Background: Ozone will be produced when the accelerator runs.Because ozone can cause corrosive damage to metal

ventilation duct has to be set in the irradiation room to discharge ozone.Purpose: Photons go into the ventilation duct

and then produce additional radiation.To do the quantitative assessment of radiation field at the outlet of duct

the simulate calculation of radiation doses at the outlet has been done.Methods: The Monte Carlo code of MCNP can analog the electronic and photon transport

so that using MCNP can solve the problem.Results: The result shows that the location and the number of backscattered interface of duct significantly affect the dosage rate while the size of duct doesn’t.Increasing a backscatter interface can lower dosage rate attenuation about three orders of magnitude.The dosage rate under the target is two orders of magnitude than that in the corner.The setting project of duct is drawn and the ratio of length and depth has been optimized.The duct should be about 3-m long and 80-cm deep when the multi-tortuous duct is placed under the target

and U-shaped duct should be about 6-m long and 1-m deep when it is set at the corner of irradiation room.Conclusions: In order to reduce dosage rates and economic costs

the shape of duct and the location of duct are designed critically.And according to the actual situation

the project is flexible.