Detailed Dose Rate Evaluations of Commercial Spent Canisters

Georgeta Radulescu and Kaushik Banerjee Reactor and Nuclear Systems Division, ORNL

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

ORNL is managed by UT-Battelle, LLC for the US Department of Energy This is a technical presentation that does not take into account the contractual limitations under the Standard Contract for Disposal of and/or High- Level (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.

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

SCALE Users’ Group Workshop, Knoxville, TN, August 19 - 21, 2019 3 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 (BWR) SNF canister

SCALE Users’ Group Workshop, Knoxville, TN, August 19 - 21, 2019 4 Dose Rate Calculation Method

• All calculations performed with SCALE 6.2.3 1.0E-12

1.0E-13 – Radiation source terms calculated with ORIGAMI

2 1.0E-14 – Dose rate calculated with MAVRIC, a Monte Carlo radiation transport code for

shielding calculations 1.0E-15 Gy per photn/m • Calculation of dose (i.e., energy absorbed per mass unit) in silicon 1.0E-16

1.0E-17 requires photon and dose response functions for silicon 1.0E-03 1.0E-02 1.0E-01 1.0E+00 1.0E+01 1.0E+02 Photon energy (MeV)

• The photon response in silicon was calculated by integrating the Photon KERMA factor for silicon as a product of the energy-dependent photon flux in dry air and the function of photon energy1 energy-dependent photon KERMA (kinetic energy released in materials) factors for silicon across photon’s energy range 1.0E-14 1.0E-15

2 1.0E-16

• The neutron response in silicon was calculated by integrating the 1.0E-17

product of the energy-dependent neutron flux in dry air and the 1.0E-18

1.0E-19 energy-dependent neutron KERMA factors for silicon across Gy per neutron/m neutron’s energy range 1.0E-20 1.0E-21 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-011.E+001.E+011.E+02 Neutron energy (MeV)

Neutron KERMA factor for silicon as a 1 1M. S. SINGH, “KERMA Factors for Neutron and Photons with Energies below 20 MeV,” UCRL- function of neutron energy 52850, Lawrence Livermore Laboratory (1979).

SCALE Users’ Group Workshop, Knoxville, TN, August 19 - 21, 2019 5 Canister Model and Selected Dose Rate Locations • Geometry model included a representative canister and its designated storage cask Geometry cell defined for • The canister contained identical fuel assemblies dose rate calculations – 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 Horizontal cross sectional view of a representative PWR SNF canister in its storage • Dose rate was calculated for selected locations on configuration 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 aBounding Co impurity concentrations found in old assemblies. The Co impurity limit in hardware materials of representative PWR SNF canister in modern assemblies is 500 ppm. its storage configuration showing dose rate locations SCALE Users’ Group Workshop, Knoxville, TN, August 19 - 21, 2019 6 Radiation Source Terms PWR SNF assembly BWR SNF assembly Bounding sources for 95% of 10 axial-zone burnup the SNF inventory1 18 axial-zone burnup 5.6E+14 5.6E+14 5.1E+14 5.1E+14 discharged from 1968 to 2013 4.6E+14 4.6E+14 4.1E+14 4.1E+14 Assembly Initial Burnup 3.6E+14 3.6E+14 3.1E+14 3.1E+14 type enrichment (GWd/MTU) 2.6E+14 2.6E+14 (wt% 235U) 2.1E+14 2.1E+14 1.6E+14 1.6E+14 1.1E+14 1.1E+14 PWR 4.8 55 6.0E+13 6.0E+13 Source strength (photons/s) strength Source Source strength (photons/s) strength Source 1.0E+13 1.0E+13 0 50 100 150 200 250 300 350 400 0 50 100 150 200 250 300 350 400 BWR 4.2 50 Active fuel length (cm) Active fuel length (cm) 5 years 10 years 20 years 30 years 50 years 70 years 5 years 10 years 20 years 30 years 50 years 70 years

