Questions on nuclear power plant

After reading news of the Japan nuclear power plant, I have some questions:

(1) If the fuel rod heats up on their own to the point of melting, how were they created to begin with, and how were they transported, or stored?

(2) What is the source of the fuel rods? If the radioactive materials were found in mines, then there should be radioactive mines that is dangerous to live near. But I have never heard of such places.

(3) Why would the water become radioactive after coming in touch with the fuel rod? Don't the radioactive elements just decay into lead? So the water should just be polluted with lead, right?

(4) Why do spent fuel rods have to be sealed in containers and buried in ground. Why not just dump them back in the mines where they were found?

(5) nuclear weapons also contain material similar to fuel rods, how come they don't heat up or need water cooling?
 
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It's the decay products that produce all the heat.

1) The uranium is more or less inert. Until an atom is bombarded with a neutron and fissions. Then the decay products heat up the fuel rod to melting unless cooled.

2) Again uranium is very slightly radiactive and uranium mines are mostly safe. It's the decay products that are highly radioactive.

3) There's all kinds of water soluable decay products like cesium and iodine.
 

QuantumPion

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The source of the fuel rods self-heating is the radioactive decay of fission products. These fission products do not exist until a large amount of fissions occur in the reactor at power. Newly manufactured fuel is only slightly radioactive (due to the natural decay of Uranium) and is safe to handle. Likewise, raw ore or a nuclear bomb core is only slightly radioactive and not enough so to cause any substantial self-heating.
 

Astronuc

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(1) If the fuel rod heats up on their own to the point of melting, how were they created to begin with, and how were they transported, or stored?
The fuel rods are heated by the fission reactions inside the ceramic fuel. Under normal operation, they are not heated to melting. Melting of the fuel is to be avoided by ensuring that the fuel is cooled during and after operation, and the power level is well controlled, and if necessary shutdown in order to prevent overheating. Fuel coolability is one of the basic mandatory requirements of a nuclear reactor design and operation.

The ceramic fuel is UO2 or (U,Pu)O2. Some burnable absorber Gd2O3 or ZrB2 (coating) is added to some of the fuel rods to control the local power peaking during operation. BWRs also use control rods, and PWRs use soluble boron in the form of boric acid to control the fission reaction during operation.

In LWRs (light water reactors), the ceramic pellets are enclosed in sealed tubes (cladding + endplugs) made of zirconium alloys such as Zircaloy-2 (BWRs) and Zircaloy-4 (PWRs). Newer alloys containing Nb are also used in PWR fuel.

(2) What is the source of the fuel rods? If the radioactive materials were found in mines, then there should be radioactive mines that is dangerous to live near. But I have never heard of such places.
The uranium used in fuel rods is extracted from ores that contain natural decay products and other metals such as vanadium. The uranium is purified, and converted from oxide to UF6. UF6 is heated to a gas and process to increase the concentration of U-235 from the natural 0.71% to values between 3 and 5%, with the reaminder being U-238 and a small amount of U-234. Recycled uranium will have some U-236. Reprocessed uranium based fuel will contain isotopes of U+Pu. Normally the U-235, U-238 and Pu-239, 240 and 241 is recovered and reused in MOX fuel.

Uranium mine tailings can be hazardous to live with. Uranium mines and their tailings are generally removed from populated areas.

(3) Why would the water become radioactive after coming in touch with the fuel rod? Don't the radioactive elements just decay into lead? So the water should just be polluted with lead, right?
Elements (nuclides) become activated if they absorb neutrons, although a proton can combine with a neutron to form nonradioactive deuterium. Deuterium may absorb a neutron and become radioactive tritium. Otherwise, the cooling water contains corrosion products of Fe, Ni, Cr and other elements, and they become radioactive upon absorbing neutrons.

(4) Why do spent fuel rods have to be sealed in containers and buried in ground. Why not just dump them back in the mines where they were found?
Spent fuel contains fission products. We use a figure of merit called burnup, GWd/tU or MWd/kgU, to describe the energy generated by nuclear fuel. It is the energy (power integrated over time) per mass of U. Roughly, 10 GWd/tU corresponds to fission of 1% of the initial U atoms.

(5) nuclear weapons also contain material similar to fuel rods, how come they don't heat up or need water cooling?
Nuclear weapons contain highly enriched (fissile) U or Pu. Pu pits are actually warm because of the alpha decay (and some gamma heating).
 

Drakkith

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I think there's a fundamental concept missing from these explanations.
The basic reason why fuel rods don't heat up and melt themselves, and the reason that the fuel is useable in the first place is because certain materials undergo a Chain Reaction.

