Inside the fuel element stainless steel cladding, the fuel is in the form of a uniform mixture of 8 wt% uranium, 1 wt% hydrogen and 91 wt% zirconium, the zirconium-hydride, being the main moderator. Since the moderator has the special property of moderating less efficiently at high temperatures, the TRIGA-reactor Vienna can also be operated in a pulsed mode (with a rapid power rise to 250 MW for roughly 40 milliseconds). The power rise is accompanied by an increase in the maximum neutron flux density from 1x10¹³ cm^-2s^-1 (at 250 kW) to 1x10¹⁶ cm^-2s^-1 (at 250 MW). This negative temperature coefficient of reactivity, as it is called, brings the power level back to the kW range after the excursion. The maximal pulse rate is 12 per hour, since the temperature of the fuel has to cool down between the single pulses.