Off-gases from decladding, voloxidation if practiced, and dissolution are passed through high-efficiency particulate filters, processed for radioiodine absorption and, in some plants, for krypton and xenon retention before discharge through the plant stack. Gases vented from downstream process equipment are also passed through high-efficiency particulate filters and radioiodine absorbers. This section describes briefly processes that have been developed for absorbing radioiodine and removing and packaging krypton and xenon. Retention of tritium and 14C may also be required in the future.

Radioiodine removal. Radioiodine removal is important because of its toxicity, the compara­tively high iodine content of fission products (0.69 w/o, Table 8.7), and the high fission yields at the mass numbers of the two principal radioiodines, 1.7 X 107 year 129I (1 percent) and 8.05-day 1311 (2.09 percent). Removal of radioiodine is complicated because of the numerous process streams in which iodine may appear and the variety of chemical forms it assumes. About 1 percent of the iodine is volatilized during decladding, some during voloxidation and a significant but incomplete amount during dissolution. If iodine is allowed to remain in the feed to solvent extraction, it reacts with solvent to form hard-to-remove compounds that eventually contaminate the entire system. It is thus important to remove as much of the iodine as possible before solvent contacting. Iodine may appear as I2, HI, HIO, or organic iodides in off-gases or aqueous or organic phases, or as HI03 in concentrated nitric acid solutions.

The preferred procedure for removing iodine is to route the gases from decladding and voloxidation to iodine absorbers and to distill iodine from dissolver solution before solvent extraction. Experiments at Oak Ridge showed that 95 percent of the iodine could be removed by distilling 2 percent of the volume from 4 M HN03 and 99 percent by distilling 20 percent [06]. Some of the remaining iodine is evolved with vent gases.

Of the numerous iodine-removal methods discussed by Goode and Clinton [G9], the most significant are characterized below.

Absorption by aqueous NaOH removes HI and I2 but not organic iodides. No good procedure is available for disposal of spent solution.

The Iodox process under development by Oak Ridge National Laboratory uses absorption in boiling 21 to 23 A/HNO3 to convert iodine and its compounds to solid, nonvolatile I2Os.

An alternative process developed by Oak Ridge [08] uses boiling 8 to 14 M HNO3 containing 0.2 to 0.4 M Hg(N03)2 to absorb all forms of iodine as Hgl2. The absorber solution is evaporated from vermiculite, which retains the iodine in stable form suitable for storage. The Barnwell plant proposes [A3] use of a similar process.

Unglazed Berl saddles coated with silver nitrate and operated at 135°C were used at Hanford [Ml] to remove HI and I2 from dissolver off-gases. In 1958, an explosion occurred, which was attributed to an unstable compound of silver and ammonia formed when the reactor was periodically cleaned by washing with ammonium sulfite. After this was replaced by sodium thiosulfate, the reactor operated for 14 years without incident.

A more effective way of using silver is to impregnate with silver a zeolite catalyst of the type used in hydrocarbon processing. With moist air at 150°C all volatile iodine species are absorbed as stable silver iodide in a form suitable for packaging and permanent storage. Silver zeolites for iodine absorption have been developed at Idaho Nuclear [P3] and Karlsruhe, Germany [W7], Wilhelm et al. [W7] give data for fractional penetration of I2 and CH3I through an amorphous silicic acid zeolite impregnated with 0.06 to 0.08 g silver/g zeolite. More than 98 percent of the silver is available for reaction, permitting loadings of 0.1 g iodine/g zeolite. Fractional penetration of iodine is a function of many variables, as described in [W7]. Decontamination factors of from 102 to 104 have been reported. Long-term management of radioiodine as a radioactive waste is discussed in Chap. 11.

Krypton and xenon removal. The number of curies of krypton and xenon per megagram (metric ton) of spent fuel from pressurized-water, liquid-metal fast-breeder, and high — temperature gas-cooled reactors from Tables 8.7, 8.8, and 8.9 are listed in Table 10.5, together with the number of standard liters per megagram, assuming atomic weights of 85 and 133 for krypton and xenon.

Table 10.5 Curies and liters of krypton and xenon in spent fuel 150 days after discharge

The volume of gas is appreciable; 80 percent or more is xenon. Practically all of the radioactivity is due to 85 Kr. One year after discharge the xenon activity would be negligible. This xenon could be a significant commercial source.

Processes that have been studied for krypton-xenon removal are listed in Table 10.6 together with comments on the process from reference [M6]. All have achieved 99 percent krypton removal.

