There have been some reports stating that the peanut shape of North Korea’s two-stage thermonuclear warhead displayed, and tested, on September 3 is unlike that of other hydrogen bombs.
This is not accurate.
The physics package of the W87 warhead also possesses a peanut like geometry. We know this because a detailed, although not complete, schematic of the W87 warhead was provided courtesy of the Congressional Cox Report on Chinese espionage in US nuclear weapons labs, written during the Clinton era. See images below.
The Report noted that a series of Chinese tests in the 1990s appeared to be of nuclear weapons more suitable for delivery by ballistic missiles, especially solid fuelled road mobile ICBMs, than China’s first generation of relatively heavy hydrogen bombs. It suggested that China was adopting the US design philosophy of producing high margin, tightly engineered, warheads with high yield-to-weight ratios precisely as the United States had done. That approach was especially associated with Lawrence Livermore. I suppose that proves the adage that it takes one to know one.
This contrasts with the Soviet design philosophy of producing more simple, robust and reliable designs which China had emulated up to the 1990s. It was surmised that the shift in approach of China’s nuclear weapons complex, revealed by the testing programme, could well have been partly the result of Chinese espionage within the US nuclear weapons complex. An investigation eventually demonstrated that China had penetrated the labs, and the Congressional Cox Report was written on providing some of the details. There was a political undertone to the Report, as it was often cited by Republicans, who largely wrote it, when they argued Clinton had gone wobbly on national security.
The highly engineered US design philosophy was one reason why some within, and without, the US nuclear weapons complex were arguing for the necessity for the Reliable Replacement Warhead in the 2000s, and why some argued the US should not adhere to the CTBT.
You can see why, in part, North Korea was keen on developing a uranium enrichment plant. Note the U235 core of the secondary on the W87. Note also the tamper of the secondary. That can consist of a U238 or a U235 blanket. The yield of the W87 is 300KT, however with a U235 tamper for the secondary that yield increases to 475KT, as using a U235 tamper produces less energetic neutrons which boosts the fission of the U235 secondary core.
The equation that I have tended to use for estimating the yield of a North Korean nuclear test does not take into account the depth of the test site tunnels, but if you use the equation that does and assume a depth of 900 metres you get about a yield of 475KT for the September 3 test.
I should stress that the W87 is a rough guide because it was especially compact, and that compactness shows how tightly engineered US warheads are. The physics package of the W87 is about 270kg and the warhead and Mark 21 RV combination has a lower bound mass of 362kg. The Mark 21 RV has a length of 175cm and a base diameter of 55cm. That’s light and that’s compact. North Korea’s Hwasong-14 RV is not so compact, and we saw images on September 3 of a the hydrogen bomb design being placed into a Hwasong-14 RV. Now we know that the hydrogen bomb will be housed in a Hwasong-14 RV but that does not mean a Hwasong-14 ICBM will deliver it, because the North Koreans have stated the hydrogen bomb will be delivered by a new ICBM, most likely one with a greater throw weight than the Hwasong-14 and probably a reconfigured Hwasong-13. Note that if the North Korean peanut shaped hydrogen bomb was of weight and compactness of the W87 it could indeed be delivered by the Hwasong-14 ICBM to the lower US 48 states so therefore obviating the need for a new ICBM.
The W87 therefore serves as a rough guide, and it does so because of the information provided in the Cox Report which is hard to find elsewhere in the public domain. The reason why the W87 was so light and compact was because it was designed for delivery by a MIRVed missile. North Korea’s hydrogen bomb is not designed for delivery by a MIRVed missile, the Hwasong-14 RV is too big and so is the hydrogen bomb warhead.
A key focus of the Cox Report was the W88 warhead, the most sophisticated in the US arsenal. The W88 is even more compact than the W87, mainly because the W88 was for the MIRVed Trident II Sea Launched Ballistic Missile whereas the W87 was for the MIRVed MX and then Minuteman III sil based ICBMs. A key design feature of the W88 warhead, we know this because of the Cox Report, is the egg geometry of its physics package as opposed to the peanut shape of the W87.
That is to say, the fissile primary of the W88 is egg shaped and the primary of the W87 is spherical. The disco ball, North Korea’s compact boosted fission weapon is spherical, and there’s no evidence to suggest that North Korea has an egg shaped primary. See below for image of the disco ball fissile primary.
The peanut shape of North Korea’s hydrogen bomb suggests that North Korea has adopted features of the US thermonuclear warhead design philosophy, and that the Cox Report has played some role, the extent of which is inherently uncertain, in the history of North Korea’s nuclear weapons programme.