Accelerator Driven Systems: Overcoming Engineering Challenges

[Azael]

I have a question on accelerator driven systems.

From everything I read this sounds like the perfect kind of reactor, minimise waste, possible to burn existing waste, no risk of meltdown and so on. It seems like this could very well be the ultra clean and safe energy source the word needs and a solution to the growing waste problem aswell??

What are the real life engineering obstacles in building accelerator driven systems? Expensive? safety issues still not solved? more neutrons flying around and weakening materials?
For a layman like me it seems like most problems of that sort would already have been solved since we have had spallation sources for quite some time.

Is just a matter of the political attitude towards nuclear power that prevents more rapid progress??

 

[Morbius]

For a layman like me it seems like most problems of that sort would already have been solved since we have had spallation sources for quite some time.

Azael,

It's a problem with magnitude. An accelerator-driven system with huge mega-amp
accelerator driving it, like a spallation source; will only give you something on the
order of a few moles of neutrons.

In a reactor, you have that many neutrons in a few cubic centimeters.

One would need to scale up the accelerator, spallation source, or whatever by
MANY, MANY, MANY... orders of magnitude before you approach what a reactor
can do.

It's like saying you know how to build a refractor telescope for your back yard; and
you now want to tackle an astronomical observing problem worthy of Palomar or
Keck.

Note these large telescopes are not scaled-up refractors. The mirrors of the Kecks are
not scaled-up versions of the 200" mirror of Palomar's Hale telescope. It is seldom
that one can scale-up by many orders of magnitude.

Primarily, there's no reason to build such a device. One can build a reactor that can
burn existing waste, that is inherently safe with zero meltdown risk... One such
design was Argonne's Integral Fast Reactor:

http://www.pbs.org/wgbh/pages/frontline/shows/reaction/interviews/till.html

Dr. Gregory Greenman
Physicist

 

[Morbius]

I have a question on accelerator driven systems.
From everything I read this sounds like the perfect kind of reactor, minimise waste, possible to burn existing waste, no risk of meltdown and so on.

Azael,

Additionally, an accelerator system is NOT necessarily meltdown proof.

The risk of a meltdown in a nuclear reactor is not caused by heat production due
to fission power - it is caused by the "decay heat". When Three Mile Island Unit 2
melted down; the fission power production had long been stopped. It was the decay
heat of the fission products that caused the TMI Unit 2 meltdown.

In an accelerator-driven system; one has the exact same problem. If you are producing
fissions in order to release energy; then you are creating the fission products that
yield the decay heat that can result in a meltdown unless provisions are made for
the cooling of the core.

This is one of the common misperceptions; that an accelerator-driven system is
less prone to meltdown risk because you can "stop" it by turning off the accelerator.

Just as in a reactor; when you drop the control rods, you only turn off the fission
power, you haven't done anything about the decay heat. Likewise with the
accelerator driven system; turning off the accelerator only turns off the fission power;
it doesn't do anything about the decay power - and THAT'S what will melt the core.

In order to have a meltdown-proof reactor, like the IFR; one needs to design the
reactor so that it can be "passively cooled" - that is the cooling system doesn't need
pumps; but only natural convection.

Dr. Gregory Greenman
Physicist

 

[Azael]

Azael,

It's a problem with magnitude. An accelerator-driven system with huge mega-amp
accelerator driving it, like a spallation source; will only give you something on the
order of a few moles of neutrons.

In a reactor, you have that many neutrons in a few cubic centimeters.

One would need to scale up the accelerator, spallation source, or whatever by
MANY, MANY, MANY... orders of magnitude before you approach what a reactor
can do.

Maby I am missunderstanding. But I thought that the bulk of the neutrons will be fission neutrons in a ADS and the neutrons from the spallation source is just to keep the reaction going since the reactor itself is subcritical. I didnt know that the spallation source has to be so immense Article like this one and on nuclear energy agency suggest that its well within reach of modern accelerators?
http://www.ictp.trieste.it/~pub_off/lectures/lns012/Kadi_002.pdf

Primarily, there's no reason to build such a device. One can build a reactor that can
burn existing waste, that is inherently safe with zero meltdown risk... One such
design was Argonne's Integral Fast Reactor:

http://www.pbs.org/wgbh/pages/frontline/shows/reaction/interviews/till.html

Dr. Gregory Greenman
Physicist

Thanks for the link. I am going to have to read more about the integral fast reactor, it looks interesting

So in your oppinion reactors like the integral fast reactor is superior to any ADS and it would be better to focuse on those designs?

