Consumption and efficiency of 10 GW Polywell fusion reactor

If polywell fusion is developed, in what ways will the world change for better or worse? Discuss.

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Mikos
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Consumption and efficiency of 10 GW Polywell fusion reactor

Post by Mikos »

I have posted this question to my previous "Spaceship Design" thread, but nobody answered, so I will try it in new topic (if this is not appropriate, then I am sorry and administrator can delete this thread).

How much (in terms of weight) hydrogen and boron will be consumed (approximately) by 10 GW Polywell reactor? And how much helium will be produced?

Btw. can you tell me how efficient (even if it is only guess) will be such reactor with direct energy conversion (and therefore how much waste heat will be probably produced)?

93143
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Post by 93143 »

The general consensus seems to be that the primary inefficiency in a high-gain reactor design is coil occlusion resulting in fusion product loss, which is expected to be about 20%. This energy is not completely lost, since you can run a steam power cycle on it, but in space you probably wouldn't bother.

Say 75%. (I'm still in grad school. MSimon is at the other end of his career and would probably give you a somewhat lower, more realistic number.) Then you have 3.3 GW of heat to get rid of, on top of your 10 GW of electricity, a substantial fraction of which may also dissipate as heat depending on how the drive works.

According to my calculations, a 10 GWe reactor (13.3 GWf by the above) at full throttle would consume about 15.1 kg/day of boron-11 and 1.4 kg/day of H2, based on a reaction energy of 8.7 MeV, or about 839 GJ/mol. Please verify for yourself, especially if you decide to change the numbers to sound more realistic.

Helium production would naturally be about 16.5 kg/day (the sum of the reactants). The mass deficit is equal to the reaction energy: 13.3 GWm, or about 0.013 kg/day.

You probably wouldn't run it at full throttle all the time, of course...

MSimon
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Post by MSimon »

I would run water through the reactor and use it as reaction mass for jumping out of gravitational wells. Or maybe LH2.

I kind of like a maglev slingshot followed by lighting off the fusion boosters at the end of the track.

You really don't want to start spewing a lot of H2 in an oxygen atmosphere from a vehicle that is standing still.
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scareduck
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Post by scareduck »

93143 wrote:You probably wouldn't run it at full throttle all the time, of course...
Really?

Nuclear fission plants want to run full bore 100% of the time. This is because of all the engineering tradeoffs needed to make them go in the first place (you're either critical or you're not), This causes all kinds of side effects, like trying to figure out what happens to energy you can't use in low utilization times (at night, for instance, in areas where daytime a/c utilization is the primary load). Sometimes they end up pumping water uphill into man-made dams, the energy thereby stored being reclaimed during peak usage.

I would imagine an energy-positive Polywell fusion device would have similarly narrow operational bands. Too dense and your core starts to generate bremsstrahlung with the electrons in the well, thermalize, or upscatter; not dense enough, and you don't have enough fusions, and the acceleration energy is eventually diffused (thermalization again). In either case, Q < 1.

93143
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Post by 93143 »

You may be right, but Polywell has an advantage fission doesn't. It doesn't take days to start up or shut down.

If you could hot-restart very very easily, a bit of extra hardware might allow PWM-type throttling of reactor output, whereby 50% power is supplied by having the reactor on full blast 50% of the time. In fact, if it turns out to operate best in pulsed mode, this may be a natural extension of normal operation.

...I thought for sure you could throttle a fission reactor... what does the Navy do?

MSimon
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Post by MSimon »

The Navy throttles its reactors.

However, commercially it makes less sense due to the high amount of invested capital.

Fission reactors take about 4 to 6 hours to start up due to the care required going from 1E-3 W to 1E8 W. A reactor designed for 1E8 W is very dangerous below operating temperature and 1E6W. You have to take the power up slowly.

In addition: unless you do a hot restart (not recommended for commercial plants) you have to wait 24 to 36 hours to restart a scrammed plant.
Engineering is the art of making what you want from what you can get at a profit.

Mikos
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Post by Mikos »

Didn't Dr. Bussard somewhere said that p-B11 Polywell reactor with direct energy conversion could have 90 - 95% efficiency? Or am I mistaken? Or was he wrong (or perhaps too optimistic)?
Last edited by Mikos on Fri Feb 01, 2008 12:16 am, edited 1 time in total.

scareduck
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Post by scareduck »

93143 wrote:You may be right, but Polywell has an advantage fission doesn't. It doesn't take days to start up or shut down.
Actually, there was a thread speculating about this recently. About three days to manage outgassing in the vacuum chamber and other things as well.

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Post by MSimon »

scareduck wrote:
93143 wrote:You may be right, but Polywell has an advantage fission doesn't. It doesn't take days to start up or shut down.
Actually, there was a thread speculating about this recently. About three days to manage outgassing in the vacuum chamber and other things as well.
Restarts would be faster and less dangerous with a Bussard Reactor.

A full start might take about as long or longer than a fission plant.
Engineering is the art of making what you want from what you can get at a profit.

93143
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Post by 93143 »

scareduck wrote:
93143 wrote:You may be right, but Polywell has an advantage fission doesn't. It doesn't take days to start up or shut down.
Actually, there was a thread speculating about this recently. About three days to manage outgassing in the vacuum chamber and other things as well.
That's why I said HOT-restart. Everything is running; all you have to do is get a wiffleball going, which should be milliseconds rather than days. The line between this sort of stop/start operation and HF pulse mode is blurry.

scareduck
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Post by scareduck »

93143 wrote:
scareduck wrote:
93143 wrote:You may be right, but Polywell has an advantage fission doesn't. It doesn't take days to start up or shut down.
Actually, there was a thread speculating about this recently. About three days to manage outgassing in the vacuum chamber and other things as well.
That's why I said HOT-restart.
I missed that in the second paragraph.

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