Tamotsu Hayase

Boiling water reactor control rod programming using heuristic and mathematical methods

OPROD, a computer code for automatic generation of control rod programming that has successfully been applied to an older boiling water reactor (BWR), has experienced some difficulties when applied to a BWR of larger power density and stronger heterogeneity. To improve the performance, a heuristic algorithm that is derived from accumulated experience has been introduced to search for a feasible rod pattern that satisfies all constraints. Application of this algorithm to an initial cycle of an 800-MW(e) BWR of high heterogeneity has been very successful. It has been demonstrated that the proposed algorithm is capable of finding a feasible rod pattern, even starting from an all-rods-out pattern. Some improvement was also made in the method of approximation programming (MAP) algorithm. The temporal constraint relaxation method is shown to be effective in finding an optimal control rod pattern in MAP starting from a guess pattern that is not feasible.

Multiregion Neutronics Model Based on Coarse Mesh Nodal Coupling Method for Transient Analyses of Boiling Water Reactors

A coarse mesh nodal coupling method, a well-known technique often used in steady-state neutronics analysis of light water reactors, is extended to a problem of transient phenomena of boiling water reactors (BWRs). Spatial collapse is attempted to develop a multiregion neutronics model and the associated axially one-dimensional and one-point models. These models are numerically solved through the use of two approximations, quasi-static and prompt jump. The results as applied to a reference BWR core for transient analyses, initiated by artificial thermal-hydraulic disturbances, are presented to show the practicality of the approach. The nature of the optimal weighting function necessary for the spatial collapse and for the quasi-static approximation is also discussed. Refs.

Three-dimensional first-order perturbation calculation method for reactivity changes due to core deformations of fast breeder reactors.

A three-dimensional diffusion calculation method has been proposed to rapidly and accurately calculate reactivity changes of LMFBRs caused by assembly displacements in accidental events. The method requires shorter computation times and provides almost the same accuracy as a conventional direct eigenvalue calculation method. In this method, changes in macroscopic neutron cross-sections and diffusion coefficient are defined so that changes in both region volume and material composition can be treated in a mesh-centered finite-difference program under the same coarse mesh division as used for the normal, non-deformed core. Reactivity changes are calculated from the above-mentioned changes by the first-order perturbation method using normal and adjoint neutron fluxes calculated beforehand for the normal core. The method was applied to deformations of a 1,000-MWe LMFBR core. Reactivity changes calculated by the method agreed within 0.4% with those by a conventional direct eigenvalue calculation method, while ...

Improvement of the Damaged Thermowell of the Monju Fast-Breeder Reactor Based on Vibration Analysis:

In December 1995, the thermowell in the secondary cooling system of the Japanese fast-breeder reactor Monju was damaged, resulting in a sodium leak. Sodium flow was thought to have produced a vibration in the thermowell that then caused high-cycle fatigue that eventually destroyed the thermowell. Using the finite element method, the natural frequency of the thermowell was found to be approximately 230 Hz, which is in the region of resonance of the fluid vibration. Next, in a theoretical study, the authors altered the position at which the diameter of the thermowell changes and examined the resulting natural frequencies. They also studied the effects of thickness and diameter on the natural frequency of the thermowell and examined the case for a truncated conical thermowell. The results indicate that production of a thermowell that is not affected by vibration due to resonance is possible. This paper recommends improved shapes for the thermowell.