Your discussions of the failure to achieve ignition at the US National Ignition Facility (NIF; Nature 491, 159 and Nature 491, 170; 2012) raise an associated concern about the US Stockpile Stewardship Program that we believe deserves the attention of everyone concerned with the effectiveness of the US nuclear deterrent.

Experimental data from the NIF reveal that lasers can compress hydrogen fuel in fusion capsules, but ignition conditions have not been obtained. Deficiencies in the simulations used to design ignition capsules meant that predictions for when fusion would be achieved were wrong. Worryingly, these deficiencies were not brought to light until experimental data from the NIF made their existence undeniable.

Our concern is that something similar could occur in the Stockpile Stewardship Program, which relies heavily on simulations to assess nuclear-weapons performance. As with ignition, an overly optimistic assessment could result from over-confidence in simulations.

The analogy ends there, because the only experimental data that could definitively expose deficiencies in the nuclear-weapons simulations would have to be obtained from nuclear tests, which are prohibited under a moratorium. The potential consequences would be very serious.