Gas cooled nuclear reactor for submarine

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ssnguy RE:Gas cooled nuclear reactor for submarine 1/20/2005 7:53:41 PM

Back in the day Gulf General Atomic built the Fort St Vain nuclear power station that was an HTGR. That was a helium cooled, graphite moderated plant. As I remember it had some interesting characterists such as the core becoming stronger as the temperature rose (within limits) and a nicely negative temperature-coefficient of reactivity. But while an HTGR could let you run some really hot steam and increase secondary plant efficiency, it would be a whopping big thing based on what I read/studied back then. There were all sorts of problems with GGR plant as I remember and it wasn't, I think, considered a success.

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Adamantine RE:Gas cooled nuclear reactor for submarine 1/21/2005 12:41:28 AM

"But while an HTGR could let you run some really hot steam and increase secondary plant efficiency, it would be a whopping big thing based on what I read/studied back then. There were all sorts of problems with GGR plant as I remember and it wasn't, I think, considered a success " Agree but HTGR is still in its infancy and it could actually use those high temperature alloy developed by aviation turbofan or state of the art civilian gas turbine. In contrast, PWR technology is quite matured. Beides, since you cannot increase the operating temperature of PWR by much, you cannot exploit on those novel and reliable high temeprature material develope for the aviation and power plant gas turbine. The R&D for all kinds of gas turbine greatly exceed the R&D of PWR reactor, it is a pity that those R&D cannot be exploited. By pursuing a HTGR, there is still alots of improvement that could be envision and inherent performance advantage waiting to be exploited. In contrast PWR has reach near theoritical performance limit, even if you throw many billins into it, you cannot improve the power density and thermal efficiency of a PWR by much. Liquid Metal is out simply because the two available coolant is either toxic (lead bismult) or corrosive (sodium) and it is too dangerous to handle them in the confine space of a submarine that reduce the ability of for easy maintenance. Personally I see HTGR is worth exploring and when HTGR is coupled to a gas turbine instead of a steam turbine, not only could you greatly reduce the size of the machinery but also capitalise on the R&D already spend on developing excellent marine turbine like LM2500+ and LM6000 and the Rolls Royce equivalence. A good HTGR could improve power density dramatically when the technology matures. PWR has a head start but is nearing the plateau of its technological development.

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french stratege RE:Gas cooled nuclear reactor for submarine 1/21/2005 1:32:02 PM

according of what I know, HTR reactor and machinery take two time less volume than conventional PWR.

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ssnguy RE:Gas cooled nuclear reactor for submarine 1/21/2005 10:37:29 PM

Okay - let's look at this from first principles. If you're doing a gas reactor you're talking about heat transfer from the solid fuel to the gas - a far less efficient transfer than from the solid fuel and a liquid where forced convection is the principle means of extracting heat. This method of heat transfer is far less efficient than forced liquid convection which has a very real meaning for the nuclear fuel: peak temperatures of the fuel will be higher - MUCH higher - than in a traditional water moderated, water cooled PWR reactor plant. Time at high temperature is a challenge for the design of long-life fuel systems because you have to deal with all those fission products. The concept of using the coolant gas directly into a gas turbine is a non-starter. To do that you have to put the gas turbine inside the shield reactor compartment - no matter how pure the gas you're using (typically, as I recall, it's helium) it can carry along radioactive impurities and you don't want those contaminating the closed atmosphere of a submarine. However, you CAN use an HTGR to run a super-heated steam plant which would give a greater enthalpy drop across the steam turbines and thus a greater efficiency. The turbines could also be smaller. Now in a different topic there was discussion of using high-temperature superconductive motors/generators for propulsion which would eliminate the heavy and (relatively) noisy reduction gears. This would allow a designer to have turbine generators that always are running at their ideal blade speeds so you'd gain efficiency that way. But I have to say that high temperature superconductive motors of the size required to drive a nuclear submarine are few and far between. AND they have a few real downsides: loss of the liquid gas coolant will cause the motor to draw more and more current, reducing its torque. This is NOT what you want to have happen to a warship. Now the degradation, I'm told, will be slow due to the mass of metal the motor itself represents, at first, but it's still very troublesome and they're going to have to do some serious testing in order to understand what happens as the motor rises above the fermi-point for the superconductors used. So I'm open-minded by very wary of either of these technologies. I know Admiral Bowman, when he was head of Naval Reactors, was a big believer in electric propulsion and was even talking, although in almost science-fiction tones, about direct electrical production from a naval reactor. Some of these ideas may see the first light of day, however, not in the oceans but in space. It's well known that NASA and NAVSEA 08 are involved in the design of a space-borne nuclear propulsion system so packages can reach long distances in a reasonable period of time - on the order of a years instead of decades. This could well be an HTGR running a gas turbine for electrical production.

