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Primary system The
Castle Bravo device was housed in a cylinder that weighed and measured in length and in diameter. A copper pit liner encased within the weapon-grade plutonium inner capsule prevented DT gas diffusion into the plutonium, a technique first tested in
Greenhouse Item.
Deuterium and lithium The device was called
SHRIMP, and had the same basic configuration (radiation implosion) as the
Ivy Mike wet device, except with a different type of
fusion fuel.
SHRIMP used
lithium deuteride (LiD), which is solid at room temperature;
Ivy Mike used
cryogenic liquid
deuterium (D2), which required elaborate cooling equipment.
Castle Bravo was the first test by the United States of a practical deliverable
fusion bomb, even though the TX-21 as proof-tested in the Bravo event was not weaponized. The successful test rendered obsolete the cryogenic design used by
Ivy Mike and its weaponized derivative, the
JUGHEAD, which was slated to be tested as the initial
Castle Yankee. It also used a -thick
7075 aluminum ballistic case. Aluminum was used to drastically reduce the bomb's weight and simultaneously provided sufficient radiation confinement time to raise yield, a departure from the heavy stainless steel casing (304L or MIM 316L) employed by other weapon projects at the time. The
SHRIMP was at least in theory and in many critical aspects identical in geometry to the
RUNT and
RUNT II devices later proof-fired in the
Romeo and
Yankee shots. On paper it was a scaled-down version of these devices, and its origins can be traced back to 1953. The
United States Air Force indicated the importance of lighter thermonuclear weapons for delivery by the
B-47 Stratojet and
B-58 Hustler.
Los Alamos National Laboratory responded to this indication with a follow-up enriched version of the
RUNT scaled down to a 3/4 scale radiation-implosion system called the
SHRIMP. The proposed weight reduction from TX-17's to TX-21's ) would provide the Air Force with a much more versatile deliverable
gravity bomb. The volume of LiD fuel used was approximately 60% the volume of the fusion fuel filling used in the wet
SAUSAGE and dry
RUNT I and
II devices, or about , corresponding to about 390 kg of lithium deuteride (as LiD has a density of 0.78201 g/cm3). The mixture cost about 4.54
USD/g at that time. The fusion burn efficiency was close to 25.1%, the highest attained efficiency of the first thermonuclear weapon generation. This efficiency is well within the figures given in a November 1956 statement, when a DOD official disclosed that thermonuclear devices with efficiencies ranging from 15% to up about 40% had been tested. The reaction would produce high-energy neutrons with 14 MeV, and its
neutronicity was estimated at ≈0.885 (for a
Lawson criterion of ≈1.5).
Possible additional tritium for high-yield As
SHRIMP, along with the
RUNT I and
ALARM CLOCK, were to be high-yield shots required to assure the thermonuclear "
emergency capability," their fusion fuel may have been spiked with additional tritium, in the form of LiT. Copper possesses excellent reflecting properties, and its low cost, compared to other reflecting materials like gold, made it useful for mass-produced hydrogen weapons. Hohlraum albedo is a very important design parameter for any inertial-confinement configuration. A relatively high albedo permits higher interstage coupling due to the more favorable azimuthal and latitudinal angles of reflected radiation. The limiting value of the albedo for high-
Z materials is reached when the thickness is 5–10 g/cm, or 0.5–1.0 free paths. Thus, a hohlraum made of uranium much thicker than a free path of uranium would be needlessly heavy and costly. At the same time, the angular anisotropy increases as the atomic number of the scatterer material is reduced. Therefore, hohlraum liners require the use of copper (or, as in other devices,
gold or
aluminium), as the absorption probability increases with the value of
Z of the scatterer. There are two sources of X-rays in the hohlraum: the primary's irradiance, which is dominant at the beginning and during the pulse rise; and the wall, which is important during the required radiation temperature's (
T) plateau. The primary emits radiation in a manner similar to a
flash bulb, and the secondary needs constant
T to properly implode. This constant wall temperature is dictated by the ablation pressure requirements to drive compression, which lie on average at about 0.4 keV (out of a range of 0.2 to 2 keV), corresponding to several million
kelvins. Wall temperature depended on the temperature of the primary's
core which peaked at about 5.4 keV during boosted-fission. also encountered as
random-phase plate in the ICF laser assemblies. This medium was a polystyrene plastic foam filling, extruded or impregnated with a low-molecular-weight hydrocarbon (possibly methane gas), which turned to a low-
Z plasma from the X-rays, and along with channeling radiation it modulated the ablation front on the high-Z surfaces; it "tamped" the
