Special Procedure 12

Recommendations for Determining Waste Disposition 
 (Top)
 
Assigning activity to waste can sometimes be a difficult process.  Recording a reasonably accurate estimate of waste disposition in the inventory record is required for each dispensation of stock material.  To facilitate this requirement, a system for calibrating a liquid scintillation counter (LSC) and determining activity in liquids, especially liquid waste, has been developed and is described herein.  To ensure that the recorded waste activities continue to be reasonably accurate, reassessing the values periodically by comparing the determined activity in liquid waste against the recorded values on inventory sheets is recommended.
 
Determining the activity in dry waste is very difficult.  However, the activity in the liquid waste can be verified with relative ease.  If the inventory records and the actual activity in the liquid waste do not agree, the values used to estimate the waste disposition can be adjusted to reflect activities more accurately.  The activities presumed to exist in liquid waste, as indicated on the inventory sheets, should be verified whenever any anomaly or inconsistency is suspected.
 
Liquid Scintillation Counting:
Liquid scintillation counting results are affected by a number of factors, e.g. pH, color, chemical constituents and variations in liquid scintillation cocktails.  The activity in the liquid waste cannot be accurately determined without first calibrating the LSC for the type of waste under consideration.  For this reason, LSCs should be calibrated for each type of liquid waste generated, e.g. for each isotope and for each container with non-similar constituents or non-similar compositions.  The LSC only needs to be calibrated for each individual isotope if the constituents and composition in the waste do not vary significantly from the sample used for calibration.  The same cocktail should be used for the calibration and for the activity calculation. The system should be recalibrated if a different cocktail is used or if waste composition changes.
 
Liquid Waste Calibration:
In order to simplify the calibration process, using the internal spike method is recommended for calibrating LSCs for liquid waste analysis.  Other calibration methods or deviations from the procedure described herein may be used, however, the method should be reviewed by EHS prior to implementation.  The spike, or activity added to a sample, can be obtained from stock material on hand.  The container of the liquid waste used for this procedure must be at least half full.
 
Data Gathering:
  1. Perform a decay correction for all stocks from which activity has contributed to the volume in the liquid waste container and from which the spike will be withdrawn.
  2. Calculate the amount of activity from each stock that is in the liquid waste container.
  3. Sum the activities in liquid waste to determine the estimated amount of activity in the liquid waste container on the date of calibration.
  4. Calculate the volume necessary to be withdrawn to obtain 0.25 mCi for the spike.  (Actual activity may range from 0.2-0.3 mCi.)  Error in measuring the volume of the spike will substantially affect the results.  Be sure to minimize the error as much as possible.
  5. Estimate the volume of liquid in the waste container in milliliters.  (1 liter = 1000 milliliters)
Sample Volume Chart
Use the sample volumes listed in milliliters (ml) under the size container being sampled from.*
 
Estimated Activity in Container (mCi)
1 Liter Bottles
2 Liter Bottles
4 Liter Bottles
10 Liter Carboys
20 Liter Carboys
0 - 0.01
2
2
2
2
2
0.01 - 0.05
0.5
2
2
2
2
0.05 - 0.1
0.5
0.5
1
2
2
0.1 - 0.25
0.1
0.5
1
2
2
0.25 - 0.5
0.1
0.1
0.25
0.5
2
0.5 - 0.75
0.1
0.1
0.25
0.5
0.5
0.75 – 1
0.02
0.1
0.1
0.5
0.5
1 – 2
0.02
0.02
0.1
0.1
0.5
2 – 5
0.01
0.02
0.02
0.1
0.1
5 – 10
0.01
0.01
0.02
0.02
0.1
10 – 20
*
0.01
0.01
0.02
0.05
>20
*
*
0.01
0.02
0.05
* Contact EHS for specific advice for high activity samples and/or container sizes not referenced in this chart.
 
Sampling and Counting
  1. Refer to the Sample Volume Chart to determine the appropriate sample volume from the estimated activity in the waste and container size.  This value is the volume of waste and water that should be placed into the calibration sample and background sample, respectively.
  2. Using great care, gently agitate, shake or stir contents of liquid waste container to assure reasonable homogeneity.  (The sample taken from the waste container needs to be representative of the total contents of the container.  Sampling from the fresh waste from the top of the container will greatly affect the accuracy of this procedure, especially for P32.)
  3. Obtain the amount of liquid waste determined in step 1 to use for the calibration and deposit it into a 20 ml liquid scintillation counting vial.
  4. Add 15 ml of liquid scintillation cocktail.  Label and cap vial.
  5. Prepare a background sample by adding the same volume of water that was used for the sample and 15 ml of liquid scintillation cocktail to a 20 ml counting vial.  Label and cap vial.
  6. Count the background sample and the waste sample in the LSC for at least one minute each.
  7. Add 0.2-0.3 mCi of that isotope to the waste sample vial.
  8. Recount the spiked sample for at least one minute.
  9. Refer to the “Efficiency and Activity Determination” section below to calculate results.
Efficiency and Activity Determination:
To simplify calculations, an Excel spreadsheet is available at www.ehs.cornell.edu to calculate results of calibrations and activities of liquid waste.  Enter the following information into the spreadsheet:
  
Date:
Enter the date that the counter is being calibrated
Permit Holder:
Enter the name of the permit holder
Isotope:
Enter the isotope that the counter is being calibrated for
Waste Constituents:
Enter the constituents in the liquid waste and their percent composition
LSC Cocktail Used:
Enter the type/brand of liquid scintillation cocktail used
LSC Sample Volume:
Enter the sample volume in milliliters (ml)
Waste Volume:
Enter the estimated volume in the liquid waste container in milliliters (ml)
Activity recorded in Inventory Records:
Enter the sum of the liquid waste activities recorded on the inventory sheets that are associated with that waste container in millicuries (mCi)
Background Counts/Minute:
Enter the results from counting the background sample in counts per minute (cpm)
Initial Sample Counts/Minute:
Enter the results from counting the waste sample prior to adding the 0.25 uCi spike in counts per minute (cpm)
Spiked Sample Counts/Minute:
Enter the results from counting the spiked waste sample in counts per minute (cpm)
Spike Activity
Enter the amount of activity added to the waste sample in mCi
Efficiency:
The efficiency will appear here in counts per disintegration (c/d)
LSC Sample Activity:
The activity in the unspiked waste sample will appear here in disintegrations per minute (dpm)
Waste Concentration:
The concentration of the unspiked waste sample will appear here in disintegrations per minute per milliliter (dpm/ml)
Waste Activity:
The total activity in the waste container will appear here in millicuries (mCi)
Difference from
Recorded Value:
 
The percent difference from the activity calculated from the inventory sheets and the activity determined from calibration will appear here.
 
Record Keeping:
Keep a copy of each of the latest calibration reports with your inventory records until the next calibration is performed.