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Advanced Reactor Simulations

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Selection of Base Software

Instead of development from scratch, extensive search in the industry has been conducted and a number of vendors have been contacted over the past two years. Evaluating according to the above areas, a PC-based transient analysis code, PCTRAN[2], developed in the U.S.A, was selected. Based upon a thermal-hydraulics model using a reduced set of fluid nodes (compared to full scope simulators) and graphical man-machine interface, various models have been installed at a wide range of plants.[3] The software was further modified considerably during the past year to meet all the IAEA set criteria. The product software's constituent models are described below:

ARS uses a point-kinetics model with one delayed neutron group. Reactivity changes due to control rod movement and boron concentration changes as well as feedback from voids, moderator temperature and fuel temperature (Doppler) coefficients are simulated. The significant of the feedback coefficients are reactor type and design specific. After reactor shutdown, a decay heat table provides the heat input.

The basic thermal hydraulics of ARS are first principle in mass and energy balance, ensuring credible and realistic simulations. A non-equilibrium pressurizer model is used for a PWR. A drift flux model is used for calculating the void fraction and water level in a BWR. Temperature distribution in the primary coolant is provided by a mechanistic model of the coolant flow covering both forced and natural circulation.

The containment model involves solution of mass and energy balance equations including the effect of non-condensable.


Man-machine Interface

Figure 1

The simulator model developed for PWRs similar to AP600 will be presented here. In the ARS the different reactor types are simulated in a similar fashion with respect to man-machine interface considering the specificity of each plant. The reactor primary coolant has two steam generators and four coolant pumps (2x4 configuration). Figure 2 is a sketch showing the primary components and the passive emergency core cooling system. It will be simulated by a plant mimic, displayed as Figure 1.

On the mimic's right hand side, there are panels for the controls of power demand, steam generators and pressurizer which are common for all PWRs. Operation is defaulted to the automatic mode. If at anytime the operator decides to take any of the control system into manual operation, just clicks on the corresponding "M" button and a window will show up. By entering a new set point, activating the manual action and closing the window, the reactor will then run on the manual mode. The Auto mode can be returned by clicking at "A".

At the bottom of the mimic, status of the Reactor Protection System (RPS) and Safety Feature Actuation System (SFAS) are displayed. Reactor will be tripped automatically upon conditions exceeding any of the RPS set points. The corresponding symbol will turn into red. The reactor/turbine can also be tripped manually by moving the mouse and clicking at the "T" (for trip) button.

At left there are panels for the passive emergency core and containment cooling systems. There is a In-Containment Refueling Water Storage Tank (IRWST). It provides heat removal from the Passive Residual Heat Removal (PRHR) system by natural circulation. Three stages of Automatic Depressurization System (ADS) on top of the pressurizer to relive steam into the IRWST during a small break loss-of-coolant accident. The fourth stage ADS valves are connected to the hot legs and relieve steam directly into the containment atmosphere. Instead of an active HPI system, which requires safety graded pumps and emergency AC power supply, there is a passive system of Core Makeup Tanks (CMT) that fill into the Direct Injection Line (DVI) by gravity. The Accumulators (ACC) are filled with borated water with pressurized nitrogen. Water in the IRWST can also be drained into the DVI when the primary coolant is completed depressurized. For containment cooling, the RB spray works on the outside surface of the containment steel lining. And the vent is opened to draw air draft from the lower part of the containment concrete wall openings.

Activation of the passive systems are indicated by the valve color changes and digitally displayed flow rates. Operators usually do not have to interfere other than verification and observation.

Figure 2

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