To the Group: A question was posted some time ago to elicit ideas relative to the unknown 559 KeV peak. Many plants have seen the peak and most seem to attribute it to 76As when other possibilities have been exhausted. The following are the results of the benchmarking and our analysis: 1) 82Br. This is predicated on the frequent occurrence of 82Br when the 559 KeV peak is present. 82Br is not always present when the 559 peak is present. 82Br also does not have a peak at 559 KeV. It is unlikely that the 559 KeV peak is due to 82Br. 2) Cosmic neutrons. At least two responses suggested that the 559 KeV peak is due to cosmic neutrons interacting with the Cd in the graded shield and producing a prompt gamma with an energy of 559 KeV: 113Cd (n, gamma) 114Cd (stable). This is described in "Low level Gamma Spectroscopy: Low, Lower, Lowest", Verplancke, Jan, Nuclear Instruments and Methods in Physics Research A312 (1992), Table 2, pp. 174-182. This phenomena occurs in low level counting applications where samples are counted for very long periods in low level background conditions. Samples are counted for just a few minutes which is too short for this effect to manifest. In addition, the 559 KeV photopeak is not seen in the daily background counts, which are one hour in duration. It is unlikely that the 559 KeV peak is due to cosmic neutron interaction with the Cd in shield. 3) 133I. There was one suggestion that the 29 KeV peak of 133I sums with the 530 KeV peak of 133I. The math works, but the aluminum end cap on the detectors would effectively eliminate the 29 KeV photon. In addition, summing is a low statistics process and it would require a lot of 133I. 133I is rarely present in any sample. It is unlikely that the 559 KeV peak is due to summing from 133I . 4) 76As. This was the most popular response but the responders were unclear of its origin. 76As is not a fission product so it must be produced by thermal neutron activation of stable 75As. Arsenic is relatively abundant in the Earth's crust. According to the Ferroalloys & Alloying Additives Handbook, [Deeley, Paul, Kundig, Konrad J.A., and Spendelow, Howard R. Jr, AMG Vanadium, Inc., Cambridge, OH. November, 2000] Arsenic is almost always present as a trace element in steel because of its natural occurrence in iron ores, coke, ferroalloys and nonferrous addition agents. Residual concentrations will be in the range of 0.005 to 0.030% for domestic steels and up to three times this amount in steels produced abroad. Although arsenic has a mildly beneficial effect on strength it is never intentionally added to steels, mainly because of its high toxicity. Also, even trace amounts are known to promote susceptibility to temper embrittlement, especially in chromium-nickel steels. Therefore, it is regarded as an unwanted "tramp" element. 76As has a principal photopeak at 559 KeV. 76As also emits multiple very low intensity gamma rays, however due to their very low intensity, none are of use in identification. A half- life determination performed at Callaway Energy Center confirms that the 559 KeV photopeak is due to 76As.
76As is sometimes seen on charcoal cartridge samples taken from the Reactor Building, but not on the particulate filter. It is odd that a particulate would be seen on the charcoal cartridge and not on the particulate filter. This would indicate that the 76As is a very small particle that is not filtered out by the 0.45 µm filter. This is different from other corrosion and erosion products, therefore the mechanism for this is unknown. REC 16.11.2.3, "Dose- 131I and 133I, 3H, and Radioactive Material in Particulate form Limiting Condition of Operation", does not apply to particulate nuclides in gaseous effluents with a half-life of less than 8 days. 76As has a half- life of 26 hours therefore it is will not be added to the gamma spec libraries used in the analysis of gaseous effluents. 76As has never been found in liquid effluent samples. Considering the relatively short half- life, the time required to process the water, and the time required to fill and prepare a DMT for discharge, it is unlikely that 76As will ever show up in liquid effluents. Therefore 76As will not be added to the libraries used to analyze liquid effluent samples. The RCS libraries were evaluated by the Chemistry Dept and it was concluded that the RCS libraries do not need to be changed. The 559 KeV photopeak is due to 76As. 76As will not be added to the gaseous or liquid effluents libraries or to the RCS libraries. Questions or comments, call or email. : : : : : : : : : : : : : : : : : : : : : : : : : Dewey Thompson Staff HP Radiation Protection Department T 314.225.1061 F 573.676.4484 E [email protected]<mailto:[email protected]> ......................... The information contained in this message may be privileged and/or confidential and protected from disclosure. If the reader of this message is not the intended recipient, or an employee or agent responsible for delivering this message to the intended recipient, you are hereby notified that any dissemination, distribution or copying of this communication is strictly prohibited. Note that any views or opinions presented in this message are solely those of the author and do not necessarily represent those of Ameren. All e-mails are subject to monitoring and archival. Finally, the recipient should check this message and any attachments for the presence of viruses. Ameren accepts no liability for any damage caused by any virus transmitted by this e-mail. If you have received this in error, please notify the sender immediately by replying to the message and deleting the material from any computer. Ameren Corporation