sputtering effect that would otherwise "choke" radiation from compressing the secondary. The reemitted X-rays from the radiation case must be deposited uniformly on the outer walls of the secondary's tamper and ablate it externally, driving the thermonuclear fuel capsule (increasing the density and temperature of the fusion fuel) to the point needed to sustain a thermonuclear reaction. (see
Nuclear weapon design). This point is above the threshold where the fusion fuel would turn opaque to its emitting radiation, as determined from its
Rosseland opacity, meaning that the generated energy balances the energy lost to fuel's vicinity (as radiation, particle losses). After all, for any hydrogen weapon system to work, this energy equilibrium must be maintained through the compression equilibrium between the fusion tamper and the spark plug (see below), hence their name
equilibrium supers. Since the ablative process takes place on both walls of the radiation channel, a numerical estimate made with ISRINEX (a thermonuclear explosion simulation program) suggested that the uranium tamper also had a thickness of 2.5 cm, so that an equal pressure would be applied to both walls of the
hohlraum. The rocket effect on the surface of tamper's wall created by the ablation of its several superficial layers would force an equal mass of uranium that rested in the remainder of the tamper to speed inwards, thus imploding the thermonuclear core. At the same time, the rocket effect on the surface of the hohlraum would force the radiation case to speed outwards. The ballistic case would confine the exploding radiation case for as long as necessary. The fact that the tamper material was uranium enriched in U is primarily based on the final fission reaction fragments detected in the radiochemical analysis, which conclusively showed the presence of U, found by the Japanese in the shot debris. The first-generation thermonuclear weapons (MK-14, 16, 17, 21, 22 and 24) all used uranium tampers enriched to 37.5% U. One of the outcomes of this diagnostic work resulted in this graphic depiction of the transport of energetic x-ray and neutrons through a vacuum line, some 2.3 km long, whereupon it heated solid matter at the "station 1200" blockhouse and thus generated a secondary fireball. The secondary assembly was the actual
SHRIMP component of the weapon. The weapon, like most contemporary thermonuclear weapons at that time, bore the same codename as the secondary component. The secondary was situated in the cylindrical end of the device, where its end was locked to the radiation case by a type of
mortise and tenon joint. The hohlraum at its cylindrical end had an internal projection, which nested the secondary and had better structural strength to support the secondary's assembly, which had most of the device's mass. A visualization to this is that the joint looked much like a cap (the secondary) fitted in a cone (the projection of the radiation case). Any other major supporting structure would produce interference with radiation transfer from the primary to the secondary and complex vibrational behavior. With this form of joint bearing most of the structural loads of the secondary, the latter and the hohlraum-ballistic case ensemble behaved as a single mass sharing common eigenmodes. To reduce excessive loading of the joint, especially during deployment of the weapon, the forward section of the secondary (i.e. the thermal blast/heat shield) was anchored to the radiation case by a set of thin wires, which also aligned the center line of the secondary with the primary, as they diminished bending and torsional loads on the secondary, another technique adopted from the
SAUSAGE. The secondary assembly was an elongated truncated cone. From its front part (excluding the blast-heat shield) to its aft section it was steeply tapered. Tapering was used for two reasons. First, radiation drops by the square of the distance, hence radiation coupling is relatively poor in the aftermost sections of the secondary. This made the use of a higher mass of fusion fuel in the rear end of the secondary assembly ineffective and the overall design wasteful. This was also the reason why the lower-enriched slugs of fusion fuel were placed far aft of the fuel capsule. Second, as the primary could not illuminate the whole surface of the hohlraum, in part due to the large axial length of the secondary, relatively small solid angles would be effective to compress the secondary, leading to poor radiation focusing. By tapering the secondary, the hohlraum could be shaped as a cylinder in its aft section obviating the need to machine the radiation case to a parabola at both ends. This optimized radiation focusing and enabled a streamlined production line, as it was cheaper, faster and easier to manufacture a radiation case with only one parabolic end. The tapering in this design was much steeper than its cousins, the
RUNT, and the
ALARM CLOCK devices. ''SHRIMP's'' tapering and its mounting to the hohlraum apparently made the whole secondary assembly resemble the body of a
