RD&D-Programme 2004 - SKB

16 Canister as barrier
SKB’s reference canister consists of an inner container of cast iron and an outer shell of copper.
The cast iron insert provides mechanical stability while the copper shell protects against
corrosion in the repository environment. The technical development of equipment and methods
for fabricating, sealing and testing the canister is described in Chapters 5–7, while design of the
encapsulation plant is dealt with in Chapter 8.
The canister is an important barrier to the escape of radionuclides. This chapter describes the
research being conducted by SKB to examine the long-term safety of the canister.
16.1 Initial state of canister
By initial state of the copper canister and the cast iron insert is meant the state of the canister at
the time of deposition. This state is described in the safety assessment with the aid of a set of
variables.
16.1.1 Variables
No changes have occurred with regard to the variables for the copper canister and the cast iron
insert. It is the same set of variables as that presented in RD&D 2001, see Table 16-1.
Table 16-1. Variables for copper canister and cast iron insert.
Variable
Geometry
Radiation intensity
Temperature
Mechanical stresses
Material composition
Definition
Geometric dimensions of the canister components. This also includes a description of
possible fabrication defects (e.g. in welding).
Intensity of α, β, γ and neutron radiation as a function of time and space in the canister
components.
Temperature as a function of time and space in the canister components.
Mechanical stresses as a function of time and space in the canister components.
Material composition of the canister components
16.1.2 Geometry
The variable “canister geometry”, which is used in the safety assessment, includes not only the
geometric shape of the canister, but also possible initial defects in the seal.
Conclusions in RD&D 2001 and its review
In RD&D-Programme 2001, SKB describes a plan for the development of acceptance criteria
and says that experience from trial welding will provide information on the types and frequency
of defects that can arise in connection with welding. SKI concurs that this is the right way to go,
but wishes to emphasize that acceptance criteria must be established for permissible defects in
the copper shell and its welds, as well as the cast inserts. Consequence analyses will show what
happens if there are more and/or larger defects than allowed by the acceptance criteria. The
effects of minor defects should also be analyzed to assess the sensitivity of the system, as is also
pointed out by SSI. When it comes to future safety assessments, SKI says that there must be a
strong link between input data to the safety assessment, acceptance criteria and test statistics
from nondestructive testing. SKB says that the work will be conducted jointly with representatives
from safety assessment, research and technology development, which SKI views very
positively.
RD&D-Programme 2004 183