Radioactive Production and Decay of SRE Reactor Gases Radionuclide Decay The rate of decay of a Ingrowth of daughter (D) and Exposure to Gases and radionuclide is a function of its radionuclides from parent (P)

Gas Decay Products half-life (T½). when T½P < T½D and AD0 = 0

Radionuclide exposure is a Activity Relationship to Half-Life function of both released function of both released 1

0.9 amounts and radioactive decay ttt1/2 1/2 1/2 amounts and radioactive decay NNNN00⎯⎯→/2 ⎯⎯→ 0 /4 ⎯⎯→ 0 /8 0.8

until the time of exposure. 0.7

0.6

0.5 Some radionuclides decay to Exponential decay results in 0.4 >99% decay after 7 half-lives and Fraction of Original Activity >99.9% decay after 10 half-lives other radionuclides but 0.3 eventually the decay chain ends 0.2 0.1

in a stable (non-radioactive) 0 012345678910 isotope. Number of Half-Lives Gases estimated to have been released after the SRE accident were primarily the isotopes of non-reactive noble gases Xenon, Krypton, and Argon identified below.

Potential Activation Product Gas Major Fission Product Gases Major Long Lived Fission Product Gas

Barium

Cesium Potassium

Xenon Argon

Iodine Krypton Potential activation gas from argon in helium purge gas

Iodine isotopes with significant yields and half-lives and their xenon gas daughters Krypton gas isotope with significant yield and half-life

Source for Table Data : National Nuclear Data Center, Brookhaven National Laboratory, http://www.nndc.bnl.gov/nudat2/ Radioactive Decay of SRE Reactor Inventory and Estimated Releases

I-131/Xe-131m Ingrowth and Decay I-133/Xe-133 Ingrowth and Decay (Activity) I-135/Xe-135 Ingrowth and Decay (Activity) Activity Units

1.00 1.00

1.00 0.90 0.90

0.90 0.80 0.80 y 0.80 0.70 0.70 0.70 0.60 0.60 I-131: T = 8.04 days 0.60 1/2 I-135: T1/2 = 6.57 hours 0.50 0.50 0.50 I-133: T1/2 = 20.8 hours Xe-131m: T1/2 = 11.9 days 0.40 0.40 0.40 Xe-133: T1/2 = 5.24 days Xe-135: T1/2 = 9.1 hours

Fraction ofOriginal Activit 0.30 0.30 0.30 Activity Original of Fraction

Combined Activity Fraction of OriginalActivity 0.20 0.20 0.20 Combined Combined 0.10 0.10 0.10

0.00 0.00 0.00 0 10203040506070 0 5 10 15 20 25 30 35 0 10203040506070 Days Days Hours

Kr-85 Decay When the exponential decay is plotted on a log scale the plot is linear. T1/2 = 10.7 years

1.00

0.90 0.80 The plots below show the amounts of noble gases estimated to have been 0.70 0.60 released after the SRE accident, the total reactor inventory of other fission 0.50 0.40 products at the time of the accident, and their decay over time. 0.30

0.20

Fraction of Original Activity (linear scale) 0.10 Acitivities of Other Significant Fission Products Sr-89 Acitivities of Gases After Shut Down of the SRE Reactor 0.00 After Shut Down of the SRE Reactor 0 102030405060708090100 (Estimated Inventory Released to the Environment) 1000000.0 (Total Reactor Core Inventory ) Ba-140 Years Ce-141

100.00 100000.0 Ru-103 Kr-85 Decay Xe-133 T1/2 = 10.7 years 10000.0 I-131 1.000

) Kr-85 I-133 10.00 1000.0 I-135 0.100

Activity (Curies) Activity 100.0 Activity (Curies Xe-135 Cs-137

1.00 Ce-144 0 102030405060708090100 10.0 0.010 Days Xe-131m Zr-95 Log scale provides a straight line for the exponential decay Fraction of Original Activity (log scale) (log Activity Original of Fraction 1.0 Sr-90 0 5 10 15 20 25 30 35 40 45 50 0.001 0.10 0 102030405060708090100 Years Cs-134 Years Radioactive Decay and Equilibrium of SRE Reactor Fuels

Radioactive Decay Decay mode depends on Half-lives are longer as decay direction to belt of stability moves closer to belt of stability

