Process for preparing polycrystalline silicon

A method for producing a polycrystalline silicon granulate in a fluidized bed reactor, comprising:

• fluidizing silicon seed particles using a fluidizing gas in a fluidized bed reactor having a reactor wall, which is heated a heating apparatus;
• adding a reaction gas comprising hydrogen, and a silane and/or a halosilane;
• and
• depositing elemental silicon, by pyrolysis, onto the silicon seed particles to form the polycrystalline silicon granulate;
• and
• discharging an offgas from a recycle gas system for workup of offgases of the fluidized bed reactor and directing hydrogen recovered from the offgas back to the fluidized bed reactor as a recycle gas, wherein the recycle gas contains nitrogen, and when the nitrogen content exceeds a predetermined threshold value between 0.01 and 1000 ppmv, the reactor is shut down, the reactor continues to operate with external hydrogen and no recycle hydrogen or a lesser amount of recycle hydrogen, or the temperature of the fluidized bed and/or the reactor wall is/are increased until the nitrogen concentration falls below the predetermined threshold value.