Who is responsible for conducting the initial written risk assessment for experiments involving biohazards?

Laboratory Supervisor/Principal Investigator.

Explanation:

The biohazard may be defined as any biological substance that can create problems for the living organisms. The substance can be either any bacteria, virus or any toxin.

The individual if working with any biohazard must read carefully and than start their work. This is the duty of principal investigator or the laboratory supervisor that they should include and show the written assessment of the experiments that might acts as biohazard.

Thus, the answer is Laboratory Supervisor/Principal Investigator.