Well, if your primary coolant pressure is 2200 psia then that is nice to have similar pin pressure.USPWR_SRO wrote:FYI, current PWR fuel pins are pressurized with 400psia at ambient temperature of helium at manufacture. After about 30,000MWD/MTU of burnup the internal pressure rises to ~1200-1400psia at ambient temperature due to fission product gases.Cyril R wrote:...Apparently this is almost mandatory for ultra high burnup fuel rods: without venting, the pressure from fission gas generation would build up and rupture the cladding...
The initial pressure is to improve heat transfer fromt the fuel pellets and to prevent collapse of the pin in the high pressure primary system when new.
With sodium cooled reactors in a pool type design, the pressure is essentially atmospheric plus hydrostatic head from the pool.
And with a metal fuel the fission gas release is bigger.
30,000 MWd/tU is quite low burnup. The TWR would require burnups more like >100,000 MWd/tU, perhaps 200,000 MWd/tU
That gives some scary amount of volatile fission products. Vented fuel starts to look really good for such a configuration.