Importance of fusion stage in thermonuclear weapon

[delsaber8]

Hi guys, I'm pretty new to the site and I have a few questions, but I'll start with the most imporatant ones or at least the ones I would like answered first.

Anyway so my understanding is that in a thermonuclear weapon the large majority of the explosive power comes from the fission reaction ( correct me if I am wrong), which leads to my question: What exactly does the fusion step do that makes these weapons so much stronger than pure fission weapons. Does the fast neutrons expelled during the fusion of deuterium and tritium split the U-238 atoms in the jacket or does it do something else?

P.S. I hope this is in the right part of the forum.

 

[jim hardy]

There's a book called "The Curve of Binding Energy" by John McPhee that is an introduction to things nuclear for us civilians.

Th answer to your question is :
Fission starts on right hand end of curve where the slope is gentle and moves left .
Fusion starts on left where slope is steep and moves right.
Both move up the curve toward the middle, but compare the respective Δenergies.

image courtesy wikipedia

 

[SteamKing]

Hi guys, I'm pretty new to the site and I have a few questions, but I'll start with the most imporatant ones or at least the ones I would like answered first.

Anyway so my understanding is that in a thermonuclear weapon the large majority of the explosive power comes from the fission reaction ( correct me if I am wrong), which leads to my question: What exactly does the fusion step do that makes these weapons so much stronger than pure fission weapons. Does the fast neutrons expelled during the fusion of deuterium and tritium split the U-238 atoms in the jacket or does it do something else?

P.S. I hope this is in the right part of the forum.

H-bombs can either be two-stage or three-stage devices.

In the ordinary two-stage device, the primary component, the fission bomb, is detonated first to provide the necessary heat and radiation to initiate the secondary device, the fusion component, which is fueled by deuterium. If the device contains a third stage, this is usually a jacket of U-238, which does not normally fission unless struck by extremely fast neutrons produced in the fusion explosion. The U-238 jacket also keeps the device from dis-assembling momentarily while the first two explosions develop.

By nature, fission reactions are limited in the amount of yield, or explosive force, they can produce. The unclassified record for a fission yield is approx. 500 Kilotons of TNT.

OTOH, fusion reactions can produce much bigger explosions than fission reactions. The largest fusion device (detonated by the USSR in 1961) had a yield estimated at 50 Megatons of TNT, or two orders of magnitude larger than the biggest fission explosion. [This latter device was reportedly an example of a three-stage weapon] Reportedly, there are no theoretical limits on the size of a fusion explosion, only practical ones in constructing the actual device. Only weapons designers know for sure.

Fusion devices can also provide a degree of flexibility in the amount of yield produced, whereas it is more difficult to adjust the yield of a fission device once it is assembled and deployed. The yield of a fusion device can reportedly be changed in a tactical situation by adding varying amounts of either deuterium or tritium fuel to the weapon before it is detonated.

http://en.wikipedia.org/wiki/Nuclear_weapon

http://en.wikipedia.org/wiki/Nuclear_weapon_yield

The latter reference includes some basic data on the physical characteristics and yields of various nuclear devices.

 

[delsaber8]

H-bombs can either be two-stage or three-stage devices.

In the ordinary two-stage device, the primary component, the fission bomb, is detonated first to provide the necessary heat and radiation to initiate the secondary device, the fusion component, which is fueled by deuterium. If the device contains a third stage, this is usually a jacket of U-238, which does not normally fission unless struck by extremely fast neutrons produced in the fusion explosion. The U-238 jacket also keeps the device from dis-assembling momentarily while the first two explosions develop.

By nature, fission reactions are limited in the amount of yield, or explosive force, they can produce. The unclassified record for a fission yield is approx. 500 Kilotons of TNT.

OTOH, fusion reactions can produce much bigger explosions than fission reactions. The largest fusion device (detonated by the USSR in 1961) had a yield estimated at 50 Megatons of TNT, or two orders of magnitude larger than the biggest fission explosion. [This latter device was reportedly an example of a three-stage weapon] Reportedly, there are no theoretical limits on the size of a fusion explosion, only practical ones in constructing the actual device. Only weapons designers know for sure.

