Urchin (detonator)
Encyclopedia
A modulated neutron initiator is a neutron source
capable of producing a burst of neutron
s on activation. It is a crucial part of some nuclear weapons
, as its role is to "kick-start" the chain reaction at the optimal moment when the configuration is prompt critical
. It is also known as an internal neutron initiator. The initiator is typically placed in the center of the plutonium pit, and is activated by impact of the converging shock wave.
One of the key elements in the proper operation of a nuclear weapon is initiation of the fission chain reaction at the proper time. To obtain a significant nuclear yield, sufficient neutrons must be present within the supercritical core at just the right time. If the chain reaction starts too soon, the result will be only a 'fizzle
yield
', well below the design specification; if it occurs too late, there may be no yield whatsoever. Low spontaneous neutron emission of the pit material is crucial to prevent predetonation.
For boosted fission weapon
s, the size of the centrally placed initiator is critical and has to be as small as possible; replacement with an external neutron source allows for more flexibility, such as variable yields.
-9 and polonium-210, separated until activation, then put into intimate contact by the shock wave. Polonium-208 and actinium-227 were also considered as alpha sources. The isotope used must have strong alpha emissions and weak gamma emissions, as gamma photons can also knock neutrons loose and can not be so efficiently shielded as alpha particles. Several variants were developed, differing by the dimensions and mechanical configuration of the system ensuring proper mixing of the metals.
generating device that triggered the nuclear detonation of the earliest plutonium
atomic bombs such as The gadget and Fat Man
, once the critical mass
had been "assembled" by the force of conventional explosives.
The detonator used in the first devices, located at the center of the bomb's plutonium pit, consisted of a 0.8 cm beryllium
pellet coated with nickel
and then a 0.1 mm layer of gold
. The coated pellet was surrounded by a beryllium shell with 2 cm outer diameter and 0.6 cm thick walls, with 15 concentric, wedge-shaped, 2.09 mm deep latitudal grooves in its inner surface that was also coated with gold and nickel. The grooves were filled with 30 Ci of polonium-210, which emits alpha particle
radiation; the inner sphere was covered with another 20 curie of polonium; its total amount was about 11 milligrams. The hollow sphere is formed from two halves made by hot pressing
. The nickel layer is deposited by exposition of the beryllium metal to nickel tetracarbonyl atmosphere. The thin layers of gold and nickel maintained the slight separation between the beryllium and polonium necessary to shield the beryllium from alpha particle
s emitted from the polonium. The whole urchin weighed about 7 grams and was attached to mounting brackets in a 2.5 cm diameter inner cavity in the pit.
When the shock wave
from the implosion of the plutonium core arrives, it crushes the detonator. Hydrodynamic forces acting on the grooved shell thoroughly mix the beryllium and polonium, allowing the alpha particles from the polonium to impinge on the beryllium atoms. Reacting to alpha particle bombardment, the beryllium atoms emit neutron
s in a rate of about 1 neutron each 5–10 nanoseconds; these neutrons then trigger the chain reaction
in the compressed supercritical
plutonium. Placing the polonium layer between two large masses of beryllium ensured contact of the metals even if the shock wave turbulence performed less than expected.
The amount of polonium generated about 0.1 watts of decay heat
, very noticeably warming the small sphere.
The grooves in the inner surface of the shell play the role of shaping the shock wave
into jets by the Munroe effect, in a way similar to a shaped charge
, for fast and thorough mixing of the beryllium and polonium. As the Munroe effect is less reliable in linear geometry, later designs used a sphere with conical or pyramidal inner indentations instead of linear grooves. Some initiator designs omitted the central sphere, being hollow instead; the advantage of a hollow design is its possible smaller size while retaining reliability.
The short half-life
of polonium (138.376 days) requires frequent replacement of initiators and continued supply of polonium for their manufacture, as their shelf life was only about 4 months. Later designs had shelf life as long as 1 year.
Since 1945, the US code name
for polonium-210 was postum
.
