Detailed Radiation Dose Rate Evaluations of Commercial

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Detailed Radiation Dose Rate Evaluations of Commercial Spent Nuclear Fuel Canisters

Georgeta Radulescu and Kaushik Banerjee

Reactor and Nuclear Systems Division, ORNL

SCALE Users’ Group Workshop

Knoxville, TN, August 19 - 21, 2019

2 SCALE Users’ Group Workshop, Knoxville, TN, August 19 - 21, 2019

This is a technical presentation that does not take into account the contractual limitations under the Standard Contract for Disposal of Spent Nuclear Fuel and/or High-Level Radioactive Waste (Standard Contract) (10 CFR Part 961). For example, under the provisions of the Standard Contract, DOE does not consider spent nuclear fuel in multi-assembly canisters to be an acceptable waste form, absent a mutually agreed to contract amendment. To the extent discussions or recommendations in this presentation conflict with the provisions of the Standard Contract, the Standard Contract provisions prevail.


Outline

• Introduction

• Calculation method

• Geometry model used in calculations

• Radiation source terms

• Dose rate results for selected locations on canister inner and outer surface

• Time-integrated dose

• Conclusions


Introduction• New passive monitoring techniques are of interest to support extended spent nuclear fuel (SNF)

storage– Material degradation (e.g., temperature, humidity, chlorine concentration, and microbes) and radioactive material release

• Electronic devices have limited radiation resistance, but they may be subjected to very high radiation levels depending on placement within the storage cask

– Normal commercial-grade chips can withstand a total dose of 50 to 100 Gy– Radiation hardened semiconductors may withstand up to 105 Gy

• Characterization of the radiation levels associated with SNF dry storage canisters is essential for designing and selecting systems that can withstand high radiation levels

• The purpose of this paper is to determine:– Dose rate in silicon as a function of fuel decay time range (from 5 years to 70 years)

– Time-integrated dose in silicon

• Dose rate in silicon is evaluated for:– Bounding SNF assembly radiation sources– A representative pressurized water reactor (PWR) SNF canister – A representative boiling water reactor (BWR) SNF canister


Dose Rate Calculation Method• All calculations performed with SCALE 6.2.3

– Radiation source terms calculated with ORIGAMI

– Dose rate calculated with MAVRIC, a Monte Carlo radiation transport code for shielding calculations

• Calculation of dose (i.e., energy absorbed per mass unit) in silicon requires photon and neutron dose response functions for silicon

• The photon response in silicon was calculated by integrating the product of the energy-dependent photon flux in dry air and the energy-dependent photon KERMA (kinetic energy released in materials) factors for silicon across photon’s energy range

• The neutron response in silicon was calculated by integrating the product of the energy-dependent neutron flux in dry air and the energy-dependent neutron KERMA factors for silicon across neutron’s energy range

1.0E-17

1.0E-16

1.0E-15

1.0E-14

1.0E-13

1.0E-12

1.0E-03 1.0E-02 1.0E-01 1.0E+00 1.0E+01 1.0E+02

Gy p

er p

hotn

/m2

Photon energy (MeV)

1.0E-21

1.0E-20

1.0E-19

1.0E-18

1.0E-17

1.0E-16

1.0E-15

1.0E-14

1.E-09 1.E-08 1.E-07 1.E-06 1.E-05 1.E-04 1.E-03 1.E-02 1.E-01 1.E+001.E+011.E+02

Gy p

er n

eutro

n/m

2

Neutron energy (MeV)

Photon KERMA factor for silicon as a function of photon energy1

Neutron KERMA factor for silicon as a function of neutron energy1

1M. S. SINGH, “KERMA Factors for Neutron and Photons with Energies below 20 MeV,” UCRL-52850, Lawrence Livermore Laboratory (1979).


Canister Model and Selected Dose Rate Locations• Geometry model included a representative

canister and its designated storage cask

• The canister contained identical fuel assemblies– Same design, initial enrichment, and burnup

• A fuel assembly was subdivided into four axial regions

– Active fuel region: pin-by-pin representation– Assembly hardware regions (lower end fitting, gas plenum,

and upper end fitting): homogeneous material mixtures and 60Co activation sources

– Assumed concentrations of cobalt impurity in steel and Inconel were 0.8 and 4.7 g/kg,a respectively

• Dose rate was calculated for selected locations on canister inner and outer surfaces

• Azimuthal dose rate variation was determined by subdividing the tally volumes into 36 angular segments (tally mesh in the MAVRIC input) Vertical cross sectional view of a

representative PWR SNF canister in its storage configuration showing dose rate locations

Horizontal cross sectional view of a representative PWR SNF canister in its storage configuration

aBounding Co impurity concentrations found in old assemblies. The Co impurity limit in hardware materials of modern assemblies is 500 ppm.

