Presently, the sg's (chart 01) get and remains voided from 300 s.
In the course of phase 4, the core (chart 02) is brought permanently to a subcritical, zero power condition.
The small peaks observed on the qcn1 plot are not relevant and are related to sg voiding model.
The slow reactivity decrease at end of transient result from the increase of boron ppm (chart 03).

u2c7 (chart 04) peaks at about the same time and level as for case 1.

Pressure p3n (chart 01) in the sg, drops steadily till a final value of only 2 bars, whilst the primary pressure p3 (chart 06) falls down to 10 bar.
Pressu starts refilling progressively from 400 s (chart 05, chart 07): remind that v2p is the volume of the pressu vapor region.
and pressure p3 (chart 06)tends to raise again to about 20 bar thanks to compression of the steam region.

(chart 08) shows the (normalized to 1) power profile fci at sec= 170, (u2c7 peak), at sec=300 (end of gv voiding) and at 310 s. The large fqc (chart 09) peak at that time is of no concern because q2c has already decreased.

Observing (chart 10) the water quality x2 in the primary system is of prime interest: dome (x2d) starts boiling firstly, followed by core upper node (x2ck(20)and core outlet (x2o).
Core bottom (x2ck(1)) remains subcooled.
At completion of sg voiding all the core is close to saturation, which explains the sharp reactivity drop.

(chart 11) exhibits an enlarged view.
At time of min margins (around 180 s), most of the power is produced in the first 7 or 8 bottom nodes, which remain sub cooled. Around 560 s, pressure increase due to pressu refill causes the whole primary to become subcooled again.

(chart 07) shows that, after refill, the pressu region becomes overheated (x2p > 1), (t2p> tl0). This is because the condensation processes have not been enabled and the pressure is still too low for triggering spray.

As the major part of core power is generated at the bottom, the representation of the bottom reflector effects should deserve attention.
Node boiling, even minor, has a self correcting effect on local power. It is therefore expected that local boiling at or near hot channel entrance will also have an effective beneficial effect on fqc and peak power. This effect could be investigated by refining and extending the neutronic tables, by enabling thermal neutron group correction and by comparing results obtained with open and closed hydraulic channels at entry.

At first sight, hot pellet temperature appears to be of more concern than dnbr (chart 04), because of the lower core location of power generation?

Another observation of interest: around the u2c7 peak time (160 s), the core is nearly critical and stationary (q2c=f2c).
This supports the feasibility of checking the safety margins by means of a design, static, fine mesh 3-d neutronic program fed with global (pressure) and inlet conditions (wec, hec) extracted from the transient run.
If the SAFPWR and design neutronic models are consistent, a critical power search should lead to same q2c of about 700 e6.

Peak value of 900 K for u2c7 (chart 04) is directly related to the assumed fxy=1.2 in hot channel, without any credit for boiling feedback, and may thus be over conservative.
If this is confirmed, we could tentatively be able to conclude that the core would be able to sustain, thanks to the sole action of its intrinsic reactivity feedback's, the limiting, hypothetical, mslb with all insolation valves failing to close.

Of course, it should also be verified if a 4% reactivity margin can be relied upon.

An additional interest of the symmetrical case is that it provides simple model for testing, in the following exercises, the effect of various parameters.