Fusion devices can also provide a degree of flexibility in the amount of yield produced, whereas it is more difficult to adjust the yield of a fission device once it is assembled and deployed. The yield of a fusion device can reportedly be changed in a tactical situation by adding varying amounts of either deuterium or tritium fuel to the weapon before it is detonated.

http://en.wikipedia.org/wiki/Nuclear_weapon

http://en.wikipedia.org/wiki/Nuclear_weapon_yield

The latter reference includes some basic data on the physical characteristics and yields of various nuclear devices.

See now, I'm slightly confused, based on what I have read from both replies I would assume that fusion is the main source of power in a hydrogen bomb. However when I went back to the original wikipedia article I found this: "It is colloquially referred to as a hydrogen bomb or H-bomb because it employs hydrogen fusion, though in most applications the majority of its destructive energy comes from uranium fission, not hydrogen fusion alone." Only to find later on in the article it states something closer to what you said: "...when they detonated the massive and unwieldy Tsar Bomba, a 50 megaton hydrogen bomb that derived almost 97% of its energy from fusion."

http://en.wikipedia.org/wiki/Thermonuclear_weapon

Now perhaps I have glossed over something important, but these two sentences from the same article seem to contradict each other, is it that the Tsar Bomba was a special case of hydrogen bomb?

Could someone provide some insight?

 

[Astronuc]

See now, I'm slightly confused, based on what I have read from both replies I would assume that fusion is the main source of power in a hydrogen bomb. However when I went back to the original wikipedia article I found this: "It is colloquially referred to as a hydrogen bomb or H-bomb because it employs hydrogen fusion, though in most applications the majority of its destructive energy comes from uranium fission, not hydrogen fusion alone." Only to find later on in the article it states something closer to what you said: "...when they detonated the massive and unwieldy Tsar Bomba, a 50 megaton hydrogen bomb that derived almost 97% of its energy from fusion."

http://en.wikipedia.org/wiki/Thermonuclear_weapon

Now perhaps I have glossed over something important, but these two sentences from the same article seem to contradict each other, is it that the Tsar Bomba was a special case of hydrogen bomb?

Could someone provide some insight?

One may be referring to a 'boosted' fission device in which fast neutrons from a small fusion system enhances the fission process. Otherwise, one may design a 'thermonuclear' device in which fusion provides the majority of the energy.

Wikipedia articles may not be entirely accurate on such technology.

 

[delsaber8]

One may be referring to a 'boosted' fission device in which fast neutrons from a small fusion system enhances the fission process. Otherwise, one may design a 'thermonuclear' device in which fusion provides the majority of the energy.

Wikipedia articles may not be entirely

So just to clarify only a boosted fission device would have the majority of its energy produced through fission whereas a hydrogen bomb would get (except in a perhaps a select few cases) the majority of its energy from Deuterium and Tritium fusion?

 

[Astronuc]

So just to clarify only a boosted fission device would have the majority of its energy produced through fission whereas a hydrogen bomb would get (except in a perhaps a select few cases) the majority of its energy from Deuterium and Tritium fusion?

Correct.

 

[SteamKing]

Fission devices can produce yields as small as 0.01 KT (10 tons) of TNT. The primary reason fusion devices were developed in the 1950s was to produce a single weapon having a large yield, on the order of megatons rather than kilotons.

After initial development of these massive devices, the yields of deployed warheads were much smaller than theoretically possible. The largest warhead deployed by the US was installed on the Titan II ICBM and had a yield of 9 MT. When submarine launched ballistic missiles were developed (the US Polaris and Poseidon missiles) and land-based ICBMs were equipped with MIRV systems, the individual warheads had yields below 500 KT.

