Radioactive waste and its latent danger

Introduction to the issue

  
GENERIC INTRODUCTION
The highly up to date topic of this multimedia CD for the Slovak Republic predetermines that the contained information relates primarily to nuclear power plants with VVER-440 reactors, or possibly to the A-1 nuclear power plant in Jaslovské Bohunice. In the case that the presented information will relate to power plants with another reactor type, it will be explicitly specified either in the text or in the graphical part.

  
Some waste has been produced from almost any human activity. Broad utilization of radioactive materials in nuclear power, agriculture, research, medicine etc. results in generation of radioactive waste (radwaste). The largest amount of radwaste is produced in activities associated with nuclear fuel cycle , i.e. beginning with its manufacture, operation in nuclear reactor up to the so-called nuclear fuel back-end.

WHAT IS RADIOACTIVITY?
Radioactivity is a natural or artificially caused transmutation of atom nucleus associated with the emission of radioactive radiation. We thus speak about either natural or artificial radioactivity. It is necessary to know that radioactive waste - as dealt with in this multimedia CD - is mainly related to the artificial radioactivity, i.e. human-made. In dictionaries, we can find an explanation that artificial radioactivity is decay of atom nucleus (of a nuclide) induced by an artificial transfer of energy to the nucleus in such a way that the nucleus becomes non-stable and decays usually into two smaller nuclides with coincident emission of alpha, beta or gamma radiation. On the contrary a spontaneous decay of radionuclides is called natural radioactivity.

  
Radioactivity is given in units Bq (Becquerel) with 1 Bq representing a decay per second. The measure of the impact of radioactive radiation on human being is determined by the amount of radiation energy absorbed in the unit of body weight. This parameter is called absorbed dose and its unit is Gy (Gray) with 1 Gy = 1 J/kg. The quantitative evaluation of the effects of various types of radiation on biological material is specified by the so called effective dose that represents the product of absorbed dose and dimensionless quality factor specified for the radiation type in question. The unit for the effective dose is Sv (Sievert) with the same dimension J/kg as Gray.
Here it is necessary to explain a few key concepts without which we cannot proceed further. A nuclide is a substance consisting of the same atoms specified by their proton number Z (it means by the number of protons - electrically positive charged structural elements of atom nucleus) and by nucleon number A (it means the total number of protons and neutrons - neutral structural elements of atom nucleus). Radionuclide is a radioactive nuclide, it means a nuclide with nuclei subject to radioactive transmutations.

And what is alpha, beta and gamma radiation? It is three kinds of radiation that are emitted during radioactive transmutations. Alpha particles are essentially nuclei of helium consisting of two protons and two neutrons emitted mostly during decays of heavy nuclei. Beta particles are electrons or positrons emitted during decays of atom nucleus. Gamma radiation differs from alpha and beta radiation by the fact that it is not a beam of particles, but electromagnetic radiation emitted due to various processes in atom nucleus.

  
Natural radioactive background (natural radioactivity of soil, atmosphere, water, cosmic radiation) is a component of our environment. It is necessary to protect themselves against any extra radioactive radiation, which is valid for all its sources starting with medical ones, up to nuclear reactors in power plants. Protection of the public and environment against radioactive and ionizing radiation is anchored in relevant laws in which usually strict limits for personal doses are specified.

WHAT IS RADIOACTIVE WASTE?
There are extremely many important sectors that commonly work with radioactivity and radioactive materials. We speak about nuclear power, industry, research, health services and a number of others. It is valid generally that various sorts of waste are generated in almost any human activity. The particular character of the above-mentioned activities associated with radioactivity is that radioactive waste is generated in them, i.e., waste contaminated by radionuclides. In comparison with wastes of other types, such as from conventional power sector, the volume of radioactive waste is relatively small. The management of radioactive waste is governed by strict rules and is subject to regulation. Radwaste still has one significant property - its danger gradually decreases in time as its radioactivity is reduced by natural decay.
Radioactive waste thus represents a non-usable material that cannot be brought into the environment due to the content of radionuclides in it or due to its contamination by radionuclides. Material containing radionuclides in the amount not exceeding the appropriate limits specified by regulations is not considered as radioactive waste and can be released to the environment.
   

  
Radioactive waste can be categorized and described from a number of points. The most significant of them is the breakdown according to its state, the level of activity, the half-time of transformation of prevailing radionuclides, radwaste origin, technology for processing, and so on. Let us have a look what sorts of radioactive waste exist.

