3. Results and discussion
3.1. Calculation of discharge limit for 125I
At present, there are no reference values in Spanish legislation to implement
declassification of radioactive wastes. In the case of liquid ef-
fluents, reference values for waste evacuation must be derived from
committed effective dose coefficients per unit of intake by ingestion
or inhalation for public members, which are defined in the “Reglamento
sobre Protección Sanitaria contra Radiaciones Ionizantes” (Real Decreto
783/2001, Spanish legislation). These coefficients are expressed in
Sv/Bq. Thus, from the annual effective dose limit for public members
(1 mSv) the maximum activity that can be evacuated for each radionuclide
can be calculated.
In the case of 125I, the value of committed effective dose is 5.7 ·
10−8 Sv/Bq. So, the maximum activity that can be evacuated for 125I
(Amax_I 125), considering annual effective dose for public members of
1 mSv is 1.75 · 104 (Bq). This value represents the limit of annual intake
by ingestion for 125I. From this value it can calculate the activity
limit per unit of volume (Bq/L) below which the liquid treated from
RIA wastes could be discharged. To do this, it has to be considered
the worst scenario from the point of view of exposure of public members.
In this case, this scenario would be the intake of water from the
release of the treated liquid, during a complete year. Assuming an
average consumption of water of 2.5 l per inhabitant and per day, and
1 Feed tank 7 Thermometer
2 Feeding valve 8-8' Pressure gauge
3 Discharge valve 9 Membrane module
4 Filter (45 m) 10 Regulation valve
5 Pressure pump 11 Rotameter
6 Security valve
µ
Fig. 2. Flowchart of the pilot plant used in the experiments.
Table 3
Characteristic parameters of the waste treated in each experiment.
Activity
(kBq/L)
Conductivity
(mS/cm)
pH Storage time
1st experiment (exp1) 2.8 12.6 6.66 Long (1.5 years)
2nd experiment (exp2) 240 14.3 6.73 Short (approx. 1 month)
M. Sancho et al. / Desalination 321 (2013) 110–118 113
considering a safety factor of 10%, the total volume of water ingested by
a member of public would be:
Vannual ingestion ¼ 2:5 liter.
day ·365 day.
year ·1:1 ¼ 1003:75 liter ð Þ: ð4Þ
Finally, dividing the maximum activity that can be evacuated in a
year by the annual ingestion volume, the discharge limit for 125I is:
Discharge limit ¼ A max I
125
Vannual ingestion
¼ 17:5 Bq.
L
¼ 0:0175 kBq.
L
: ð5Þ
Therefore, in order to discharge to sewer the liquid from RIA after
treatment by reverse osmosis, it must have an activity value lower
than 0.0175 kBq/L. The liquid could be discharge only if chemical
and biological characteristics were in accordance with legislation,
and always after authorization.
3.2. R