The other common design is when there is two-phase boiling flow with direct steam to the turbine. When steam is fed directly to the turbine, again there is a direct relation of the maximum power output to the secondary (turbine stop valve) pressure. The maximum heat removal is due to the circulation of two-phase mixture when the downcomer is liquid and the core is two-phase and heat removal is then totally evaporative

The maximum power output in a natural circulation boiling system without a HX is not derived on the basis when the natural circulation driving head is equal to the two-phase losses. Instead, as we have noted above, the ultimate or maximum power output is set by the onset of flow instability and hence subsequent CHF.

The form of the natural circulation line has been found above to be Np/Ns~ constant for a given downcomer head to core height ratio, L*. The limiting maximum power solutions for the unstable case are, from Equation (12) and (13) and the data comparisons in Figure 3,

Np/N~ 3 (19)

with a residual dependency on the loss coefficients. When there is a natural circulation loop, then the intersection of the stability region with the natural circulation flow is very nearly, for typical design values, when,

Np/Ns~ 2 (20)

By comparing a wide range of parallel or multichannel instability data at high pressure (5MPa) on a Np versus Ns plot, the data do indeed group around a line given by N*= 3+10/Ns (Rohatgi and Duffey, 1994).

Now the maximum two-phase flow in the whole natural circulation loop at intermediate inventories, is given by Equation 15:

1/3

Combining the equations for the maximum flowrate at the instability limiting case, we find the hypothetical maximum core power at this maximum flowrate to be:

This result clearly shows to optimize the design power output, the minimum loss coefficients, and the maximum elevation (driving) head and flow area should be obtained.

The maximum unstable channel power is less than that ultimately obtainable from the boiling flow in a channel which is given by:

which is typically of order 10MW(t). It is therefore important to compare the various limits to see which may be the design constraint.