Answer to Question #12732 Submitted to "Ask the Experts"
Category: Radiation Basics — Radionuclides
The following question was answered by an expert in the appropriate field:
For our whole-body count/internal exposure program at an operating nuclear power plant, we use Class Y for 95Zr, annual limit of intake (ALI) = 1 × 107 Bq. However, Class D for 95Zr has a more restrictive ALI, 5 × 106 Bq. I'm not sure why Class Y was chosen and is used. I understand the decay/parent relationship of Zr/Nb, but is this use of Class Y for 95Zr appropriate and does it provide the adequate level of protection for our workers?
While I cannot say with certainty why your facility opted to use the seemingly less restrictive value of the ALI for Class-Y 95Zr rather than the ALI you cite for Class D, I believe there is a sound argument that can be made for using the Class Y value.
If we review the table values of 95Zr inhalation ALIs, as given in International Commission on Radiological Protection (ICRP) Publication 30 Part 1, page 80, we see that the Class-D lowest cited ALI value of 5 × 106 Bq is the ALI based on limitation of dose and effects to bone surfaces—i.e., a nonstochastic based ALI intended to restrict dose to a specific tissue, in this case bone surfaces. The ALIs for the Class W and Class Y 95Zr are each based on limitation of committed effective dose—i.e., stochastic-based ALIs. In addition, all of the inhalation ALIs for daughter 95Nb are stochastic-based. It is also the case that the Class Y ALI for 95Zr has the same 1 × 107 Bq ALI as the stochastic-based ALI for Class D 95Zr. We could argue that occupational protection would be promoted by using the effective dose protection limits common to parent and daughter radionuclides. Beyond this, we should note that it is the current position of the ICRP that the proper ALI to be used for radiation protection purposes is the stochastic-based ALI; there is no longer a requirement or need to calculate and implement non-stochastic based ALIs. The ALIs that are cited in Code of Federal Regulations 10 CFR 20 are based on earlier ICRP recommendations (ICRP Publications 26 and 30), dating back about 40 years, that have been supplanted by more recent recommendations.
I hope this is helpful to you.
George Chabot, PhD