A SCOPING CFD EVALUATION OF RIA CONSEQUENCES
by
W.W. Yuen and T.G. Theofanous
Introduction
We were asked by Dave Diamond (BNL) to examine pressure pulses potentially generated by fine scale fragmentation of fuel in the core of a nuclear reactor undergoing a postulated RIA. The core region involved was given as ~ 0.9 m, near the core exit, of 20 central fuel assemblies. The amount of fuel fragmented was initially specified as an outside rim region of the fuel pellet 0.1 mm thick. Later on, Ralph Meyer (NRC) requested a calculation involving a much more significant fraction of the pellet, perhaps even 100%. Meanwhile, on our own initiative, we had carried out calculations for a 50% fraction. Since these results are indicative, in the interest of time we go forward with this report. A 100% fragmentation case can be added later, if desirable.
Specification of the Calculations
Calculations were carried out with an adapted version of the ESPROSE.m code. The code is a multidimensional, multiphase dynamics code, originally developed for the calculation of energetics in steam explosions (Theofanous et al., DOE/ID-10503). The key characteristics of the calculation are as follows.
Figure 1. The geometry considered. The fuel/coolant ratio in the core is taken as 1:1. (The "dark" area represents the disrupted region.)
Results and Discussion
The results of main interest are pressure wave dynamics, coolant and fuel motions, and fuel displacements. The cases considered are denoted by the main parameter, which is the fraction of fuel fragmented. Thus we have Run 2% and Run 50%.
Sample evolutions of radial pressure distributions on a horizontal chord going through the center of the disrupted region are shown in Figures 2a and 2b. For complete animations, click here P2%, P50%. The key difference is that Run 2% remains (the mixing region) in single phase, so the pressure actually undershoots as a result of the outwards imparted motion, while Run 50% goes two-phase, which sustains the pressure at the saturation level (determined by the amount of thermal energy in the fragmented fuel). We note that the pressure gradients are very significant in magnitude, and highly transient.
Sample evolutions of radial coolant and fuel velocities can be seen in Figures 3a and 3b. We can readily see that fuel motions in Run 2% are very slow and of very limited duration, while for Run 50%, they are quite substantial. Fuel displacements for various radial positions and times for Run 50% are shown in Figure 4. It is obvious from these that the 50% case could lead to rather significant distortion of the core rod geometry and hence, potentially, a significant impact on control rod operation.
Concluding Remarks
As expected, the consequences of RIA on core geometry depend on the amount of fuel assumed to have been dispersed. The 2% case seems to be rather inconsequential, while the 50% case could be very severe. Potential "relief" of this severity could be found by relaxing any of the following conservative assumptions (they are listed in perceived order of priority).
On the other hand, it may be of interest to examine the structural integrity of the core barrel and core supports (at both ends) under the action of pressure waves.