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Advisory Council on the Environment

Environmental Performance of Daya Bay

 

(ACE Paper 47/98)
for information

1. An Introduction to Guangdong Daya Bay Nuclear Power Station

 

The Guangdong Daya Bay Nuclear Power Station (GNPS) is operated by Guangdong Nuclear Joint Venture Company (GNPJVC), which is owned by Guangdong Nuclear Investment Company (75%) and Hong Kong Nuclear Investment Company (25%).

 

GNPS, commonly known as the Daya Bay Nuclear Power Station, is located at Dakeng on the coast of Daya Bay and is about 45 km from Shenzhen and 50 km from Tsimshatsui, Hong Kong. It is equipped with two Pressurised Water Reactor units each rated at 984 MW. Reactor equipment was supplied by France and conventional island equipment was supplied by the United Kingdom.

 

Construction of the station began in 1987. It was completed in 1994 with Unit 1 entering commercial operation in February and Unit 2 in May in the same year.

 

The station sent out 11.8 billion units of electricity in 1997. Hong Kong Nuclear Investment Company, wholly owned by CLP Holdings Company, receives about 70% of the electricity output of GNPS, corresponding to about 20% of the electricity consumption of Hong Kong. The remaining 30% of the output of GNPS is consumed by Guangdong, meeting about 4% of the requirement of the province.

 

The production and release of small quantities of radioactive products is a feature in the normal operation of a nuclear power station. These radioactive products can be classified as gaseous or liquid release into the environment, or solid waste that needed to be stored outside the station boundaries in a repository site. The ability of a station to control the discharge of this waste is the key measure of its environmental performance. The smaller amount of gaseous and liquid discharges into the environment and the less space taken up for the permanent disposal of solid waste, the lower is the impact on the environment of the station.

 

There are regulatory limits for the quantity of radiological releases from a nuclear power station, and they are formulated based on international criteria. The regulatory limits for GNPS have made reference and are comparable to those for established utilities in the West.

 

The station has been operating smoothly since overcoming some initial technical problems after its commercial operation in 1994. Its radiological discharge into the environment is well within the criteria set by national monitoring authorities.

2. Origin of Radioactive Waste

 

A nuclear power station uses nuclear fuel to produce heat which in turn produces electricity. The heat produced in the nuclear fuel is taken away by cooling water in the primary cooling system. This system transfers the heat to a secondary cooling system to raise steam to drive a turbine-generator to produce electricity.

 

During nuclear heat production, the contents of the nuclear fuel are transformed through nuclear fission process into a mixture of substances, all of which are radioactive, and nuclear radiation is produced. The radiation may turn a very small quantity of material near the nuclear fuel radioactive. This radioactive material may be picked up by the cooling water circulating in the primary cooling system. It is necessary to operate a process of continuous purification of the primary cooling water in order to control this amount of radioactivity. The purpose is to minimise any unwarranted radioactivity release to the environment should an accident occur, and to reduce the radiation exposure of the personnel working at the station.

 

No purification process is completely efficient and at Daya Bay, there remains a very small quantity of gases and liquids containing radioactive products that will not be captured by the purification process. These gases and liquids need to be released to the environment, as for all nuclear power stations. These releases are governed by stringent regulations.

3. Radioactivity release during normal operation

3.1 Gaseous release

 

Contaminated gases are passed through treatment systems installed with filters and iodine absorbers to reduce radioactivity. They are then kept in storage tanks at the station for typically several weeks to take advantage of the fact that the radioactivity of this gaseous waste will reduce naturally with time.

 

This gaseous waste contains mostly noble gases, halogens and aerosols. There are regulations to cover the concentration of the radioactivity in the release and the annual quantity of release. Stringent checks are carried out to ensure that the gaseous waste meets statutory requirements, before it is released with the exhaust ventilating air of the station through the ventilation stack, where radioactivity is monitored and the release can be stopped if necessary.

 

Over its five years of operation, Daya Bay has been reducing its gaseous discharge into the environment. See table below. Daya Bay has plans to reduce further this release and thereby its environmental impact.

Percentage of annual gaseous release to annual regulatory limits for GNPS

  1994 1995 1996 1997
Noble gases
Aerosol & halogens
1.99
1.12
7.04
1.90
3.82
0.60
2.72
0.30

3.2 Liquid release

 

Radioactive liquids are filtered and further processed in an ion-exchanger or an evaporator. The liquids are held in tanks for typically several weeks for its radioactivity to decrease with time and will be given further treatment if necessary. When their contents meet statutory requirements, they will be discharged into the sea after diluting with the cooling sea water for the power station in the sea water discharge channel of the station.

 

The radioactivity of the discharge is monitored. There are regulations covering the concentration of the radioactivity in the release and the annual quantity of release.

 

Daya Bay has been able to reduce the total gamma activity (except tritium) in its liquid discharge over the years. See table below. GNPJVC management is taking measures to improve station operation to limit this figure to 1.0% by 2002.

 

Tritium is produced during normal reactor operation mainly by the collision of neutrons with boron (for reactor power control) and lithium (found in a chemical compound to regulate water chemistry) in soluble chemicals in the primary cooling water. It is a radionuclide of hydrogen and has been widely used as a weak light source for timepiece or emergency exit illumination. This radionuclide has a low radio-toxicity and does not accumulate in the human body if ingested. For these reasons, it is given a higher annual regulatory limit for release than other radioactive substances and the quantities produced at Daya Bay is typical for normal operation.

