Introduction

The dynamic characteristics of the Canadian SCWR are different from those of the CANDU

reactor or ACR. To examine its unique dynamics, it is necessary to have a dynamic model. The

dynamics of the Canadian SCWR depend on the reactor core, and require coupling of the reactor

kinetics with the feedback reactivity. The dynamic model of the Canadian SCWR can be divided

into a thermal-hydraulic model and a reactor kinetics model. The thermal-hydraulic model is

focused on the hydraulic flow influenced by the external heating. The reactor kinetics model is to

calculate the reactor power, that is, the heat generation rate in the fuel rods. The reactor kinetics

model supplies the reactor power to the thermal-hydraulic model as the heat source, while the

thermal-hydraulic model provides the reactivity feedback, which is related to the thermal

parameters of the reactor, to the reactor kinetics model.

The Canadian SCWR is still at the conceptual development stage. There is no real plant built yet.

CFD method has been extensively adopted as an effective and powerful approach for thermal-

hydraulic behaviour analysis at the supercritical conditions. The results obtained by the CFD

simulation are widely accepted. Therefore, CFD numerical simulation is adopted here as a

verification tool for the thermal-hydraulic model.

The modeling process consists of several steps. Firstly, the thermal-hydraulic model of the

reactor is constructed based on the fundamental conservation equations of mass, energy and

momentum. Secondly, the transient behaviour of the Canadian SCWR is obtained with the

transient simulation of the thermal-hydraulic model, a set of linear dynamic models are obtained

using a least-square system identification method. The development of the thermal-hydraulic

model is performed separately for the following reasons:

a) The thermal-hydraulic model can be separated from the reactor kinetics model. The

reactor power is considered as an input to the thermal-hydraulic model.

b) It is convenient to use the CFD method to verify the thermal-hydraulic model. The CFD

method is mainly focused on the thermal-hydraulic analysis. User defined function will be

needed and computational time may be increased if the reactor kinetics model is included in the

CFD simulation.

c) It is convenient for the linear dynamic model development using system identification

techniques. The system identification is carried out on the thermal-hydraulic model only. If the

system identification is applied to the dynamic model with the coupling of the thermal-hydraulic

and reactor kinetics, the existence of the internal feedback will cause difficulties during the

system identification process.

For the thermal-hydraulic modeling, a moving boundary method is adopted to solve the problems

due to the unique characteristics of thermal properties and heat transfer of supercritical water.

The supercritical water exhibits significant difference in its thermal properties before and after

the pseudo-critical point as shown in Section 1.1.1. In the modeling process, the thermal

properties inside each control volume should be as consistent as possible. If the boundary of a

control volume is constant, that is, the length of the control volume does not change, it is

possible that there are two thermodynamic states in one single control volume. Normally, for

each thermodynamic state, there are different equations to calculate the related parameters. In

For the unique thermal physical properties of supercritical water, the empirical correlations to

predict heat transfer coefficient are divided into several sub-correlations by the correction factor.

As shown in Section 1.1.2, the correction factor F usually depends on the specific heat ratio and

the density ratio. The discontinuity on the heat transfer coefficient can be caused by the

correction factor F. When the boundary of a control volume is close where the discontinuity may

happen, numerical oscillation can occur. Therefore, the moving boundary method is adopted.

The boundary positions are set at the pseudo-critical point and where the discontinuity in the heat

transfer function may happen. In every control volume, there is only one thermodynamic state.

Thus, the calculation errors are reduced. Under the same accuracy requirement, the amount of

control volume can be reduced and the computational burden can be reduced. The numerical

oscillation can be avoided.

The rest of this chapter is organized as follows: in Section 3.2, a detailed thermal-hydraulic

model of the Canadian SCWR using moving boundary method is developed. In Section 3.3, the

thermal-hydraulic model is validated using commercial CFD code: FLUENT 6.1. In Section 3.4,

the benefits of adopting the moving boundary method are illustrated by comparisons with the

fixed boundary method. In Section 3.5, the dynamic model of the Canadian SCWR is obtained

by coupling the thermal-hydraulic model with its reactor kinetics model, and the steady-state and

transient simulation results are also presented. To illustrate the uniqueness of Canadian SCWR,

the dynamics of the Canadian SCWR are compared with those of ACR-700. In Section 3.6, a set

of linear dynamic models around the full power operating point are developed using system