the frequency of mechanical resonances, which increases the danger of catastrophic failure during start-up (as it happened during the Stuxnet event in Iran). Interrupting the cyclindrical rotor by flexible bellows controls the low-frequency vibrations, and careful speed control during start-up helps to ensure that the centrifuge does not operate too long at speeds where resonance is a problem. But more (proprietary) measures seem necessary. Therefore Russia stayed with "subcritical" centrifuges (i.e., with small lengths around 0,5-1 m), whereas those of Urenco have lengths up to 10 m. The Zippe-type centrifuge is difficult to build successfully and requires carefully machined parts. However, compared to other enrichment methods, it is much cheaper and is faster to set up, consumes much less energy and requires little area for the plant. Therefore it can be built in relative secrecy. This makes it ideal for covert nuclear-weapons programs and increases the risk of nuclear proliferation. Centrifuge cascades also have much less material held in the machine at any time than gaseous diffusion plants. Global usage Pakistan's atomic bomb program developed the P1 and P2 centrifuges based on early designs of Urenco; the first two centrifuges that Pakistan deployed in larger numbers but reduce it after 1981 based on estimation require for critical mass. The P1 centrifuge uses an aluminum rotor, and the P2 centrifuge uses a maraging steel rotor, which is stronger, spins faster, and enriches more uranium per machine than the P1. In Pakistan, the Zippe-type centrifuge had a local designation and was known as Centrifuge Khan (after Abdul Qadeer Khan). Russian sources dispute the account of Soviet centrifuge development given by Gernot Zippe. They cite Max Steenbeck as the German scientist in charge of the German part of the Soviet centrifuge effort, which was started by German refugee Fritz Lange in the 1930s. The Soviets credit Steenbeck, Isaac Kikoin and Evgeni Kamenev with originating different valuable aspects of the design. They state Zippe was engaged in building prototypes for the project for two years from 1953. Since the centrifuge project was top secret the Soviets did not challenge any of Zippe's claims at the time. Zippe-type centrifuge facilities Khan Research Laboratories in Pakistan National Enrichment Facility of Los Alamos National Laboratory in the United States Urenco Group in Great Britain, the Netherlands and Germany Russia (where it is called Kamenev centrifuge) Iran, North Korea France, India, China, Japan See also Gas centrifuge Ultracentrifuge