1.1 Activity in Irradiated Fuel

For irradiation in a constant neutron flux, the activity of any fission-product nuclide can be evaluated from the equations in Chap. 2. When fissions occur at a constant rate and when neutron-absorption reactions in the fission product and its precursors can be neglected, the activity of a nuclide with relatively short-lived precursors can be evaluated by applying Eq.

(2.37) :

N = Fy(-e-KTR)e-^Tc (8.1)

where F = fission rate, flssions/s

N = atoms of long-lived fission product present after cooling for a time Tc TR = irradiation time, s Tc = cooling time, s

у = cumulative fission yield, atoms/atom fissioned X = decay constant for the nuclide, s’1

When the half-lives of a fission product and of its decay precursors are short compared to the irradiation time (Г1/2 < TR), the fission-product nuclide reaches saturation prior to the end of the irradiation. Its saturation activity per unit of reactor power is a constant, so that

?y=ye-KTc (when TV2<TR) (8.2)

The saturation activity is conveniently expressed in curies per watt of thermal power, or

Curies _ _XTc /disintegration^ / fission

Watt fission/s /200MeV/

	w ( MeV	( Сі
	» Vl.6X КГ13 W-s )	ЗЛ X 1010 disintegrations/s)
or	Curies_ 0 845 — лгс Watt	(when T1/2<TR)	(8.3)

Practical irradiation periods for fuel in power reactors are in the range of about 1 to 4 years. Most of the fission-product nuclides reach saturation in this period. An example is 8.05-day 1311, which is formed in 2.93 percent of 23SU fissions. Its saturation activity is 0.023 Ci/W.

Curies _ 0.586у7де *Tc Watt Ty г

Many radioactive fission-product nuclides have half-lives that are long compared to reactor irradiation periods, i. e., 3H, 8SKr, ’“Sr, 129I, and I37Cs. In these cases, Eq. (8.1) simplifies to

Because these long-lived nuclides do not reach saturation in the reactor fuel, their yearly production rate is important. This is obtained by dividing Eq. (8.5) by TR and setting Tc equal to zero:

Curies _ 0-586y

Watt X time “ Ty2 ( J

The short-lived daughter of a long-lived parent nuclide contributes significantly to the activity even after long cooling periods because it is constantly being formed from the parent (e. g., 90Y from 90Sr). If the half-life of the daughter is very small relative to that of its parent, the two are in secular equilibrium and the daughter activity is equal to that of the parent.

Only a few fission-product nuclides have half-lives too long for saturation but too short for the assumption of linear buildup that led to Eq. (8.4). Examples are 106 Ru, 144Ce, and 147Pm. A few radionuclides, such as ^Nb, 140La, and 147Pm, have precursors that must be considered in the calculation of activity after a few months of postirradiation cooling.

In Table 8.1 are listed those fission-product nuclides that contribute appreciably to the activity of fission products formed after long irradiation and cooled for periods of a few months or more. Fission-product activities have been calculated for uranium fuel irradiated for 3 years in the 1000-MWe pressurized-water reactor (PWR) operating as shown in Fig. 3.31. Activities are listed for fuel at the time it is discharged from the reactor and after

