Heat Transfer in Pressurized Water Reactor Components Most Often Subject to Thermal Shock

Experimental studies have been performed to quantitatively analyze heat transfer in the three pressurized water reactor (PWR) cooling system components subject to the greatest thermal loads (i.e., charging, safety, and pressurizer nozzles). Flows are so complex in these components that thermal boundary layer theory cannot be used to predict heat transfer analytically. Heat transfer parameters must be ascertained

1. to calculate stresses so that component service life can be predicted
2. to select the thermal sleeve design that provides the greatest thermal protection
3. to determine flow rates resulting in minimum thermal shock.

Heat transfer coefficients and fluid temperatures have been determined for all possible flow conditions during PWR operation. In charging line and injection line nozzles, flowinduced turbulence prompts injection and main fluids to mix by reducing the amplitude of the thermal shock. However, this turbulence gives rise to temperature fluctuations that may cause a thermal fatigue cycle, thus producing component failure. The greatest thermal protection (at acceptable vibration levels) is provided by a thermal sleeve whose main fluid end is saddle shaped. When fluid is injected into the pressurizer, injection and pressurizer fluids do not mix. During inlet, there is a horizontal interface between the two fluids that rises over a given period of time. Every point in the pressurizer head is subjected to a thermal shock as the horizontal interface passes by. When inlet flow follows outlet flow, the interface descends gradually, causing renewed thermal shock in the opposite direction. Since forces of gravity have a greater effect on flow than forces of inertia, a nonleaktight thermal sleeve has no part to play in protecting the pressurizer nozzle as regards temperature. On the other hand, it reduces the value of the heat transfer coefficient.