The megaton devices were used primarily for demonstration purposes, to let each side know that the other had the capability of making such a device. Once the Test Ban Treaty went into effect in 1963 and atmospheric testing of nuclear devices was no longer permitted, much of the propaganda value of staging tests of of extremely large-yield bombs evaporated, and resources were concentrated on the manufacture of warheads with more modest yields.

 

[etudiant]

The 'boosted' hydrogen bombs use a small fission bomb to set off the hydrogen fusion, which generates a flood of neutrons that will cause the bomb case, made of U238, to fission. The Tsar Bomba case was made of lead instead of uranium, which eliminated that half of the potential yield. Most bombs are fission, fusion, fission devices, because U238 is such a cheap way to get the bomb case to contribute.

 

[jim hardy]

The largest fusion device (detonated by the USSR in 1961) had a yield estimated at 50 Megatons of TNT, or two orders of magnitude larger than the biggest fission explosion.

Just a piece of trivia...

My Dad was a weather forecaster at Miami Airport office back then. The shock wave from that thing gave two blips on the Miami barometer chart as it went by , one from each direction i guess.

He took us kids to the office to see it.

The idea of something man-made perturbing the whole Earth was mind boggling at the time.
Memory of that lent an air of immediacy to Cuban crisis the next year.

 

[mheslep]

... because U238 is such a cheap way to get the bomb case to contribute.

I believe the main reason for the addition of that third fission stage, instead of simply adding more fusion fuel, is that fission has qualities that produce a more destructive effect than does fusion. That is, something about fission reaction (high mass of fission products?) do more damage than the neutrons given off by fusion, even if the net energy is the same. The neutron bomb has that intention: smaller blast effect, large lethal radiation dose.

 

[mheslep]

Hi guys, I'm pretty new to the site and I have a few questions, but I'll start with the most imporatant ones or at least the ones I would like answered first.

Anyway so my understanding is that in a thermonuclear weapon the large majority of the explosive power comes from the fission reaction ( correct me if I am wrong), which leads to my question: What exactly does the fusion step do that makes these weapons so much stronger than pure fission weapons. Does the fast neutrons expelled during the fusion of deuterium and tritium split the U-238 atoms in the jacket or does it do something else?

P.S. I hope this is in the right part of the forum.

As others have indicated, the initial, or primary, stage of a nuclear weapon is limited in its yield to the kiloton range. The reason for this is that the primary explosion works on the well known nuclear chain reaction principle. The weapon is detonated with a few neutrons which initiate fission. Each fission event produces more than two fast neutrons moving at a fraction of the speed of light, so that the number of fission events doubles every 10ns or so. At the same time, the kinetic or average thermal energy of the fissionable mass rapidly increases tending to blow the mass apart, at which point fission of the primary stops. The race between fission and explosive dis-assembly ends after some fifty generations of fission or so, depending on design factors like the containment (or 'tamper'). Most of the energy release of the weapon occurs in the last couple generations.

However, the fusion stage of the weapon does not rely on neutron chain reaction but is ignited by output energy of the primary in a thermal manner, i.e. more simultaneously, hence the term thermonuclear. Consequently, the fusion stage of these weapons could theoretically be made much larger than that which has been tested so far (Tsar Bomba).

The ignition of the fusion stage renders its energy in the form a colossal quantity of neutrons. Thus, an enclosure around the fusion stage, made of the fissionable material U-238, no longer depends only a chain reaction to reach large yields, and the fission cross section of U-238 is in the range of the neutrons produced by the fusion explosion.

Teller-Ulam three stage thermonuclear design is shown here

 

[delsaber8]

As others have indicated, the initial, or primary, stage of a nuclear weapon is limited in its yield to the kiloton range. The reason for this is that the primary explosion works on the well known nuclear chain reaction principle. The weapon is detonated with a few neutrons which initiate fission. Each fission event produces more than two fast neutrons moving at a fraction of the speed of light, so that the number of fission events doubles every 10ns or so. At the same time, the kinetic or average thermal energy of the fissionable mass rapidly increases tending to blow the mass apart, at which point fission of the primary stops. The race between fission and explosive dis-assembly ends after some fifty generations of fission or so, depending on design factors like the containment (or 'tamper'). Most of the energy release of the weapon occurs in the last couple generations.