Use of polonium for the neutron initiator was proposed in 1944 by Edward Condon
. The initiator itself was designed by James L. Tuck.
uranium bomb. Its design was simpler and it contained less polonium. It was activated by the impact of the uranium projectile to the target. It was added to the design as an afterthought and was not essential for the weapon's function.
in January 1950, and tested in May 1951. The TOM design used less polonium, as the number of neutrons per milligram of polonium was higher than of the Urchin. Its outer diameter was only 1 cm. A series of calibration experiments for initiation time vs yield data of the TOM initiators was done during the Operation Snapper
, during the Fox test on 25 May 1952.
performed the Smiling Buddha
nuclear test. The initiator, codenamed "Flower", was based on the same principle as the Urchin. It is believed the polonium was deposited on lotus-shaped platinum
gauze
to maximize its surface and enclosed in a tantalum
sphere surrounded by uranium shell with embedded beryllium pellets. According to other sources, the design was yet more similar to the Urchin, with a beryllium shell shaped to create beryllium jets upon implosion. The initiator outer diameter is reported as 1.5 cm, or "about 2 cm".
Boosted fission weapon
s and weapons using external neutron sources offer the possibility of variable yield
, allowing selection of the weapon's power depending on the tactical needs.
under the leadership of Charles Allen Thomas
. The Dayton Project
was one of the various sites comprising the Manhattan Project
In 1949 Mound Laboratories
in nearby Miamisburg, Ohio
opened as a replacement for the Dayton Project
and the new home of nuclear initiator research & development. Polonium-210 was produced by neutron irradiation of bismuth
. Production and research of polonium at Mound was phased out in 1971.
Polonium from Dayton was used by the G Division of Los Alamos in initiator design studies at a test site in Sandia Canyon. The initiator group built test assemblies by drilling holes in large turbine ball bearings, inserting the active material, and plugging the holes with bolts. These test assemblies were known as screwballs. The test assemblies were imploded and their remains studied to examine how well the polonium and beryllium mixed.
The production of the beryllium-polonium TOM initiators ended in 1953; the initiators were replaced with a different design, which slightly reduced the weapon yield but its longer shelf life reduced the complexity of the logistics. The sealed neutron initiator, brought into inventory in late 1954, still required a periodic disassembly to access its capsule for maintenance checks. The capsules were phased out completely in 1962.
Urchin style initiators were later superseded by other means of generating neutrons such as pulsed neutron emitters
that do not use polonium
. Another possibility is a boosted fission weapon
, using a hollow pit, injected with gaseous deuterium
and tritium
mixture; tritium, while also radioactive, has much longer half-life (13.2 years) than polonium-210, allowing longer shelf life of the weapons.
Iran was suspected
of producing polonium for neutron initiators. Documents proving that have yet to be authenticated, however.
Neutron source
A Neutron source is a device that emits neutrons. There is a wide variety of different sources, ranging from hand-held radioactive sources to neutron research facilities operating research reactors and spallation sources...
capable of producing a burst of neutron
Neutron
The neutron is a subatomic hadron particle which has the symbol or , no net electric charge and a mass slightly larger than that of a proton. With the exception of hydrogen, nuclei of atoms consist of protons and neutrons, which are therefore collectively referred to as nucleons. The number of...
s on activation. It is a crucial part of some nuclear weapons
Nuclear weapon design
Nuclear weapon designs are physical, chemical, and engineering arrangements that cause the physics package of a nuclear weapon to detonate. There are three basic design types...
, as its role is to "kick-start" the chain reaction at the optimal moment when the configuration is prompt critical
Prompt critical
In nuclear engineering, an assembly is prompt critical if for each nuclear fission event, one or more of the immediate or prompt neutrons released causes an additional fission event. This causes a rapid, exponential increase in the number of fission events...
. It is also known as an internal neutron initiator. The initiator is typically placed in the center of the plutonium pit, and is activated by impact of the converging shock wave.
One of the key elements in the proper operation of a nuclear weapon is initiation of the fission chain reaction at the proper time. To obtain a significant nuclear yield, sufficient neutrons must be present within the supercritical core at just the right time. If the chain reaction starts too soon, the result will be only a 'fizzle
Fizzle (nuclear test)
In nuclear weapons, a fizzle occurs when the testing of a nuclear bomb fails to meet its expected yield. The reason for the failure can be linked to improper bomb design, poor construction, or lack of expertise. All countries that have had a nuclear weapons testing program have experienced fizzles...
yield
Nuclear weapon yield
The explosive yield of a nuclear weapon is the amount of energy discharged when a nuclear weapon is detonated, expressed usually in the equivalent mass of trinitrotoluene , either in kilotons or megatons , but sometimes also in terajoules...
', well below the design specification; if it occurs too late, there may be no yield whatsoever. Low spontaneous neutron emission of the pit material is crucial to prevent predetonation.