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Adamantine RE:Gas cooled nuclear reactor for submarine 1/22/2005 2:07:29 AM

"Now the degradation, I'm told, will be slow due to the mass of metal the motor itself represents, at first, but it's still very troublesome and they're going to have to do some serious testing in order to understand what happens as the motor rises above the fermi-point for the superconductors used" A superconductor motor is much more smaller than conventional motor and hence the VOLUME of wire winding is much smaller. During the superconducting state, zero or little heat is generated through despite the massive current load, so its safe. BUT if the temperature raise above critical temperature due to coolant lose or malfunction of cooling unit, such a massive current density (amount of energy running across per cm3 of the wire) will probably melt the superconductor and may even cost explosion and the sudden release of the energy of the strong electric current into heat and kinetic energy of exploding wire fragment. Of course the failure and degration maybe GENTLE and non catastrophic if the wire manage to retain high conductivity after critical temperature is exceeded. But many ceramic baed HTS tend to lose conductivity rapidly once critical temperature is exceeded ( correct me if I am wrong, I am not a physicist). "So I'm open-minded by very wary of either of these technologies. I know Admiral Bowman, when he was head of Naval Reactors, was a big believer in electric propulsion and was even talking, although in almost science-fiction tones, about direct electrical production from a naval reactor" This concept is no science fiction. The basic idea was invented long ago. Its a machine called the MHD (magnetohydrodynamic). Basically the nuclear reactor or other heat source will produce a movement of electrically charged and conducting liquid metal. The moving liquid metal will directly cause the electrons in the surrounding superconducting wire to move. Theoretical efficiency can be 30% higher than normal generator. As I have said before, even a good submarine nuclear reactor could at most convert only 23 to 24% of its thermal energy into usable shaft energy, a good HTS motor or MHD could greatly increase the shaft output without increaing the thermal energy of the nuclear reactor and thus reduce the need for refuelling and also permit more distance to be travelled or higher avearge travelling speed without increasing the rate of burn of the nuclear fuel. In thenear future if HTS of MHD becomes practical, we may be able to squeeze 40% of the thermal energy of a reactor into usable shaft power or motive force (there is no shaft or moving parts in a MHD). A MHD electric generator can directly coupled to a MHD propulsor and hence no moving part is needed at both the electric power generating end (no rotating electric generator or steam turbine) and the motive power generating end (no propellor). BUT the magnetic signature of both a MHD electric generator and propulsor is very very high. It is possible to shield the magnetic signature of the MHD electric power generator BUT IT IS NOT possible to shield the magnetic signature of the electrically charged seawater at the MHD propulsor end. Hence the most practical design for stealthy submarine is a MHD electric generator coupled directly to a nuclear reactor powering a series of CONVENTIONAL electric motor which runs some very quiet DISTRIBUTED WATER PUMP. "If you're doing a gas reactor you're talking about heat transfer from the solid fuel to the gas - a far less efficient transfer than from the solid fuel and a liquid where forced convection is the principle means of extracting heat" Maybe you are right. But the latest helium cool reactor seems much more compact than those old carbon dioxide cooled reactor. Well we can brain storm more along this line :)

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Adamantine Direct drive turbine and radioactivity 1/23/2005 2:39:15 PM

The main attractiveness of gas cooled turbine is its high thermal efficiency (enable the same amount of fuel to provide greater mileage) and the ELIMINATION of the secondary circuit (steam generator in PWR) as the coolant gas (such as helium) could drive a turbine directly in a Brayton cycle. The elimination of the secondary circuit greatly reduce the size of the gas cooled reactor although the primary is probably larger tha PWR. ssnguy correctly points out that radioactivity in the turbine may post problems. I agree. But some commercial design seems to address this problems. BTW, boiling water reactor seems to use superheated steams from the reactor coolants to drive the turbine directly (if i am not wrong) and there is no major safety issue. Quite a few GE advanced boiling reactor (ABWR) are operating in Japan and Taiwan utilities.

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ssnguy RE:Direct drive turbine and radioactivity 1/25/2005 12:41:37 AM

Commerical BWRs aren't operating in a seal environment like a submarine. I also believe that BWRs are saturated steam systems.

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Nanheyangrouchuan RE:Direct drive turbine and radioactivity 1/25/2005 1:23:52 PM

wouldn't it be possible to scub the radioactive particles from the gas stream? Maybe not immediately after the gas has left the reactor but on the return side after the gas has lost suffcient energy? To lessen the chance of radioactive exposure what about adding a second gas loop that actually powers the turbine like the PWRs do now?

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Adamantine RE:Direct drive turbine and radioactivity 1/25/2005 1:27:37 PM

"To lessen the chance of radioactive exposure what about adding a second gas loop that actually powers the turbine like the PWRs do now? " That defeats the purpose of using gas cooled reactor. It will make the whole reactor bulky and hence reduce its advantage over a PWR.

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Nanheyangrouchuan RE:Direct drive turbine and radioactivity 1/25/2005 5:50:46 PM

Yes, but then you irradiate the turbine as well. Regarding helium and other gases as coolants, if exposed to the levels of thermal energy that reactors give off, the helium atoms will probably glow. In lighting, eventually the bulb is depleted and you must replace it, but theorectically a helium bulb should last forever, unless there is a quantum drain on the electrons to the point they can no longer give up photons when exposed to sufficiently high energy. If "quantum degredation" is possible and does in fact exist, does that mean that the ability of gases, inert or otherwise, to absorb energy is limited? The gas in such a cooling system would have to be replaced regularly. But how often?

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