This article didn't explain it at all, so you're right to ask.
tl;dr: Gaseous propellant (I'm guessing hydrogen) is heated with fission then pointed in the opposite direction of intended travel.
The uranium in this design is not a propellant, but a heat source. Aside: you can use photons/heat as a propellant, but it's thrust is very low https://en.wikipedia.org/wiki/Pioneer_anomaly. Ideal propellants typically have a high exit velocity and low mass. That gives you the longest amount of "burn" time, and the greatest amount of control for the weight. https://en.wikipedia.org/wiki/Specific_impulse
Back in the day when the US was building more of these nuclear rockets, the propellant of choice was typically hydrogen https://en.wikipedia.org/wiki/NERVA. Old timey video explaining it https://youtu.be/eDNX65d-FBY?t=238. I'm assuming this proposed design would also use hydrogen, but I couldn't find any sources on the propellant for their design.
Liquid hydrogen served to keep the reactor cool as it transitioned from liquid to gas as that phase change absorbs energy. The gas is the directed through the reactor core where the gas heats up. As gases heat up, they absorb energy, their average particle velocities increase.
Eventually, the hydrogen molecules (mostly H2 or H-H gaseous hydrogen), makes it to the nozzle and is ejected. The high-velocity hydrogen is what actually provides the bulk of the thrust to the spacecraft.
Compare this to Project Orion (https://en.wikipedia.org/wiki/Project_Orion_(nuclear_propuls...) which intended to detonate nuclear warheads and the craft essentially rode the shock wave into the stars. I would classify this method of propulsion, not safe.
The primary failsafe mode for an NTR would be to insert control rods to stop fission. Without the hydrogen the core wouldn't be able to cool itself and would melt down. There are however NTR core designs with closed circuit cooling. The core would be kept at a low critical state (hot but not melting) and circulate a coolant through the core and into a generator and from there to radiator panels. When the NTR wasn't providing thrust it would provide electrical power. When thrust is needed the coolant loop would cut off and hydrogen would be pumped through the core. Provided no mechanical breakdown in the coolant/generator loop an NTR could provide power for years.
I guess one could still build single burn/single use reactors. The thing would be a bit lighter than one that can survive multiple burns & it should not pose a Hazzard as long as you plan the resulting orbit of the discarded reactor accordingly.
For a Hohmann transfer orbit you need at least two burns, the perigee burn to put you into the elliptical transfer orbit and the apogee burn to circularize that orbit at your destination. Even free return trajectories can require a secondary burn. So in many situations throwing your engines away is not a great idea.
An NTR can be designed such that the engine and spacecraft "chassis" are reusable over multiple missions. NASA has/has an NTR concept with such a reusable vehicle. The fuel tanks are disposable and slot into the central frame like AA batteries. The crew portion would be a TransHab-like habitation module with a docked crew capsule and Mars lander. Propellant tanks would be disposed of during the mission and the vehicle parked in Earth orbit between missions. For a new mission propellant tanks would be fitted along with a new crew and off it goes. It's an interesting design but a little passed the current bleeding edge of in-orbit construction.
Uhh space itself is about 2.7K. Seems like cooling shouldn’t be a problem. There’s probably all sorts of ways to avoid a problem. Even just using a different element altogether.
You'd think cooling would be no problem, but it turns out that when your only option is to radiate away heat, that's _very_ slow. We get spoiled here on Earth by conduction and convection, both much easier and faster.
Natural gas conversion mostly reduces particulates. The EIA data says natural gas produces more CO2/MkWh than coal:
Electricity generation CO2 emissions
million kWh million metric tons million short tons pounds per kWh
Coal 1,124,638 1,127 1,240 2.21
Natural gas 1,246,847 523 575 0.92
Petroleum 21,860 21 23 2.11
Ironically, particulates decrease insolation and actually slightly reduce warming (CO2's half life is a lot longer than particulates, so the "cooling" is short-lived and will go away when you stop burning coal, net warming, overall). Natural gas doesn't produce as many particulates, so I would expect that converting to natural gas would increase global warming in the short and long term compared to continue to burn coal.
It's far better to switch to solar/wind/geo/nuclear than natural gas in the long run, but there are health benefits to removing the coal particulates:
"Coal impacts: air pollution
When coal is burned it releases a number of airborne toxins and pollutants. They include mercury, lead, sulfur dioxide, nitrogen oxides, particulates, and various other heavy metals. Health impacts can range from asthma and breathing difficulties, to brain damage, heart problems, cancer, neurological disorders, and premature death." https://www.ucsusa.org/resources/coal-power-impacts#:~:text=....
I presume that the primary driver of this conversion isn't the environment, but economics. The price of coal/kWh is more expensive than the volume of gas required to generate the same kWh. The Total system LCOE is usually the number people want to minimize.
https://www.eia.gov/outlooks/aeo/pdf/electricity_generation....
tl;dr: No. Burning gas is worse from a climate perspective.
You are correct, parent commenter misunderstood the table they are referring to.
