Storage and disposal of radwaste and spent nuclear fuel

Storage and disposal of spent nuclear fuel

   
According to the Atomic Law (1.5) (the Act No. 130/1998 on the peaceful utilization of nuclear energy), all activities related to radioactive waste management have to be directed toward its safe storage. It is thus obvious that storage and disposal of radwaste provides a very significant activity.
    

   
By the term storage of radioactive waste or spent nuclear fuel, temporary location of this material in areas, premises or facilities providing its isolation, control and at the same time protection of the environment is understood. On the contrary disposal of radwaste or spent nuclear fuel represents its permanent location into repository. The definition of the repository of radwaste or spent nuclear fuel says that it is an area, premise or facility on the ground, or underground, designed for the disposal of radwaste, or of spent nuclear fuel and providing its isolation, control and protection of the environment. The term permanent disposal includes important conditions to minimize transfer of the care and responsibility for the material disposed on future generations. The principle is thus that the disposal of radwaste and spent fuel is the task for generations directly benefiting from the use of nuclear fuel.

Disposal of spent nuclear fuel and high-level active radwaste is carried out separately into special, deep underground repositories. Low-level active and medium-level active radwaste can be disposed in repositories of either surface or underground type; however, designs for deep underground disposal of this radwaste also exist.
    

    
SPENT NUCLEAR FUEL
Even if fuel is called spent fuel, in fact it still contains about 1% of fissionable isotope of uranium 235U and about 1% of fissionable isotope of plutonium 239Pu which is still a very significant amount with regard to fuel reprocessing (see section 5 "Future of radwaste"). In the fuel, however, there is still a number of other fission products (about 3%) significant with regard to radioactivity. The main fraction among these radionuclides has mainly the isotope of cesium 137Cs and the isotope of strontium 90Sr following tens years after fuel discharge from reactor. Besides this the so-called residual heat is still released from fuel. The residual heat is generated in the ongoing nuclear reactions (however, not in chain fission reaction). However, due to radioactive decay the radioactivity and production of residual heat are decreased. Even though this decrease is significant, it is necessary to provide cooling of spent nuclear fuel and its safe storage within about 50 years. The ultimate disposal of spent nuclear fuel is in a deep underground repository.    

  
STORAGE OF SPENT FUEL
Spent nuclear fuel - after its burn-up in reactor - is transported below water into a special spent fuel pool that is situated next to the reactor. The pool is filled with water, the fuel is cooled in it and is situated there within the time period of 3 to 5 years. Within this time, fuel radioactivity drops down to about half of the original value and the spent fuel can be transported in special containers into the so called interim spent fuel storage.  

   
The Interim Spent Fuel Storage is a facility making possible to store spent nuclear fuel in a safe manner for the time interval of up to 50 years. During this time, the radioactivity of the fuel drops significantly and the production of residual heat can be considered very low. The significant interim storage has been built on plant site, but can be also located outside it. During plant operation, the interim storage is being gradually filled and with regard to the time period of storage, the fuel essentially "survives" the plant itself. 
Depending on the fact whether the storage containers are cooled by water or air, two types of interim storage - dry and wet - are differentiated.    

  
In the wet way of storage of spent fuel, the storage containers are situated in water pools either directly next to the reactor, or outside the reactor hall, in a special facility equipped with the technology needed. Water provides the removal of residual heat from the spent fuel and at the same time it provides sufficient protection against radioactive radiation. The disadvantage of wet storage is the necessity to cool and clean water continually due to the generation of liquid radwaste. The significant is that the technology for the wet storage of spent nuclear fuel is more complex than for the dry one.

   
Dry storage of spent fuel is a newer method. In this case, fuel is used in special concrete constructions or in storage containers made out of either concrete or stainless steel. The cooling fluid is not water, but air, or other gas. The simplicity of methods of dry storage makes it possible to use a number of known technical solutions.  

  
For example, fuel can be situated in tubes with noble gas, which are laid vertically in nests of concrete structures with air circulation (Fort Saint Vrain in USA, or Wylfa in UK). Another possibility is to store fuel in enclosed steel baskets situated in concrete vessels cooled by air flowing through special channels (Oconee in USA, or Gentilly in Canada). The simplest is the dry storage of spent fuel in special storage containers the structure of which makes it possible to sufficiently cool the fuel by natural circulation of air. These containers are situated in light assembly halls with simple additional equipment, without the necessity of permanent presence of personnel (Dukovany), or even directly in the open area. The latter method is used for example in Canada. The dry storage of spent fuel is simpler in comparison with the wet one, with cheaper operation and essentially zero production of radwaste. On the other hand, it has very high safety and financial demands on storage containers for dry storage. 

DISPOSAL OF SPENT FUEL
For the disposal of spent nuclear fuel, only the use of deep underground repositories is applicable. Deep underground repository is a specially built or a modified underground work in a deep geological formation. In the repository, spent nuclear fuel should be laid in such a way that its protection against human sabotage actions, airplane crashes, fires, floods and other consequences of extreme climatic conditions is assured permanently. The selection of a site for deep underground repository is thus very responsible, complex and relatively long activity, and the site itself has to meet a number of strict criteria. As spent fuel should be disposed in the deep underground repository for a sufficiently long time period even with regard to geological times, it is likely that permanent information transfer about the repository and its site to future generations may not be successful. A concept was thus accepted to prevent intentional or accidental contact of future generations with the material disposed.    

  
Repositories have been built in geologically quiet areas where minor earthquakes, floods, and so on do not threat. The concept of deep underground repository contains several safety barriers against the release of radioactivity from spent nuclear fuel. The first barrier is the fixing of fuel in the internal structure of special storage container. Another barrier is the container itself with the hermeticallity estimated up to several hundreds thousands years. The containers are situated in sealing and shielding materials of the civil construction of the repository that represent another independent barrier. The ultimate barrier is the geologic formation of the deep underground repository. Suitable geological formations are stable rock formations (massive clay, or granite rocks). The stability of these formations is estimated up to tens millions years.

Deep underground repository of spent nuclear fuel has not yet been in operation anywhere, but designs have been under way in a number of countries. These activities are necessarily associated with extensive geologic works that are demanding both from financial and time points of view. It is estimated that on the survey and design of deep underground repository, more than 50% from the total costs on the construction of repository needs to be spent. The construction of repository should last for about 10 years. During this entire time period up to the closure of the repository and abandonment of the site, it is necessary to carry out hydrological, hydrogeological, geochemical, seismic and radiation monitoring, and to observe evolutionary changes in the conditions and behavior of the particular barriers against the leak of radioactivity.
    

    
In several countries intensive research and design has been carried out of deep underground repository of spent nuclear fuel and also of high-level radwaste that will be - according to expectations - disposed essentially everywhere in the same way as spent fuel. Research and designs of deep underground repository have been underway in France, Belgium, Germany, Sweden, USA, Japan, in some countries of the former Soviet Union and in a number of other countries. Not only investigation of the repository sites themselves is carried out, but - thanks to advanced computers - it is possible to model the repository, geologic formation and biosphere and to use the model for the safety assessment, performance of various deterministic and probabilistic analyses, and so on.

As examples of deep underground repositories under design, the Gorleben site (salt formation) in Germany, project KBS-3 (rock formation) in Sweden, or repository Yucca Mountain in USA (tuff - glass rock of volcanic origin) can be shown. The depth of disposal (from 300 m up to 1000 m), the capacity of the repository (thousands up to tens thousands tons of spent nuclear fuel and high-level radwaste), and the lifetime (up to millions years) are common features of these, but also of other deep underground repositories under design.