Does a ADS have any advantage over a fast reactor if the accelerator problem is solved?

 

[Azael]

Azael,

Additionally, an accelerator system is NOT necessarily meltdown proof.

The risk of a meltdown in a nuclear reactor is not caused by heat production due
to fission power - it is caused by the "decay heat". When Three Mile Island Unit 2
melted down; the fission power production had long been stopped. It was the decay
heat of the fission products that caused the TMI Unit 2 meltdown.

In an accelerator-driven system; one has the exact same problem. If you are producing
fissions in order to release energy; then you are creating the fission products that
yield the decay heat that can result in a meltdown unless provisions are made for
the cooling of the core.

This is one of the common misperceptions; that an accelerator-driven system is
less prone to meltdown risk because you can "stop" it by turning off the accelerator.

Just as in a reactor; when you drop the control rods, you only turn off the fission
power, you haven't done anything about the decay heat. Likewise with the
accelerator driven system; turning off the accelerator only turns off the fission power;
it doesn't do anything about the decay power - and THAT'S what will melt the core.

In order to have a meltdown-proof reactor, like the IFR; one needs to design the
reactor so that it can be "passively cooled" - that is the cooling system doesn't need
pumps; but only natural convection.

Dr. Gregory Greenman
Physicist

I see thanks for clearing up that missconception. I always thought that meltdown was because of uncontrolled reaction.

 

[Morbius]

Maby I am missunderstanding. But I thought that the bulk of the neutrons will be fission neutrons in a ADS and the neutrons from the spallation source is just to keep the reaction going since the reactor itself is subcritical.

Azael,

The power density of the reactor or ADS is proportional to the neutron density.

A reactor has a neutron density that is orders of magnitude higher than what you
can get out of an accelerator. The only way for the ADS to get anywhere close
to the neutron densities of a reactor is via sub-critical multiplication as is proposed
in the link you cited. The device that the accelerator is driving is ALMOST a
critical reactor. It is a slightly sub-critical reacor with a source.

You could do the same thing with a reactor and a neutron source; leave the reactor
slightly subcritical; but drive it with a naturally radioactive neutron source.

So in your oppinion reactors like the integral fast reactor is superior to any ADS and it would be better to focuse on those designs?

Does a ADS have any advantage over a fast reactor if the accelerator problem is solved?

Yes - the IFR would be "infinitely" superior to an ADS. Argonne National Lab
already has the IFR technology well in hand; too bad research was stopped in the
early '90s - they'd be so much farther along.

From what I've seen of these ADS systems; they are basically a feeble attempt to
justify the building of an accelerator.

Dr. Gregory Greenman
Physicist

 

[Morbius]

I see thanks for clearing up that missconception. I always thought that meltdown was because of uncontrolled reaction.

Azael,

It's actually quite difficult to get the fission reaction to runaway uncontrolled.

There are many prompt feedback mechanisms that will terminate a fission power
excursion.

For example, in a light water reactor; the water coolant also serves as moderator.
If the reactor fission power starts to runaway - the water gets hotter, and hence less
dense. Its moderator function is thereby decreased - and that drops the reactivity and
hence terminates the runaway.

Additionally, there is Doppler broadening of absorption resonances. This is less
important in LWRs than the moderator effect above; but it is the primary feedback in
the IFR [ which doesn't have a moderator ].

Look at a plot of the absorption tendencies of practically any heavy material; and you
will see a series of "spikes" called "resonances" [see attached]

When the materials of the reactor heat up; these resonances broaden - because
the increased thermal motion of the material can better compensate for any
"mismatch" between a neutron's energy and the energy of the resonance - the
energy at which neutrons are most highly absorbed. This increases non-productive
absorption of neutrons; and again lowers reactivity and terminates the excursion.

Dr. Gregory Greenman
Physicist