That table says there's slightly more kWh generated from natural gas (1,246,847) than from coal (1,124,638). I assumed they read this wrongly and thought this was CO2 emissions per kWh.
The best number to read from that table is the last column, "CO2 emissions - pounds per kWh":
Natural Gas: 0.92
Coal: 2.21
So yeah, natural gas's CO2 emissions are much lower.
https://www.electricitymap.org/map illustrates this for the world, it offers estimates for types of electricity generation in each country and where a country or region offers real-ish time data on power generation it reflects that.
Countries/ regions that show dark brown are mostly relying heavily on coal. Getting on for 1 gram per watt-hour of CO2, which is ludicrous.
In a few cases they've managed to find something even less environmentally responsible to burn than coal, such as oil or the most pants-on-head crazy electrical generation method - peat†, which unless somebody starts a national project of shooting endangered animals and then burning the corpses as fuel ought to stand as the least responsible way to make power.
† In theory burning wood could be sustainable because you really could grow enough wood quickly enough to power a not insubstantial electricity plant forever. You probably shouldn't but you could. Peat does not form quickly enough for that to ever be practical.
It's definitely economics; and that's before you factor in carbon cost and looming potential for expensive law suits because of people filing for damages (think tobacco industry). Any remaining coal plants will be under a lot of scrutiny in a few years.
Gas plants are much better from that point of view but still too expensive. Renewables are really killing it on the cost front and with viable energy storage solutions coming online even a role as peaker plants is not going to be a long term thing for gas plants.
Economics are also the reason nuclear is not happening. It's just too expensive and risky for operators to get involved in. When prices below 0.02$/kwh are becoming normal for new solar bids, that kind of puts the squeeze on everything else. Even gas. And there is no sign of this being a final price, this will likely dip well below 0.01$ at some point.
The only reason for investments like this is short term gains while production capacity for clean energy simply can't cover the whole market just yet. So converting coal plants makes sense to replace expensive capacity with slightly less expensive capacity while enough cheap capacity to replace it is short term just not there yet.
>It's far better to switch to solar/wind/geo/nuclear than natural gas in the long run...
Solar and wind require gas plants as balance when the sun is not shining and the wind is not blowing. All other "infinite" energy sources such as coal or nuclear don't have enough dispatchability to perform solar/wind load following. Hydroelectric has a high dispatchability but is not infinite (the lake level gets too low and you have to wait for the next rain).
I worked for a boss that pretty much demanded that we move to the cloud. I showed them the costs for the then 2 providers (GCP/AWS) and arrived at the exact same conclusion on server hosting alone, as bandwidth wasn't the main driver of our application.
The rationale was that we'd save so much money by not having to manage the servers ourselves, but we honestly spent much, much more time in software deployments than managing hardware migrations and failures.
I interpreted it as: prior to moving to the cloud, server maintenance was not a very significant cost, so the justification for moving to the cloud was weak.
This sort of anti-hype is not useful. I've seen a number of people say such things recently, so I figure it might be worth correcting, sarcasm notwithstanding.
First, yes, ingesting large amounts of bleach would likely be hazardous to one's health. The article that was linked suggests that there is a drug which is FDA approved, and used in a therapeutic context today, which may also be effective at least in vitro against the new SARS virus. It's not "haha bleach," or aggressive pulverizing, or any other dramatic form of cell death you can easily imagine. It's a drug [that was linked].
I'm interested in the engineering details, not the message board punditry. If you've got details to contribute --- especially if I'm wrong about something --- let's see 'em.
tl;dr: Gaseous propellant (I'm guessing hydrogen) is heated with fission then pointed in the opposite direction of intended travel.
The uranium in this design is not a propellant, but a heat source. Aside: you can use photons/heat as a propellant, but it's thrust is very low https://en.wikipedia.org/wiki/Pioneer_anomaly. Ideal propellants typically have a high exit velocity and low mass. That gives you the longest amount of "burn" time, and the greatest amount of control for the weight. https://en.wikipedia.org/wiki/Specific_impulse
Back in the day when the US was building more of these nuclear rockets, the propellant of choice was typically hydrogen https://en.wikipedia.org/wiki/NERVA. Old timey video explaining it https://youtu.be/eDNX65d-FBY?t=238. I'm assuming this proposed design would also use hydrogen, but I couldn't find any sources on the propellant for their design.
Liquid hydrogen served to keep the reactor cool as it transitioned from liquid to gas as that phase change absorbs energy. The gas is the directed through the reactor core where the gas heats up. As gases heat up, they absorb energy, their average particle velocities increase.
Eventually, the hydrogen molecules (mostly H2 or H-H gaseous hydrogen), makes it to the nozzle and is ejected. The high-velocity hydrogen is what actually provides the bulk of the thrust to the spacecraft.
Compare this to Project Orion (https://en.wikipedia.org/wiki/Project_Orion_(nuclear_propuls...) which intended to detonate nuclear warheads and the craft essentially rode the shock wave into the stars. I would classify this method of propulsion, not safe.