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This diagram should explain the basic problem. Prior to the accident, the control rods were in position II - that is to say, in the normal fully withdrawn position. The actual neutron absorber was sandwiched between two graphite displacer sections - so when the SCRAM button was pushed, the graphite displacer descended into the active region of the core and ...


10

Nuclear fission is spontaneous. In fact, if you have a critical mass of fuel assembled, you need to take explicit steps to keep it from going into chain reaction, typically by adding neutron-absorbing material, AKA "control rods" to the assembly. When you remove the control rods, the reactor starts up by itself.


5

Atomic Weapons The critical mass of metallic U-235 is about 50 kg[1] and the only U-235-based fission weapon deployed in combat--the Little Boy weapon--had about 64 kg of U-235[2]; there's more U-235 than that in nuclear power plants. Why this is not a good comparison Weapons-grade U-235 is highly enriched (think 80%+), meaning it contains mostly U-235 and ...


5

Disclaimer: I am not a nuclear engineer. From what I've read, nuclear weapon design is a very complex science. I don't think that you could reliably describe a scaling rule. What you could do is look at some historical data and extrapolate from there. However, extrapolation is dangerous™ and you should not assume that something will continue to scale in the ...


4

The primary use/purpose of water in light water reactors is neutron flux moderation and cooling/steam generation. Water is only used for shielding during re-fueling. In land based PWR's (pressurized water reactors) the thickness of water to effectively protect workers from the radiation in the core when it is open for refueling is around 23 feet. Basically ...


4

I'm going to answer this for a LWR, since I guess that's what you're thinking of. First of all, the premise of the question is not quite correct. The power distribution of an LWR is generally sinusoidal, with a peak towards the middle of the fuel. This is true with or without control rods inserted. The most common power reactor design in operation--a ...


4

I work at a small research reactor---power reactors are significantly different, and I'm not qualified to say much about them. Since we don't operate constantly, our fuel burns up very slowly (the reactor's been there for ~50 years and the only change was when we swapped our original fuel for fuel from a decommissioned research reactor, for reasons unrelated ...


4

In the US, Nuclear is used to base load the grid. As implemented in the USA, Nuclear energy has very high fixed costs and relatively low variable cost. So it is in the utility's interest to run their nuclear capacity as close to 100% as they can. This may be different in places (France?) with a more significant investment in nuclear or other electrical ...


3

Chernobyl does power its own cooling system during normal operation. It would be very strange if it did not. It uses the grid as backup and then uses diesel generators as a backup for the backup. This report here supports that: https://www.rri.kyoto-u.ac.jp/NSRG/reports/kr79/kr79pdf/Malko1.pdf On page 8, it states that before the incident, the four main pump ...


3

To produce precious metals via fission, you need a starting metal with about twice the atomic weight of the desired end product. In practical terms this means the starting metal is more precious than the end product. Furthermore, the fission process releases neutrons and gamma rays in deadly quantities, and the fission products are themselves often ...


3

This is a really good question and your question is answered directly here: https://en.wikipedia.org/wiki/Corium_(nuclear_reactor) Basically the corium is so hot that it will react with concrete producing water vapor and carbon dioxide which may further react into just hydrogen gas and carbon monoxide and that the corium can melt the concrete to about a ...


3

I am also not a nuclear engineer, but here are my thoughts on it: My impression is that you might have trouble scaling a weapon in an arbitrary dimension but that they should scale basically linearly with mass assuming an acceptable configuration of the weapon. In practice you reach a maximum size for your h-bomb based on the input energy available and ...


3

I just thought that I would clarify something that Fred said. In an emergency shutting down the nuclear chain reaction is the easy part. The main issue is the decay heat. The decay heat comes from the residual radioactive components in the core. This decay heat starts off at approximately 7% of the total reactor power. However, it drops off exponentially ...


2

There are different types of nuclear reactors. The most commons ones are pressure water reactors and boiling water reactors. The World Nuclear Association has published the following information: Pressure Water Reactors (PWR) are currently used in: the US, France, Japan, Russia & China. The number of such reactors is 277. There are 80 Boiling Water ...


2

According to the Wikipedia entry: [The earthquake] produced maximum ground g-forces of 0.56, 0.52, 0.56 (5.50, 5.07, and 5.48 m/s2) at units 2, 3, and 5 respectively. This exceeded the earthquake tolerances of 0.45, 0.45, and 0.46 g (4.38, 4.41, and 4.52 m/s2). The shock values were within the design tolerances at units 1, 4, and 6. When the ...