However, the fusion stage of the weapon does not rely on neutron chain reaction but is ignited by output energy of the primary in a thermal manner, i.e. more simultaneously, hence the term thermonuclear. Consequently, the fusion stage of these weapons could theoretically be made much larger than that which has been tested so far (Tsar Bomba).

The ignition of the fusion stage renders its energy in the form a colossal quantity of neutrons. Thus, an enclosure around the fusion stage, made of the fissionable material U-238, no longer depends only a chain reaction to reach large yields, and the fission cross section of U-238 is in the range of the neutrons produced by the fusion explosion.

Teller-Ulam three stage thermonuclear design is shown here

Since you brought up the staging I suppose it wouldn't be a bad time to ask, what would the third stage would be? See I was under the impression that the U-238 Jacket did not count as a stage. Also I was reading that the jacket stage can produce roughly half the yield in large bombs, why is this?

 

[mheslep]

Since you brought up the staging I suppose it wouldn't be a bad time to ask, what would the third stage would be? See I was under the impression that the U-238 Jacket did not count as a stage. Also I was reading that the jacket stage can produce roughly half the yield in large bombs, why is this?

The weapon design I described has the common label "fission-fusion-fission"; that second fission refers to the U-238 jacket I described above.

 

[nikkkom]

I believe the main reason for the addition of that third fission stage, instead of simply adding more fusion fuel, is that fission has qualities that produce a more destructive effect than does fusion. That is, something about fission reaction (high mass of fission products?) do more damage than the neutrons given off by fusion, even if the net energy is the same.

No, there won't be more destruction per unit energy.

Fission has one difference: it generates fallout. I would not classify it as "destruction" per se, but it does make aftermath even worse.

The neutron bomb has that intention: smaller blast effect, large lethal radiation dose.

Neutron bombs are tiny nukes which use as _little_ fission as possible, so that fusion neutron flux is high, yet the explosion is relatively small.

 

[etudiant]

No, there won't be more destruction per unit energy.

Fission has one difference: it generates fallout. I would not classify it as "destruction" per se, but it does make aftermath even worse.

Neutron bombs are tiny nukes which use as _little_ fission as possible, so that fusion neutron flux is high, yet the explosion is relatively small.

The search for a non fission initiated fusion device was certainly spurred by the neutron bomb concept, as the result could be a neutron bomb without the messy and expensive fission trigger.

Apart from the fallout aspect, which is military undesirable because it is so unpredictable, there is no fundamental difference between fission and fusion bomb impacts. The fusion bombs tend to be much bigger, so the initial thermal pulse is much longer, which justifies immediate sheltering just to avoid getting cooked alive. The main destruction is the heat pulse and the subsequent blast in both cases. The militarily attractive feature of the neutron bomb of course is that lethality is maintained despite the much smaller heat and blast elements.

 

[delsaber8]

[/QUOTE]Neutron bombs are tiny nukes which use as _little_ fission as possible, so that fusion neutron flux is high, yet the explosion is relatively small.[/QUOTE]

So how exactly do they achieve a higher neutron production? I understand that the bomb's shell that might otherwise absorb neutrons is replaced with thinner materials, that allow neutrons to escape, but is their another aspect to them that causes higher neutron flux via fusion?

 

[SteamKing]

Neutron bombs are tiny nukes which use as _little_ fission as possible, so that fusion neutron flux is high, yet the explosion is relatively small.

So how exactly do they achieve a higher neutron production? I understand that the bomb's shell that might otherwise absorb neutrons is replaced with thinner materials, that allow neutrons to escape, but is their another aspect to them that causes higher neutron flux via fusion?

