I.-When the installation is commissioned and after any significant change in working methods and conditions likely to affect the health and safety of workers, the employer shall carry out measurements in the delimited areas and in the workplaces adjoining these areas in accordance with Article R. 4451-24, to the initial verification of:
1° The level of external exposure;
2° Where applicable, the concentration of radioactive activity in the air or surface contamination;
3° The concentration of radon activity in the air, where the area is delimited for radon.
If necessary, it will check the effectiveness of the protection and alarm systems put in place to prevent exposure to ionising radiation.
II.-These initial checks are carried out by an accredited body under the conditions set out in Article R. 4451-51.