Radioactive Material Thresholds

[Greg.Wells@aeat.co.uk (Greg Wells)]

     Greetings Radsafers
     
     I've just joined this discussion group and already have a question to 
     ask. It's one of those straight forward questions that never seems to 
     get a straight forward answer I'm afraid - at least not if based on 
     the UK legislation anyway.
     
     The problem I have is this: In the current Radioactive Substances Act 
     (RSA93) there is a schedule (Schedule 1) listing certain elements 
     associated with certain values (in Bq/g) above which the elements (or 
     at least the radioactive isotopes of the element) are considered to be 
     radioactive. This is fine for the elements listed (Ac, Pb, Po, Pa, Ra, 
     Rn, Th, U), but its not very useful for radioactive materials 
     consisitng of any other elements. According to RSA93 all other 
     elements (or rather their relevant radioactive isotopes) are deemed to 
     be radioactive if they have been produced artificially. With this 
     definition there is no threshold value below which the elements can be 
     considered non-active.
     
     In the Approved Code of Practice 'The Protection of Persons against 
     Ionising Radiation arising from any Work Activity' the definition of 
     radioactive material is anything above 100Bq/g with the exception of 
     certain elements (such as Th and U).
     
     This creates a problem when trying to decide legally and safely 
     whether an item is considered to be a radioactive or not. Here at 
     Harwell Laboratories in the UK a certain amount of interpretation and 
     extrapolation of the RSA93 values has been implemented to create 
     threshold values for two groups of radionuclides: Those which emit 
     alpha particles (0.4Bq/g) and those which do not (4.0Bq/g).
     
     This broad classification may seem strange for some as the 
     radiotoxicity of some radionuclides is far greater than others (e.g. 
     Alpha emitter Pu-239 is 100 times more radiotoxic than alpha emitter 
     Po-210 according to ICRP 30). However, I believe the threshold values 
     used at Harwell (and at other sites around the UK) are conservative 
     ones.
     
     My question is this:
     Is there a definitive legal threshold activity per mass value defining 
     the limit below which material can be considered non-active (either 
     generically or individually)? i.e. have I missed something?
     
     (This question relates mainly to the UK legislation but information 
     with regard to other country's laws will be useful and appreciated. 
     Perhaps the IAEA has some guidance on threshold values?)
     
     Apologies if this subject has been brought up before. I'm having 
     trouble accessing the Radsafe archive. What's the current address by 
     the way?
     
     Greg Wells
     
     P.S. I've got an even more tricky question to follow!!!