Enhanced Radiation weapons tend to use tritium in place of deuterium to increase the neutron flux when the device is detonated:

http://en.wikipedia.org/wiki/Neutron_bomb

These weapons need to be tended carefully because of the short half-life of tritium (approx. 12.3 years). These devices must have the tritium replenished periodically. Otherwise, the outer casing is made as lightly as possible, with no uranium tampers or such, so the flux of neutrons coming from the blast is not inhibited.

 

[delsaber8]

Enhanced Radiation weapons tend to use tritium in place of deuterium to increase the neutron flux when the device is detonated:

http://en.wikipedia.org/wiki/Neutron_bomb

These weapons need to be tended carefully because of the short half-life of tritium (approx. 12.3 years). These devices must have the tritium replenished periodically. Otherwise, the outer casing is made as lightly as possible, with no uranium tampers or such, so the flux of neutrons coming from the blast is not inhibited.

Hmm I see, so would using tritium get an extra neutron per fusion reaction?

 

[mheslep]

No, there won't be more destruction per unit energy.

Fission has one difference: it generates fallout...

No? I can't find the reference, so take this with a grain of salt, but I thought it has something to do with penetrating nature of fast neutrons - that they may encounter some 900 light element nuclei, possibly over large distances, gradually giving up their energy. Where as the relatively massive charged fission nuclei must have a far shorter mean free path, and thus give up their kinetic energy in much more confined space. The net effect was that fission was supposed to yield more of its energy in shock waves and blast effect.

 

[SteamKing]

Hmm I see, so would using tritium get an extra neutron per fusion reaction?

Each T-T fusion produces 2 neutrons.

http://en.wikipedia.org/wiki/Nuclear_fusion

 

[delsaber8]

Each T-T fusion produces 2 neutrons.

http://en.wikipedia.org/wiki/Nuclear_fusion

Actually while I was reading the wiki article, I came across across some sort of lithium-7 fission. Now I've actually never heard of this before now. How major is the role this reaction plays, in modern thermonuclear weapons. Because the Wiki claims it boosted the expected yield to 150% of what was expected. The reason I ask is because I had trouble understanding the next paragraph regarding its usefulness.

 

[etudiant]

Once the yield gets to be more than a few kilotons, the blast pressure will do pretty much the same damage whether from fission or fusion.
The B-61 fusion bomb yield can be modulated quite substantially, at least over a 5:1 yield range, presumably by increasing the tritium component, thus allowing a single bomb to serve both for tactical as well as selected strategic uses.
The principal fusion component afaik is usually lithium deuteride. Tritium was used in the initial H bomb demonstrator, which was correspondingly huge and entirely ground bound. It would not even have fit on a ship, much less an airplane. I don't know if using lithium 7 in the lithium deuteride would help the fusion reaction.
Fallout has usually been from dirt that is sucked up by the fireball and that gets irradiated in the process.
I don't think the nuclear materials themselves are the major factor, even though most of it survives the nuclear reaction. While nuclear bombs are pretty inefficient, the volumes of dirt mobilized just swamp the couple of tons of residual bomb material. This is why strikes against hardened missile silos are much dirtier than air bursts, even though the latter may do more extensive blast and heat damage.

 

[mheslep]

Each T-T fusion produces 2 neutrons.

http://en.wikipedia.org/wiki/Nuclear_fusion

I'm not sure how much T-T burn would take place. D-T is 100x more likely to occur at its peak temperature (~60keV) , though I suppose much higher temperatures might occur in a weapon.

 

[SteamKing]

Actually while I was reading the wiki article, I came across across some sort of lithium-7 fission. Now I've actually never heard of this before now. How major is the role this reaction plays, in modern thermonuclear weapons. Because the Wiki claims it boosted the expected yield to 150% of what was expected. The reason I ask is because I had trouble understanding the next paragraph regarding its usefulness.