Room-temperature adsorption is used for off-gases from nuclear power plants to delay escape of krypton and xenon long enough for all radionuclides except 85 Kr to decay to innocuous levels. Retention of 85Kr would require very large bed volumes and a more complex system for bed regeneration. There is a fire hazard when treating reprocessing off-gases with charcoal, so that 02 and NO* must be removed from the feed.

In cryogenic adsorption, smaller bed volumes suffice, but the feed must be pretreated to remove condensibles. The fire hazard with charcoal remains and may be worse, because of the possibility of adsorption of ozone produced by radiolysis of oxygen.

Development of permselective membranes is only at the laboratory stage. For reprocessing

Table 10.6 Processes for removal of 85 Kr from reprocessing off-gas

Process Development status Comments

off-gases, disadvantages are the serious consequence of mechanical failure and deterioration from radiation and exposure to ozone and NO*.

The last two processes are the ones favored for reprocessing plants.

Cryogenic distillation has been extensively operated at Harwell [W8] and the Idaho Chemical Processing Plant [В9]. The principal concerns are (1) plugging of low-temperature equipment by condensed ice, solid C02 or Xe, or solid nitrogen oxides and (2) possible explosion from accumulation of solid hydrocarbons in the presence of condensed oxygen and ozone. To deal with them, feed gases must be pretreated for removal of impurities before condensing the krypton and xenon. At the Idaho plant [B9], N02 and C02 were removed from feed gas by scrubbing with sodium hydroxide solution. No attempt was made to package the C02 containing 14 C, but this could have been done by precipitation as CaC03 with lime. N20 was dissociated into N2 and 02 by passage over a rhodium catalyst at 650°C. The hydrocarbon content of feed gas was low enough that hazardous accumulation in low-temperature equipment was prevented by warm-up once per shift. More generally applicable practice would be to oxidize hydrocarbons by passing feed gas over copper oxide at 600°C.

After purification the feed gas at Idaho was cooled to -160°C by passage through regenerators, precooled by outflowing cold gas, in which H20 and remaining traces of C02 and nitrogen oxides were condensed and removed. Finally, the purified feed gas was washed with liquid nitrogen to condense krypton and xenon, which were then concentrated by fractional distillation. The concentrate was separated periodically by batch distillation into an oxygen fraction, which was recycled to prevent loss of small amounts of accompanying krypton, and a krypton fraction and a xenon fraction, which were bottled separately for storage.

Absorption in halogenated solvents, such as refrigerant R-12, CF2C12, has been extensively studied at Brookhaven [S21], Harwell [T3], and Oak Ridge [M6, V4]. The process has several advantages. Fire or explosion hazards are minimal, and gas purification prior to absorption is not required. The process is flexible and does not use extremely low temperatures. Dis­advantages are operation at 8 to 10 bar pressure, a fairly complex flow sheet, and the need for an auxiliary system to separate krypton from xenon and C02.

Figure 10.7 shows one of the flow sheets for removing radioisotopes from reprocessing plant off-gases by refrigerated absorption tested by Oak Ridge National Laboratory [V4]. Contaminated feed gas, consisting of H20, C02, N20, Xe, Kr, Ar, N2, and 02, and possibly containing I2, CHjI, and N02 not previously removed, is compressed to 8 bar (100 psig) and cooled to —28°C in a cold trap. This removes most of the H20, N02, I2, and iodine compounds. The gas is then fed to a 5-m absorber-fractionator column refluxed with refrigerant R-12 at —28°C at the top and reboiled at 31°C at the bottom. Decontaminated Ar, N2, and 02 are taken off the top, and a solution of Kr, Xe, N20, and C02 in R-12 is taken off the bottom. This solution also contains traces of Ar, N2, 02, N02, and water, and iodine compounds if present. In the stripper, Kr, Xe, N20, and C02 diluted with small amounts of Ar, N2, and 02 are taken off as overhead product, together with some R-12. Solvent from the bottom of the stripper is distilled to separate it from small amounts of water and other less volatile impurities prior to recycle to the absorber-fractionator.

Overhead product from the stripper, although greatly reduced in volume from feed gas, requires further treatment (not shown) to separate and package C02 containing 14 C, Kr, and Xe. One possible sequence of operations would be

1. Absorb C02 for permanent storage on solid soda lime.

2. Remove R-12 for recycle with a selective molecular sieve.

3. Decompose N20 over a rhodium catalyst at 650°C.

4. Remove 02 with copper at 600° C.

5. Condense Xe, Kr, and some Ar in a cold trap refrigerated with liquid nitrogen.

6. Separate the condensate by low-temperature distillation into (a) an Ar-Kr fraction to be bottled for permanent storage as radioactive waste and (b) an Xe fraction.