Table 8.1 Long-lived radioactive fission products’*’ In discharge fuel 106 Ci/yr Elemental boiling temperature, °c§ Radionuclide Half-life At discharge* 150-day decay 10-yr decay 3H 12.4 yr 1.93 X 10*2 1.88 X 10*2 1.09 X 10’2 100 (as tritiated water) 19 Se <6.5 X 104 yr 1.08 X 10_s 1.08 X 10’5 1.08 X 10‘s Total^ 10.0 1.08 X 10“s 1.08 X 10_s 657 MKr 10.76 yr 0.308 0.300 0.162 Total 85.0 0.300 0.162 -153.4 86 Rb 18.66 days 1.34 X 10’2 5.18 X 10’3 0 Total 1.34 X 10‘2 5.18 X 10’3 0 705 89 Sr 52.7 days 19.6 2.65 0 90 Sr 27.7 yr 2.11 2.09 1.65 Total 1.38 X 102 4.74 1.65 1357 90 Y 64.0 h 2.20 2.09 1.65 91Y 58.8 days 25.5 4.39 0 Total 2.08 X 102 6.48 1.65 3337 93 Zr 1.5 X 106 yr 5.15 X 10’s 5.15 X 10~5 5.15 X 10-5 93 Zr 65.5 days 37.3 7.54 0 Total 96.2 7.54 5.15 X 10"s 4325 93mNb 13.6 yr 3.95 X 10’6 4.98 X 10’6 2.3 X 10’s 9SmNb 90 h 0.762 0.160 0 95 Nb 35.0 days 37.6 14.2 0 Total 2.30 X 102 14.4 2.3 X 10~s 4842 99 Tc 2.12 X 10s yr 3.90 X 10’4 3.90 X 10‘4 3.90 X 10’4 Total 29.7 3.90 X 10~4 3.90 X 10‘4 3927 103 Ru 39.5 days 33.2 2.41 0 106 Ru 368 days 14.8 11.2 1.50 X 10‘2 Total 75.7 13.6 1.50 X 10’2 4227 103 mRh 57.5 min 33.2 2.41 0 106 Rh 30 s 20.2 11.2 1.50 X 10’2 Total 1.17 X 102 13.6 1.50 X 10’2 3667 toipd 7 X 106 yr 3.00 X 10‘6 3.00 X 10~6 3.00 X 10’6 Total 9.10 3.00 X 10’6 3.00 X 10_e 3112 110m Ag 255 days 0.100 6.64 X 10’2 4.52 X 10’6 110 Ag 24.4 s 4.33 8.65 X 10’3 5.88 X 10’7 111 Ag 7.5 days 1.08 1.03 X 10’6 0 Total 10.4 7.51 X 10’2 5.11 X 10‘6 2163 u3mCd 13.6 yr 2.86 X 10‘4 2.81 X 10~4 1.74 X 10’4 nSmCd 43 days 0.0150 1.34 X 10"3 0 Total 0.981 1.62 X 10’3 1.74 X 10’4 770 111mSn 14.0 days 1.62 X 10’3 9.65 X 10’7 0 1I9mSn 250 days 4.47 X 10’4 2.95 X 10’4 1.79 X 10‘8

