In sodium, boiling exhibits quite different characteristics than in water and stable boiling does not appear possible at reactor flow rates. The specific volume of the vapor at low pressure is high and it requires very high

f See Tilbrook and Macrae (10).

velocities for stability; higher than can be provided by available pump designs.

So in a sodium cooled system, inadvertent boiling is presumed to lead rapidly to a bulk-boiling region in which an annular flow representation is the best model. In this model the sodium vapor expands against the inertia of sodium liquid slugs above and below it. The value of Tc is constant at the saturation value because all the heat goes into converting more liquid into vapor. The bubble can extend outside the core into a region of signifi­cant condensation (Fig. 1.10).

The mathematical representation uses the conservation equations of mass and energy for the saturated region to define the size of the bubble, the velocity of the interface between vapor and liquid, and the pressure within the bubble. This model has no need of special boundary representations between boiling and nonboiling regions, but there are considerable un­certainties. The uncertainties include:

(a) The superheat value at which boiling occurs, which may range as high as 500°F but is expected to be as low as 30-50°F in a reactor environ­ment.

(b) Heat transfer data and the conditions under which a liquid film is retained on the fuel-pin cladding adjacent to the bubble.

(c) Condensation effects within a real system.

The condensation that the bubble experiences outside the core is most important since this defines the mode of sodium-vapor bubble collapse, which allows the sodium liquid slugs to reenter the core. This gives rise to
presumed chugging motion (see also Section 4.4.2) in which the vapor bubble alternately grows and collapses until fuel failure results from the reduced heat transfer.

However boiling within a sodium-cooled breeder is an accident condition and may give rise to rapid fuel failure. Thus, in a design model of the core, sodium boiling would not be included and the design would almost certainly have considerable margins before boiling might be initiated. For a 35 psi cover gas system the sodium might boil at 1850°F whereas the highest coolant temperature would be approximately 1200°F, leaving a temperature margin of over 600°F.