Percentage of annual liquid release to annual regulatory limits for GNPS

 

  1994 1995 1996 1997
Tritium
Total gamma activity except Tritium
39.9
12.7
18.17
3.85
39.7
1.46
51.3
1.61

4 Intermediate/Low Level Radioactive Solid Waste

4.1 Waste production and packaging

 

The purification for the primary circuit of the reactor system collects radioactive substance as sludge, resin and in filter cartridges. This substance is classified as Intermediate Level Waste (ILW) which is solidified and packed in concrete drums. A concrete drum is over 1 m in diameter and height, having 2 cubic metres of total volume and 5 tonnes in weight. There are several drum thicknesses, from 0.15 m to 0.4 m to suit contents having various degree of radioactivity.

 

It is also necessary for the station to replace spent nuclear fuel with new fuel once a year, and to carry out regular maintenance on its systems and components, for which most of the activities will coincide with the fuel replacement period. Because the maintenance work will cover those systems exposed to radiation, a certain amount of radioactive waste consisting mainly of used tools and protective clothing is generated.

 

This type of waste is classified as Low Level Waste (LLW) and is packed in standard 55 gallon metal drums. The metal drum measures 0.6 m in diameter and 0.9 m high, having 0.2 cubic metre of total volume and typically 0.2 tonne in weight when full.

 

GNPS produced about 200 cubic metres of these wastes per year, packed and ready for disposal.

 

Spent nuclear fuel is at present stored inside the nuclear island which is built to withstand conceivable hazards. Spent fuel is not classified as a waste since Chinese policy is to reprocess the spent fuel to extract useful constituents for further use. Reprocessing for spent nuclear fuel will be carried out by an external service company for Daya Bay though it will not take place in the near future.

4.2 Disposal of solid waste

 

GNPS produced about 750 cubic metres of solid waste up to 1997 and this waste is kept at the GNPS premises in an interim storage building having a 2,500 cubic metre capacity. The waste will be deliver to an external repository for permanent disposal.

 

A repository for this waste is now being built several kilometres away from Daya Bay using a repository method that has been in use in France since 1969. The repository is scheduled to enter trial operation next year, to be operated by a subsidiary of China National Nuclear Corporation. It is understood that the repository will be operated based on international practices and monitored by the regulatory authorities to similar standards.

5 Environmental Monitoring at Daya Bay

5.1 Monitoring authorities

 

The duties of the National Environmental Protection Administration (NEPA) include environmental and radiation protection around nuclear installations. This covers specifying the limits to radiation dosage of the public and the radiological releases of these installations and carrying out environmental monitoring. Independent monitoring of radiological releases at Daya Bay is carried out by the regional environmental bureau.

 

National Nuclear Safety Administration (NNSA) is a regulatory body within NEPA for the safety of nuclear installations. Its duties include supervising the safe operation of Daya Bay. NNSA has a regional office in Guangdong for on-site activities including safety inspection.

5.2 Monitoring programme

 

A complete study was made before the commissioning of Daya Bay to provide a reference position for any change in radiation level in the environment during station operation. The study included:

Food chain study. This investigated the production and consumption of foodstuff within 10 km on land, and within 20 km at sea from the station.

Ocean ecology study. This studied the hydrology, marine biology and fish farming near the station.

Background radiation study. This studied the normal (or ambient) background radiation level of the environment around the station, as well as the natural radiation level of sea water, marine life, sediment, land life, soil, air and water.

 

Daya Bay set up the following stations for monitoring the environment after station operation had commenced.

3 atmospheric monitoring stations. These stations are located 1 km from GNPS, sampling ambient radiation and meteorological conditions.

4 environmental monitoring stations. These stations are built 5 km from GNPS, sampling gamma radiation near the ground.

 

Apart from the above sampling, Daya Bay collects another a thousand odd environmental samples per year at specific locations and regular intervals (from daily to yearly) according to its monitoring programme, to measure radiation levels and compare with the results obtained before station operation.

 

Results are independently verified by NEPA. So far they indicated no significant increase in radioactivity around Daya Bay.

 

Measurements have also been made on the impact of thermal discharge in the sea around Daya Bay. The effect are found to be not significant, with an increase in sea water temperature within 1oC over a sea surface area of around 5 square kilometres and no significant change in the concentration of chemicals, oxygen and bacteria.

5.3 Impact on the local inhabitants

 

NEPA requires that a single nuclear power station should produce no more than a radiation dosage of 0.25 mSv/year for a member of the public. The figure is below the annual regulatory limit of 5 mSv/year for the public from all artificial radiation sources. A conservative assessment indicated that even with radiological releases reaching their regulatory limits at the station, the most exposed member of the public will receive no more than 2.8% of the permitted radiation dose for the public (5 mSv/year). See table below.

Dosage of the most exposed local inhabitants as a percentage of their annual limits

  Liquid discharge Gaseous discharge
External exposure & Inhalation
Foodstuff ingestion
Total
 
1.8%
1.0%
2.8%
 
0.6%
0.3%
0.9%
 
Type of local inhabitants Fisherman Farmer

 

Bearing in mind the actual releases of Daya Bay are no higher than 3% of their annual limits, the effective dose for the most exposed member of the public should be no higher than 0.005 mSv/year (0.1% of the annual dose limit of 5 mSv/year). This small dosage is equivalent to watching around 1/2 hour of TV per day.

6 Conclusions

  • The environmental releases of GNPS have been small in quantities and have been decreasing. They have also been well within the criteria set by national monitoring authorities.

     

  • Monitoring results have indicated that the effects of these releases on the environment are small. This has been supported by independent monitoring by the regulatory authorities.

     

  • Conservative estimates indicate that the impact of these releases on the most exposed population is small and remains substantially within well-founded regulatory limits.

Hong Kong Nuclear Investment Company
November 1998


 

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