Table 8.1 Long-lived radioactive fission products’*’ (Continued) Radionuclide Half-life In discharge fuel 10* Ci/yr Elemental boiling temperature, °C§ At discharge* 150-day decay 10-yr decay 123 Sn 125 days 0.242 1.05 3.87 X io-10 125 Sn 9.4 days 0.368 5.81 X 10’6 0 126 Sn 10s yr 1.49 X 10‘5 1.49 X 10’5 1.49 X 10’s Total 72.2 1.05 1.49 X 10’5 2722 124 Sb 60.4 days 1.11 X 10~2 1.95 X 10’3 0 125 Sb 2.71 yr 0.237 0.215 1.85 X 10‘2 126m Sb 19.0 min 6.13 X 10’4 1.49 X 10~s 1.49 X 10’s 126 Sb 12.5 days 1.55 X 10‘3 1.50 X 10‘s 1.47 X 10’5 Total 1.31 X 102 0.217 1.85 X 10’2 1625 mm те 117 days 1.66 X 10’s 6.82 X 10~6 0 125Шгре 58 days 8.47 X 10’2 8.69 X 10‘2 7.66 X 10‘3 127mTe 109 days 0.420 0.167 0 127 Те 9.4 h 1.96 0.62 0 129m Te 34.1 days 1.56 7.38 X 10"2 0 129 Те 68.7 min 9.18 3.87 X 10~2 0 Total 1.63 X 102 0.986 7.66 X 10~3 1012 129 j 1.7 X 107 yr 1.01 X 10‘6 1.02 X 10‘6 1.02 X 10~6 131 j 8.05 days 23.5 5.94 X 10’5 0 Total 2.66 X 102 6.04 X 10’5 1.02 X 10~6 183 131mXe 11.8 days 0.174 8.50 X 10~5 0 133 Xe 5.270 days 43.9 1.46 X 10‘7 0 Total 1.78 X 102 8.51 X 10‘5 0 -108.2 134 Cs 2.046 yr 6.70 5.83 0.228 135 Cs 3.0 X 106 yr 7.79 X 10’6 7.79 X 10‘6 7.79 X 10‘6 136 Cs 13.7 days 1.66 5.42 X 10-4 0 137 Cs 30.0 yr 2.94 2.92 2.33 Total 1.56 X 102 8.75 2.56 686 mmBa 2.554 min 2.75 2.72 2.18 140 Ba 12.80 days 39.5 1.18 X 10’2 0 Total 1.51 X 102 2.73 2.18 1634 140 La 40.22 h 40.9 1.34 X 10‘2 0 Total 1.49 X 102 1.34 X 10’2 0 3370 141 Ce 32.5 days 37.9 1.53 0 144 Ce 284 days 30.2 21.0 4.11 X 10’3 Total 1.48 X 102 22.5 4.11 X 10’3 3470 143 Pr 13.59 days 32.7 1.85 X 10’2 0 144 Pr 17.27 min 30.5 21.0 4.11 X 10"3 Total 1.23 X 102 21.0 4.11 X IO’3 3017 147 Nd 11.06 days 16.0 2.58 X 10’3 0 Total 24.9 2.58 X 10’3 0 3111 147 Pm 4.4 yr 2.78 2.65 0.211 mm Pm 41.8 days 1.06 8.91 X 10’2 0

Table 8.1 Long-lived radioactive fission products* (Continued) In discharge fuel 10* Ci/yr Elemental boiling temperature, °C§ Radionuclide Half-life At discharge* 150-day decay 10-yr decay 148 Pm Total 5.4 days 5.42 31.6 7.08 X 10*3 2.74 0 0.211 3200 151 Sm Total «87 yr 3.41 X 1СГ2 11.5 3.41 X 10’2 3.41 X 10-2 3.16 X 10-2 3.16 X 10’2 1670 152 Eu 134 Eu 155 Eu 156Eu Total 12.7 yr 16 yr 1.811 yr 15.4 days 3.41 X 10’4 0.191 0.204 6.16 6.56 3.32 X 10~4 0.187 0.174 5.94 X 10’3 0.367 1.92 X 10~4 0.123 4.44 X 10*3 0 0.127 1430 160 ^ Total 72.1 days 3.49 X 10’2 4.01 X 10~2 8.24 X 10’3 0 2470 Total, all fission products 3.76 X 103 1.14 X 102 8.66 *Uranium-fueled 1000-MWe PWR, З-year fuel life. *Total elemental activities for fuel at discharge include short-lived radionuclides not listed here. §G. V. Samsonov [SI], ^ Total activity of the element whose principal radionuclide(s) is (are) listed above.

postirradiation cooling periods of 150 days and 10 years. The variation of beta activity of the long-lived fission products with cooling time is shown in Fig. 8.1.

Gaseous fission products are important when possible releases of radioactive species to the air are to be considered. At the reactor site such releases can result when gaseous fission products diffuse from the fuel material and escape through defects in the fuel cladding. These radioactive nuclides are still confined within the coolant circuit of the reactor. However, coolant leaks and the need for occasional venting of insoluble and noncondensable gases from a liquid coolant system result in some handling of radioactive fission gases at the reactor site. Gaseous radioiodine is removed by adsorption in activated carbon. Radioactive noble gases are held for radioactive decay for periods of time varying from over a week to a month, after which the 8SKr and possibly some remaining 133Xe are discharged to the atmosphere. Atmospheric dilution brings the concentration of these radionuclides to levels well below tolerance. Alternatively, these vented gases may be treated by various means, such as absorption, adsorption, condensation, and/or compression into storage cylinders, for removal and long-term storage.

