Nuclear Engineering 470

Modeling a Steam Generator (SG)

with TRACE

STEAM GENERATORS

• 3 Types of steam generators• Once Through (Babcock and Wilcox)

• Horizontal (Russian VVER)

• U-Tube (Westinghouse, Combustion Engineering, Canada, Japanese, French, German …. )

STEAM GENERATORS

• It is important to model steam generators in LOCA analysis.

• Steam generators provide a significant amount of cooling during a LOCA event.• It is possible for the primary side pressure to be lower than the secondary

side pressure causing the secondary side to become a heat source.

• Steam generators provide reflux cooling. Steam flows from the core, through the hot leg, and into the steam generator. The steam is condensed in the Tubes and flows back down the tubes (counter current flow) the hot leg into the reactor vessel (horizontally stratified counter current flow).

STEAM GENERATORS

• Has anyone found a steam generator component in TRACE?

• There isn’t one, you have to build a steam generator model like you would build a plant model.• Pipes Components

• Separator Components

• Heat Structures

• A good steam generator model will closely approximate:• Steady State Performance

• Transient Performance

STEAM GENERATORS

• Primary-Side• Performance Parameters of Interest: Pressure (~15 MPa) and Temperature

change from the Inlet to the Outlet (40 K).

• Straight forward modeling: The primary-side flow field is typically modeled using a single effective flow channel modeled with a pipe component.

• For some accidents (voiding at the top of the tube bundle) you should use more 1-D components to model the flow path for the short, medium, and long tubes in a bundle.

• 1-D components model the inlet and outlet plenums as well as an average tube geometry.

• Hydraulic diameter input is for one tube.

• Flow area and wall area input is the total for all tubes.

• Because of stress-corrosion cracking, the total number of tubes in use may not equal the total number of tubes.

STEAM GENERATORS

• Secondary-Side• Performance Parameters of Interest: Outlet Pressure (~6 MPa),

Temperature, Moisture Content, Recirculation Mass Flow, Steady-State Liquid Inventory

• The secondary-side is typically modeled using a pipe or tee component and a separator component. However, it can be modeled using a vessel component.

• For some accidents it is possible to get non-physical thermal stratification when 1-D components are used. Look for Lower temperatures at higher elevations in the bundle region. Create circulation paths with 1-D or 3-D components

• Components model the boiling tube bundle, steam dome, downcomer, feedwater connection, and beginning of the steam line.

• An adjustable gate at the bottom of the downcomer allows for the tuning of the recirculation flow. This gate is modeled with a loss coefficient at the bottom of the downcomer to match the recirculation ratio (mdot recirculated/mdot steam > 1 typically).

STEAM GENERATORS