Sir

You are correct to state that the Reliable Replacement Warhead (RRW) programme would do nothing to improve the reliability of the US nuclear arsenal (Nature 461, 11; 2009). It is possible that the “powerful figures within Obama's own administration” are being advised by people associated with the weapons laboratories who may have an interest in derailing a ratification of the Comprehensive Test Ban Treaty.

Existing nuclear weapons are already very reliable and their safety features are adequate, as a series of reports by R. E. Kidder of Lawrence Livermore National Laboratory affirmed. (For further details, see http://go.nature.com/yCmG4W.) As US weapons are comfortably tolerant of small variations that may occur in materials or the manufacturing process, they can be remanufactured without explosive proof-testing.

Those in favour of the RRW claim that it would not need nuclear testing. This is because the RRW would be composed of an existing primary stage, which is where many of the uncertainties of nuclear weapons design reside, and probably also an existing secondary stage. However, very few people outside the weapons-design community would trust deployment of a weapon that has not been tested.

As a result, the pressure to test any RRW could well derail the ratification of the test ban treaty. Failing to ratify this treaty (not to mention resuming nuclear testing) would do great harm to US national interests as well as those of the world at large.

Others argue that the RRW programme is needed to maintain expertise in weapons physics. This can be done without the RRW. There is no need for it or any continued nuclear proof-testing.