The NIST natural-matrix radionuclide standard reference material program for ocean studies

In 1977, the Low-level Working Group of the International Committee on Radi onuclide Metrology met in Boston. MA (USA) to define the characteristics of a new set of environmental radioactivity reference materials. These refere nce materials were to provide the radiochemist with the same analytical cha llenges faced when assaying environmental samples. It was decided that radi onuclide bearing natural materials should be collected from sites where the re had been sufficient time for natural processes to redistribute the vario us chemically different species of the radionuclides. Over the succeeding y ears, the National Institute of Standards and Technology (NIST), in coopera tion with other highly experienced laboratories, certified and issued a num ber of these as low-level radioactivity Standard Reference Materials (SRMs) for fission and activation product and actinide concentrations. The experi ence of certifying these SRMs has given NIST the opportunity to compare rad ioanalytical methods and learn of their limitations. NIST convened an inter national workshop in 1994 to define the natural-matrix radionuclide SRM nee ds for ocean studies. The highest priorities proposed at the workshop were for sediment, shellfish, seaweed, fish flesh and water matrix SRMs certifie d for mBq per sample concentrations of Sr-90, Cs-137 and Pu-239 + Pu-240. T he most recent low-level environmental radionuclide SRM issued by NIST, Oce an Sediment (SRM 4357) has certified and uncertified values for the followi ng 22 radionuclides: K-40 Sr-90, I-129, Cs-137, Eu-155, Pb-210, Po-210, Pb- 212, Bi-214, Ra-226 Ra-228 Th-228, Th-230, Th-232, U-234, U-235, Np-237, U- 238, Pu-238(,) Pu-239, + Pu-240, and Am-241. The uncertainties for a number of the certified radionuclides are non-symmetrical and relatively large be cause of the non-normal distribution of reported values. NIST is continuing its efforts to provide the ocean studies community with additional natural matrix radionuclide SRMs. The freeze-dried shellfish flesh matrix has been prepared and recently sent to participating laboratories for analysis and we anticipate receiving radioanalytical results in 2000. The research and d evelopment work at NIST produce well characterized SRMs that provide the wo rld's environment-studies community with an important foundation component for radionuclide metrology.