This means that the fuel absorbs a neutron that has been emitted from a decaying atom, and this splits apart, releasing more neutrons to do the same thing. The core uses fuel rods and neutron moderators to control the reaction to avoid too much or too little. However, as the fuel is decaying and reacting, it is releasing the decay products. These build up inside the core during normal operation and can impede the chain reaction if they readily absorb neutrons themselves.

Normal operation of the core results in an equilibrium where the decay products build up to the point where they are decaying as fast as they are being produced. (Some can absorb neutrons and turn into products that no longer affect the core as well)

The problem, like in japan recently, was that these decay products produce heat during their natural decay. Normally the reactor has cooling systems that remove this heat from both the fuel and the decay products and use it to power turbines. Even during a core shutdown the cooling system is active because of the need to control the decay heat. However, if the cooling system doesn't work, then the heat builds up to the point that the fuel starts to melt and bad things happen.

(1) If the fuel rod heats up on their own to the point of melting, how were they created to begin with, and how were they transported, or stored?
A fuel rod on it's own does not have enough material in the right physical configuration to sustain a chain reaction. It most likely produces a small amount of heat, but not much.
(3) Why would the water become radioactive after coming in touch with the fuel rod? Don't the radioactive elements just decay into lead? So the water should just be polluted with lead, right?
The water in contact with the fuel in the core is contaminated by the decay products. Each decay product is another element with its own half life. Some are very short, on the order of days, others are very long, on the order of years or hundreds of years or more. An isotope of Iodine is commonly produced with a halflife of somewhere around a week I believe. While very dangerous if ingested or inhaled, it decays fairly quickly, so if you can keep it isolated for a short period of time it will decay to a much less hazardous material.

(4) Why do spent fuel rods have to be sealed in containers and buried in ground. Why not just dump them back in the mines where they were found?
To avoid leaking the radioactive waste products AND the fuel itself into the environment. Normally the material that is used to make fuel rods is in a very low concentration, so it does relatively little harm. However in a fuel rod it is highly concentrated and any leakage can result in a much higher than normal amount contaminating the environment.
 

Astronuc

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I think there's a fundamental concept missing from these explanations.
The basic reason why fuel rods don't heat up and melt themselves, and the reason that the fuel is useable in the first place is because certain materials undergo a Chain Reaction.
The reason that the fuel doesn't melt is that cooling is provided, i.e., there is an adequate flow of coolant such that the thermal energy is carried away from the fuel rods (core) and deposited elsewhere (turbines (~32-37%) and condensers (68-63%) for most steam plants) during normal operation.

This means that the fuel absorbs a neutron that has been emitted from a decaying atom, and this splits apart, releasing more neutrons to do the same thing. The core uses fuel rods and neutron moderators to control the reaction to avoid too much or too little. However, as the fuel is decaying and reacting, it is releasing the decay products. These build up inside the core during normal operation and can impede the chain reaction if they readily absorb neutrons themselves.
In a fission chain reaction, the fast or prompt neutrons are released from the fissioning nucleus. Those account for about 0.993 of neutrons born. The smaller fraction (~0.007) of 'delayed' neutrons actually do come from decay of certain fission products. It is the presence of delayed neutrons that allows for 'control' of the fission reaction rate. At criticality, k = 1 (k = k effective), the amount of neutrons produced from fission and decay is balanced by the neutrons absorbed and leaking from the system. When k = 1, the reactor power is constant. When k < 1, power decreases, and when k > 1, power increases. The difference, Δk = k-1, determines the rate at which power changes. When k-1 > 0.0065, or the fraction of delayed neutrons, the reaction is prompt critical and power can increase orders of magnitude in a matter of seconds. This situation (the basis of a reactivity insertion accident) is to be avoided.

Normal operation of the core results in an equilibrium where the decay products build up to the point where they are decaying as fast as they are being produced. (Some can absorb neutrons and turn into products that no longer affect the core as well)
More or less correct. Those nuclides with short half-lives, i.e., shorter than the operating time of the fuel, reach equilibrium. Nuclides such as Cs-137 and Sr-90 continue to accumulate with burnup (or time of operation).
The problem, like in japan recently, was that these decay products produce heat during their natural decay. Normally the reactor has cooling systems that remove this heat from both the fuel and the decay products and use it to power turbines. Even during a core shutdown the cooling system is active because of the need to control the decay heat. However, if the cooling system doesn't work, then the heat builds up to the point that the fuel starts to melt and bad things happen.
The problem at Fukushima was loss of cooling. The decay power is much lower than operating power, but cooling is still required.


A fuel rod on it's own does not have enough material in the right physical configuration to sustain a chain reaction. It most likely produces a small amount of heat, but not much.
The first statement is correct. Fuel rods are arranged in regular arrays that constitute fuel assemblies, and a collection of fuel assemblies constitutes the core. The core is arranged to allow for criticality, energy generation and cooling.

