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Conference Article: Background iodine in a commercial advanced gas-cooled reactor

Garland, J.A.; Wiffen, R.D. (AERE Harwell, Oxon (United Kingdom)); Webster, D. (Hinkley Point Power Station, Somerset (United Kingdom))

Abstract

Measurements have been made of the 131I present in the coolant gas of Hinkley Point R3, using two sampling methods. One uses charcoal traps which can be connected to many access points on the reactor circuit. The second uses copper gauzes or charcoal loaded filters followed by a charcoal trap, to obtain some information on speciation. This method has been used at a specially installed sampling point on the duct to the gas by-pass plant. The measurements discussed here were made in normal operation and following a planned reactor trip. All measurements using the second method indicated the presence of two iodine species. One (the reactive species) was absorbed efficiently by copper gauzes and charcoal loaded filter papers. The other form (the penetrating species) was retained only weakly by charcoal loaded filter papers but was absorbed quantitatively by the charcoal trap. It is not possible to identify these forms unambiguously. I2, I and HI are possible candidates for the reactive form. The penetrating form does not appear to be pure CH3I but may be (at least in part) higher alkyl iodides. In normal operation 10.4 +- 3.1% of the iodine reaching the sampler is in the penetrating form. Losses of the reactive form in the sampling pipe could be substantial, despite its short length (approx. = 2 m, 9.5 mm bore) and the high flow rate (approx . = 80 g s-1). At other locations used for iodine sampling, much longer sampling pipes could absorb essentially all the reactive iodine. This would account for the order of magnitude greater total concentration (284 +- 83 pci/kg) at the special sampling point. Following the reactor trip, large fluctuations were observed in 131I and 133I concentrations were observed in charcoal pack samples collected at two of the circulators. Peaks in I concentration were observed at about 30 min and between 2 and 3 h following the trip. Corresponding changes in the penetrating iodine form were seen at the special sampling point. The concentration of the reactive form decreased three-fold following the trip but regained some of the loss subsequently.

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key words: Gas Cooled Reactor, Nuclear Technology
Reference:
Specialists' meeting on fission product release and transport in gas-cooled reactors Berkeley (United Kingdom) 22-25 Oct 1985
International Atomic Energy Agency, International Working Group on Gas-Cooled Reactors, Vienna (Austria)
IWGGCR--13, pp:81-91