Radionuclide Decay Radionuclide Decay and Exposure to Gases and and Exposure to Gases and

Gas Decay ProductsGas Decay Products

Radionuclide exposure is a Radionuclide exposure is a function of both released function of both released

amounts and radioactive decay amounts and radioactive decay until the time of exposure. until the time of exposure.

Major Fission Product Gases

Xenon

Iodine

Iodine isotopes with significant yields and half-lives and their xenon gas daughters

Cesium

Barium

Major Long Lived Fission Product Gas

Krypton

Krypton gas isotope with significant yield and half-life

Activity Relationship to Half-Life

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

0 1 2 3 4 5 6 7 8 9 10

Number of Half-Lives

Frac

tion

of O

rigin

al A

ctiv

ity

1/ 2 1/ 2 1/ 20 0 0 0/ 2 / 4 /8t t tN N N N⎯⎯→ ⎯⎯→ ⎯⎯→

Exponential decay results in >99% decay after 7 half-lives and >99.9% decay after 10 half-lives

Ingrowth of daughter (D) Ingrowth of daughter (D) radionuclides from parent (P) radionuclides from parent (P)

when Twhen T½½PP < T< T½½DD and Aand AD0D0 = 0= 0

Gases estimated to have been released after the SRE accident werGases estimated to have been released after the SRE accident were primarily the e primarily the isotopes of nonisotopes of non--reactive noble gases Xenon, Krypton, and Argon identified below.reactive noble gases Xenon, Krypton, and Argon identified below.

The rate of decay of a The rate of decay of a radionuclide is a function of its radionuclide is a function of its

halfhalf--life (Tlife (T½½).).

Potential Activation Product Gas

Potential activation gas from argon in helium purge gas

Source for Table Data : National Nuclear Data Center, Brookhaven National Laboratory, http://www.nndc.bnl.gov/nudat2/

Argon

Potassium

Some radionuclides decay to Some radionuclides decay to other radionuclides but other radionuclides but

eventually the decay chain ends eventually the decay chain ends in a stable (nonin a stable (non--radioactive) radioactive)

isotope. isotope.

Radioactive Production and Decay of SRE Reactor GasesRadioactive Production and Decay of SRE Reactor Gases

Radioactive Decay of SRE Reactor Inventory and Estimated ReleaseRadioactive Decay of SRE Reactor Inventory and Estimated Releasess

I-131/Xe-131m Ingrowth and DecayActivity Units

0.00

0.10

0.20

0.30

0.40

0.50

0.60

0.70

0.80

0.90

1.00

0 10 20 30 40 50 60 70

Days

Frac

tion

of O

rigin

al A

ctiv

ity

I-131: T1/2 = 8.04 days

Xe-131m: T1/2 = 11.9 days

Combined Activity

Kr-85 DecayT1/2 = 10.7 years

0.00

0.10

0.20

0.30

0.40

0.50

0.60

0.70

0.80

0.90

1.00

0 10 20 30 40 50 60 70 80 90 100

Years

Frac

tion

of O

rigin

al A

ctiv

ity (l

inea

r sca

le)

Acitivities of Other Significant Fission Products After Shut Down of the SRE Reactor

(Total Reactor Core Inventory )

1.0

10.0

100.0

1000.0

10000.0

100000.0

1000000.0

0 5 10 15 20 25 30 35 40 45 50

Years

Act

ivity

(Cur

ies)

Sr-89

Ba-140

Ce-141

Ru-103

I-131

I-133

I-135

Cs-137

Ce-144

Zr-95

Sr-90

Cs-134

Acitivities of Gases After Shut Down of the SRE Reactor(Estimated Inventory Released to the Environment)

0.10

1.00

10.00

100.00

0 10 20 30 40 50 60 70 80 90 100Days

Act

ivity

(Cur

ies)

Xe-133

Kr-85

Xe-135

Xe-131m

Kr-85 DecayT1/2 = 10.7 years

0.001

0.010

0.100

1.000

0 10 20 30 40 50 60 70 80 90 100

Years

Frac

tion

of O

rigin

al A

ctiv

ity (l

og s

cale

)

Log scale provides a straight line for the exponential decay

WhenWhen the exponential decay is plotted on a log scale the plot is linethe exponential decay is plotted on a log scale the plot is linear. ar.

The plots below show the amounts of noble gases estimated to havThe plots below show the amounts of noble gases estimated to have been e been released after the SRE accident, the total reactor inventory of released after the SRE accident, the total reactor inventory of other fission other fission

products at the time of the accident, and their decay over time.products at the time of the accident, and their decay over time.