2

In Nuclear Thermal Rockets (NTRs) the heat from a nuclear reaction replaces the chemical energy of the propellants in a chemical rocket. In an NTR, a working fluid, usually liquid hydrogen, is heated to a high temperature in a nuclear reactor and then expands through a rocket nozzle to create thrust. The external nuclear heat source theoretically allows ...


2

The claim to "use nuclear waste as fuel" is often misleading. In theory, almost all reactors can reuse the waste, but it requires reprocessing. For example, in current LWR reactors, natural uranium is enriched and put into the reactor as fuel. When the fuel is discharged, it still has some enriched U-235 in it, along with fissionable plutonium, ...


2

Fusion bombs are typically boosted by encasing them in a fissionable (uranium) housing. The gigantic neutron flux produced by the fusion reactions then fissions the housing and boosts the yield considerably. Since the housing also acts to hold in the neutrons which help produce fusible fuel, the housing also enhances the yield of the fusion reactions as well,...


2

Slowing a neutron down in a reactor in called moderation and the material that is responsible for moderation is the moderator. Thermal Reactors In a thermal reactor, moderation is essential as neutrons tend to be born with a lot of energy (fast) but thermal reactors relying on slow (also called thermal) neutrons to perform fission. To more efficiently slow ...


2

The half life of Radium 214 is 2.46[s]. You can check half times at the following wikipedia article. I am not sure what the 3 stands for, but I suspect that this is some sort of annotation/footnote that was not properly inserted in the webpage.


2

Thorium is a non-issue, it can't go supercritical. The issue here is that we're talking about neutron-induced fission, and a Thorium nucleus that fissions will only release a single neutron. Since the some of those neutrons escape, it's necessarily subcritical. Uranium will release 3 neutrons per fission, so this can cause chain reactions. But the critical ...


1

Re: load following, from Wikipedia Modern nuclear plants with light water reactors are designed to have maneuvering capabilities in the 30-100% range with 5%/minute slope. Nuclear power plants in France and in Germany operate in load-following mode and so participate in the primary and secondary frequency control. Some units follow a variable load program ...


1

The spent fuel rods stay first for around five years under water, in a Spent fuel pool (SPF), until radiation and heat reduced enough that they can be transported. For transport, they get placed in a special capsule. In case of the CASTOR (CAsk for Storage and Transport Of Radioactive material), made by German GNS, these capsules consist of a 40 cm thick ...


1

I am not aware of any technological challenges. There have been several sodium fast reactors that have operated, the latest being the Experimental Breeder Reactor II (EBR-II) in Idaho and the Fast Flux Test Facility (FFTF) in Washington. The next step in the development is to build one and gain operational experience. I think the answer you are looking for ...


1

Your understanding is correct. In a PWR reactor, the core usually operates with the control blades fully withdrawn, but some reactors operate with the rods slightly inserted into the top of the core. In both cases, the tips of the control rods will burn out faster than the rest of the control rod. Even when the rods are fully withdrawn, the tip of the ...


1

You are correct that the average cross sections are much higher in a thermal reactor than in a fast reactor. However, it is not the magnitude of the cross sections that matter, but instead the ratio of the fission to absorption cross sections in the fuel that matter for breeding. This ratio is often referred to as the greek letter "eta" $\eta$. (...


1

We dont know. Simply we lack data beyond a certain point just before the first explosion. We therefore have to resort to mathematical modeling for deducing what happened. Now this mathematical modeling depends heavily on what data we have, and the stability of the reactor was very bad at the moment of the accident. The modeling accuracy decreases from that ...


1

Chernobyl did not have a containment structure which was Russian practice at the time. The graphite moderator caught fire which was very difficult to control without containment. The heat from the fire carried the radioactive ash material high into the atmosphere causing the extreme area of contamination. I doubt anything would contain a nuclear explosion. ...


1

One way is to use massive pond systems for evaporate cooling. Turkey Point nuclear plant. Closed loop system explained here - https://youtu.be/qt9dtYC7RDE


1

I think this is a question better for the bean counters instead of Engineering. I worked as an Intern at a natural gas fired power plant (about 500 MW total capacity) and profits were the critical factor in determining what happens at a plant. Fuel was a pass-thru cost but capital and maintenance projects were different. Capital was considered re-investment ...


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