"Hi gb

Water - in quantity - could be used as reaction mass and as cosmic-ray shielding. Using a low temperature reactor to heat the comet mass, capturing the water driven off, then using it to boost tens of thousands of tons back to LEO, GEO and L1/2 for use as space-station mass-shielding and propellant on a BIG Mars vehicle... well there's a lot of positives with the idea. A reactor only has to run at about 1900 C and can use condensed water instead of hydrogen cracked from the water.

If there's water available then perhaps we should look at BIG scale ISRU."

I think I might still be missing a point or two.

For instance the part about "only has to run at about 1900 C" is a bit puzzling.
And I think you are suggesting using water as propellent instead of Hydrogen for the nuclear powered rocket. And I'm not convinced that is a good idea- though I've heard of this type of idea. Example:
http://www.neofuel.com/moonicerocket/

But I don't agree with assessment in above link.

In my example, what I thought was a reasonable price for water which mined on the Moon would be $500 per lb.
And I thought a reasonable price for electrical power was $10 per kwh.
So, per lb of rocket fuel [LH&LOX] basically I'm saying it is about 5 times harder [cost more] to mine the water than provide power to split the water.

So in the neofuel thingy the ISP for steam is "between 100 and 230" ISP.
That sucks.
LH&LOX gives around 450 ISP, so you using at least twice the propellent for same thrust.

Let me put it this way, the advantage of nuclear reactors is one can make propellants with higher ISP than chemical, e.g 900 ISP [and provide comparable thrust] has been demonstrated [LH&LOX of 460 is about highest ISP for chemical engine I've seen].

Now the engine for chemical rocket is relatively tiny compared the rocket's mass. With a nuclear reactor one is dealing with much bigger/massive engine- but the added mass of reactor can easily make it back with the higher ISP.

If with a nuclear propulsion one is having 1/2 the ISP as chemical and a larger engine mass, I don't see much of an advantage with it.

Now, I realize you are talking about NEOs and not the Moon.
Let me put this way.
You pick the NEO you think can be mined for water. I put a water tank on that rock. Now all you have to do, is put the water in the tank.
You don't have to transport it anywhere. How much would you need to charge as a price for one hundred thousand tonnes of water? Per lb delivered to the tank.
In other words if you charge a $1 per lb, that's $220 million worth of water [no where near the price] and of course $100 per lb is 22 billion.
Question what is from time of blast off, all money spent to that point [btw how much would that be?] until you put a lb water in my tank- how long does this take, and thereafter how fast will you fill that tank?

Now, the buyer of NEO water has to get this stuff somewhere to sell it. If seller is on the Moon he doesn't have to move all of it off the lunar surface, but to sell higher quantities [reach more potential customer] the seller on the moon might move, say, 50% of the stuff off the lunar surface. The water mining company will be buyer of the rocket fuel, on the surface and rocket fuel in lunar orbit- by selling him rocket fuel his costs are lowered, and he can increase his production by bringing more stuff from Earth [more bulldozers, or whatever]. Same goes for company providing power. The water miner and power company is making a market for stuff they need which can be made on the Moon- so more rocket fuel customers.

Now back to the NEO. If water buyer is say spending $100 per lb for water, that means if he uses an inefficient propulsion system that uses say twice as much water [at cost of $100 per lb] that could add considerable to his cost. Now, I need to know the delta-v to this NEO before even try to give anything more precise. But you did say you were shipping to LEO, and so at least I can say about this inefficient rocket is that you will need get out of LEO. Assuming is it's going to be reused.
So, that alone is about 3 km/sec, so it will need a lot water propellent to get the nuclear engine and tanks back to the NEO.