In fission reactions, the concept of the 'cross-section' is important in understanding how likely a particular reaction will occur. For the most common type of uranium fission reaction, U-235, this isotope is likely to fission when the uranium nucleus is struck by a neutron have a certain minimum energy, whereas, if a U-238 nucleus were struck by a neutron with the same energy, no fission reaction would occur. In a hydrogen bomb, the fusion reactions produce much more energetic neutrons, which, when they strike U-238 nuclei, cause this isotope to fission.

For the early hydrogen weapons tests, cryogenically stored deuterium was used as the fusion fuel, but this meant the weapon weighed many tons and was impractical to deliver in a combat situation. The development of the so-called 'dry' fusion weapons required that the deuterium fuel be compounded chemically with another element which would not inhibit fusion reactions and which would keep the weight of the device to a minimum and be chemically stable at room temperature for a long time.

When it was decided that lithium was the ideal choice to compound with deuterium, physicists thought that the Li-6 isotope would fuse readily given the conditions produced by detonating the primary. There was a program to separate Li-6 from the more common Li-7 isotope, much in the same way that U-235 was separated from U-238. In the first test of a lithium-deuteride device, the lithium-6 isotope had been enriched from the natural 7.5% to approx. 40%.

http://en.wikipedia.org/wiki/Castle_Bravo

The reactions occurring in the lithium were poorly understood when this device was designed and tested, which was why the extra yield came as a big surprise to the observers at the test site. In analyzing the results of the test to figure out why the device had such a greater yield, it was discovered that Li-7, rather than remaining inert, can actually capture very energetic neutrons, like the type produced in a fusion blast, and then decay into an alpha particle, a tritium nucleus, and emit another neutron. The tritium is additional fuel which can fuse during the detonation, and the neutron, being quite energetic, can go on to cause additional fission reactions in the U-238 tamper.

The original designers had overlooked the fact that while Li-7 has a very small 'cross-section' for the neutrons emitted in the fission blast, once the more energetic neutrons from the fusion reactions start to be produced, the 'cross-section' of Li-7 is much larger, and this isotope is more likely to capture and react with these energetic neutrons. The designers' lack of understanding on this point can be understood, since Castle Bravo was only the second H-bomb test but the first to use lithium-deuteride as the fuel.

This was an important discovery since a.) it meant that isotope separation of lithium into Li-6 and Li-7 would not be required, causing a considerable saving in time and money when it came to mass-producing hydrogen weapons, and b.) each weapon could be smaller for a given yield than originally thought, due to the extra energy which the Li-7 reactions would produce during detonation.

 

[delsaber8]

In fission reactions, the concept of the 'cross-section' is important in understanding how likely a particular reaction will occur. For the most common type of uranium fission reaction, U-235, this isotope is likely to fission when the uranium nucleus is struck by a neutron have a certain minimum energy, whereas, if a U-238 nucleus were struck by a neutron with the same energy, no fission reaction would occur. In a hydrogen bomb, the fusion reactions produce much more energetic neutrons, which, when they strike U-238 nuclei, cause this isotope to fission.

For the early hydrogen weapons tests, cryogenically stored deuterium was used as the fusion fuel, but this meant the weapon weighed many tons and was impractical to deliver in a combat situation. The development of the so-called 'dry' fusion weapons required that the deuterium fuel be compounded chemically with another element which would not inhibit fusion reactions and which would keep the weight of the device to a minimum and be chemically stable at room temperature for a long time.

When it was decided that lithium was the ideal choice to compound with deuterium, physicists thought that the Li-6 isotope would fuse readily given the conditions produced by detonating the primary. There was a program to separate Li-6 from the more common Li-7 isotope, much in the same way that U-235 was separated from U-238. In the first test of a lithium-deuteride device, the lithium-6 isotope had been enriched from the natural 7.5% to approx. 40%.

http://en.wikipedia.org/wiki/Castle_Bravo

The reactions occurring in the lithium were poorly understood when this device was designed and tested, which was why the extra yield came as a big surprise to the observers at the test site. In analyzing the results of the test to figure out why the device had such a greater yield, it was discovered that Li-7, rather than remaining inert, can actually capture very energetic neutrons, like the type produced in a fusion blast, and then decay into an alpha particle, a tritium nucleus, and emit another neutron. The tritium is additional fuel which can fuse during the detonation, and the neutron, being quite energetic, can go on to cause additional fission reactions in the U-238 tamper.

