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When a reactor is shut down the core produces much less heat, but they do still produce heat through a mechanism known as decay heat. The fact that the core is producing less heat means that the coolant temperature is going to drop, but how far that temperature drops depends on the decay heat generation rate. This in turn is based on operating history, or ...

13

Xenon is a result of the nuclear reaction and a neutron poison. If you don't wait for the xenon to decay, it eats up too many neutrons and you can't go critical. They always say "there are not enough rods to pull." If you have a nice new reactive core, you can get started sooner. If the core is old, you will have to wait a long time before enough xenon (...

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.

9

The RM-1 Russian submarine reactor had a core of less than one cubic metre. It had about 100kg fuel load, which was 90% enriched (i.e. 90kg) Uranium 235. This was liquid-metal cooled [specifically a "eutectic lead-bismuth alloy (44.5 wt% lead, 55.5 wt% bismuth)" - source as below, p40], so didn't need a moderator. Submarine 901 had in its right-...

9

Carriers are ships intended to project power. The USA sits on a continent without any nearby enemies. So, American military doctrine is expeditionary in nature. The assumption is that if America fights a war, it will be abroad and therefore it needs the means to project force abroad. Therefore, a strong navy with a nuclear powered carrier element was seen as ...

7

The answer really boils down to two factors: safety and testing. I'm going to give a generic summary of these two things below, but the real answer is quite complicated. The crux of nuclear plant operation revolves around nuclear safety. I'm not talking about personal safety, which is the purview of the Occupational Safety and Health Administration (OSHA),...

7

The Oak Ridge National Laboratory had an experimental thorium reactor in the 1960s, but the program was terminated in the 1970s. This can be partially be attributed to the needs of both the military and businesses, at the time. Unlike $^{235}{U}$, thorium is not naturally fissile, it needs to be bombarded with slow neutrons to ultimately produce $^{233}{U}$,...

6

Fission Products Extraction The seperation of plutionium and uranium from the other fission products is done with the organic molecule tributyl phosphate by liquid-liquid-extraction. You have two phases, one organic and one aqueous. The fission products will solve in the aqueous phase and uranium/plutonium will solve in the organic phase with the tributyl ...

6

You could try Wikipedia, but they note that the article may not meet there quality standards, so I'll stay away from there as a source. The World Nuclear Association seems more credible, anyway. Their page discusses the process. Quoting the section entitled "Reprocessing today – PUREX" (emphasis mine): All commercial reprocessing plants use the ...

5

The likely reason is that simply pouring concrete over it would not have contained it. The site is smoldering and emits gases of various types. It also has voids and shifts around, various areas collapsing into pockets. If it were covered with concrete, the concrete would develop cracks and radioactive particles would be emitted. The sarcophagus structure ...

5

As I wrote over on a similar question on our sister Physics site, the German THTR-300 Thorium High-Temperature Reactor operated for about 16,000 hours and the IAEA has data on it. So, a thorium-fuelled nuclear reactor has been developed and operated. So it's possible. High-grade uranium ore hasn't been in short supply, and uranium ore is a tiny part of ...

5

The barometric height formula is defined as: $$p(z) = p(0)\exp(-Mgz/(RT))$$ For a centrifuge the upper expression is transformed to: $$p(r) = p(0)\exp(M\omega^2r^2/(2RT))$$ As you see the $g$ is transformed to the radial acceleration $a = \omega^2 r$. The factor $2$ stems from the integration which you have to do during the derivation of the barometric ...

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 ...

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 ...

4

The type of machine that runs the simulation (e.g. super computer, slide rule, pen and paper, etc.) is less important than the physical and mathematical models that describe the process being simulated. Throwing more computing resources at a simulation whose fundamental equations are flawed will not improve the accuracy of the solution. To answer your ...

4

Pu238 is not usually isotopically separated from spent nuclear fuel for exactly the reason you pointed out, it would be very difficult. Instead most of what we have comes from one of two different processes. The first is bombardment of Np237 (also made in a nuclear reactor) with neutrons. It will become Np238 and then undergo a beta emission (fancy way of ...

4

The original sarcophagus built around 1986 has some issues and it was estimated in 1988 that it would only last 20-30 years (here we are). Check out this link that explains some of the main issues with the current sarcophagus; namely structural instability. Trying to repair the current sarcophagus is impractical. The new confinement structure is designed ...

3

The reality of generating electricity from nuclear fusion on an industrial scale is still a very long ways off. It is speculative high cost, high risk venture, with a potential high payoff far off into the future. Test reactors alone are >$100 million. The existing technical problems and challenges with nuclear fusion are very complex. We're talking ... 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 Radioisotope thermoelectric generators have been used in space for decades, both on the moon missions and on deep-space probes where solar cells would not be practical. 3 The accuracy of a simulation depends more on how well your equations and assumptions model reality, than the resolution of the simulation (computing power, as Carlton has stated). More computing power does let you have your solution in much less time however. A model, by definition, is not the real thing. No model can be trusted unless it is tested in the ... 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 It largely depends on the critical mass of the fuel - that is, the smallest amount of fuel that is needed for a controlled, sustained reaction to occur. This is often on the order of ten or so kilograms, but can vary with the other aspects of the design (see, for example, this paper on$^{238}\text{Pu}\$). The Wikipedia page also has a nice summary from ...

2

This is a rather complex question as there are multiple variables and many proposed designs for thorium fuel cycles to consider as well; but it seems your primary interest is in whether or not leaving the Pa-233 in solution will adversely affect the thorium nuclear cycle to the point where it makes more sense to remove this isotope to be reintroduced after ...

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 ...

2

Fusion is widely extolled by advocates as being safe (but is that collective opinion justified)? I would prefer to begin my discussion comparing fusion with renewable energy systems with an examination of safety as in my opinion, safety should always come first when we compare energy alternatives. Some Reasons offered why Fusion should be Safe - - Only ...

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

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

I'm not an expert on this so this is an "educated guess". It boils down to what the advantage of nuclear power is. To laypeople it sounds cool, but for the military you need a tangible advantage. That advantage is (see the Wikipedia article on nuclear submarines): The performance advantages of nuclear submarines over "conventional" (...

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