1.1 General
This Safety Guide describes and supplements HAF 101 "Code of Practice on Safety in Nuclear Power Plant Siting" (hereinafter referred to as "the Code").
This is only for guidance, so in the practical works, specific methods and procedures shall be selected according to the problems difficulties and complexity as well as the feasibility of engineering measures.
Radioactive materials may directly pollute the ground water once they enter underground due to the accidental release. The ground water may also by indirectly polluted by radioactive material which is released to the atmosphere or surface water due to the release. The radioactive material will enter into the ground water in both manners. In either case, the contaminated water may flow at the water-intaking point so that the public is harmed by the radiation.
1.2 Scope
This Guide provides guidance and method for determination of concentration of radioactive nuclide in the ground water due to the release of radioactive material by nuclear power plant as imagined.
This Guide highlights methods of radioactive material in ground water and methods of evaluating its radionuclide migration in all kinds of hydrogeological units.
The radionuclide migration in the underground depends on mainstream ground water movement (transportation), diffusion of pollution peak (hydrological diffusion) and detention and release of radionuclide in solid phase (alternative distribution).
In any evaluation of consequence when the radioactive material is released into the ground water by nuclear power plant, the following two points are the most important: concentration of radioactive material at the nearest water-intaking point to the area and required time for radioactive material arriving at the point. The variable value may be estimated by description of physical or mathematical model for radioactive material action in the ground water.
For evaluating hydrogeological features at a plant, make a careful study of all the relevant parameters to estimate the consequence once the release happens at the water consuming point; if the consequence due to the hypothetical accident release is likely to be unacceptable and cannot be solved by proper project methods, the site must be judge to be unacceptable, which is the general requirement.
Some suggestions are given in this Guide with regards to the items below:
(1) Collect and investigate complete data information on radioactive material by accidental release in the movement with ground water at different stages for nuclear power plant siting;
(2) Select proper mathematical model or physical model for hydrological diffusion and two phase distribution of radioactive material;
(3) Formulate proper surveillance outline.
This Guide briefly discusses the model validity.
This Guide does not discuss the source of hypothetical accident release because such source depends on sort and design of nuclear power plants under consideration. Similarly, this Guide does not discuss the direct and indirect approaches for the radioactive material leaked into the ground water. What shall be recognized is that the complexity required for investigation depends on choice of sources and site conditions under consideration.
See Annex I for consideration about availability of ground water and guidance for study and investigation thereof to evaluate the suitability of an aquifer as water supply source of safety-related system.
The ground conditions for nuclear power plant are detailed in Safety Guide HAD 101/12 "Ground Safety Problems of Nuclear Power Plant" in terms of hydrological geology issues.
1.3 Hydrogeological Features
The hydrogeological features are characterized mainly by hydrological features, diffusion and retention characteristics of hydrogeological units in hydrological geology system. For example, hydrological conductivity values differ greatly from one site to another in different hydrogeological units, even a difference of 1010 times is possible. Therefore, the radioactive material released at some place may be retained within a range near to the nuclear power plant (hundreds of meters likely) for a long term.
In terms of hydrological geology, there are no accurate quantitative criteria to judge whether the site is acceptable in overall. In extreme cases, it is generally explicit between acceptability and irreceivability. However, the cases situated between the two will not be clear. In this case, the methods stated in this Guide must be used to evaluate different conditions one by one. The following items shall be considered:
(1) Source term (construction, geometrical shape and other features related to the released substance);
(2) Nearest position for getting ground water;
(3) Main points of ground water drained to surface water body;
(4) Depth of regional and local water level;
(5) Flow direction and hydrological gradient necessary to determine the manner and time of ground water into the environment;
(6) Minimum range from the site to main regional aquifer and its supply area;
(7) Ground water-involved human activities with an influence on the site.
The following two extreme cases describe the roles of hydrogeological factors played in the site suitability in the view of pollutants movement.
The first example has very favorable conditions:
——Water utilization: important ground water intaking point is available near to the nuclear power plant.
——Ground water: water level is at a proper depth under the ground surface; ground water moves at a low speed (several centimeters a day); the phreatic aquifer is not connected with other aquifers far away from the site.
——Subsurface geology: under the site, there is a consolidated, uniform and competent formation with many highly adsorbed minerals; the unsaturated zone is featured by low porosity and medium- and low-permeability rate.
The second example has very unfavorable conditions and more detailed hydrogeological survey and analyses are needed:
——Water utilization: ground water nearby nuclear power plant is used widely if no other water sources are available.
——Ground water: high water level is under the nuclear power plant (several meters from the ground surface) and the ground water flows at a high speed (several meters a day).
——Subsurface geology: porous pervious strata under the site has poor retention capacity for radionuclide; large fracture and crack are seen on the rock under the porous surficial deposits.
Many site features related to the hydrological geology may be affected by construction and installation of nuclear power plant and possible consequences arising from such a change shall be evaluated.
If the radioactive surface water is likely to pollute the ground water, the hydrological geology located between related surface water body and ground water intaking point shall be surveyed.
1.4 Hydrogeological Survey
1.4.1 Site survey stage
At the beginning of site survey, the hydrogeological survey mainly is deskwork, including collection of useful data related to the hydrological geology at the area and identification of main water users. If the compared sites are appraised, and then reduced to some number (candidate sites), the diffusion characteristic of sites may be estimated in the rough for further screening. See Chapter 2 for the data acquisition and analyses detail at the stage, and Safety Guide HAD 191/07 "Site Survey for Nuclear Power Plant" for the site survey methods.
1.4.2 Site evaluation stage
The hydrological geology shall be studied in details at site evaluation stage. See Chapter 3 for the collected data and investigation. In this stage, the model for movement of radioactive material in the ground water shall be proposed and the diffusion of radionuclide shall be evaluated. The selection of proper models is detailed in Chapter 4.
1.4.3 Pre-operational stage
After the site evaluation, the pre-operation research shall be carried out. The laboratory and field study shall be continued to modify the models suggested at site evaluation stage so that they are suitable for specific end use, such as emergency plan. The development of hydrological geology surveillance outline shall be considered at this stage and during the operation of nuclear power plant. The details are described in Chapter 5.
1 Introduction 1
1.1 General 1
1.2 Scope 1
1.3 Hydrogeological Features 2
1.4 Hydrogeological Survey 3
2 Site Survey Stage 3
2.1 General 4
2.2 Data Collection 4
3 Site Evaluation Stage 5
3.1 General 5
3.2 Original Parameters for Release of Radioactive Materials 6
3.3 Regional Hydrogeological Information 6
3.4 Regional Study and Survey 7
3.5 Survey and Information Collection of Relevant Hydrological Geology System 9
3.6 Water Utilization 12
3.7 Investigation Outline 12
4 Establishment of Diffusion and Retention Model for Radionuclide in Ground Water 15
4.1 General 15
4.2 Model Selection 16
4.3 Verification of Model Validity 19
5 Ground Water Surveillance Outline 20
5.1 Objective 20
5.2 Methods 20
5.3 Monitoring Programme 21
Appendix A Establishment of Model for Radionuclide Migration in Ground Water 24
Annex I Consideration about Availability of Ground Water 34
Annex II Analytic Solution of Advection and Hydrological Diffusion Equations 36
Annex III Accuracy and Reliability of Hydrogeological Survey 41
Annex IV Examples of Measurement Methods and Frequency in the Surveillance Outline 43
