Ulam still only gave the design a "50–50 chance" of success in February 1950. At the end of March, he reported that it would not work at all. Scientists like Hans Bethe and George Gamow felt that Teller had committed the nation to an expensive crash program on the basis of a model that he knew was flawed. However, in February 1951, Ulam had a new idea, in which the shock wave from an atomic bomb "primary" stage, through an arrangement he called "hydrodynamic lensing", would compress a "secondary" stage of deuterium fusion fuel wrapped around a plutonium rod or "spark plug". On being informed, Teller immediately grasped the potential for using the X-rays produced by the primary explosion for hydrodynamic lensing. This arrangement, which made thermonuclear weapons possible, is now known as the Teller–Ulam design. Although it was not what Truman had approved, the design did work, and was capable of producing multi-megaton explosions. "Rarely in the history of technology", wrote Howard Morland, "has such a seemingly daunting problem turned out to have such a nifty solution."