I-133/Xe-133 Ingrowth and Decay (Activity)

0.00

0.10

0.20

0.30

0.40

0.50

0.60

0.70

0.80

0.90

1.00

0 5 10 15 20 25 30 35

Days

Frac

tion

of O

rigin

al A

ctiv

ity

I-133: T1/2 = 20.8 hours

Xe-133: T1/2 = 5.24 days

Combined

I-135/Xe-135 Ingrowth and Decay (Activity)

0.00

0.10

0.20

0.30

0.40

0.50

0.60

0.70

0.80

0.90

1.00

0 10 20 30 40 50 60 70

Hours

Frac

tion

of O

rigin

al A

ctiv

ity

I-135: T1/2 = 6.57 hours

Xe-135: T1/2 = 9.1 hours

Combined

Radioactive DecayRadioactive Decay

Due to nuclear instabilityDue to nuclear instabilityMode depends on direction to Mode depends on direction to belt of stability. Possible belt of stability. Possible Modes:Modes:

Alpha decay (Alpha decay (αα))Beta decay (Beta decay (ββ--))Positron Decay (Positron Decay (ββ+)+)Electron capture (EC)Electron capture (EC)Spontaneous FissionSpontaneous Fission

HalfHalf--lives are longer as decay lives are longer as decay moves closer to belt of stabilitymoves closer to belt of stability

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

Decay mode depends on Decay mode depends on direction to belt of stabilitydirection to belt of stability

Radioactive Decay and Equilibrium of SRE Reactor FuelsRadioactive Decay and Equilibrium of SRE Reactor Fuels

< 10-15

10+00

10-15 10+01

10-07 10+02

10-06 10+03

10-05 10+04

10-04 10+05

10-03 10+07

10-02 10+10

10-01 > 10+15

Seconds

Number of Neutrons

Num

ber o

f Pro

tons

Radionuclide Decay Half-Life Patterns

238U4.5E9 y

234Th24 d

234mPa1.2 m

234U4E5 y

230Th7.7E4 y

226Ra1600 y

222Rn3.82 d

218Po3.05 m

214Pb26.8 m

214Bi19.9 m

214Po1.6E-4 s

210Pb22.3 y

210Bi5.01 d

210Po138 d

208PbStable

Beta

Alpha

Uranium-238 Decay Chain

232Th1.4E10 y

228Ra5.8 y

228Ac6.13 h

228Th1.91 y

224Ra3.66 d

220Rn55 s

216Po0.15 s

212Pb10.6 h

212Bi60.6 m

212Po3E-7 s

208Tl3.1 m

208PbStable

Beta

Alpha

Thorium-232 Decay Chain

1/3

2/3

Number of NeutronsN

umbe

r of P

roto

ns

Radionuclide Decay Mode Patterns

Decay mode depends on Decay mode depends on direction to belt of stabilitydirection to belt of stability

Radioactive Decay and Equilibrium of SRE Reactor FuelsRadioactive Decay and Equilibrium of SRE Reactor Fuels

The time it takes for the The time it takes for the daughter to reach equilibrium daughter to reach equilibrium

with the parent depends on the with the parent depends on the halfhalf--life of the daughter. life of the daughter.

Because of the long halfBecause of the long half--life of life of intermediate members in the intermediate members in the

UU--238 and U238 and U--235 decay chains, 235 decay chains, equilibrium is only established equilibrium is only established

for early short halffor early short half--life life daughters. All members of the daughters. All members of the ThTh--232 decay chain approach 232 decay chain approach

early equilibrium.early equilibrium.

Ingrowth of daughter (D) Ingrowth of daughter (D) radionuclides from parent (P) radionuclides from parent (P) when Twhen T½½PP >> T>> T½½DD and Aand AD0D0 = 0= 0

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

0.0

0.2

0.4

0.6

0.8

1.0

1.2

0 20 40 60 80 100 120 140

Days

Frac

tion

of U

-238

Orig

inal

Act

ivity

U-238Th-234 and Pa-234m

U-238: T1/2 = 4.5E9 years

Th-234: T1/2 = 24 daysPa-234m: T1/2 = 1.2 min

Only these two short half-life daughters grow significantly toward equilibrium in 50 years.

Ingrowth of early decay chain Ingrowth of early decay chain short halfshort half--life daughters from life daughters from

UU--238238

Ingrowth of early decay chain Ingrowth of early decay chain short halfshort half--life daughters from life daughters from

UU--235235

Ingrowth of long halfIngrowth of long half--life life daughters from Udaughters from U--234234

Ingrowth of decay chain short Ingrowth of decay chain short halfhalf--life daughters from Thlife daughters from Th--232232

Th-231 Ingrowth from U-235

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

1.1

0 1 2 3 4 5 6 7 8 9 10

Days

Frac

tion

of U

-235

Orig

inal

Act

ivity

U-235Th-231

U-235: T1/2 = 7.04E8 years

Th-231: T1/2 = 1.06 days

Only this one short half-life daughter grows significantly toward equilibrium in 50 years.

Th-230 through Po-210 Ingrowth from U-234

0.0000001

0.0000010

0.0000100

0.0001000

0.0010000

0.0100000

0.1000000

1.0000000

0 20 40 60 80 100 120 140

Years

Frac

tion

of U

-234

Orig

inal

Act

ivity

U-234Th-230 > Po-210

U-234: T1/2 = 4E5 years

Th-230: T1/2 = 7.7E4 years

None of the U-234 decay chain daughters grow significantly toward equilibrium in 50 years.

Ra-228 and Th-228 through Tl-208 Ingrowth from Th-232

0.0

0.2

0.4

0.6

0.8

1.0

1.2

0 10 20 30 40 50 60 70 80 90 100

Years

Frac

tion

of T

h-23

2 O

rigin

al A

ctiv

ity

Th-232

Ra-228

Th-228 > Tl-208

Th-232: T1/2 = 1.4E10 years

Ra-228: T1/2 = 5.8 yearsTh-228: T1/2 = 1.9 yearsRa-224 > Tl-208 < 3.66 days

All of the Th-232 decay chain daughters grow significantly toward equilibrium in 50 years.