Most of the long-lived radioactive fission gases are still present in the fuel when it is processed to recover the uranium and plutonium. In many separation processes the first step involves mechanical chopping of the fuel rods, followed by acid dissolution of the fuel material. Gaseous and volatile fission products liberated in these steps must be disposed of safely. Of the noble fission gases, 85 Kr is the only radionuclide that is present in significant quantities after reprocessing cooling periods of a few months. At many reprocessing plants 85 Kr is discharged directly to the atmosphere through a tall release stack provided to ensure sufficient mixing with the air. Alternatively, krypton can be recovered from the off-gases by condensation, adsorption, or absorption, as discussed in Chaps. 10 and 11.

In addition to the fission-product tritium listed in Table 8.1, additional tritium is produced in the reactor by neutron reactions with boron control absorbers, with lithium contaminants, and with deuterium in the water coolant-moderator. A portion of the tritium that is produced in the coolant of light-water reactors (LWRs) is released to the environment as diluted tritiated water at the reactor site. Solid boron control absorbers containing tritium are ultimately stored as solid radioactive wastes. During fuel reprocessing a portion of the fission-product tritium is evolved as gaseous hydrogen and the remainder appears as tritiated water (НТО) or as zirconium tritide in the chopped fuel cladding. If not collected prior to fuel dissolution, the tritiated water follows the water carrier in fuel reprocessing and at present is released to the environment as tritiated water vapor or liquid.

Although most of the fission-product radioiodine will have decayed away during the preprocessing cooling period, the extremely low tolerance concentration of radioiodine requires that 1311 and 129I be removed from reprocessing effluents. Also, radioactive iodine remaining in the dissolved-fuel solution extracts readily and reacts with the organic extracting solvents. Only about 1 Сі/year of 129I is formed in a 1000-MWe reactor, but its long half-life and relatively high biological toxicity make 1291 an important long-term environmental hazard. Special processes for recovering and sequestering radioactive iodine from the off-gas in fuel reprocessing are discussed in Chaps. 10 and 11.

I31Cs and 90Sr, elements of groups I and II of the periodic table, are important in determining the radioactivity of fission products after long decay periods. They are both easy

Figure 8.1 Radioactivity of fission pro­ducts and actinides in high-level wastes produced in 1 year of operation of a uranium-fueled 1000-MWe PWR.

to remove from uranium in aqueous processing because of their very low solubility in organic solvents.

Yttrium and the lanthanides, which are grouped together under group IIIB, likewise are easily separable from uranium in aqueous processing, with the possible exception of cerium. The troublesome activity from cerium contamination is due to the beta and gamma decay of 144Pr, the short-lived daughter of 144Ce. 140La emits penetrating gamma radiation and is one of the most important rare-earth fission products to be considered if the decay period is of the order of 30 days or less. 147Nd is relatively short-lived, and its long-lived daughter 147Pm emits no gammas; both are easily removed in aqueous processing.

Zirconium and niobium, of groups IVB and VB, are both amphoteric1" in character, and their complex hydrolytic behavior makes zirconium and niobium two of the most difficult fission products to separate by aqueous processing. Group VI fission products have either very short or very long half-lives, and the most troublesome fission product in this group, "Mo, will be present in appreciable activity only for very short cooling periods. Its group VII decay daughter, 2.12 X 10s year "Tc, contributes to the long-term radioactivity of stored fission-product wastes. "Tc may be important to the long-term transport of fission products stored in geologic media.

106Ru, of group VIII, is one of the most important fission-product contaminants in fuel reprocessing because of its multiple valence states and complex chemistry in aqueous solutions. In the presence of strong oxidizing agents ruthenium may appear in gaseous form as Ru04.