Other actions include dismantling weapons into various components, which are then verified as actual nuclear material. This may take years and require highly intrusive verification that may be unacceptable to the party complying with disarmament.

A better solution is to take action immediately upon presentation of the weapon to the inspecting party. Actions that will demonstrably show that a weapon is no longer usable. A novel proposal of the AMC working group on technical verification ensures that the most critical part of the weapon, the fissile pit, is irreversibly destroyed and cannot be used as is under any circumstance. Additionally, the pit can only be recycled through the entire sequence of chemical and metallurgical processes to have produced it in the first place. The disabled weapon is truly removed from inventories of military items.

Before we look at the method, a couple of caveats should be noted. First, rapid early disablement reduces the urgency and invasiveness of verification, but it does not replace it.

Second, neither disablement, dismantlement nor denuclearization are barriers to rearmament. Rearmament represents a failure in treaty compliance, not a failure of disarmament. If the weapon was to be completely recycled and rebuilt, that would be considered rearmament.

How it works

The proposal presented by our working group takes advantage of the fact that modern strategic warheads in the inventories of the permanent five Security Council members, coincidentally the Nuclear Weapons States (NWS), rely on tritium boosting to get their very high destructive yields. It is extremely likely that every modern strategic weapon targeted on another P5 rival uses tritium boosting. Boosting involves injecting tritium, an isotope of hydrogen, into the plutonium (or uranium) fission stage (called the pit) of the weapon. The tritium is stored outside the pit and is injected moments before it is exploded. Because tritium is radioactive and has a half-life of only 12 years, the injection container, or ‘bottle’ in jargon, is located outside the nuclear package for regular periodic replacement. This provides easy access and ensures that technicians replacing the ‘bottle’ do not see the sensitive innards of the weapon.

In our proposal, cooperative disarming parties and observers will remove the tritium bottle and replace it with one containing ordinary hydrogen. They will then open the valve and inject the pit with ordinary hydrogen. The hydrogen will react strongly with the plutonium metal in the pit causing it to form plutonium hydride (PuH2). The PuH2 flakes, spalls and falls to the bottom of the pit as a dense but fluffy powder. If the high explosive surrounding the pit were to be detonated in this configuration it is likely there would be no nuclear yield in the pit. Plutonium pits are manufactured to an extremely high metal standard and a pit riddled with holes in the metal and several hundred grams of PuH2 on one side is not a precision object. It cannot produce a symmetrical nuclear explosion. Neutronic properties are destroyed. If the pit gives no yield, then the much larger thermonuclear stages will give zero yield.

The process is irreversible. There is no way to undo this reaction without taking the pit apart completely and subjecting it to complicated steps of chemistry and metallurgy. The weapon is thus disabled in minutes and within a very short time of being presented for disablement and observation.

Note that this reaction takes place in the short space of minutes to hours. In a military explosion the heavy hydrogen isotope (tritium) is only in contact with the plutonium metal for a few seconds so the reaction has no time to start.

Our proposal also includes suggestions on verifiable measurements. These would give some confidence that the object being disabled is a plutonium pit. It should be possible to make very simple measurements of radiation. These measurements would not be of the high standard necessary for serious verification but would be a first step in confidence building. Secondly, tritium is a very expensive material which releases heat as it radioactively decays. If a bottle removed from a military system produces an expected amount of heat it is a useful, though not foolproof indication, that the bottle contains tritium and the inspected object is likely to be a real nuclear weapon, not a fake. This can be done in a field calorimeter.

Once the disablement is completed, verification and dismantlement can proceed. It can be rapid or even take years. In the interim, an object that cooperative parties have agreed is a nuclear weapon is truly disabled. Furthermore, the disablement process provides indications that the object had a high probability of being a nuclear weapon offered for disarmament. Security issues are greatly reduced because the procedure does not require access to the most sensitive nuclear components, the pit itself and the thermonuclear secondary.

Irreversible disablement could and should be the first step in the final disarmament of a stockpile. It provides very early assurance that weapons are being irreversibly removed from military stocks and that the weapon can never be used again without complete disassembly and remanufacture.