Various Methodologies for Calculating the Impacts on Delta-T From Independent Spent Fuel Storage Installations (ISFSI) Jim Holian and Steve Mirsky, SAIC NUMUG Meeting San Francisco October 2009 NUMUG Meeting San Francisco October 2009

Background Original study performed for Cooper Nuclear Station (Nebraska Public Power) Results presented at 2008 NUMUG in Charlotte Study based on very conservative assumptions Floods delayed new met tower construction Additional study requested to reduce conservativeness in impact analyses

Three Methods Analyzed Single-node adiabatic plume (initial study) Multi-node plume with heat transfer (dilution) Multi-node plume with heat transfer and buoyancy

Baseline Based on NUHOMS Horizontal Storage Module (HSM) ISFSI Design Other designs, i.e. Historm, VSC-24, NAC, may provide varying results due to different material, geometries, and flow areas Calculations provided for comparisons between methods and are not specific to any plant

Factors Impacting Results Independent of Method Distance from ISFSI to the met tower Stability class distribution Wind speed Wind direction frequency ISFSI heat load

Single Node Adiabatic Plume First quick estimate of thermal impacts at the meteorological tower (if any) No mixing with air outside plume No heat loss/gain with surrounding air Buoyancy is assumed zero Only calculation node is at the cross section of plume where it intersects met tower

Key Assumption HSM exit air temperature (HSM ext ) above ambient is linearly proportional to HSM heat load (HL) H ext = Actual HL /Design HL X Design HSM ext Example: 50°F = 12 kW/24 kW X 100°F

Method 1 Calculation The effect on air temperature at the met tower is the ratio of the flow rate at the HSMs (FR HSM ) and the flow rate at the tower (FR TWR ) along the insulated plume times HSM ext : ΔT twr = FR HSM / FR TWR X HSM ext FR TWR has huge influence on ΔT twr

What Influences FR TWR ? Greatest is Stability - G stable plume cross section at a met tower 600 feet away is over 70 times smaller than an unstable A plume Wind Speed Combination of stable plume and light winds (typical nighttime scenario) results in large ΔT twr

Met Tower ΔT (°F) at 500 Feet from ISFSI (8 HSMs – G & D Stability) HSM Heat kW Wind Speed (m/s) G G D D

Met Tower ΔT (°F) at 150 Feet from ISFSI (10 HSMs – G & E Stability) HSM Heat kW Wind Speed (m/s) G G E E

Multi-Node Plume With Heat Transfer Includes heat transfer with air outside plume Calculation node every 50 feet from ISFSI to met tower No heat loss/gain with ground Buoyancy is still assumed zero Reduces some conservatism from Method 1

Heat Transfer Methodology Determine flow area, heat transfer area, and plume volume for each node Calculate the natural convection heat transfer correlation Calculate the average and exit temperature of each node

Heat Transfer Calculations Natural Convection Heat Transfer Coefficient Selected: HTC = 0.18 Δt For each node, the heat transfer Q is: Q i = HTC i x Area i x Δt i Temperature change across the node is: T i+1 =T i -Q i /V i ρ i c p where V i is volume, ρ i is density, and c p is air specific heat.

Heat Transfer Methodology Results Reduced overall temperature change at tower by approximately 23% from simple adiabatic method (at 600 feet and 12 kW heat load for HSM geometry) Demonstrates the impact of temperature change versus distance

Multi-Node Plume With Heat Transfer and Buoyancy Utilizes same approach as the Heat Transfer Methodology except buoyancy is calculated into plume height Reduces additional conservatism from first two methodologies

Buoyancy Methodology Archimedes Law applied to each node Calculate the vertical displacement of each node Sum displacements to determine height of plume when it reaches met tower

Buoyancy Calculations Determine Buoyancy Force for each node i: F i =(ρ out -ρ iplume ) V iplume g Heated air buoyancy acceleration is: A ib = F i / V i ρ iplume Vertical plume displacement for each node is: Z i =0.5 A ib t 2

Buoyancy Results Including buoyancy reduced the overall temperature change at tower by over 40% from the simple adiabatic method In some instances, plume may go over tower, impact sensors at another level, or traverse between sensor levels Extremely dependent on HSM heat load and wind speed

ISFSI Effects on Radiological Dose Assessment

Impacts Thermal impacts of HSM designed heat load dissipation – (direct thermal 24/7) Structural – thousands of square feet of solid concrete in ISFSI HSMs sitting on a large concrete base often rimmed with gravel (passive thermal – urban heat island) Physical – ISFSI size impacting wind speed and direction if inside 10:1 ratio (Reg. Guide 1.23). Usually not a large impact

NRC XOQDOQ Comparison Ran standard Χ/Qs using 5 years of data Added 0.4°F to ΔT to account for passive thermal impacts holding all other parameters equal Added 3.0°F to ΔT only when wind is blowing from ISFSI to account for direct thermal impacts holding all other parameters equal Calculated the % change in sector Χ/Qs

Impacts on Delta-T Stability Distribution

Χ/Q Impact (%) When Lower Temperature Increased by 0.4°F Distance (Miles) SECTOR S SSW SW WSW W WNW NW NNW N NNE NE ENE E ESE SE SSE

Χ/Q Impact (%) When Lower Temperature Increased by 3.0°F for ISFSI to South of Tower Distance (Miles) SECTOR WNW NW NNW N NNE NE ENE E

Conclusions Various methods exist for calculating the change in temperature at the met tower from the ISFSI The conservative adiabatic method provides a quick first estimate The heat transfer method is less conservative and reduces the overall estimate by around 23% Adding buoyancy to the adiabatic and heat transfer method reduces the estimate by over 40%, is the least conservative, and approximates the height of impact at the tower Distance from the tower, stability, wind speed, and heat load have huge impacts on the results

Conclusions (cont’d) Heat Transfer is most important in unstable to neutral environments with wind speeds > 2 m/s Buoyancy has significant impacts in stable light wind conditions The impact on downwind dose assessment is large for changes in temperature at the tower >0.4°F In general, the analyses suggest that ISFSIs should be located further than 600 feet from the meteorological tower

Questions?