For boosted fission weapon
Boosted fission weapon
A boosted fission weapon usually refers to a type of nuclear bomb that uses a small amount of fusion fuel to increase the rate, and thus yield, of a fission reaction. The neutrons released by the fusion reactions add to the neutrons released in the fission, as well as inducing the fission reactions...
s, the size of the centrally placed initiator is critical and has to be as small as possible; replacement with an external neutron source allows for more flexibility, such as variable yields.
Design
The usual design is based on a combination of berylliumBeryllium
Beryllium is the chemical element with the symbol Be and atomic number 4. It is a divalent element which occurs naturally only in combination with other elements in minerals. Notable gemstones which contain beryllium include beryl and chrysoberyl...
-9 and polonium-210, separated until activation, then put into intimate contact by the shock wave. Polonium-208 and actinium-227 were also considered as alpha sources. The isotope used must have strong alpha emissions and weak gamma emissions, as gamma photons can also knock neutrons loose and can not be so efficiently shielded as alpha particles. Several variants were developed, differing by the dimensions and mechanical configuration of the system ensuring proper mixing of the metals.
Urchin
Urchin was the code name for the internal neutron initiator, a neutronNeutron
The neutron is a subatomic hadron particle which has the symbol or , no net electric charge and a mass slightly larger than that of a proton. With the exception of hydrogen, nuclei of atoms consist of protons and neutrons, which are therefore collectively referred to as nucleons. The number of...
generating device that triggered the nuclear detonation of the earliest plutonium
Plutonium
Plutonium is a transuranic radioactive chemical element with the chemical symbol Pu and atomic number 94. It is an actinide metal of silvery-gray appearance that tarnishes when exposed to air, forming a dull coating when oxidized. The element normally exhibits six allotropes and four oxidation...
atomic bombs such as The gadget and Fat Man
Fat Man
"Fat Man" is the codename for the atomic bomb that was detonated over Nagasaki, Japan, by the United States on August 9, 1945. It was the second of the only two nuclear weapons to be used in warfare to date , and its detonation caused the third man-made nuclear explosion. The name also refers more...
, once the critical mass
Critical mass
A critical mass is the smallest amount of fissile material needed for a sustained nuclear chain reaction. The critical mass of a fissionable material depends upon its nuclear properties A critical mass is the smallest amount of fissile material needed for a sustained nuclear chain reaction. The...
had been "assembled" by the force of conventional explosives.
The detonator used in the first devices, located at the center of the bomb's plutonium pit, consisted of a 0.8 cm beryllium
Beryllium
Beryllium is the chemical element with the symbol Be and atomic number 4. It is a divalent element which occurs naturally only in combination with other elements in minerals. Notable gemstones which contain beryllium include beryl and chrysoberyl...
pellet coated with nickel
Nickel
Nickel is a chemical element with the chemical symbol Ni and atomic number 28. It is a silvery-white lustrous metal with a slight golden tinge. Nickel belongs to the transition metals and is hard and ductile...
and then a 0.1 mm layer of gold
Gold plating
Gold plating is a method of depositing a thin layer of gold onto the surface of another metal, most often copper or silver , by chemical or electrochemical plating...
. The coated pellet was surrounded by a beryllium shell with 2 cm outer diameter and 0.6 cm thick walls, with 15 concentric, wedge-shaped, 2.09 mm deep latitudal grooves in its inner surface that was also coated with gold and nickel. The grooves were filled with 30 Ci of polonium-210, which emits alpha particle
Alpha particle
Alpha particles consist of two protons and two neutrons bound together into a particle identical to a helium nucleus, which is classically produced in the process of alpha decay, but may be produced also in other ways and given the same name...
radiation; the inner sphere was covered with another 20 curie of polonium; its total amount was about 11 milligrams. The hollow sphere is formed from two halves made by hot pressing
Hot pressing
Hot pressing is a high-pressure, low-strain-rate powder metallurgy process for forming of a powder or powder compact at a temperature high enough to induce sintering and creep processes. This is achieved by the simultaneous application of heat and pressure....
