Atmospheric Sources of Tritium and Potential Implications to Surface and Groundwater Monitoring Efforts Ken Sejkora Entergy Nuclear Northeast – Pilgrim

Atmospheric Sources of Tritium and Potential Implications to Surface and Groundwater Monitoring Efforts

Ken SejkoraEntergy Nuclear Northeast – Pilgrim Station

Presented at the EPRI & NEI Nuclear Plant Groundwater Monitoring Workshop

Providence, RI / 12-13 Sep 2006

Reason for Concern Recent issue of tritium in groundwater

has resulted in more plants looking for tritium in more places, at lower limits of detection

Increased probability of seeing “natural” atmospheric tritium in water samples

Recognize the presence of atmospheric tritium, expected levels, and implications to monitoring efforts

Sources of Atmospheric Tritium

Spallation from cosmic ray interactions in the upper atmosphere

Atmospheric testing of nuclear weapons in 1940s through 1970s

Airborne effluents from nuclear power plants, laboratories, biological waste incinerators, manufacturing, etc.

Cosmogenic Tritium Spallation: high energy cosmic ‘rays’ split nitrogen

and oxygen nuclei in the upper atmosphere “Primordial” source of tritium… it’s always been

here, at a relatively constant level Subject to seasonal and latitudinal variations as

the tropopause opens in spring and fall Tropopause = boundary layer between stratosphere

and troposphere, 11-16 km altitude Seasonal opening centered at 43 to 48 degrees latitude

Cosmogenic Tritium - continued

Expected concentration of 1 to 5 tritium units (TU) in atmospheric precipitation

Tritium Unit - TU 1 tritium atom per 1E+18 hydrogen atoms 1 TU = 0.118 Bq/L = 3.19 pCi/L 1 to 5 TU = 3 to 15 pCi/L

Relatively constant “baseline” in precipitation

Weapons-related Tritium Prior to weapons testing, global equilibrium H-3

inventory in atmosphere was estimated at 7.3 kg, or about 80 megaCuries

Nuclear weapons testing increased baseline approximately 200-fold

Atmospheric tritium levels peaked in early 1960s at slightly over 1000 TU (~3000 pCi/L)

Weapons inventory continues to decay with half-life of 12.3 years… modern-day levels are in the range of 30 to 100 TU (100 to 300 pCi/L)

Also subject to tropopause influence – seasonal, latitude

“Industrial” Tritium Nuclear power plants

1E+1 to 1E+3 Ci/yr per facility Research facilities, universities Incinerators of biological/medical wastes

containing tritium Manufacturing: explosives detectors,

illuminated signs, etc. Air conditioner condensate from such facilities

Where does condensate go? Storm drain vs. sanitary?

Localized, ground-level sources of tritium

EPA Laboratory Monitoring National Air and Radiation Environmental

Laboratory (NAREL), Montgomery, AL www.epa.gov/narel/radnet/erdonline.html Network of 41 monitoring stations, nationwide, that

collect precipitation quarterly for pollutant assessment Nominal MDC of 120 to 140 pCi/L

Most results are non-detectable, but many positive results are in the range of 75 to 300 pCi/L, with some results as high as 2900 pCi/L Excel spreadsheet of 2000 through 2006 data available

There is detectable tritium in precipitation!

Precipitation Washout Most airborne tritium is released from plant

as tritiated water vapor Precipitation falling through air column

above the plant while airborne release is occurring will “washout” tritium, and bring it down to ground in liquid form

Can result in relatively high concentrations of tritium in precipitation, surface water, runoff, and storm water… implications for infiltration and “contamination” of groundwater

Precipitation Washout

Precipitation Washout Example #1

Assumptions: Protected area is within 400m radius circle =

500,000 m2 area 24-hour rainfall event, 1 cm (0.4”) precipitation Total volume of precip over area= 5E+6 liters

H2O 1 Ci/day release rate from plant vent 10% “scrubbing” efficiency as precipitation

removes tritium from air… (is this too low?) Resulting concentration = 20,000 pCi/L