„ Due to nuclear instability Radionuclide Decay Mode Patterns Radionuclide Decay Half-Life Patterns

„ Mode depends on direction to Decay mode depends on belt of stability. Possible direction to belt of stability

Seconds Modes: > 10+15 10-01

10+10 „ Alpha decay (α) 10-02 10+07 Number of Protons 10-03 Number of Protons

10+05 10-04

„ Beta decay (β-) 10+04 10-05

10+03 10-06

10+02 „ Positron Decay (β+) 10-07

10+01 10-15

10+00 < 10- „ Electron capture (EC) 15 „ Spontaneous Fission Number of Neutrons Number of Neutrons

Decay Chains Associated with SRE Reactor Fuel Elements (Thorium-232 only used in SRE Core II)

Uranium-238 234U 238U 228Th 232Th 4E5 y 4.5E9 y Thorium-232 Decay Chain 1.91 y 1.4E10 y 234m Decay Chain Pa 228Ac 1.2 m 6.13 h

Beta 230 234 Th Th Beta 224 228 7.7E4 y 24 d Ra Ra Alpha 3.66 d 5.8 y Alpha

226Ra 1600 y 220Rn 55 s

222Rn 3.82 d 212Po 216Po 3E-7 s 0.15 s

2/3 212Bi 210Po 214Po 218Po 60.6 m 138 d 1.6E-4 s 3.05 m 208Pb 212Pb 210 214 1/3 Bi Bi Stable 10.6 h 5.01 d 19.9 m

208 208 210 214 Tl Pb Pb Pb Stable 22.3 y 26.8 m 3.1 m Radioactive Decay and Equilibrium of SRE Reactor Fuels

Ingrowth of early decay chain Ingrowth of daughter (D) Ingrowth of early decay chain short half-life daughters from radionuclides from parent (P) short half-life daughters from U-238 when T½P >> T½D and AD0 = 0 U-235

Th-234 and Pa-234m Ingrowth from U-238 Th-231 Ingrowth from U-235

1.2 1.1 U-235: T1/2 = 7.04E8 years U-238: T1/2 = 4.5E9 years 1.0 1.0 0.9

0.8 0.8 0.7

U-238 0.6 U-235 0.6 Th-231: T1/2 = 1.06 days Th-234 and Pa-234m Th-231 0.5 Only this one Th-234: T = 24 days 1/2 0.4 short half-life 0.4 Pa-234m: T1/2 = 1.2 min

Fraction of U-238Original Activity daughter grows Only these two short Fraction of U-235 Original Activity 0.3 significantly half-life daughters 0.2 toward 0.2 grow significantly equilibrium in 50 toward equilibrium in 0.1 years. 50 years. 0.0 0.0 0 20 40 60 80 100 120 140 012345678910 Days Days

The time it takes for the Ingrowth of long half-life daughter to reach equilibrium Ingrowth of decay chain short daughters from U-234 with the parent depends on the half-life daughters from Th-232

Th-230 through Po-210 Ingrowth from U-234 half-life of the daughter. Ra-228 and Th-228 through Tl-208 Ingrowth from Th-232

1.0000000 1.2 U-234: T = 4E5 years Th-232: T1/2 = 1.4E10 years 1/2 Because of the long half-life of 0.1000000 intermediate members in the 1.0 0.0100000 intermediate members in the

0.8

0.0010000 U-238 and U-235 decay chains, U-234 Ra-228: T1/2 = 5.8 years Th-232 Th-230 > Po-210 0.6 Th-230: T1/2 = 7.7E4 years 0.0001000 Th-228: T1/2 = 1.9 years equilibrium is only established Ra-224 > Tl-208 < 3.66 days Ra-228 None of the U-234 0.4 All of the Th-232 Th-228 > Tl-208 Fraction of U-234 Original Activity 0.0000100 decay chain decay chain for early short half-life Activity Original of Th-232 Fraction daughters grow daughters grow significantly 0.0000010 0.2 significantly toward equilibrium daughters. All members of the toward equilibrium in 50 years. in 50 years. 0.0000001 0.0 0 20 40 60 80 100 120 140 Th-232 decay chain approach 0 102030405060708090100 Years early equilibrium. Years