Lesson 13 - Unit 11 Summary

This Unit described the use of a special element in the RT Module called a Source. 

In previous Units we described how you could introduce mass into a system by directly specifying an initial mass and/or a rate of addition to one or more pathways in the system.  For many kinds of systems, such an approach is all you will need.

Some systems, however, particularly those involving the simulation of proposed or existing waste disposal facilities, have as part of their design engineered packages or containers in which the waste is placed prior to disposal. The rate at which the disposed contaminant mass is released from such facilities can be very complex.  This is because in order to do so it is often necessary to directly model the mechanisms controlling these releases, such as failure of barriers surrounding the disposed waste and/or degradation of a matrix (e.g., grout) in which the contaminants are encapsulated. The Source element can be used to simulate these kind of complex release mechanisms.

In general, if you can't accurately describe the introduction of mass into your system by using an initial or boundary condition in a pathway, you should use a Source element. This will often be the case if 1) your source term involves barriers that fail over time; 2) your source term includes one or more matrix materials that release species mass as they degrade; and/or 3) the source term is modified by reactions (e.g., decay and ingrowth) prior to and during release. If any of these processes are active, describing the source term using simple algebraic equations becomes difficult, if not impossible (since in these situations, representing the source term accurately involves the solution of integrals and/or coupled systems of equations). These processes, however, can be explicitly represented using a Source element.

In this Unit, we discussed the following:

• A Source element is a complex element that is also a specialized type of Container (i.e., it can contain other elements).
• Sources can simulate containment barriers that must be breached before any contaminants can be released.  We described how loss of containment (failure of barriers) can be simulated.
• A Source can represent a single container or package (e.g., a large vault) or a collection of packages (e.g., a large number of drums). 
• Source barriers can fail according to very complex failure distributions, can be failed by sudden (random) events, and can have multiple barriers (i.e., a package within a package). 
• Contaminant mass can be encapsulated within a waste matrix material (such as grout, glass or in the case of used nuclear fuel, uranium dioxide) such that even after the barrier in which it is contained fails, no release may occur until this matrix material degrades. We described how the waste degradation process can be simulated.
• Once mass in a Source is exposed (i.e., accessible to the environment due to failure of barriers and/or degradation of matrix material), it must be transported out of and away from the Source.  We described how you can use Associated Cells inside a Source element to model these processes. By doing so, we can represent all of the complex transport processes we have discussed in previous Units inside a Source.

We worked through a number of Examples and Exercises, and these illustrated that the combination of mass exposure (package failure and matrix degradation), mass transport out of the Source, and decay and ingrowth within the Source can result in extremely complex release rates that would be difficult, if not impossible, to represent using a simple equation. It is precisely these kinds of source terms that require use of the Source element.