Discharge Monitoring

The release of radioactive materials from nuclear power plants is routinely monitored. Under normal operating conditions such releases are very small and are difficult to measure against background levels, even with modern instruments.

Since the early days of nuclear power the routes for radioactive pathways to man have become much better understood. The sensitivity of instruments and measurement techniques has markedly improved. The same is true for the techniques for analysing results.

With improved monitoring techniques have come more rigorous monitoring standards imposed by nuclear regulators. In the early days, the available instruments were in some cases, not able to distinguish station releases from the background and standards were less rigorous, reflecting this limitation. For example, the Federal Standard in the US was 5 mGy per year (Mounfield, 1991) up until 1971. After this time, more stringent limits were introduced at site boundaries; the limits were set down to 0.1 mGy per year for external gamma radiation from nuclear effluents.

For monitoring higher levels, passive devices are used, e. g. thermoluminescent or film dosimeters. For greater sensitivity, high-pressure ionisation chambers are available for measuring gamma-emitting radionuclides such as Iodine-131. Gamma ray spectroscopy using multichannel analysers provides a capability to analyse large numbers of environmental samples.

Routes for release that are monitored include, e. g. ventilation stacks, routes for discharging wastewater through cooling waters to the sea, rivers, etc. Data are supplied

to national radiological protection bodies and also international bodies, e. g. United Nations Committee on the Effects of Atomic Radiation, UNSCEAR (U. S. Regulatory Commission, 1980).

Releases to the environment during normal operation are now set to very low limits. For example, Swedish Regulations (U. S. Regulatory Commission, 1980) require that any releases to the environment during normal operation must result in a dose equivalent of less than 0.1 milli-Sievert (mSv) per year to nearby residents. Releases to the environment are a subject of continuing interest to the IAEA (IAEA/NSR/2002, 2003). The IAEA is promoting an international conference on the protection of the environment from the effects of ionising radiation to be held in Sweden in 2003 (International Conference, 2003). This will aim to achieve an international consensus to form the basis of the agency safety standards in this field.

In addition to offsite releases, onsite releases and exposures are also monitored. For workers in the nuclear industry, there has been a general reduction in operational exposures since the start of nuclear power plant operation. In the UK (Hughes, 1996) and elsewhere, this has been driven by changes in regulatory requirements including the as low as reasonably achievable (ALARA) principle and more restrictive dose limits. It has also been driven by the support of industry and the availability of improved radiation protection methods. Table 3.3 shows the reduction in UK worker dose and discharges during the early 1990s. Figure 3.3 shows the reducing trend of maximum individual dose at the UK Aldermaston establishment during the 1990s.

The 1990 International Commission on Radiological Protection (ICRP) recommended the main limit as an annual average of 20 mSv over a five-year period, not exceeding 50 mSv in any single year. Annual dose limits are currently 20 mSv per year with a lifetime dose of a few hundred mSvs. In the UK, now only a handful of workers receive doses over 15 mSv per year (Hughes, 1996). In addition to individual dose, measures of

Table 3.3. Worker dose and discharges

Dose measure

1990

1996

UK classified workers receiving more than 10mSv (excluding miners) (HSE, 1998)

~ 1500

~ 100

UK annual collective Dose for classified

persons (excluding miners) receiving more than 0.1 mSv (man Sieverts) (HSE, 1998)

~ 80

~ 35

Typical annual UK individual dose from civil discharges (mSv)

~ 35

~1

Collective UK dose truncated to 500 years from UK civil discharges (mSv) (Bexon, 2000)

~ 128

18

image026

Figure 3.3. Maximum individual dose at AWE Aldermaston. Source: Sallit (2002).

occupational exposure at a particular power station are also used, by considering the total dose of all personnel who have received a measurable exposure.

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