# Tag Info

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To answer the question simply, $E=mc^2$. Energy is a manifestation of mass, and mass is a manifestation of energy. In a fusion or fission process, the total "energy" of the system remains constant, it just changes shape. By "energy" I mean the totality of the already present energy, and the bound energy of the mass that takes part in the reaction.

8

The WKB approximation states that in one dimension, the tunneling probability $P$ can be approximated as $\ln P=-\frac{2\sqrt{2m}}{\hbar}\int_a^b \sqrt{V-E} dx$ , where the limits of integration $a$ and $b$ are the classical turning points, $m$ is the reduced mass, the electrical potential $V$ is a function of $x$, and $E$ is the total energy. Setting ...

8

The Sun obviously produces far more energy per second than is required to fuse an iron nucleus with some other nucleus. The problem is concentrating all that energy on the iron nucleus. It's not enough to known that it takes the energy from $n$ hydrogen fusions to fuse one iron nucleus, it's getting the energetic products from those $n$ hydrogen fusion ...

7

The problem with attempting to fuse two protons is that there is no bound state $^2$He, for the rather obvious reason that there are no neutrons present to hold the two protons together. The fusion of two protons requires one of them to undergo beta plus decay while the two protons are close, and the probability of this is vanishingly small. It happens in ...

6

Well, if you search the internet it seems there are kids out there that make the claim of having built a fusion reactor . I watched this link. Note that in .56 minute he gives a small description, and does not claim breaking even, but that he demonstrated fusion. It is a plasma that he obviously creates and manages to fusion some deuterium that is not ...

6

[Rewrote the answer because I found out that my initial approximation was too crude.] In the WKB approximation, the tunneling probability is $\exp[-\int_a^b dx \sqrt{(2m/\hbar^2)(V-E)}]$, where the integral is over the classically forbidden region from $a\sim10^{-15}$ m to $b\sim 10^{-10}$ m. The first obvious thing to try is approximating the integrand as ...

6

The problem with proton-proton fusion is that there is no bound state of two protons. For the fusion to occur one of the protons has to turn into a neutron by beta plus decay. This is mediated by the weak force so it's a slow process and the probability of it happening while the protons are close enough to form a deuteron is very low. By contrast a deuteron ...

5

Yes, heavy shielding is needed primarily for gamma radiation. Neutron radiation (with energies seen in fission reactors) is easily stopped with boron-10 (isotopically enriched boric acid in water). While alpha and beta radiation is easier to shield, it is even more dangerous if alpha and beta active particles (dust) is consumed by human, because they will ...

5

From what I have read in "American Prometheus: The Triumph and Tragedy of J. Robert Oppenheimer" Teller was the first one to express this concern before the Trinity test. Also quoting from: http://www.sciencemusings.com/2005/10/what-didnt-happen.html Physicist Edward Teller considered another possibility. The huge temperature of a fission explosion -- ...

5

All three isotopes of Hydrogen can undergo fusion under the right conditions. The main reason to use D or T is that they fuse more easily than H. For example, H-H fusion is primarily what drives our sun, but in the lab D-D or D-T reactions are much easier to initiate. The D-T reaction gives off 17.6 MeV of energy, D-D actually has 3 different reactions it ...

5

The basic problem of modelling a star is covered in a number of textbooks and lecture notes. Try searching for "stellar structure and evolution" or something along those lines. The best readily available lecture notes, IMO, are those of Onno Pols, available here. There was also a similar post on Quora, which you can read too. In the mean time, here's the ...

4

ITER is aiming for 150.000.000K. Please note that this temperature of the plasma, i.e. average kinetic energy of the ions is in electron volts For example, a typical magnetic confinement fusion plasma is 15 keV, or 170 megakelvins . 15 KeV is enough to assure that the plasma does not neutralize itself and the bare nuclei have a high enough statistical ...

4

While D-He3 fusion reaction rate peaks at smaller energies than D-D (see this picture), and produce more energy (18MeV for D-He3 vs. 3-4MeV for D-D reaction), this is not the main reason why some people think He3 is a 'better' fuel. The main reason is that D-He3 fusion fuel cycle is aneutronic. That is, all fusion products (if we disregard auxiliary ...

4

Fusion happens between light nuclei. It cannot happen in room temperatures and pressures, it needs very high energies in order to strip the electrons from the nucleus and to overcome the electromagnetic repulsion of the positive charges . The fusion reaction rate increases rapidly with temperature until it maximizes and then gradually drops off. The DT ...

