What are the benefits of using fusion power over fission power?

[Stephanus]

Dear PF Forum,
I'm interested in environment.
And I want to know about nuclear power plant waste.

In fission reaction:
Neutron + U235 -> Kr 92 + 141 Ba + 3 free neutrons.
What is the radioactive waste in fission power?
Is it in the result Kr92 and 141 Barium?
Is it in the 3 free neutrons?
Where do those free neutrons hit? Another uranium? I read that the a large number of the neutrons are absorbed in a rod to prevent the reaction goes at geometric sequence.

Fusion reaction:
In fusion reaction, I read that the reaction is
D + T -> He + 1 free neutron.
What are the radioactive waste from fusion reaction?
I read that Tritium is radioactive and its half life is 12 years old. But it's the fuel right, not the waste.
Or the radioactive waste comes from the free neutron which hit the tokamak wall?

Thanks for the explanations.

[Edit: I read some links in internet but I haven't had a satisfactory answers.
http://fusionforenergy.europa.eu/understandingfusion/merits.aspx
https://en.m.wikipedia.org/wiki/Nuclear_fusion
https://en.wikipedia.org/wiki/Nuclear_fission
https://en.wikipedia.org/wiki/Radioactive_waste][Edit: in https://en.wikipedia.org/wiki/Radioactive_waste it explains that there many radioactive waste:

The back end of the nuclear fuel cycle, mostly spent fuel rods, contains fission products that emit beta and gamma radiation, and actinides that emit alpha particles, such asuranium-234, neptunium-237, plutonium-238 and americium-241, and even sometimes some neutron emitters such as californium (Cf). These isotopes are formed in nuclear reactors.

But where do they come from?
Since the fision reaction is: Neutron + U235 -> Kr92 + Ba141]

 

[e.bar.goum]

Since the fision reaction is: Neutron + U235 -> Kr92 + Ba141

That's not correct. That is one channel through which ²³⁵U will fission. In reality, fission produces a range of possible fission products. For ²³⁵U, the mass distribution looks like the red curve:

(Credit: Wikimedia)

For isotopes such as 234U, 237Np, 238Pu, 241Am, it's important to know that while 235U will fission in a neutron induced reaction quite a lot of the time, this isn't the only reaction that will occur. For example, 237Np is formed by successive neutron capture from 235U +2n -> 237U, which beta decays into 237Np. In addition, 237Np will form in spent fuel from 241Am, or from neutron knockout reactions from 238U or Pu. In turn, 241Am is formed. Neutron capture followed by decay is the explanation for the rest of the actinides you are asking about.

The neutrons resulting from fission be moderated, thermalised, and will either hit another uranium, inducing another reaction, or will be otherwise absorbed. The key part of the running of a nuclear reactor is keeping the neutron flux to what you want it to be - too low, the reactor will stop, too high, the reactor will run away. The control rods control the neutron flux, yes.

 

[Stephanus]

Thanks e.bar.goum
So, that's the fission reaction waste product. I know that fission causes radioactive wastes but just don't know the process, yet.
What actually interest me is fusion power. I read that fusion is the energy of the future (apart from the joke that in the future fussion is still the energy of the future)
What is the radioactive waste from fusion plant (if someday it is built)?
D +T -> He4 + free neutron.
Is it the helium?
What about the free neutron, does the radioactive matrial come from the free neutron hits the wall?
If it's tritium, isn't tritium the fuel?

Thanks for any replies.

 

[e.bar.goum]

Thanks e.bar.goum
So, that's the fission reaction waste product. I know that fission causes radioactive wastes but just don't know the process, yet.
What actually interest me is fusion power. I read that fusion is the energy of the future (apart from the joke that in the future fussion is still the energy of the future)
What is the radioactive waste from fusion plant (if someday it is built)?
D +T -> He4 + free neutron.
Is it the helium?
What about the free neutron, does the radioactive matrial come from the free neutron hits the wall?
If it's tritium, isn't tritium the fuel?

Thanks for any replies.

Yes, the fast free neutrons hitting the walls/other components and inducing nuclear reactions will be the major contribution to radioactive waste from fusion power. You can carefully choose the materials of the walls in order to minimise the impact of this waste (low n capture cross section, low lifetime). Tritium would indeed be recaptured for the fuel.