. The nickel layer is deposited by exposition of the beryllium metal to nickel tetracarbonyl atmosphere. The thin layers of gold and nickel maintained the slight separation between the beryllium and polonium necessary to shield the beryllium from alpha particle
Alpha particle
Alpha particles consist of two protons and two neutrons bound together into a particle identical to a helium nucleus, which is classically produced in the process of alpha decay, but may be produced also in other ways and given the same name...
s emitted from the polonium. The whole urchin weighed about 7 grams and was attached to mounting brackets in a 2.5 cm diameter inner cavity in the pit.
When the shock wave
Shock wave
A shock wave is a type of propagating disturbance. Like an ordinary wave, it carries energy and can propagate through a medium or in some cases in the absence of a material medium, through a field such as the electromagnetic field...
from the implosion of the plutonium core arrives, it crushes the detonator. Hydrodynamic forces acting on the grooved shell thoroughly mix the beryllium and polonium, allowing the alpha particles from the polonium to impinge on the beryllium atoms. Reacting to alpha particle bombardment, the beryllium atoms emit neutron
Neutron
The neutron is a subatomic hadron particle which has the symbol or , no net electric charge and a mass slightly larger than that of a proton. With the exception of hydrogen, nuclei of atoms consist of protons and neutrons, which are therefore collectively referred to as nucleons. The number of...
s in a rate of about 1 neutron each 5–10 nanoseconds; these neutrons then trigger the chain reaction
Nuclear chain reaction
A nuclear chain reaction occurs when one nuclear reaction causes an average of one or more nuclear reactions, thus leading to a self-propagating number of these reactions. The specific nuclear reaction may be the fission of heavy isotopes or the fusion of light isotopes...
in the compressed supercritical
Critical mass
A critical mass is the smallest amount of fissile material needed for a sustained nuclear chain reaction. The critical mass of a fissionable material depends upon its nuclear properties A critical mass is the smallest amount of fissile material needed for a sustained nuclear chain reaction. The...
plutonium. Placing the polonium layer between two large masses of beryllium ensured contact of the metals even if the shock wave turbulence performed less than expected.
The amount of polonium generated about 0.1 watts of decay heat
Decay heat
Decay heat is the heat released as a result of radioactive decay. This is when the radiation interacts with materials: the energy of the alpha, beta or gamma radiation is converted into the thermal movement of atoms.-Natural occurrence:...
, very noticeably warming the small sphere.
The grooves in the inner surface of the shell play the role of shaping the shock wave
Shock wave
A shock wave is a type of propagating disturbance. Like an ordinary wave, it carries energy and can propagate through a medium or in some cases in the absence of a material medium, through a field such as the electromagnetic field...
into jets by the Munroe effect, in a way similar to a shaped charge
Shaped charge
A shaped charge is an explosive charge shaped to focus the effect of the explosive's energy. Various types are used to cut and form metal, to initiate nuclear weapons, to penetrate armor, and in the oil and gas industry...
, for fast and thorough mixing of the beryllium and polonium. As the Munroe effect is less reliable in linear geometry, later designs used a sphere with conical or pyramidal inner indentations instead of linear grooves. Some initiator designs omitted the central sphere, being hollow instead; the advantage of a hollow design is its possible smaller size while retaining reliability.
The short half-life
Half-life
Half-life, abbreviated t½, is the period of time it takes for the amount of a substance undergoing decay to decrease by half. The name was originally used to describe a characteristic of unstable atoms , but it may apply to any quantity which follows a set-rate decay.The original term, dating to...
of polonium (138.376 days) requires frequent replacement of initiators and continued supply of polonium for their manufacture, as their shelf life was only about 4 months. Later designs had shelf life as long as 1 year.
Since 1945, the US code name
Code name
A code name or cryptonym is a word or name used clandestinely to refer to another name or word. Code names are often used for military purposes, or in espionage...
for polonium-210 was postum
Postum
Postum was a powdered roasted grain beverage sold by the Kraft Foods company as a coffee substitute. The caffeine-free beverage mix was created by Postum Cereal Company founder C. W. Post in 1895 and marketed as a healthful alternative to coffee...
.
Use of polonium for the neutron initiator was proposed in 1944 by Edward Condon
Edward Condon
Edward Uhler Condon was a distinguished American nuclear physicist, a pioneer in quantum mechanics, and a participant in the development of radar and nuclear weapons during World War II.-Early life and career:...
. The initiator itself was designed by James L. Tuck.