Precipitation Washout Example #2

Assumptions: Calm wind, misty rain, washout is within 30m

radius circle (building roof)= 2,800 m2 area 24-hour rainfall event, 1 cm (0.4”) precipitation Total volume of precip = 28,000 liters H2O 1 Ci/day release rate from plant vent 100% “scrubbing” efficiency as misty

precipitation removes tritium from air Resulting concentration = 36,000,000

pCi/L

Precipitation Washout Reasonableness of Assumptions

Source term = 1 Ci/day For BWRs, most airborne tritium results from

evaporation from spent fuel pool Consider 5E-2 uCi/mL in pool water, 2000

gal/d evaporation rate = 0.4 Ci/d release rate

HEPA filters will do nothing to remove tritiated water vapor from air stream prior to release


Rainfall rate Duration of rainfall event is more important

than rate… 1 cm in 1 hour will pick up 1/10th of tritium compared to 1 cm over 10 hours

Slower rates may yield higher concentrations… 0.5 cm in 24 hours will likely be as effective as 5 cm in “washing” tritium from the air. However, tritium will be “diluted” in ten times the volume in the case of the 5 cm rainfall event


Scrubbing Efficiency Probably dependent on distribution of raindrop

size, educated guess that it is between 30% and 90%

Example: Hospital or school on cool, calm morning – how far does the “plume” disperse from the building? If it were raining, how much would you envision to washout within 50 meters of building? 500 meters? Will any get offsite?

Groundwater “Contamination” Importance of infiltration

Infiltration vs. Runoff Factors important to infiltration

Slope of land Land cover – asphalt vs. bare ground vs. vegetated

field… immediate vicinity of plant is probably paved Permeability of soil Network of storm drains

What doesn’t infiltrate into ground will leave site as runoff… potential for offsite “contamination”?


Infiltration vs. Groundwater Existing volume of groundwater will “dilute”

tritiated water after entering the ground… concentrations will be lower in groundwater than in precipitation

Subsurface flows and gradients may result in precipitation tritium moving away from one well, or toward another Underground rock formations, sand lenses, pipe

runs, utilities, etc.


Infiltration vs. Groundwater Concentrations will decrease with increasing

distance from point of infiltration Think 3-D: vertical as well as horizontal distances Shallow well may indicate precipitation tritium,

whereas deep well won’t Wells close to a lawn or unpaved switchyard may

indicate precipitation tritium due to immediate proximity of point of infiltration. Wells farther away may be below detection due to dilution.


Runoff and Storm Drains Storm drains are not water tight. Infiltration

can occur at leaks in seams and catch basins. Leaks can result in point sources of

infiltration, that may “look” like a point source such as a leaking pipe.

Consider highly concentrated water draining from a building roof at concentration of >1E6 pCi/L.

Groundwater “Contamination” Importance of Detection Level

NRC vs. EPA EPA drinking water standard is 20,000 pCi/L; EPA

established required LLD at 1/10th of this, or 2,000 pCi/L

NRC adopted LLD of 2,000 pCi/L in REMP tables in NUREG-1301/1302; plants implement through ODCM

“Baseline” precipitation levels of 100 to 300 pCi/L are well below required LLD of 2,000 pCi/L; precipitation washout can be readily detectable

“Aggressive” LLDs of <500 pCi/L may occasionally detect “baseline” atmospheric tritium; will certainly detect precipitation washout

Summary Tritium exists in the atmosphere, and can

be detected in precipitation Cosmogenic: 3 to 15 pCi/L Weapons testing: 100 to 300 pCi/L Industrial: Highly localized, can yield very high

concentrations immediately adjacent to facility EPA monitoring network can provide

baseline values detectable in environment

Summary - continued

Tritium in precipitation can result in detectable tritium in surface water, runoff, storm drains, and groundwater; could potentially be mistaken for underground leaks

Localized washout can result in very high concentrations, possibly even exceeding drinking water standards

Summary - continued

Ability to detect tritium is largely a function of selected LLD. LLDs much lower than required may result in identification of “nuisance” levels of tritium, inseparable from precipitation tritium

Consider sampling program for precipitation, roof drainage, and surface runoff to characterize potential impacts from washout of airborne effluents

Documents

Atmospheric Sources of Tritium and Potential Implications to Surface and Groundwater Monitoring Efforts Ken Sejkora Entergy Nuclear Northeast – Pilgrim