Discussion below https://space.stackexchange.com/q/50365/12102 including a comment that suggests that the following quote makes it sound like a "solid core nuclear thermal rocket", and that it is likely to have similar performance "as every other solid core nuclear thermal rocket."

From their October 19, 2020 press release [Ultra Safe Nuclear Technologies Delivers Advanced Nuclear Thermal Propulsion Design To NASA](https://usnc.com/ultra-safe-nuclear-technologies-delivers-advanced-nuclear-thermal-propulsion-design-to-nasa/):

>NTP systems achieve expanded payload mass capabilities due to their two-fold increase in specific impulse compared with chemical propulsion systems. 

From kerolox to LOX LH2 Isp's range from roughly 360 to 440 seconds.

**Quesiton:** Are nuclear thermal designs in the ballpark of roughly 700 to 900 seconds? If so, what is the limiting factor? Why can't they easily go higher?

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related and potentially helpful:

 - https://space.stackexchange.com/q/49606
 - https://space.stackexchange.com/q/48504
 - https://space.stackexchange.com/q/48166 answer is the intuitive dimensional analysis for thermal velocity $I_{sp} \propto \sqrt{T/M}$ but it's only an unsourced estimate
 - https://space.stackexchange.com/q/46314
 - https://space.stackexchange.com/q/41666 currently unanswered