Geometry cell defined for dose rate calculations


Radiation Source Terms

Gamma source strength axial profile

Neutron source strength axial profileNeutron source strength axial profile


PWR SNF assembly18 axial-zone burnup

BWR SNF assembly10 axial-zone burnup

SNF assembly photon and neutron source strength variation as a function of fuel type, assembly region, and decay time

1.E+07

1.E+08

1.E+09

1.E+10

1.E+11

1.E+12

1.E+13

1.E+14

1.E+15

1.E+16

0 10 20 30 40 50 60 70 80Sour

ce st

reng

th (p

artic

les/

s)

Decay time (years)PWR; active fuel; photon BWR; active fuel; photon BWR; LEF; photonPWR; UEF; photon PWR; LEF; photon BWR; UEF; photonPWR; GP; photon BWR; GP; photon PWR; active fuel; neutronBWR; active fuel; neutron

1.0E+136.0E+131.1E+141.6E+142.1E+142.6E+143.1E+143.6E+144.1E+144.6E+145.1E+145.6E+14

0 50 100 150 200 250 300 350 400

Sour

ce st

reng

th (p

hoto

ns/s

)

Active fuel length (cm)5 years 10 years 20 years 30 years 50 years 70 years

1.0E+055.1E+061.0E+071.5E+072.0E+072.5E+073.0E+073.5E+074.0E+074.5E+075.0E+07

0 50 100 150 200 250 300 350 400

Sour

ce st

reng

th (n

eutro

ns/s

)


1.0E+136.0E+131.1E+141.6E+142.1E+142.6E+143.1E+143.6E+144.1E+144.6E+145.1E+145.6E+14

0 50 100 150 200 250 300 350 400

Sour

ce st

reng

th (p

hoto

ns/s

)


1.0E+045.0E+061.0E+071.5E+072.0E+072.5E+073.0E+073.5E+074.0E+074.5E+075.0E+075.5E+076.0E+076.5E+077.0E+077.5E+078.0E+07

0 50 100 150 200 250 300 350 400

Sour

ce st

reng

th (n

eutro

ns/s

)


Assembly type

Initial enrichment(wt% 235U)

Burnup (GWd/MTU)

PWR 4.8 55

BWR 4.2 50

Bounding sources for 95% of the SNF inventory1

discharged from 1968 to 2013

1J. HU, I. C. GAULD, J. L. PETERSON and S. M. BOWMAN, US Commercial Spent Nuclear Fuel Assembly Characteristics: 1968-2013, NUREG/CR-7227, ORNL/TM-2015/619, US Nuclear Regulatory Commission (2016) .


Examples of Dose Rate Variations on Canister Surface

30

35

40

45

50

55

60

65

70

0 1 2 3 4 5 6 7

Dose

rate

(Gy/

h)

! (radians)

Axial dose rate variation

0.1

1.0

10.0

100.0

0 100 200 300 400 500

Dose

rate

(Gy/

h)

Height (cm)

Azimuthal dose rate variation

Canister top surface

Dose rate variation as a function of radius

Radial surface

1.0E+136.0E+131.1E+141.6E+142.1E+142.6E+143.1E+143.6E+144.1E+144.6E+145.1E+145.6E+14

0 50 100 150 200 250 300 350 400

Sour

ce st

reng

th (p

hoto

ns/s

)



𝜋 0

3𝜋/2

𝜋/2

2𝜋

36 equal azimuthal segments


Dose Rate in Silicon as a Function of Location and Decay Time

Photon dose rate as a function of decay time for selected locations

PWR SNF Canister

Neutron dose rate as a function of decay time for selected locations

Photon dose rate as a function of decay time for selected locations

Neutron dose rate as a function of decay time for selected locations

Geometry model showing dose rate locations

BWR SNF Canister

1.E-03

1.E-02

1.E-01

1.E+00

1.E+01

1.E+02

1.E+03

0 10 20 30 40 50 60 70 80

Dose

rate

(Gy/

h)

Decay time (days)

D B I A E J C H G F

1.E-05

1.E-04

1.E-03

1.E-02

0 10 20 30 40 50 60 70 80

Dose

rate

(Gy/

h)

Decay time (days)

I D E J B G A C H F

1.E-06

1.E-05

1.E-04

1.E-03

1.E-02

0 10 20 30 40 50 60 70 80

Dose

rate

(Gy/

h)

Decay time (days)

I D J B E G A H C F

1.E-03

1.E-02

1.E-01

1.E+00

1.E+01

1.E+02

1.E+03

0 10 20 30 40 50 60 70 80

Dose

rate

(Gy/

h)

Decay time (days)

D I B E A J C H G F


Total Dose Rate in Silicon as a Function of Canister Location and Decay Time

Decay time (years)

5 10 20 30 50 70

Location Dose rate (Gy/h)

A 41.80 21.53 6.75 2.38 0.92 0.55

B 85.62 49.81 23.58 14.93 7.56 5.28

C 22.56 12.05 5.22 3.14 1.60 0.95

D 480.51 225.59 134.23 100.25 61.00 37.79

E 63.83 33.37 18.02 13.18 7.39 4.69

F 0.19 0.10 0.05 0.03 0.01 0.01

G 4.86 2.77 1.15 0.66 0.31 0.18

H 9.52 5.02 1.88 1.00 0.44 0.26

I 233.22 109.13 65.14 47.18 28.79 17.89

J 26.22 13.93 7.21 4.82 2.79 1.72

Decay time (years)

