Lesson 2 – Using the Species Element in the Radionuclide Transport Module
We first discussed the Species element in Unit 5, Lesson 4. We noted that if you were using the CT Module, the Species element would look like this:
However, if you are using the RT Module, the Species element looks like this:
The items outlined in red are present when using the RT Module, but not the CT Module.
When using the CT Module, if you double-click (to edit an individual species), the dialog will look like this:
However, if you are using the RT Module, the dialog looks like this:
The items outlined in red are editable when using the RT Module, but are grayed out when using the CT Module.
So there are three items in the Species element dialog (or the dialog for editing a single species) that are available in the RT Module, but not the CT Module:
- The Radioactive checkbox is available to indicate that the species is radioactive. We will discuss the significance of this below.
- You can specify Daughter products (i.e., reaction products), and the Stoichimetry for those daughters. These are discussed in the next Lesson.
- A number of fields are available to allow you to automatically populate the Species element with species from an extensive database of radionuclide decay data (rather than editing the species manually). These are discussed in Lesson 5.
You will note that the Isotope check box is available in both the CT Module and the RT Module. It is rarely necessary when using the CT Module, but often required when using the RT Module. We will discuss this topic in Lesson 7.
In the previous Units, we have entered and displayed inventories, concentrations, quantities and release rates for species in terms of mass. However, when simulating radioactive species, you will often want to enter and/or display these in terms of activity. As used here, the activity of a radioactive species represents the number of disintegrations per unit time (and should not be confused with the thermodynamic activity of a species). Activity (disintegrations per unit time) is measured in units of Becquerels (Bq) or Curies (Ci). So this means you may want to enter or display inventories, concentrations, quantities and release rates for species in terms of Ci or Bq instead of mass (e.g., grams).
To facilitate this, GoldSim allows you to specify that a species is radioactive. If the Radioactive checkbox is checked, GoldSim computes the specific activity for that species. The specific activity has units of activity per unit mass (e.g., Bq/g or Ci/g) and is computed as:
(Avogadro's Number)(Decay Rate) / (Atomic Weight)
Note: As mentioned when we first introduce the Species element, what we refer to in GoldSim as the Molecular (or Atomic) Weight is actually the molar mass (the mass of one mole of the substance). Strictly, the term “molecular (or atomic) weight” refers to the mass of a single molecule or atom (typically expressed in terms of atomic mass units).
When you specify that a species is Radioactive, GoldSim computes and displays the specific activity:
A drop-down list allows you to specify whether to display these here as Bq/g or Ci/g.
More importantly, the Species element has outputs that are vectors of the specific activities and molecular (or atomic) weight for each of the species:
Note that, by definition, the specific activity of a non-radioactive species is assumed to be zero.
The various Contaminant Transport Module elements that we have been discussing in the previous Units all require their inputs to be in terms of mass (and display their outputs in terms of mass). The specific activity vector allows you to easily convert an input or output vector of mass to activity (or activity to mass):
In this example, the key inputs for the species used to compute the specific activity, the Molecular (or Atomic) Weight and the Half-life, were entered manually. But as we will discuss in Lesson 5, The RT Module includes an extensive database of radionuclide decay data that you can use to automatically populate the Species element with this data (as well as their daughter products).
As noted previously, one of the key features of the RT Module is the ability to define daughter products for a species when it decays (in the example above, we manually specified that U238 decayed to U234). We will discuss this in detail in the next Lesson.