Talk-Polywell.org

Hi all

This is my first post on this site, although I've been a serial lurker here for over a year

As there's a current news dearth, I thought I'd share an article I found on newscientist on fusion-fission hybrids:

http://www.newscientist.com/article/mg2 ... ption.html

Edit: Fixed link

Interesting statement. I wonder if it is true for thermoneuclear fusion.

The author of the subject article wrote:To make a fusion reaction self-sustaining requires a plasma volume of about 3300 cubic metres, more than three times the proposed volume of ITER, the world's most advanced fusion project now under construction in France.

Guess its a good thing that Polywell isn't thermonuke!

Isn't it sort of a logical fallacy to assume that a plasma itself needs to be self sustaining, and not the overall mechanism used to create the plasma/fusion reaction?

EricF wrote:Isn't it sort of a logical fallacy to assume that a plasma itself needs to be self sustaining, and not the overall mechanism used to create the plasma/fusion reaction?

That is the premise of Polywell. However with toks you have all that alpha energy which is best put to use heating the plasma. Since collecting it in an orderly way from a tok is very difficult.

I have my own concerns about this scheme. There are good things, but also potentially bad things. The fusion reactions drive the fission ones without any automatic feedback mechanism. So, for example, if you have some type of coolant failure on the fission blanket, you have to rely on sensors to realize this to shut down the fusion system before you melt a hole in your containment.

kcdodd wrote:I have my own concerns about this scheme. There are good things, but also potentially bad things. The fusion reactions drive the fission ones without any automatic feedback mechanism. So, for example, if you have some type of coolant failure on the fission blanket, you have to rely on sensors to realize this to shut down the fusion system before you melt a hole in your containment.

Nuke plants operate on this premise.

Not necessarily. If water is used as a moderator then there is an automatic negative feedback. Overheat = boiling = no moderator.

kcdodd wrote:Not necessarily. If water is used as a moderator then there is an automatic negative feedback. Overheat = boiling = no moderator.

There is significant residual heat generation from radioactive isotopes. Which is why reactors require emergency cooling systems. About 3 days of cooling is required before natural convection (in a pump cooled reactor) can handle the job.

What kind of heat load are we talking about? Residual heat power is probably several orders less than full level power, which is what I am talking about. In a fusion-fission hybrid, there is nothing (physically) to keep the blanket from staying at full output, which could mean thermal failure within a very short time-span (seconds?). It just seems the design is less safe then todays fission plants, which seems a bit ridiculous to consider. If they designed in some natural safety measures I would like the idea more.

kcdodd wrote:What kind of heat load are we talking about? Residual heat power is probably several orders less than full level power, which is what I am talking about. In a fusion-fission hybrid, there is nothing (physically) to keep the blanket from staying at full output, which could mean thermal failure within a very short time-span (seconds?). It just seems the design is less safe then todays fission plants, which seems a bit ridiculous to consider. If they designed in some natural safety measures I would like the idea more.

About 10% of reactor full power.

MSimon wrote:

kcdodd wrote:What kind of heat load are we talking about? Residual heat power is probably several orders less than full level power, which is what I am talking about. In a fusion-fission hybrid, there is nothing (physically) to keep the blanket from staying at full output, which could mean thermal failure within a very short time-span (seconds?). It just seems the design is less safe then todays fission plants, which seems a bit ridiculous to consider. If they designed in some natural safety measures I would like the idea more.

About 10% of reactor full power.

You must be talking about more than beta decay, such as delayed and spontaneous fissioning. Those reactions that have a half life of one second or less are virtually done 10 seconds after shutdown. As far as beta decay, it can be managed in fail-safe ways (unlike the current generations of Pressurized water reactors). We can use better designs.

Helius wrote:

MSimon wrote:

kcdodd wrote:What kind of heat load are we talking about? Residual heat power is probably several orders less than full level power, which is what I am talking about. In a fusion-fission hybrid, there is nothing (physically) to keep the blanket from staying at full output, which could mean thermal failure within a very short time-span (seconds?). It just seems the design is less safe then todays fission plants, which seems a bit ridiculous to consider. If they designed in some natural safety measures I would like the idea more.

About 10% of reactor full power.

You must be talking about more than beta decay, such as delayed and spontaneous fissioning. Those reactions that have a half life of one second or less are virtually done 10 seconds after shutdown. As far as beta decay, it can be managed in fail-safe ways (unlike the current generations of Pressurized water reactors). We can use better designs.

It is not just beta decay. It is all kinds of things from the fission fragments. It is inherent in fission although the proportions will vary according to the fuel. Look up operational characteristics of a fission reactor.

What I am telling you is based on my experience as a reactor operator.

Isn't it sort of a logical fallacy to assume that a plasma itself needs to be self sustaining, and not the overall mechanism used to create the plasma/fusion reaction?

Magnetic confinement schemes generally revolve around the concept of "ignition," which basically means the conditions are such that the reaction is self-sustaining, like a bonfire -- you just need to keep the wood dry enough, the oxygen content high enough, and get the initial spark hot enough. Polywells are more like an internal combustion engine: you need a constant current both to start it and to keep it running, which is why your car has a battery and alternator.

There are two basic applications for the fusion/fission hybrid as a source of neutrons; the Carlo Rubbia style electric power reactor and the thorium fuel factory as proposed by Hans A. Bethe.

I do not favor the hybrid power reactor because of the material damage that very high energy neutrons inflect on the structure of the reactor. The availability of such a reactor will be low since much time will be spent replacing parts heavily degraded by energetic neutrons.

The hybrid fuel factory can tolerate low availability because the production of fuel is not time critical, so frequent down time can be tolerated.

Both hybrid approaches are highly proliferation sensitive and is similar in concept to a uranium enrichment plant. The NRC and the IAEA will require an army of inspectors to police the fissile product of the hybrid. This can be tolerated when one fuel factory can support 500 dependent fission reactors.

This monitoring by the regulatory bodies is very unwieldy for large numbers of power hybrids. Imaging 30,000 small fusion or hybrid power reactors each with an army of regulators interfering in there operation. These regulators will in effect be de facto managing the reactor facility. But it would make for plenty of nuclear jobs.

Having two or three massive hybrid fuel factories would be no different than the current uranium enrichment plant paradigm that exists today.

Also the PR fallout on pure fusion research if there was an accident at a fusion/fission hybrid facility. Most people don't know the difference as it is.