In the past 18 months, North Korea sank a South Korean warship, the Cheonan, and bombarded Yeonpyeong Island in South Korea. Together, these two attacks killed 50 South Koreans. Moreover, it seems like further provocations might be on the way. These brazen acts threaten the credibility of US security commitments in Northeast Asia and have led to calls in South Korea for the United States to base nuclear weapons there and in Japan for building an independent nuclear deterrent. One Japanese academic has even posed the question, ‘Did deterrence against North Korea fail in 2010?’
Japanese and South Korean strategists have long worried that North Korea views its small nuclear deterrent as a shield. They argue that from behind this shield, and with the protection afforded by his hardened and deeply buried bunkers, a risk-taking Kim Jong-il believes he can launch limited offensive military operations against his neighbours with impunity. Strategic stability, our Asian allies argued during the 2010 Nuclear Posture Review, isn’t the same thing as regional security. On the contrary, a stable ‘balance of terror’ may embolden Pyongyang to continue attacks like the ones against the Cheonan and Yeonpyeong.
In response to North Korea's provocations, the United States and South Korea have agreed upon a series of measures to provide effective conventional military responses to North Korean provocations. Although such conventional retaliatory options are necessary, they aren’t sufficient. The United States must also find ways to make Kim significantly less confident in the protection he is provided by his elaborate tunnelling campaign. In particular, the United States must find a way to make him worry about the thing he most values: himself.
Getting at Kim means putting at risk North Korea’s growing number of hard and deeply buried targets. North Korea has sought to preserve its leadership and other valued assets, such as its nuclear and missile forces, by constructing underground facilities ranging from relatively shallow ‘cut and cover’ facilities to complexes buried beneath hundreds of meters of hard rock. Although most of these facilities can be threatened with conventional earth penetrators, a few may be too deep for conventional options, especially those where Kim himself might plan to hide. For the near future, only nuclear weapons could hold such targets at risk.
Yet the current earth penetrating warhead in the United States nuclear arsenal, the B61-11, is ill-suited for certain North Korean targets. The United States has long had an official requirement for a hard rock penetrator, but this requirement has been unmet since the Clinton administration – rightly in our view – decided to retire the aging B53 penetrator warhead because of safety and reliability concerns. The Clinton administration developed the B61-11 to replace the B53, but this newer warhead was designed to penetrate frozen tundra, like that found in Russia, and not hard rock like that found in North Korea.
Today, however, we don’t have to choose between safety and meeting reasonable, existing military requirements to hold at risk targets in hard rock. This is because the United States could meet the military requirements previously met by the B53 by modifying an existing warhead, the B83. (The National Nuclear Security Administration had planned to begin looking at B83 lifetime extension options soon, but it now appears this has been unnecessarily deferred to the mid-2020s.) Specifically, this military requirement could be met by simply changing the external casing of the warhead, providing better attitude control, and confirming that the configuration of internal components would survive the rapid deceleration accompanying penetration into a few meters of hard rock. As a first step, we would recommend that Sandia National Laboratories conduct a ‘sled test’ of a B83 (using a test assembly with no nuclear material) to assess the ruggedness of the non-nuclear components in a penetrator mode.