Prospects for in-containment cesium capture for light water reactors

Radioactive cesium and radioiodine are the two most important fission products to consider in the event of a large release. We examine options to trap cesium on sorbents located in containment during an accident to (1) reduce potential cesium releases from containment and (2) reduce in-plant radiation levels to enable greater operator plant access during an accident. The half-life for Cs-134 is 2 years, and the half-life for Cs-137 is 30 years. Consequently, radiocesium will exist in the environment for a number of years, and its dose will be delivered over a long period of time. Methods and materials for the passive capture of aqueous Cs released to the reactor containment during severe accidents were evaluated for 2 BWRs and 2 PWRs. Successful substrates for Cs capture are insoluble and may mimic naturally occurring Cs (pollucite) for improved stability over wide ranges ofpH, temperature, and water chemistry. Titanate nanotubes were shown to have good selectivity and high Cs capacity above pH 4. Conservatively, 1 to 2 metric tons of the nanotubes were calculated to be sufficient to absorb all Cs released from even the largest LWRs. Copper ferrocyanide on mesoporous silica was compared against Prussian Blue (iron hexacyanoferrate) over a pH range of 0.1 to 7.3 in three different aqueous chemistries. While the Cs capacity of the two materials was similar, they both exhibit a Cs capacity about 10 times less than the titanate nanotubes. The copper ferrocyanide proved to have greater Cs selectivity and improved stability versus Prussian Blue, which tended to degrade over time. A naturally occurring zeolite known as clinoptilolite was analyzed for its Cs capacity and selectivity over a range ofpH (from 1 to 11), temperatures (from 25 °C to 60 °C), and competing cation concentrations. Clinoptilolite proved to have a good Cs capacity (similar to that of the titanate nanotubes) and good Cs selectivity. It can also be heat-treated in order to fix the Cs to the clinoptilolite substrate. A potential challenge for the clinoptilolite is that it is slightly soluble, and its Cs capacity decreases with increasing temperature. Additionally, recent findings indicate that Cs capture on oxides of Zr, Mo, Si, and others may also be viable. Solid, insoluble sorbents for Cs are a promising approach to Cs capture, but three of the remaining challenges are determining how to best manufacture and place the sorbents in the containment, how to ensure their chemical compatibility/stability during a severe accident, and to prove their ability to withstand decay heat from the Cs.