Breakdown of radwaste according to its state:

• solid radwaste,
• liquid radwaste,
• gaseous radwaste.

Breakdown of solid radwaste according to possible technologies of its processing:

• combustible radwaste,

• compressible and non-combustible radwaste,

• others.

  
Breakdown of radwaste according to the level of activity:

• low-level radwaste,
• medium-level radwaste,
• high-level radwaste.

Breakdown of radwaste according to the half-time of transmutation of prevailing radionuclides:

• radwaste with short-lived radionuclides,
• radwaste with long-lived radionuclides.

Very important, especially with regard to the permanent storage of radioactive waste is its breakdown according to the level of activity, half time of decay of predominant nuclides and production of residual heat:

  
Category I - high level activity, long-lived sources, high production of heat (permanent storage in deep underground repositories),
Category II - medium level activity, long-lived sources, small production of heat (permanent storage in deep underground repositories),
Category III - low-level activity, long-lived sources, insignificant production of heat (permanent storage in deep underground repositories),
Category IV - medium-level activity, short-lived sources, low production of heat (permanent storage in surface and sub-surface repositories),
Category V - low-level activity, short-lived sources, insignificant production of heat (permanent storage in surface or under-surface repositories).

Note:

Regulation of the Nuclear Regulatory Authority (ÚJD) of the Slovak Republic gives the following categorization of radwaste:

a) Transient radioactive waste, the activity of which drops during its storage below the limiting value making possible its release into the environment,2)
b) Low-level active waste and medium-level active waste, the activity of which is higher than the limiting value making possible its release into the environment and its residual heat released is lower than 2 kW/m3:
1. Short-lived radioactive waste that meets the acceptance criteria for surface repositories of radioactive waste after conditioning and the average mass activity of alpha nuclei is lower than 400 Bq/g,
2. Long-lived radioactive waste that fails to meet the limits and acceptance criteria for surface repositories of radioactive waste after conditioning or the average mass activity of alpha nuclei is higher than or equal to 400 Bq/g,
c) High-level waste the produced residual heat of which is higher than or equal to 2 kW/m3.

  
Breakdown of radwaste according to its origin:

• institutional radwaste,
• radwaste from nuclear power plants,
• radwaste from other sources (for example, from military ones).

  
Spent nuclear fuel is sometimes considered as radwaste from nuclear power plants. Spent fuel after its reprocessing becomes a valuable source of raw materials as it can be used for the production of new nuclear fuel. A number of elements known from the periodic table are produced by fission reactions in fuel in nuclear power plants. Spent fuel is significant mainly from the point of view that it contains more than 90% of the total activity of all waste, even though its production from nuclear reactors is only about 1% of the volume of all radwaste worldwide.
Radwaste from other sources, such as from the production of nuclear arms, does not occur in the Slovak Republic and that is why this multimedia CD will not deal with it.

  
According to radioactive waste generation, we categorize waste into four groups. Firstly it is the radwaste generated as waste during the production of another, mostly radioactive material. As an example, the generation of various sorts of radwaste during the production of nuclear fuel, starting with uranium ore mining, up to the technology process of fuel production can be shown. The second group is represented by originally non-radioactive materials that were exposed to radioactive radiation in the course of operation of a certain facility and activated to such level, that we can designate as radwaste. The third group consists of originally non-radioactive materials that were contaminated by radionuclides due to intensive contact with radioactive fluid and thus become waste. The last group is represented by originally radioactive materials that become waste only after the completion of their operation (e.g., sources).

  
Storage and permanent disposal of radioactive waste represents the last but one and ultimate phase of the radwaste management. Storage is a temporary location of radwaste or spent nuclear fuel in the areas or premises of facilities making possible waste isolation and protection of the environment. By disposal of radioactive waste or spent nuclear fuel, its permanent location into repositories of various types (surface, below surface and deep underground repositories) is understood, with the objective to prevent unauthorized release of radioactive substances into the environment during a sufficiently long time interval (up to millions years for high-level radwaste).
In order to be complete, it is necessary to mention another way of radwaste management. By an artificial transmutation of long-lived radionuclides to short-lived ones (by the so-called transmutation) it is possible to significantly shorten the time period during which it is possible to consider the material as a radwaste. However, it is necessary to note that this way is now only at the beginning stage of practical utilization and only future will show whether it finds a wide application.