1.E+16 1.E+15 Gamma source strength axial profile Gamma source strength axial profile 1.E+14 1.E+13 1.E+12 5.0E+07 8.0E+07 1.E+11 4.5E+07 7.5E+07 7.0E+07 1.E+10 4.0E+07 6.5E+07 1.E+09 6.0E+07 3.5E+07 5.5E+07 1.E+08 3.0E+07 5.0E+07 4.5E+07 1.E+07 2.5E+07 4.0E+07 Source strength (particles/s) strength Source 0 10 20 30 40 50 60 70 80 3.5E+07 2.0E+07 3.0E+07 Decay time (years) 1.5E+07 2.5E+07 2.0E+07 PWR; active fuel; photon BWR; active fuel; photon BWR; LEF; photon 1.0E+07 1.5E+07 1.0E+07 PWR; UEF; photon PWR; LEF; photon BWR; UEF; photon 5.1E+06 5.0E+06 Source strength (/s) strength Source

PWR; GP; photon BWR; GP; photon PWR; active fuel; neutron 1.0E+05 (neutrons/s) strength Source 1.0E+04 BWR; active fuel; neutron 0 50 100 150 200 250 300 350 400 0 50 100 150 200 250 300 350 400 Active fuel length (cm) Active fuel length (cm) SNF assembly photon and neutron 5 years 10 years 20 years 30 years 50 years 70 years 5 years 10 years 20 years 30 years 50 years 70 years source strength variation as a function of fuel type, assembly Neutron source strength axial profile Neutron source strength axial profile region, and decay time

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) . SCALE Users’ Group Workshop, Knoxville, TN, August 19 - 21, 2019 7 Examples of Dose Rate Variations on Canister Surface Radial surface Canister top surface Axial dose rate variation

100.0 5.6E+14 5.1E+14 4.6E+14 4.1E+14 10.0 3.6E+14 3.1E+14 2.6E+14 2.1E+14 1.6E+14 1.0 1.1E+14 6.0E+13 Source strength (photons/s) strength Source Dose rate (Gy/h) rate Dose 1.0E+13 0 50 100 150 200 250 300 350 400 Active fuel length (cm) 0.1 5 years 10 years 20 years 30 years 50 years 70 years 0 100 200 300 400 500 Dose rate variation as a Height (cm) Gamma source strength axial profile function of radius Azimuthal dose rate variation �/2 36 equal azimuthal 70 segments 65

60

55

50 � 0 45

Dose rate (Gy/h) rate Dose 40 2�

35

30 0 1 2 3 4 5 6 7 ! (radians)

3�/2 SCALE Users’ Group Workshop, Knoxville, TN, August 19 - 21, 2019 8 Dose Rate in Silicon as a Function of Location and Decay Time PWR SNF Canister BWR SNF Canister

1.E+03 1.E+03 D I B E A J C H G F D B I A E J C H G F 1.E+02 1.E+02

1.E+01 1.E+01

1.E+00 1.E+00

1.E-01 1.E-01 Dose rate (Gy/h) rate Dose Dose rate (Gy/h) rate Dose

1.E-02 1.E-02

1.E-03 1.E-03 0 10 20 30 40 50 60 70 80 0 10 20 30 40 50 60 70 80 Decay time (days) Decay time (days)

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

1.E-02 1.E-02 I D E J B G A C H F I D J B E G A H C F

1.E-03 1.E-03

1.E-04

1.E-04 Dose rate (Gy/h) rate Dose 1.E-05 Dose rate (Gy/h) rate Dose

1.E-05 1.E-06 0 10 20 30 40 50 60 70 80 0 10 20 30 40 50 60 70 80 Decay time (days) Decay time (days) Geometry model showing Neutron dose rate as a function of Neutron dose rate as a function of dose rate locations decay time for selected locations decay time for selected locations

SCALE Users’ Group Workshop, Knoxville, TN, August 19 - 21, 2019 9 Total Dose Rate in Silicon as a Function of Canister Location and Decay Time

PWR SNF Canister BWR SNF Canister

Decay Decay time 5 10 20 30 50 70 time 5 10 20 30 50 70 (years) (years) Location Dose rate (Gy/h) Location Dose rate (Gy/h)

