieved in the coupled
methods. Thus, not only do the OIC and MAC techniques converge for small values of
the relevant convergence criteria, but the computational expense of these methods is a
predictable function of these criteria. The degree of stability of the iterative techniques
is enhanced by a higher level of coupling, but the efficiency of these methods tends to
decrease as a higher degree of coupling is achieved. This is apparent in the diminished
efficiency of the MAC procedure. Seeking an optimum balance of efficiency and
stability, the OIC technique is demonstrated to be the optimum method for coupled
neutronic/thermal hydraulic reactor calculations. / Graduation date: 1994
| Identifer | oai:union.ndltd.org:ORGSU/oai:ir.library.oregonstate.edu:1957/36090 |
| Date | 26 April 1994 |
| Creators | Betts, Curt M. |
| Contributors | Kulas, Mary M. |
| Source Sets | Oregon State University |
| Language | en_US |
| Detected Language | English |
| Type | Thesis/Dissertation |
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