Each fuel rod produces a fair amount of heat. Each foot (30 cm) of fuel produces between 3 to 6 kW of thermal energy - on average. Some section of fuel operate at a power of up to 10 kW/ft in a 17x17 PWR fuel rod, or up to 14 kW/ft for a 10x10 BWR fuel rod. The maximum allowable linear heat generation rate is determined by design (enrichment, enrichment distribution, fuel rod geometry) and operation.

In a PWR the fuel rod surface temperature can be as high as 350°C, but usually slightly lower, and the surface of BWR fuel rod cladding about 290°C (saturated steam conditions). During operation, the surface of the ceramic fuel pellet is around 370-400°C, while the centerline temperature around 900-1300°C.

The water in contact with the fuel in the core is contaminated by the decay products. Each decay product is another element with its own half life. Some are very short, on the order of days, others are very long, on the order of years or hundreds of years or more. An isotope of Iodine is commonly produced with a halflife of somewhere around a week I believe. While very dangerous if ingested or inhaled, it decays fairly quickly, so if you can keep it isolated for a short period of time it will decay to a much less hazardous material.
The water in core is only contaminated with fission products if the fuel rod cladding is breached. Otherwise, the corrosion products that accumulate on the fuel become 'activated' by neutron absorption, and some small amount of hydrogen may become tritium.

To avoid leaking the radioactive waste products AND the fuel itself into the environment. Normally the material that is used to make fuel rods is in a very low concentration, so it does relatively little harm. However in a fuel rod it is highly concentrated and any leakage can result in a much higher than normal amount contaminating the environment.
Ideally, during operation, fuel does not fail, i.e., the cladding is not breached. However, fuel rods do occassionally fail. There are treatment systems to collect the fission products and hold them until they decay.

This page describes the fission reaction and the distribution of fission energy.
http://hyperphysics.phy-astr.gsu.edu/Hbase/nucene/u235chn.html

The part about Neutrons 12 should be Neutrinos 12.
 

Drakkith

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The reason that the fuel doesn't melt is that cooling is provided, i.e., there is an adequate flow of coolant such that the thermal energy is carried away from the fuel rods (core) and deposited elsewhere (turbines (~32-37%) and condensers (68-63%) for most steam plants) during normal operation.
Of course! But as was explained by someone a while back, fuel rods that are outside the core aren't in a physical arrangement with other fuel rods to allow them to sustain a chain reaction, correct? No chain reaction means no cooling, correct? (Ignoring built up waste products in the core of course)

The problem at Fukushima was loss of cooling. The decay power is much lower than operating power, but cooling is still required.
Yes, I never meant to imply otherwise.

Each fuel rod produces a fair amount of heat. Each foot (30 cm) of fuel produces between 3 to 6 kW of thermal energy - on average. Some section of fuel operate at a power of up to 10 kW/ft in a 17x17 PWR fuel rod, or up to 14 kW/ft for a 10x10 BWR fuel rod. The maximum allowable linear heat generation rate is determined by design (enrichment, enrichment distribution, fuel rod geometry) and operation.
I meant a fuel rod on its own, removed from the core or prior to install. How much heat would a brand new fuel rod in transport produce? (Lets say an "average fuel rod", if there is one)

The water in core is only contaminated with fission products if the fuel rod cladding is breached. Otherwise, the corrosion products that accumulate on the fuel become 'activated' by neutron absorption, and some small amount of hydrogen may become tritium.
Ah, ok. That makes sense.

Ideally, during operation, fuel does not fail, i.e., the cladding is not breached. However, fuel rods do occassionally fail. There are treatment systems to collect the fission products and hold them until they decay.
Are you talking about during operation, or during storage of the rods? Or both?

Thanks Astronuc.
 

QuantumPion

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Of course! But as was explained by someone a while back, fuel rods that are outside the core aren't in a physical arrangement with other fuel rods to allow them to sustain a chain reaction, correct? No chain reaction means no cooling, correct? (Ignoring built up waste products in the core of course)

I meant a fuel rod on its own, removed from the core or prior to install. How much heat would a brand new fuel rod in transport produce? (Lets say an "average fuel rod", if there is one)
No, chain reactions have nothing to do with decay heat or cooling, they are completely unrelated concepts. You could expose a non-fissile material like Nickle to a high neutron flux and make it so radioactive that it can melt under its own heat. Fuel assemblies, once exposed, are always generating heat and so are be cooling one way or another.

As I mentioned previously, fresh fuel rods are barely radioactive at all and are safe to handle. The decay heat of a fresh assembly is something like a a few milliwatts.

Are you talking about during operation, or during storage of the rods? Or both?