5 10 20 30 50 70

Location Dose rate (Gy/h)

A 139.59 71.94 19.71 5.26 0.59 0.24

B 312.32 163.86 51.63 20.73 7.19 4.31

C 52.39 27.29 7.90 2.22 0.33 0.13

D 366.01 163.83 93.70 69.58 42.90 26.33

E 107.50 59.39 21.25 10.40 4.69 2.84

F 0.23 0.12 0.04 0.02 0.01 0.004

G 12.42 6.32 1.88 0.79 0.23 0.13

H 26.66 13.76 3.84 1.15 0.23 0.11

I 177.45 81.38 46.74 34.22 20.06 12.54

J 54.73 29.62 12.33 6.96 3.65 2.23

PWR SNF Canister BWR SNF Canister


11

Comparison to As-Loaded Canister Dose Rates

• For approximately same decay times, dose rates based on bounding radiation sources are

– higher (~8 to 80%) than those of a as-loaded canister containing high-burnup fuel assemblies

– much higher (3 to 4 times) than those of a as-loaded canister containing low-burnup assemblies

Representative canister

As-loaded canister #1

As-loaded canister #2

SNF assembly burnup (GWd/MTU) 55 35 – 52 14 – 39

Average SNF assembly burnup (GWd/MTU) 55 46 22

SNF assembly discharge years 2000 2000 – 2003 1983 – 2000

Dose rate range at location D (Gy/h) 115.6 – 134.2 84.6 –124.4 26.0 – 30.3

Dose rate range at location I (Gy/h) 49.0 – 67.0 28.9 – 60.6 8.4 – 14.8


Time-Integrated Dose in Silicon as a Function of Canister Location

Time intervala(years)

5 to 10 10 to 20 20 to 30 30 to 50 50 to 70

Location Time-integrated dose (Gy)

A 1.3E+06 1.1E+06 3.7E+05 2.7E+05 1.3E+05

B 2.9E+06 3.1E+06 1.6E+06 1.9E+06 1.1E+06

C 7.4E+05 7.1E+05 3.6E+05 4.0E+05 2.1E+05

D 1.5E+07 1.5E+07 1.0E+07 1.4E+07 8.5E+06

E 2.1E+06 2.0E+06 1.4E+06 2.0E+06 1.0E+06

F 6.3E+03 6.4E+03 3.2E+03 3.3E+03 1.9E+03

G 1.6E+05 1.6E+05 7.8E+04 8.1E+04 1.0E+06

H 3.1E+05 2.8E+05 1.2E+05 1.2E+05 5.9E+04

I 7.2E+06 7.4E+06 4.9E+06 6.4E+06 4.0E+06

J 8.5E+05 8.9E+05 5.2E+05 6.6E+05 3.9E+05

PWR SNF Canister BWR SNF Canister


Time intervala(years)

5 to 10 10 to 20 20 to 30 30 to 50 50 to 70

Location Time-integrated dose (Gy)

A 4.5E+06 3.6E+06 9.6E+05 3.8E+05 6.8E+04

B 1.0E+07 8.6E+06 3.0E+06 2.2E+06 9.6E+05

C 1.7E+06 1.4E+06 3.9E+05 1.8E+05 3.8E+04

D 1.1E+07 1.1E+07 7.1E+06 9.7E+06 6.1E+06

E 3.5E+06 3.2E+06 1.3E+06 1.2E+06 6.5E+05

F 7.5E+03 7.2E+03 2.4E+03 2.0E+03 9.8E+02

G 4.0E+05 3.2E+05 1.1E+05 8.1E+04 3.1E+04

H 8.6E+05 6.8E+05 2.0E+05 1.0E+05 2.9E+04

I 5.4E+06 5.5E+06 3.5E+06 4.6E+06 2.7E+06

J 1.8E+06 1.7E+06 8.3E+05 9.1E+05 5.2E+05

aRelative to fuel discharge date aRelative to fuel discharge date


Conclusions

• Dose rate in silicon was evaluated as a function of decay time (5 to 70 years from fuel discharge) for selected locations on the inner and outer surfaces of representative PWR and BWR SNF canisters using bounding radiation sources

• At a 5-year decay time, the estimated maximum dose rate values are 480 Gy/h (inner surface of the PWR canister) and ~366 Gy/h (inner surface of the BWR canister) and the estimated minimum dose rate is ~0.2 Gy/h for both canisters (outer top surface)

• Over a 65-year period (i.e., from 5 to 70 years after fuel discharge), the dose rate decreases by a factor between 13 and 580, depending on location

• The cumulative dose can be very large (e.g., estimated maximum value over a 5-year interval is 1.5E+07 Gy) depending on location and the time interval over which dose is accumulated

• The dose rates based on bounding radiation sources are slightly conservative for as-loaded canisters containing high-burnup fuel assemblies and more conservative for canisters containing low-burnup assemblies


Thank you for your attention!

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Detailed Radiation Dose Rate Evaluations of Commercial