ML19263F409
| ML19263F409 | |
| Person / Time | |
|---|---|
| Issue date: | 12/17/1979 |
| From: | Kim K NRC OFFICE OF NUCLEAR REGULATORY RESEARCH (RES) |
| To: | |
| Shared Package | |
| ML19263F408 | List: |
| References | |
| NUDOCS 8001280131 | |
| Download: ML19263F409 (6) | |
Text
.
A SENSITIVITY STUDY OF LEACHABILITY K. S. Kim Presented at ANS Annual Meeting, 1980 I
1817 162 8 001280 131
SUMMARY
Leaching of radioactive materials from a waste form is the first step of nuclide migration to the biosphere.
Since the current approach in design of a high level waste repository is a " multi-barrier system approach," each barrier :;hould be considered as a component of the total system.II) Therefore, the leachability of a waste form should be evaluated together with the characteristics of other components of the total system. The system approach to the evaluation of leachability is particularly important in cases when (1) leaching of a waste form affects the performance of another barrier directly, or (2) leachability of a waste form and another isolation barrier are affected by a comon event.
Leachability is one of the most important performance c.haracteristics of solidified nuclear waste.
The measure of leachability is customarily expressed as leach rate in units of gm/cm2 - day. This unit corresponds to " normalized to congruent dissolution," and may not represent the actual physical and chemical processes during leaching.
In this study, leach rate (or release rate) in a unit of yr 1 is used in the sensitivity analysis, thus geometric dependency (e.g., surface area) does not appear explicitly in the mathematical formalism.
In this brief sensitivity analysis, the leach rate of a waste form and subsequent migration of radionuclides through the hydrogeologic system are evaluated together in a simple two-node model.
Node 1 represents a waste form containing N(t) radionuclide with a leach rate of 1(t).
Node 2 represents a biosphere or an aquifer accumulating M(t) radionuclide released from the waste form. The travel t'me of radionuclides from 1817 163
- Node 1 to Node 2 is denoted as T years.
The rate of radionuclide inventory change in these nodes can be expressed as following for a simple decaying radionuclide:
dN/dt = -(A+1) dt
-A' dM/dt = Ni e
-AM Here, A is a decay constant.
From the above rate equations, M(t) can be expressed as a fraction F(t) of the initial inventory N in Node 1.
o F(t)aM(t)/Ng rt A
l
= exp [-A(t+r)] No 1(t')exp[-
1(t) dt] dt' Ingeneral,F(t)cannotbesolvedinaclosedform.
In a simple case where 1(t) is a constant i, F(t) can be written as:
g F(t) = exp [-A(t+T)] [1 - exp (-t t)]
g The above simple relationship between F(t), t.A and t is applied to o
~
silicate waste forms to illustrate the sensitivity of leachability.
The leach rate of silicate material is known'to increase dramatically above a certain pH level of a leachate.(2) The pH level of a leachate tends to increase with increased cation concentration.
A relatively faster groundwat~' flow around a waste form dilutes the leached cations from the waste form more quickly.
Therefore, a faster groundwater flow can reduce the leach rate of a silicate waste form by keeping low pH level of leachates. However, increased groundwater flow can also shorten the travel time, r, so that radionuclides can reach an aquifer more quickly hence having decayed less.
In the following example the inventory
~
1817 164
This document is. solely the work of the Commission staff and does not necessarily represent the views of the President's Commission or any member of the Commission.
This pre-publication copy is a final document and will be subject only to minor editorial changes in its published form.
/$ (
,. G.
,tu,
4
, fraction of Cs-137 (A=0.023 yr 1) is calculated for two cases.
Case 1 represents higher groundwater flow with a slower leach rate (t = 10 years, t=10 3 yr1). Case 2 represents a slower groundwater flow and a higher leach rate (t = 100 years, t = 10 2 yr 1). As can be seen in Figure 1, case 2 shows a higher Cs-137 level in the aquifer during the first 100 years, but as time goes on the Cs-137 level in case 1 becomes greater than that of case 2.
Leach rate, groundwater flow rate, and travel time are all related, however, the relationship between those parameters is not well understood, thus mathematical formulations of the parameters can not be well established.
Although the above example is a simplification of an extremely complex situation, this example illustrates that the leach rate of waste forms alone does not uniquely determine the performance of a waste form, but it should be evaluated together with related parameters.
This example also illustrates that a " flooding condition" is not necessarily the worst condition with respect to nuclide migration.
References (1) Draf t 10 CFR Part 60 (2)
A. Paul, J. of Material Science 12, pp2246 (1977) 1817 166
.m.,
,,e lQil IVJ
6 Case 12 - Greater Deterioration of the Iodine Filters in the Auxiliary Building.
Case 13 - All Hydrogen from Core Damage Burned in the Containment Building.
Case 14 - Effect on the TMI Accident if an Adequate Hydrogen Recombiner had been Available.
Case 15 - Effect of Different Meterological Conditions in the Vicinity of the Plant Site.
' Case 16 - Criticality of the THI-2 Core.
Case 17 - The Effect on the THI-2 Accident if the Core Fuel were in Equilibrium at End of Cycle.
IV. Hypothetical Fuel Melting Accidents.
e6
.:ol l: o '
1817 1.67
FIGURE 1 FRACTIONAL NUCLIDE IN AQUIFER vs. REPOSITORY TIME 1817 168
~
l CS-137
~.
10-2 s
\\
,/
\\
\\
/
\\
/
/
\\
\\
F(t)
\\
/
\\
/
\\
/
\\
/
\\
10-3
~
CASE 1
\\
\\
CASE 2 g
i t
i 1
10-4
_,. t i
i 1
1 100 t (YEARS) 1817.169
.