Occupational Intakes of Radionuclides Part 1 - ICRP

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DRAFT REPORT FOR CONSULTATION
(285) The probability of exposure and the likely time pattern of intake are often
dependent on the tasks being performed. For example, exposures may be chronic for
workers in the mining industry. On the other hand, workers in nuclear power plants
are not expected to receive significant intakes except in the rare event of an accident.
(286) The required frequency of measurements in a routine monitoring programme
depends upon the retention and excretion of the radionuclide and the sensitivity of the
measurement techniques available. Selection of monitoring intervals should also take
into account the probability of occurrence of an intake; where the risk of intake is
high, the frequency of monitoring may need to be increased to reduce the uncertainty
in the time of intake. The measurement technique should be selected so that
uncertainties in the measured value are small in relation to the major sources of
uncertainty.
(287) For situations where an acute exposure situation may be expected, Publication
78 (ICRP, 1997b) provides a simple rule that limits the possible error on the estimate
of intake arising from the unknown time of exposure. Monitoring intervals are
selected so that any underestimation introduced by the unknown time of intake is no
more than a factor of three. In practice, this is a maximum underestimate because the
actual distribution of the exposure in time is unknown. The error in assessed intake
can take on both positive and negative values, depending on the probability
distribution of the exposure over the monitoring interval, with the result that the mean
value of any underestimate is less than a factor of three. However, if a substantial part
of the intake occurs just before sampling or measurement, the intake could be
overestimated by more than a factor of three. This may be particularly important in
the case of excreta monitoring, since the fraction excreted each day may change
rapidly with time in the period immediately following the intake.
(288) An alternative, graphical approach has been developed by Stradling et al
(2004), which takes into account uncertainties in material-specific parameters such as
those describing absorption and particle size distribution, as well as time of intake.
Information on the minimum detectable amount for a particular measurement
technique is used to determine a monitoring interval appropriate for the dose level of
interest.
(289) When chronic exposures are expected, the monitoring programme should be
chosen taking into consideration that the amount present in the body and in excreta
will increase in time until equilibrium is reached. In each monitoring interval,
measurements will reflect the activity accumulated in body organs as a result of
chronic intakes received in earlier years. The monitoring programme should take into
account the workers’ assignment of duties. For certain radionuclides there may be a
significant difference between measurements taken before and after the weekend, or
before and after an absence from work.
5.3.2 Confirmatory Monitoring
(290) One method of confirming that working conditions are satisfactory (typically
for annual effective doses less than 1 mSv) is to carry out occasional individual
monitoring. Unexpected findings would give grounds for further investigation.
Confirmatory monitoring of this type is most useful for those radionuclides that are
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