Thanks Astronuc.
During operation. The flow rate in the core during operation is very high and thus any movement of the fuel rod or contact with debris will quickly damage the thin cladding. Fuel rods can fail due to rubbing against the frame of the assembly if they are not tightly held in place, or more commonly due to debris in the core (metal shavings, small bolts, etc).
 

Drakkith

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No, chain reactions have nothing to do with decay heat or cooling, they are completely unrelated concepts. You could expose a non-fissile material like Nickle to a high neutron flux and make it so radioactive that it can melt under its own heat. Fuel assemblies, once exposed, are always generating heat and so are be cooling one way or another.

As I mentioned previously, fresh fuel rods are barely radioactive at all and are safe to handle. The decay heat of a fresh assembly is something like a a few milliwatts.
Lol, I'm not talking about decay heat here. I'm saying that the fuel rods don't heat themselves up to melting right after assembly and during transport because there isn't a chain reaction like there is in the core and there isn't a high amount of decay products. This is answering question 1. of the OP's post.



During operation. The flow rate in the core during operation is very high and thus any movement of the fuel rod or contact with debris will quickly damage the thin cladding. Fuel rods can fail due to rubbing against the frame of the assembly if they are not tightly held in place, or more commonly due to debris in the core (metal shavings, small bolts, etc).
ah ok.
 

Astronuc

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Are you talking about during operation, or during storage of the rods? Or both?
Ideally, fuel rods do not fail - before use, during use, and after use. Failure = breach of cladding, or loss of hermeticity.

The cladding is the first barrier between fission products and the environment. The primary function of the cladding is to transfer heat (thermal energy) from the ceramic fuel in which fission occurs to the coolant while retaining any fission products, typically fission product gases (Xe, Kr), volatiles (Cs, I), transuranic nuclides (Np, Pu, Am, Cm, . . . ), and recoil fission products. The cladding also keeps the water coolant from chemically reacting (oxidation) with the fuel and fission products.

When the cladding or end plug is breached, the fuel rod is considered failed.

If it fails before operation - it cannot be used. That's an economic loss.

If it fails during operation, the fission products and possibly fuel particles (if the breach is severe) contaminate the coolant and parts of the primary system where the coolant travels. There are filters to collect fission products and corrosion particulates, but they are not 100% efficient, and some f.p.s and fuel particles will collect in the crud (corrosion products) on the fuel. Any residual fuel particles on the outside of the cladding then release fission products directly into the coolant when that fuel operates in the core.

If it fails after operation, e.g., in the SFP, it contaminates the SFP water and could provide exposure to workers. If it fails in dry storage, it contaminates the dry storage cask, which then complicates retrieval if retrieval is necessary.

Ideally, once the fuel rod is manufactured, it does not fail.

However as QuantumPion pointed out, fuel rods do fail occassionally, due to debris fretting (foreign material, e.g., wire bristles from a cleaning brush, or metal shavings from a machining operation), grid-to-rod fretting (the fuel rods or assembly(s) experiences flow-induced vibration which induces wear on cladding (tribology)), primary hydriding (hydrogenous material left inside the rod hydrides the cladding reducing its ability to maintain hermeticity), or PCMI/PCI (thermal expansion of pellets causes cladding strain and stress such that in the presence of iodine a crack initiates and propagates).

A fresh fuel rod or assembly has very little radioactivity, which is really in the fuel pellet, and does not undergo fission in the absence of a critical configuration outside the core. Spent fuel does generate heat, and spent fuel assembly or group of assemblies does need to be cooled.
 

OmCheeto

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After reading news of the Japan nuclear power plant, I have some questions:

(2) What is the source of the fuel rods? If the radioactive materials were found in mines, then there should be radioactive mines that is dangerous to live near. But I have never heard of such places.
An interesting, somewhat related read: http://www.scientificamerican.com/article.cfm?id=ancient-nuclear-reactor"

Two billion years ago parts of an African uranium deposit spontaneously underwent nuclear fission. The details of this remarkable phenomenon are just now becoming clear
For those curious of the workings of the universe. :smile:

----------------------------------
Before PhysFor, there was only SciAm.
 
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OmCheeto

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ps. welcome to Physics Forums, anti_matter.
 

Drakkith

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Astronuc

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link to Global Public Square interview with Nathan Myhrvold putting a smiley face on nuclear waste...

http://www.cnn.com/video/#/video/world/2011/06/05/gps.nathan.myhrvold.cnn?iref=allsearch

Question about nuclear waste...is it possible that it can be used as a "fuel source"?
It depends on what one considers nuclear waste. If by nuclear waste, one is actually referring to spent fuel, then there is unused fuel, but is must be reconfigured for a fast reactor or other design.

If by nuclear waste, one means the fission products, then that is not useful as a nuclear fuel, but it could be useful for processes requiring radiation.
 

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