Abner
A different initiator (code named ABNER) was used for the Little BoyLittle Boy
"Little Boy" was the codename of the atomic bomb dropped on Hiroshima on August 6, 1945 by the Boeing B-29 Superfortress Enola Gay, piloted by Colonel Paul Tibbets of the 393rd Bombardment Squadron, Heavy, of the United States Army Air Forces. It was the first atomic bomb to be used as a weapon...
uranium bomb. Its design was simpler and it contained less polonium. It was activated by the impact of the uranium projectile to the target. It was added to the design as an afterthought and was not essential for the weapon's function.
TOM initiator
An improved construction of the initiator, probably based on conical or pyramidal indentations, was proposed in 1948, put into production by Los AlamosLos Alamos National Laboratory
Los Alamos National Laboratory is a United States Department of Energy national laboratory, managed and operated by Los Alamos National Security , located in Los Alamos, New Mexico...
in January 1950, and tested in May 1951. The TOM design used less polonium, as the number of neutrons per milligram of polonium was higher than of the Urchin. Its outer diameter was only 1 cm. A series of calibration experiments for initiation time vs yield data of the TOM initiators was done during the Operation Snapper
Operation Tumbler-Snapper
Operation Tumbler-Snapper was a series of atomic tests conducted by the United States in early 1952 at the Nevada Test Site. The Tumbler-Snapper Series of tests followed Operation Buster-Jangle, and preceded Operation Ivy....
, during the Fox test on 25 May 1952.
Flower
In 1974, IndiaIndia
India , officially the Republic of India , is a country in South Asia. It is the seventh-largest country by geographical area, the second-most populous country with over 1.2 billion people, and the most populous democracy in the world...
performed the Smiling Buddha
Smiling Buddha
The Smiling Buddha, formally designated as Pokhran-I, was the codename given to Republic of India's first nuclear test explosion that took place at the long-constructed Indian Army base, Pokhran Test Range at Pokhran municipality, Rajasthan state on 18 May 1974 at 8:05 a.m....
nuclear test. The initiator, codenamed "Flower", was based on the same principle as the Urchin. It is believed the polonium was deposited on lotus-shaped platinum
Platinum
Platinum is a chemical element with the chemical symbol Pt and an atomic number of 78. Its name is derived from the Spanish term platina del Pinto, which is literally translated into "little silver of the Pinto River." It is a dense, malleable, ductile, precious, gray-white transition metal...
gauze
Gauze
Gauze is a thin, translucent fabric with a loose open weave.-Uses and types:Gauze was originally made of silk and was used for clothing. It is now used for many different things, including gauze sponges for medical purposes. When used as a medical dressing, gauze is generally made of cotton...
to maximize its surface and enclosed in a tantalum
Tantalum
Tantalum is a chemical element with the symbol Ta and atomic number 73. Previously known as tantalium, the name comes from Tantalus, a character in Greek mythology. Tantalum is a rare, hard, blue-gray, lustrous transition metal that is highly corrosion resistant. It is part of the refractory...
sphere surrounded by uranium shell with embedded beryllium pellets. According to other sources, the design was yet more similar to the Urchin, with a beryllium shell shaped to create beryllium jets upon implosion. The initiator outer diameter is reported as 1.5 cm, or "about 2 cm".
Other designs
Uranium deuteride (UD3) can be used for construction of a neutron multiplier.Boosted fission weapon
Boosted fission weapon
A boosted fission weapon usually refers to a type of nuclear bomb that uses a small amount of fusion fuel to increase the rate, and thus yield, of a fission reaction. The neutrons released by the fusion reactions add to the neutrons released in the fission, as well as inducing the fission reactions...
s and weapons using external neutron sources offer the possibility of variable yield
Variable yield
Variable yield — or dial-a-yield — is an option available on most modern nuclear weapons. It allows the operator to specify a weapon's yield, or explosive power, allowing a single design to be used in different situations...
, allowing selection of the weapon's power depending on the tactical needs.
Development
The Polonium used in the urchin initiator was created at Oak Ridge and then extracted and purified as part of the Dayton ProjectDayton Project
The Dayton Project was one of several sites involved in the Manhattan Project to build the first atomic bombs. Charles Allen Thomas, an executive of the Monsanto Company corporation, was assigned to develop the neutron generating devices that triggered the nuclear detonation of the atomic bombs...
under the leadership of Charles Allen Thomas
Charles Allen Thomas
Charles Allen Thomas was a noted American chemist and businessman, and an important figure in the Manhattan Project....
