Estimation of effective dose for individual persons of the population by natural radionuclides in case of discharges and landfilling of residues.

Transcription

1 Estimation of effective dose for individual persons of the population by natural radionuclides in case of discharges and landfilling of residues. Harry Friedmann University Vienna, Nuclear Physics Veszprém, May 2012

2 Motivation As a consequence of directives of the EU which consider NORM and TENORM as important within the protection of the general public against the dangers arising from ionizing radiation: This act deals with radiation protection against natural terrestrial radiation sources. In this act, radiation protection concerns only individuals of the public. It does not deal with radiation protection for radiation workers (workers which are physically and medically surveyed). Veszprém, May /12

3 The reason for exposure to members of the public can be: Discharges via the air (dust, gases); Discharges via the water; Landfilling; Gamma radiation from the plant. The exposure is caused by: Internal exposure by incorporation (breathing, food, drinking water); External exposure from deposited radionuclides or direct exposure from the plant. It is extremely difficult to estimate the possible annual dose to members of the public and to compare it with the 1 msv/a limit of the act. This can be done only by specialists, but the results may differ substantially when asking different experts. Therefore, a common procedure to judge and calculate the exposure to members of the public was laid down in an Austrian Standard (S 5252) to avoid contradictory expertises. Veszprém, May /12

4 The Austrian Standard S 5252 Contents: 1. Scope of application 2. Legislative references 3. Definitions 4. Activity distribution in the environment 5. Control of residues Appendix A: Example: Dosis from discharged air Appendix B: Example: Dosis from deposition Appendix C: Example: Dosis from U and Th deposition via ground water Veszprém, May /12

5 General Procedure START Calculate the effective dose in the simplest possible way (worst case, default values,...). Improve radiation protection. Additional mitigation procedures. no < 1 msv/y? no better information available? yes STOP yes Improve the calculation of the effective dose with better data Veszprém, May /12

6 Activity distribution in the environment Via air: Emission without dilution: Relevant parameters: activity concentration in Bq/m³ in the plume (determinable from filter dust), emission rate. Emission with dilution: Using Gaussian diffusion factor with certain default values: Relevant parameters: Height of the point of emission, distance to emission point, diffusion parameter, mean velocity of the wind in direction of the point of interest. Result: Mean activity in the air at the point of interest. Deposition from the atmosphere: Relevant parameters: dry and wet deposition rates (if unknown default values have to be used). Result: Deposition rate at the point of interest. Veszprém, May /12

7 Via Water: Dispersion by flowing waters: Relevant parameters: mean activity concentration at the point of emission, emission rate, mean flow rate, sedimentation rate, decay rate, distance to emission point. Exchange between liquid phase and suspended particles: Relevant parameters: distribution coefficient (default values for different radionuclides). Result: Mean activity in the water and mean activity concentration in the sediments at the point of interest. Activity distribution in plants (from atmospheric deposition and soil contamination): Relevant parameters: deposition rate, contamination, decay rate, time until consumption, effective reduction rate, transfer factors etc. (default values). Result: Mean activity in plants at consumption (animals). Veszprém, May /12

8 Activity dispersion in food products Terrestrial Transfer: forage crops, milk, meat. Fish: Bioaccumulation factor. Result: Mean activity in food. Activity dispersion from deposition Dispersion via ground water: Relevant parameters: activity concentration, chemical behavior, dumping site characteristics (class, water content,...), distribution coefficient. Result: Mean activity concentration in the dump water = max. concentration in drinking water. Veszprém, May /12

9 Control of residues Default values: Percentage of time inside houses; Breathing rate; Mean food consumption (distribution); Dose conversion coefficients (inhalation, ingestion). How to combine multiple discharges Sampling procedures (air, water) Sampling procedures for temporal deposition Veszprém, May /12

10 Calculation of the effective dose (for 1 y) Definition of exposition pathways: Gamma exposition from deposits; Inhalation of dust; Inhalation of radon progeny; Ingestion via food (including drinking water); Direct ingestion from soil (children). Calculation of the effective dose for each pathway for 1 year: Conservative but realistic assumptions must be used; Future use of deposits must be taken in consideration; All nuclides within the relevant natural decay chain must be taken into consideration unless the concentration of certain nuclides is not relevant for the effective dose because of production conditions (default = radioactive equilibrium). General: Default values can be used if no information is available; If better information is available, special information can be used. Veszprém, May /12

11 Examples: Dose from discharged air Input data: Chimney small medium large Xlarge h=hight (m) d=diameter (m) s=shape V=Flow rate (m³/h) T=Temperature ( C) p=pressure (hpa) c=dust (mg/m³) a) Calculation without dilution: Probability for wind direction to point of exposure w = 1 Concentration in plume C i = w c a i Effective dose (inhalation) E inh = v a Σ i C i h(g*) inh,i v a...annual breathing volume (age dependent: m³) h(g*) inh,i...dose conversion coefficient for inhalation (weighted for age dependance: Sv/Bq) Radio- nuclide u...mean wind speed Veszprém, May /12 #i Activity Concentration a in Bq/g Dust #1 Dust #2 Th-234 0,1 0,1 Th-230 0,1 0,1 Ra-226 0,2 0,1 Pb-210 1,5 0,5 U-235 0,01 0,01 Pa-231 0,05 0,05 Ac-227 0,01 0,1 Ac-228 0,1 0,1 Th-228 0,1 0,1 b) Calculation with dilution: C i = w c a i G V/u with G...Gauss dilution factor (dependent from distance to emission point and height)

12 Veszprém, May /12