 

[Stephanus]

Yes, the fast free neutrons hitting the walls/other components and inducing nuclear reactions will be the major contribution to radioactive waste from fusion power. You can carefully choose the materials of the walls in order to minimise the impact of this waste (low n capture cross section, low lifetime). Tritium would indeed be recaptured for the fuel.

So the radioactive waste comes from the wall.
What if we made the wall from lithium for tritium breeding, so the radioactive wastes are minimized?

 

[e.bar.goum]

There are other considerations- lithium melts at 180 C

 

[Vanadium 50]

There are other considerations- lithium melts at 180 C

And boils at a low temperature compared to structural metals. Now you have highly reactive lithium plating out in random places. This happened with the Fermilab lithium lens some years back. Made a heck of a mess. How do you clean it up? Not with water, that's for sure!

 

[Stephanus]

There are other considerations- lithium melts at 180 C

And boils at a low temperature compared to structural metals. Now you have highly reactive lithium plating out in random places. This happened with the Fermilab lithium lens some years back. Made a heck of a mess. How do you clean it up? Not with water, that's for sure!

I read that (can't give you the link. Forgot about it) the radioactive waste from fusion power has 100 years half life. Care to tell me what elements are they?
Can IFIF (IF I remember the initial) produce a good wall? IFIF is the partner of ITER
[EDIT: https://en.wikipedia.org/wiki/International_Fusion_Materials_Irradiation_Facility
IFMIF: International Fusion Materials Irradiation Facility

The International Fusion Materials Irradiation Facility, also known as IFMIF, is a projected materials test facility in which candidate materials for the use in an energy producing fusion reactor can be fully qualified

]

 

[mfb]

I read that (can't give you the link. Forgot about it) the radioactive waste from fusion power has 100 years half life. Care to tell me what elements are they?

It is unclear which materials a fusion power plant would use, but certainly multiple different elements. There are several options for each part, some of them will be tested in ITER, some in other fusion reactors. You need lithium, you probably want some additional neutron multiplication (on average, you have to make more than one tritium nucleus per neutron, as there are losses in the processing chain), you need a very heat and radiation tolerant first wall, you need materials that transport the heat to the cooling systems, and so on.
Many isotopes will have very short liftimes (below a day) - they can make maintenance breaks tricky but they are no problem in terms of waste. Some others decay within years, so waiting a few years before disassembling a reactor helps a lot. Afterwards the remaining parts can be processed: separate the non-radioactive components from those still radioactive. The small radioactive rest needs long-term storage, but with a careful choice of materials in the reactor the radioactivity will be negligible after 100 years.

In general, every isotope in the reactor will lead to the production of many new isotopes: a nucleus can capture a neutron, then it can capture another neutron or (if it is unstable) decay, some isotopes even have more than one possible decay mode, and so on.

 

[Stephanus]

Dear staffs, mentors, science advisors.
In the next 100 years,
1. Will the energy from fusion power be available?
2. Will the energy from fusion power be very cheap compared to gasoline now, say about 5%.
It seems that there are inevitable dismantling process. So, can the technology someday cope that problem, at least make it easy?
Just want to know if in the future humanism have a bright future. No global warming, unlimited (if not abundant) clean and cheap energy and very little polution.

 

[mfb]

As someone quoted recently: we are a physics forum, not a psychics forum.
Wendelstein 7-X, ITER and so on explore those points.

 

[eXorikos]

We can say for sure that theavailable energy will not be unlimited, though.

 

[Bandit127]

Stepanus - please keep to the topic you started. This thread is about nuclear waste and not fusion. If you wish to discuss fusion, please can you do that in a new thread. Thanks.

 

[berkeman]

Stepanus - please keep to the topic you started. This thread is about nuclear waste and not fusion. If you wish to discuss fusion, please can you do that in a new thread. Thanks.

As has been already pointed out, there is indeed nuclear waste from fusion reactors. But yes, if the OP wants to discuss the viability of fusion power, that would fit best in a different thread. (Although I'm pretty certain there are already such threads on the PF...)

 

[Stephanus]

Yeah, sorry. You're right, actually I want to know about the viability of fusion power. What fusion power has advantages over fission are:
1. The availability of fuel (Hydrogen, you can find everywhere in the world, if not the universe , while Uranium mines are located only in certain areas)
2. The nuclear waste...
So it's 100 years as mfb points out.

It is unclear which materials a fusion power plant would use, but...
...The small radioactive rest needs long-term storage, but with a careful choice of materials in the reactor the radioactivity will be negligible after 100 years

I like that.
Thanks.