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On an Improved Design of a Fluidized Bed Nuclear Reactor - II: Linear Stability and Transient Analysis

Alexander Agung, Danny Lathouwers, Tim H. J. J. van der Hagen, Hugo van Dam, Christopher C. Pain

Nuclear Technology / Volume 165 / Number 2 / February 2009 / Pages 133-144

Technical Paper / Fission Reactors /

A new design of a fluidized bed has been proposed and it has been shown that under steady condition the reactor is able to produce power up to 120 MW. To study the behavior of the reactor under transient conditions as well as its stability, a model describing the coupling of neutronics, thermal hydraulics, and fluidization is applied. The objective of this study is to comprehend whether the reactor is stable under its operational range. Further, knowledge of the extent of operational parameters under large perturbations is necessary for a safe operation.

The stability of the system is investigated by numerical means and is performed by linearizing and perturbing the system around its equilibrium points to form Jacobian matrices. The resulting matrices are further used to obtain the eigenvalues of the system. The system is investigated under variation of mass flow rate, and it is found that within the operational range the eigenvalues are located in the negative part of the phase plane, implying linear stability. Further, the calculated decay ratios indicate a strongly damped system.

Simulations of transient conditions are performed, namely, a step change in coolant flow rate and inlet temperature, representing situations that might occur in real operations of the reactor. The coolant flow rate is varied by ±1 kg/s and the inlet gas temperature is varied by ±10 K from their steady state of 33 kg/s and 543 K, respectively. Another transient is also simulated, i.e., a transient related to noise resulting from stochastic movements of the fuel particles. For this purpose, an additional term is included in the reactivity feedback and modeled as a time-dependent external reactivity. Magnitude of the variance for this simulation is obtained from the preceding static calculations. These simulations show that the total power of the reactor may fluctuate and reach high values. However, the fuel temperature, thanks to passive reactivity feedback, is well below safety limits at all times.

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