The implication is that to get 12V output you need to trigger the SCRs in the correct sequence and at a very precise time just before zero crossing occurs. As you will only get a brief 'pulses' of power with relatively long gap between them, the charging current to the capacitor (C4) and current into the load will be extremely high. Your load resistor suggests about 50A load is expected, adding the charging curent to get anything like a clean output would easily double that to 100A and might be considerably more.
Not only does your schematic trigger all SCRs simultaneously, which you have already be warned will short the incoming power, but you also wire the LED sides of all the optocouplers in parallel which is a bad idea. Use a current limiting resistor in series with each LED. You also have no feedback mechanism to regulate the voltage, you will almost certainly find it's output is unstable under different input voltage, load and temperature conditions. The input voltage to U1A also exceeds it's common mode rating by about 50% !
SCR regulated supplies are better suited to high current power supplies where the output voltage is close to the input voltage so the SCR is conductive over a larger part of the AC half cycle. Often they are used as a pre-regulator and followed by a more precise regulator to give stable output.
Brian.