4

If you search the ITER site, ITER being the international prototype fusion reactor which will demonstrated the possibility of getting megawat useful energy from fusion, one sees that their main aim is to demonstrate this feasibility: The main carrier of energy out of the plasma is the neutron, and methods to efficiently use this energy have not been ...

4

As you correctly stated in normal situation the star cannot sustain the process. This doesn't mean that there are no such reactions going on in the core. The difference is that during the pre-supernova phase of the star the production of iron is negligible compared to the star. When it goes supernova, it produces a comparable amount of iron.

3

Just few more points. A blob of water few times larger than sun would already be a supergiant.(I'll ignore this in the rest of my answer) The water will not stay still; it will be attracted by star's gravitational field. This will cause the star's mass to increase. The water which is falling on the star, even if it's initially freezing cold, will heat up ...

3

Following through to previous news articles such as this one: “This is my Inertial Electrostatic Confinement Fusion Reactor. It works on the property of inertial electrostatic confinement,” Conrad says. See Wikipedia on inertial electrostatic confinement. The actual design is a Farnsworth-Hirsch Fusor. Note Conrad's last name is Farnsworth which is a ...

3

The bulk of physics obeys the time reversal symmetry. In QM that means that a process which is allowed in one direction is allowed in the other direction and that the total probability for it is the same in both directions (assuming you can set up the time-reversed final state as an initial state). So a way to understand the delicacy of trying to assemble a ...

3

In a fission bomb, the fallout consists of fission-decay fragments, which are nuclei that can have long enough half-lives to be transported by winds. Fusion bombs are basically the same idea, because they use fission triggers. and is ionizing radiation capable of radiating materials for a long time ? In theory, yes, e.g., exposure to neutrons in ...

2

By that logic, a battery violates the law also. "Look, all I did was put in enough energy to flip a switch, and now an LED keeps shining and shining! I got more energy out than I put in!" In nuclear fusion, we are releasing some latent energy which is present in the materials, by changing the nuclear structure into other materials that contain less energy. ...

2

Just to summarize the correct comments by dmckee and Alan.SE: The second law prevents you from using the Sun (or anything) to heat an object to greater than the surface temperature of the Sun. Otherwise you could take a box of gas at equilibrium split it into two halves, use lenses and mirrors to focus the radiation from the left half on the right half, ...

2

There is another way to do what you are suggesting. And that is to use an already concentrated form of energy -- lasers. There are a variety of forms of high energy laser fusion projects underway. Inertial Confinement Fusion is the fancy term for the leading method. See ICF details here I would post some illustrations, but I'm not allowed to post images ...

2

Visible part of spectrum does not get involved to nucleus. Maybe electrons. Which can only make plasma or increase pressure if it is constrained in a cube. If pressure can reach the critical level, then deuterium can be used for fusion(like pressurizing through lazer bombardment). So, you need sudden increase of pressure right(to increase probability of ...

2

Note that the definition of "nuclear reactor" or "nuclear power" can be quite broad. Actually, you could build a nuclear reactor with stone age technology, without even understanding the physics behind it: you find some strange rocks, let water run trough it, the water will be heated, now you can cook with it or use the heat for other purposes. (don't try ...

2

Stars have multiple phases to their life cycles, but the typical phase one thinks of is known as the "main sequence."1 This is defined by the stage when nuclear burning of hydrogen is balancing the gravitational tendency to collapse. That is, the outer layers of the star would fall toward the center but for the fact that there is a pressure gradient, and ...

2

Another, non mainstream thought is the use of Helium-3 instead Helium-2 of as a fuel: 3He + 3He → 2 1H + 4He + 12.9MeV or: 2H + 3He → 1H + 4He + 18.4MeV Here the reaction products are all charged, which means that they could work directly on an electrostatic field, thus transferring their kinetic energy directly to a current. Depending on the reactor's ...

2

The basic idea in all three cases is that the energy becomes heat, and is extracted using turbines, just like in a fossil fuel burning power station. On a microscopic level the energy is released in the form of kinetic energy of the helium nucleus and the neutrons that are given off. These then collide with other particles, so that this kinetic energy ...

2

I'm not an expert and I know we have some people on the site who are much closer to being experts, but there are several points that I can offer to tide you over. A large fraction of the precautions taken in reactor design are for protection in the event that things have gone wrong, rather than necessary in the course of typical running. Much thought also ...

2

You're totally misinformed... atomic bombs raise from an exponentially expanding interaction, where in Uranium, for examples, a neutron is fired initially, and each neutron hits another Uranium atom and releases another 3 neutrons, and the energy released grows exponentially to create a horrible explosion. And about the speed you can reach, this depends ...

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