shrimp. The secondary's length is defined by the two pairs of dark-colored diagnostic
hot spot pipes attached to the middle and left section of the device. These pipe sections were in diameter and long and were butt-welded end-to-end to the ballistic case leading out to the top of the shot cab. They would carry the initial reaction's light up to the array of 12 mirror towers built in an arc on the artificial shot island created for the event. From those pipes, mirrors would reflect early bomb light from the bomb casing to a series of remote high-speed cameras, and so that Los Alamos could determine both the
simultaneity of the design (i.e. the time interval between primary's firing and secondary's ignition) and the thermonuclear burn rate in these two crucial areas of the secondary device. This secondary assembly device contained the
lithium deuteride fusion fuel in a stainless-steel canister. Running down to the center of the secondary was a 1.3 cm thick hollow cylindrical rod of
plutonium, nested in the steel canister. This was the
spark plug, a tritium-boosted fission device. It was made from plutonium rings and had a hollow volume inside that measured about 0.5 cm in diameter. This central volume was lined with copper, which like the liner in the primary's fissile core prevented DT gas diffusion in plutonium. The spark plug's boosting charge contained about 4 grams of
tritium and, imploding together with the secondary's compression, was timed to detonate by the first generations of neutrons that arrived from the primary. Timing was defined by the geometric characteristics of the sparkplug (its uncompressed annular radius), which detonated when its criticality, or
k, transcended 1. Its purpose was to compress the fusion material around it from its inside, equally applying pressure with the tamper. The compression factor of the fusion fuel and its adiabatic compression energy determined the minimal energy required for the spark plug to counteract the compression of the fusion fuel and the tamper's momentum. The spark plug weighed about 18 kg, and its initial firing yielded . Then it would be completely fissioned by the fusion neutrons, contributing about to the total yield. The energy required by the spark plug to counteract the compression of the fusion fuel was lower than the primary's yield because coupling of the primary's energy in the hohlraum is accompanied by losses due to the difference between the X-ray fireball and the hohlraum temperatures. The neutrons entered the assembly by a small hole through the ≈28 cm thick U blast-heat shield. It was positioned in front of the secondary assembly facing the primary. Similar to the tamper-fusion capsule assembly, the shield was shaped as a circular frustum, with its small diameter facing the primary's side, and with its large diameter locked by a type of
mortise and tenon joint to the rest of the secondary assembly. The shield-tamper ensemble can be visualized as a
circular bifrustum. All parts of the tamper were similarly locked together to provide structural support and rigidity to the secondary assembly. Surrounding the fusion-fuel–spark-plug assembly was the
uranium tamper with a standoff air-gap about 0.9 cm wide that was to increase the tamper's momentum, a levitation technique used as early as
Operation Sandstone and described by physicist
Ted Taylor as
hammer-on-the-nail-impact. Since there were also technical concerns that high-
Z tamper material would mix rapidly with the relatively low-density fusion fuel—leading to unacceptably large radiation losses—the stand-off gap also acted as a buffer to mitigate the unavoidable and undesirable
Taylor mixing.
Use of boron Boron was used at many locations in this dry system; it has a high cross-section for the absorption of slow neutrons, which fission U and Pu, but a low cross-section for the absorption of fast neutrons, which fission U. Because of this characteristic, B deposited onto the surface of the secondary stage would prevent pre-detonation of the
spark plug by stray neutrons from the primary without interfering with the subsequent fissioning of the U of the fusion tamper wrapping the secondary. Boron also played a role in increasing the compressive plasma pressure around the secondary by blocking the sputtering effect, leading to higher thermonuclear efficiency. Because the structural foam holding the secondary in place within the casing was doped with B, the secondary was compressed more highly, at a cost of some radiated neutrons. By contrast, the
Castle Koon MORGENSTERN device did not use B in its design; as a result, the intense neutron flux from its
RACER IV primary predetonated the spherical fission spark plug, which in turn "cooked" the fusion fuel, leading to an overall poor compression. The plastic's low molecular weight is unable to implode the secondary's mass. Its plasma-pressure is confined in the boiled-off sections of the tamper and the radiation case so that material from neither of these two
walls can enter the radiation channel that has to be open for the radiation transit. == Detonation ==