A 41.80 21.53 6.75 2.38 0.92 0.55 A 139.59 71.94 19.71 5.26 0.59 0.24

B 85.62 49.81 23.58 14.93 7.56 5.28 B 312.32 163.86 51.63 20.73 7.19 4.31

C 22.56 12.05 5.22 3.14 1.60 0.95 C 52.39 27.29 7.90 2.22 0.33 0.13

D 480.51 225.59 134.23 100.25 61.00 37.79 D 366.01 163.83 93.70 69.58 42.90 26.33

E 63.83 33.37 18.02 13.18 7.39 4.69 E 107.50 59.39 21.25 10.40 4.69 2.84

F 0.19 0.10 0.05 0.03 0.01 0.01 F 0.23 0.12 0.04 0.02 0.01 0.004

G 4.86 2.77 1.15 0.66 0.31 0.18 G 12.42 6.32 1.88 0.79 0.23 0.13

H 9.52 5.02 1.88 1.00 0.44 0.26 H 26.66 13.76 3.84 1.15 0.23 0.11

I 233.22 109.13 65.14 47.18 28.79 17.89 I 177.45 81.38 46.74 34.22 20.06 12.54

J 26.22 13.93 7.21 4.82 2.79 1.72 Geometry model showing J 54.73 29.62 12.33 6.96 3.65 2.23 dose rate locations

SCALE Users’ Group Workshop, Knoxville, TN, August 19 - 21, 2019 10 Comparison to As-Loaded Canister Dose Rates

Representative As-loaded As-loaded canister canister #1 canister #2 SNF assembly burnup 55 35 – 52 14 – 39 (GWd/MTU) Average SNF assembly 55 46 22 burnup (GWd/MTU) SNF assembly discharge 2000 2000 – 2003 1983 – 2000 years Dose rate range at 115.6 – 134.2 84.6 –124.4 26.0 – 30.3 location D (Gy/h) Dose rate range at 49.0 – 67.0 28.9 – 60.6 8.4 – 14.8 location I (Gy/h)

• 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

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

PWR SNF Canister BWR SNF Canister

Time Time intervala 5 to 10 10 to 20 20 to 30 30 to 50 50 to 70 intervala 5 to 10 10 to 20 20 to 30 30 to 50 50 to 70 (years) (years) Location Time-integrated dose (Gy) Location Time-integrated dose (Gy)

A 1.3E+06 1.1E+06 3.7E+05 2.7E+05 1.3E+05 A 4.5E+06 3.6E+06 9.6E+05 3.8E+05 6.8E+04

B 2.9E+06 3.1E+06 1.6E+06 1.9E+06 1.1E+06 B 1.0E+07 8.6E+06 3.0E+06 2.2E+06 9.6E+05

C 7.4E+05 7.1E+05 3.6E+05 4.0E+05 2.1E+05 C 1.7E+06 1.4E+06 3.9E+05 1.8E+05 3.8E+04

D 1.5E+07 1.5E+07 1.0E+07 1.4E+07 8.5E+06 D 1.1E+07 1.1E+07 7.1E+06 9.7E+06 6.1E+06

E 2.1E+06 2.0E+06 1.4E+06 2.0E+06 1.0E+06 E 3.5E+06 3.2E+06 1.3E+06 1.2E+06 6.5E+05

F 6.3E+03 6.4E+03 3.2E+03 3.3E+03 1.9E+03 F 7.5E+03 7.2E+03 2.4E+03 2.0E+03 9.8E+02

G 1.6E+05 1.6E+05 7.8E+04 8.1E+04 1.0E+06 G 4.0E+05 3.2E+05 1.1E+05 8.1E+04 3.1E+04

H 3.1E+05 2.8E+05 1.2E+05 1.2E+05 5.9E+04 H 8.6E+05 6.8E+05 2.0E+05 1.0E+05 2.9E+04

I 7.2E+06 7.4E+06 4.9E+06 6.4E+06 4.0E+06 I 5.4E+06 5.5E+06 3.5E+06 4.6E+06 2.7E+06

J 8.5E+05 8.9E+05 5.2E+05 6.6E+05 3.9E+05 Geometry model showing J 1.8E+06 1.7E+06 8.3E+05 9.1E+05 5.2E+05 dose rate locations aRelative to fuel discharge date aRelative to fuel discharge date

SCALE Users’ Group Workshop, Knoxville, TN, August 19 - 21, 2019 12 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

SCALE Users’ Group Workshop, Knoxville, TN, August 19 - 21, 2019 13 Thank you for your attention!

Questions?

SCALE Users’ Group Workshop, Knoxville, TN, August 19 - 21, 2019 14