The original designers had overlooked the fact that while Li-7 has a very small 'cross-section' for the neutrons emitted in the fission blast, once the more energetic neutrons from the fusion reactions start to be produced, the 'cross-section' of Li-7 is much larger, and this isotope is more likely to capture and react with these energetic neutrons. The designers' lack of understanding on this point can be understood, since Castle Bravo was only the second H-bomb test but the first to use lithium-deuteride as the fuel.

This was an important discovery since a.) it meant that isotope separation of lithium into Li-6 and Li-7 would not be required, causing a considerable saving in time and money when it came to mass-producing hydrogen weapons, and b.) each weapon could be smaller for a given yield than originally thought, due to the extra energy which the Li-7 reactions would produce during detonation.

Now this cross section you speak of, how exactly does it increase? See what I picture is that there are several components to this cross section: One that is wide reaching but only captures fast neutrons, and a second component that is much smaller, however captures the thermal and slow neutrons. I apologize if I'm mistaken but the more we discuss, the further I'm pushed out of my comfort zone of knowledge (which is a good thing).

 

[SteamKing]

Now this cross section you speak of, how exactly does it increase? See what I picture is that there are several components to this cross section: One that is wide reaching but only captures fast neutrons, and a second component that is much smaller, however captures the thermal and slow neutrons. I apologize if I'm mistaken but the more we discuss, the further I'm pushed out of my comfort zone of knowledge (which is a good thing).

There are a couple of different 'cross-section' values used in nuclear contexts.

http://en.wikipedia.org/wiki/Nuclear_cross_section

http://en.wikipedia.org/wiki/Neutron_cross-section

The first article is a general discussion of the concept, while the second discusses the details of how the 'cross-section' is affected by different physical properties, like temperature, or the amount of energy.

'Cross-section' is an unfortunate term because it implies that there is some physical property which changes; the 'cross-section' value is a term which has meaning only in comparison across changing conditions. It is more an estimate of the probability of the occurrence of a particular type of reaction rather than a physical size, although it carries the units of area.

Similar concepts are used when referring to things like radar being able to detect objects: a radar 'cross-section', which also is measured in units of area, indirectly measures the strength of the return signal of a radar scan. A large radar 'cross-section' can indicate several different things: the object is relatively close to the transmitter, the object is made of metal or other material which readily reflects radar signals, the shape of the object, etc. Stealth technology for aircraft was aimed at using various means to reduce the radar cross-section of a plane to inhibit, if not eliminate entirely, the reflection of the radar tracking signal. These means might include materials applied to the outside of the plane which would absorb the radar signal, or designing a special shape which would deflect the radar signal away from the receiver.

 

[delsaber8]

There are a couple of different 'cross-section' values used in nuclear contexts.

http://en.wikipedia.org/wiki/Nuclear_cross_section

http://en.wikipedia.org/wiki/Neutron_cross-section

The first article is a general discussion of the concept, while the second discusses the details of how the 'cross-section' is affected by different physical properties, like temperature, or the amount of energy.

'Cross-section' is an unfortunate term because it implies that there is some physical property which changes; the 'cross-section' value is a term which has meaning only in comparison across changing conditions. It is more an estimate of the probability of the occurrence of a particular type of reaction rather than a physical size, although it carries the units of area.

