Topic 7 - Dose Limits

Example calculations using internal dose conversion factors

Annual Limit on Intake

• Activity of a radionuclide, which if taken in alone, would irradiate an individual to the limit set by the ICRP for each year of occupational exposure
• Intake rate: quantity per year
• Pure parent assumption
• Can have an ALI each year
• No time constraint set on exposure period
  • (rate can be instantaneous or up to a year)
• Two limits – stochastic and nonstochastic considered.

• Secondary limit
  • designed to meet the basic limits for occupational exposure
  • derived from the previous two relationships.
  • The ALI is the greatest value of the annual intake, I, which satisfies both of the following inequalities:
    
    

• Where
  • I (Bq) is the annual intake of the specified radionuclide (by ingestion or inhalation).
    • S = stochastic limit
    • N = nonstochastic limit
  • H_T,50 per unit intake (Sv Bq^-1) is the committed dose equivalent in tissue (T) from the intake of unit activity of the nuclide by the specified route.

• If I_s not exceeded, then stochastic limits met
• If I_n not exceeded, then nonstochastic limits met
• Select value of I which satisfies both inequalities to determine limiting value.

Calculating an ALI inhalation- example ²³⁹ Pu intake by inhalation

ICRP 30

Calculating ALI (stochastic)

Calculating ALI (nonstochastic)

Calculating ALI: ²³⁹Pu

• Stochastic limit
  • 5.6 x 10² Bq
• Nonstochastic limit
  • 5.3 x 10² Bq
• Uncertainites in metabolic models result in listed ALI at one significant figure:
  • 5 x 10² Bq

Derived Air Concentrations

• Revised version of ICRP 2 MPC_air
  • Maximum permissible concentration (air)
  • Old MPCs were misused. 
  • They were maximum permissible concentrations intended to control exposure over prolonged periods (> 3 mos).
  • They have been used to infer over-exposure for even short exposure times.
  • The limit for inhalation of a radionuclide is the appropriate ALI.
  • The concentration of a radionuclide in air during any year is limited as follows:
• That concentration of a radionuclide in air, which if breathed for one working year, would result in one ALI by inhalation:
  
  • 2000 hrs per working year
  • 1.2 m³ h^-1 working breathing rate
• Note:  ALI is the main limit and the DAC is a derived limit.

DAC’s, alternate calculation

• Calculated based on annual breathing rate:
  
  • 2400 m³  annual breathing rate
• Again Note:  ALI is the main limit and the DAC is a derived limit.

Use of Dose Conversion Factors

• Tables generated based on ICRP methodologies
• Printed by several agencies
  • EPA
  • DOE
• Units may vary (SI vs standard)
• Methodology may vary
  • ICRP 2
  • ICRP 26/30
  • ICRP 60
• Example to follow

50-Yr committed dose equivalent factors -- rem/ƒΚCi intake: ²³⁹Pu

^* Indicates that <90% of total dose is received in year of intake

GI Absorption & Lung Retention

Class examples to work

• Show how CEDE obtained
• Demonstrate particle size correction  
  • PuO₂
  • Use 9.6 μm

Respiratory Model

Fig. 5.2. Mathematical model used to describe clearance from the respiratory system. The values for the removal half-times, T_a-1 and compartmental fractions, F_a-1 are given in the tabular portion of the figure for each of the three cleasses of retained materials. The values given for D_N-P, D_T-B and D_P (left column) are the regional depositions for an aerosol with an AMAD of 1 μm. The schematic drawing identifies the various clearance pathways from compartments a-i in the four respiratory regions, N-P, T-B, P and L.
n.a. = not applicable.

Fig. 5.1. Deposition of dust in the respiratory system. The percentage of activity or mass of an aerosol which is deposited in the N-P, T-B and P regions is given in relation to the Activity Median Aerodynamic Diameter (AMAD) of the aerosol distribution. The model is intended for use with aerosol distributions with AMADs between 0.2 an 10 μm and with geometric standard deviations of less than 4.5. Provisional estimates of deposition further extending the size range are given by the dashed lines. For an unusual distribution with an AMAD of greater than 20 μm, complete deposition in N-P can be assumed. The model does not apply to aerosols with of greater than 20 μm, complete deposition in N-P can be assumed. The model does not apply to aerosols with AMADs of less than 0.1 μm.

Particle Size Correction

Where D_N-P, etc are the deposition probabilities in the respiratory region for a given AMAD (see fig 5.1), and f_N-P etc are the fraction of the committed dose in the reference tissue arising from deposition in the N-P, T-B, and P regions (see conceptual model).

Tissue Weighting Factors