. The Dayton Project
Dayton Project
The Dayton Project was one of several sites involved in the Manhattan Project to build the first atomic bombs. Charles Allen Thomas, an executive of the Monsanto Company corporation, was assigned to develop the neutron generating devices that triggered the nuclear detonation of the atomic bombs...
was one of the various sites comprising the Manhattan Project
Manhattan Project
The Manhattan Project was a research and development program, led by the United States with participation from the United Kingdom and Canada, that produced the first atomic bomb during World War II. From 1942 to 1946, the project was under the direction of Major General Leslie Groves of the US Army...
In 1949 Mound Laboratories
Mound Laboratories
Mound Laboratories in Miamisburg, Ohio was an Atomic Energy Commission facility for Nuclear weapon research during the Cold War....
in nearby Miamisburg, Ohio
Miamisburg, Ohio
Miamisburg is a city in Montgomery County, Ohio, United States. The population was 20,181 at the 2010 census. It is part of the Dayton Metropolitan Statistical Area...
opened as a replacement for the Dayton Project
Dayton Project
The Dayton Project was one of several sites involved in the Manhattan Project to build the first atomic bombs. Charles Allen Thomas, an executive of the Monsanto Company corporation, was assigned to develop the neutron generating devices that triggered the nuclear detonation of the atomic bombs...
and the new home of nuclear initiator research & development. Polonium-210 was produced by neutron irradiation of bismuth
Bismuth
Bismuth is a chemical element with symbol Bi and atomic number 83. Bismuth, a trivalent poor metal, chemically resembles arsenic and antimony. Elemental bismuth may occur naturally uncombined, although its sulfide and oxide form important commercial ores. The free element is 86% as dense as lead...
. Production and research of polonium at Mound was phased out in 1971.
Polonium from Dayton was used by the G Division of Los Alamos in initiator design studies at a test site in Sandia Canyon. The initiator group built test assemblies by drilling holes in large turbine ball bearings, inserting the active material, and plugging the holes with bolts. These test assemblies were known as screwballs. The test assemblies were imploded and their remains studied to examine how well the polonium and beryllium mixed.
The production of the beryllium-polonium TOM initiators ended in 1953; the initiators were replaced with a different design, which slightly reduced the weapon yield but its longer shelf life reduced the complexity of the logistics. The sealed neutron initiator, brought into inventory in late 1954, still required a periodic disassembly to access its capsule for maintenance checks. The capsules were phased out completely in 1962.
Urchin style initiators were later superseded by other means of generating neutrons such as pulsed neutron emitters
Neutron generator
Neutron generators are neutron source devices which contain compact linear accelerators and that produce neutrons by fusing isotopes of hydrogen together. The fusion reactions take place in these devices by accelerating either deuterium, tritium, or a mixture of these two isotopes into a metal...
that do not use polonium
Polonium
Polonium is a chemical element with the symbol Po and atomic number 84, discovered in 1898 by Marie Skłodowska-Curie and Pierre Curie. A rare and highly radioactive element, polonium is chemically similar to bismuth and tellurium, and it occurs in uranium ores. Polonium has been studied for...
. Another possibility is a boosted fission weapon
Boosted fission weapon
A boosted fission weapon usually refers to a type of nuclear bomb that uses a small amount of fusion fuel to increase the rate, and thus yield, of a fission reaction. The neutrons released by the fusion reactions add to the neutrons released in the fission, as well as inducing the fission reactions...
, using a hollow pit, injected with gaseous deuterium
Deuterium
Deuterium, also called heavy hydrogen, is one of two stable isotopes of hydrogen. It has a natural abundance in Earth's oceans of about one atom in of hydrogen . Deuterium accounts for approximately 0.0156% of all naturally occurring hydrogen in Earth's oceans, while the most common isotope ...
and tritium
Tritium
Tritium is a radioactive isotope of hydrogen. The nucleus of tritium contains one proton and two neutrons, whereas the nucleus of protium contains one proton and no neutrons...
mixture; tritium, while also radioactive, has much longer half-life (13.2 years) than polonium-210, allowing longer shelf life of the weapons.
Iran was suspected
Nuclear program of Iran
The nuclear program of Iran was launched in the 1950s with the help of the United States as part of the Atoms for Peace program. The support, encouragement and participation of the United States and Western European governments in Iran's nuclear program continued until the 1979 Iranian Revolution...
of producing polonium for neutron initiators. Documents proving that have yet to be authenticated, however.