Similar concepts are used when referring to things like radar being able to detect objects: a radar 'cross-section', which also is measured in units of area, indirectly measures the strength of the return signal of a radar scan. A large radar 'cross-section' can indicate several different things: the object is relatively close to the transmitter, the object is made of metal or other material which readily reflects radar signals, the shape of the object, etc. Stealth technology for aircraft was aimed at using various means to reduce the radar cross-section of a plane to inhibit, if not eliminate entirely, the reflection of the radar tracking signal. These means might include materials applied to the outside of the plane which would absorb the radar signal, or designing a special shape which would deflect the radar signal away from the receiver.

I'm thinking I have a better handle on the whole cross section thing, and the temperature dependence. Now forgive my if I'm wrong, but is this the reason U-238 can be used in the jacket stage of a nuclear weapon, because the temperature is so high?

 

[SteamKing]

The U-238 is used then because it will fission from being struck by more energetic neutrons coming from the fusion part of the blast. Remember, all of the reactions involved in a fusion detonation occur in a fraction of a second.

I should make an important distinction here about U-238 fission. Unlike U-235, U-238 cannot sustain a chain reaction, so each U-238 nucleus which fissions does not release other solitary neutrons which can continue to cause fission in other U-238 nuclei. In order for a significant amount of U-238 fission to occur, the material must be bombarded by a large flux of high-energy neutrons. U-235 nuclei fission, on the other hand, by being able to capture lower energy, so-called thermal, neutrons. This neutron capture causes an instability in the now U-236 nucleus which results in fission with the creation of two extra neutrons. These extra neutrons are ejected from the fissioning U-236 nucleus to cause other U-235 nuclei to fission, and so on, creating an uncontrolled chain reaction.

 

[jim hardy]

interesting article here about pits...

http://cryptome.org/2014/03/nuke-pits.pdf

 

[mheslep]

... Unlike U-235, U-238 cannot sustain a chain reaction, so each U-238 nucleus which fissions does not release other solitary neutrons which can continue to cause fission in other U-238 nuclei. ...

Well U-238 fission does release neutrons, just not enough (1.7) and the energy of those neutrons produced by fission is too low to fission other U-238 nuclei. The result being exactly as you say, that U-238 can not sustain a chain-reaction.

The result then of a U-238 jacket around the fusion portion of weapon is that every 14 MeV neutron produced by fusion, that then causes a fission event in the jacket, 'boosts' the overall yield to ~200 MeV per event.

These circumstances are the motivation behind the renewed interest so called hybrid fusion-fission designs for controlled fusion power reactors (see LIFE) The use of U-238 in hybrids would be unique and unavailable to fission-only reactors.

 

[delsaber8]

interesting article here about pits...

http://cryptome.org/2014/03/nuke-pits.pdf

I only managed to skim the article but does it mention anything about the explosive lenses used in the pit, to compress the plutonium into a super critical mass?

 

[jim hardy]

I only managed to skim the article but does it mention anything about the explosive lenses used in the pit, to compress the plutonium into a super critical mass?

no it doesn't. (caveat - i didn't scrutinize it either, was just interested in the parts about longevity and makeup)You can see Klaus Fuchs' hand-made sketches(the ones he sent to Stalin) in Rhodes' "Dark Sun", with dimensions removed of course.
That was plenty for me.

 

[SteamKing]

interesting article here about pits...

http://cryptome.org/2014/03/nuke-pits.pdf

This brings up an interesting side issue: for all the efforts thru the years at devising various disarmament treaties, the US seems bent on disarming by neglect. Much of the nuclear weapons infrastructure which was created in the 1940s and 1950s has either been demolished or closed due to safety and environmental hazards. (The massive K-25 gaseous diffusion plant which once dominated Oak Ridge, TN is now gone.) The human capital which formerly worked at these installations has long since retired or died, so that much institutional knowledge associated with weapons production has been lost. In another 25 or 30 years, will there be enough technical capability remaining to replenish or renew even the current stockpiles permitted under arms control agreements?

 

[mheslep]

Are there reports that weapons in the US stockpile are in need of replacement? I have not seen anything near term.

All published US weapon designs rely on Pu not HEU for the primary.