Effect of Saltstone Vault Roof Configuration on the Rate of Contaminant Transport Page: 4 of 10
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EFFECT OF SALTSTONE VAULT ROOF CONFIGURATION ON
THE RATE OF CONTAMINANT RELEASE
Robert H. Hsu
Andrew D. Yu
Westinghouse Savannah River Company
P.O. Box 616
Aiken, SC 29802
At the Savannah River Site, low-level radioactive
decontaminated salt solution is mixed with slag, flyash, and
cement to form a grout-like material called "Saltstone."
The Saltstone is poured into concrete vaults constructed at
the Saltstone Disposal Facility (SDF) [Langton 1988]. The
impact of SDF on groundwater has been studied in a radio-
logical performance assessment (PA) [Westinghouse 1992;
Cook 1993]. Sophisticated groundwater models were used
to predict the groundwater flow and contaminant transport
problems [Yu 1993b]. The modeling effort was divided
into two parts: the unsaturated-zone model and the satu-
rated zone model. The unsaturated-zone model consists of
the domain above the water table. The saturated-zone
model comprises the aquifers and aquitards underlying the
SDF. The unsaturated-zone model predicts mass flux rates
(in units of grams/year or Curies/year) to the water table as
a function of time. These flux rates are used as the source
term for the saturated-zone model. The saturated-zone
model predicts the spatial contaminant concentrations in
the groundwater as a function of time.
This paper describes .the unsaturated-zone model-
ing using the ECLIPSE code [Intera 1993] to evaluate the
performance of three different roofing options: 1) the Worst
Scenario, only 30 cm (I ft) clean grout and no additional
pour or roof, 2) a proposed New Design of a 1.33 percent
slope concrete pour over the clean grout; and 3) the Origi-
nal Design vault, with an engineered concrete roof of two
percent slope. ECLIPSE is a finite-difference petroleum
reservoir engineering code with an environmental tracer
option. Nitrate was used as the "tracer" contaminant be-
cause it does not absorb or decay and is most abundant in
the decontaminated salt solution. In this study, ECLIPSE
solves the two-phase (air-water) two-dimensional (x-z ver-
tical slice) flow and transport problem up to 10,000 years.
The properties of all materials are assumed to remain un-
The predicted rate of nitrate released to the water
table (g/yr) was divided by the total initial nitrate inventory
in the modeling domain tb obtain the fractional release rate
(yr). When the release rate histories were used as source
terms for the saturated-zone model, the calculated peak
groundwater nitrate concentrations at the compliance point
are 1.01, 0.85, and 0.37 mg/liter, respectively, for the three
cases. Because the maximum concentration limit (MCL)
for nitrate is 45 mg/liter, we conclude that a configuration
change from the Original Design to the New Design will
not adversely impact the performance of the SDF for
groundwater protection. Thus, the modeling results indi-
cate that an engineered concrete roof originally proposed
may be over-designed. Eliminating the roof and substitut-
ing it with a thin layer of concrete will reduce the project
cost by approximately $2,000,000 per vault.
The Saltstone Disposal Facility
The purpose of Saltstone Disposal Facility (SDF)
is to protect human health, the environment, and the
groundwater resources. It is designed for the release of con-
taminants in a slow, controlled manner over thousands of
years. The impact of SDF on groundwater has been studied
in a radiological performance assessment (PA) [Westing-
house 1992]. The PA addresses the performance require-
ments or objectives mandated by DOE Order 5820.2A [US
DOE 1988]. One of the major performance objectives is to
show that the impacted groundwater will be in compliance
with the Safe Drinking Water Act (US DOE 1990].
The SDF will consist of 15 concrete vaults. The
first vault (Phase I), completed in 1990, is 100-feet wide by
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Hsu, R. H.; Yu, A. D. & Lam, Poh-Sang. Effect of Saltstone Vault Roof Configuration on the Rate of Contaminant Transport, article, December 28, 1994; Aiken, South Carolina. (https://digital.library.unt.edu/ark:/67531/metadc698985/m1/4/: accessed March 23, 2019), University of North Texas Libraries, Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.