That depends on so many things I won't attempt an estimate: whether the detector is immersed in the water or not, volume (and speed of replacement) of water resp. steam, is it filtered, is the radioactivity uniform within the water, what kind of isotopes are involved, how the radionuclides can leave the damaged fuel rod (is one crack in the cladding enough for volatile breakdown products to leave entirely, resp. for water to enter and wash out more?), are meltdown products present.

In fact, I'm not sure how nitrogen injection is supposed to increase concentration, other than by increasing emission from the pressure chamber due to increased pressure.

(Minor note: that r in the unit made me scratch my head at first, until I realised that it may be part of a non-standard and in the second case typo-lade abbreviation of hour. The standard SI symbol of hour is h, "hr" is the same kind of imprecision like "kph" in place of km/h.)

_{*Lunatic*, n.
One whose delusions are out of fashion.}

How much damaged fuel rod material would be needed to generate readings of 100 Sv/hr

Enough to get near to re-criticality should do nicely. Intermittent re-criticality would also do the trick.

"It is not necessary to have hope in order to persevere."