Long-Term Behavior of Bitumen Waste Form
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ABSTRACT Waste blocks were produced in a bituminization plant using NPP-operational and other liquid wastes of low and intermediate level activity and tested under laboratory and near-surface wet disposal conditions. Leach rates of radioactive and non-radioactive waste components and depths of radionuclide penetration into the host loamy soil were estimated. Bituminized waste seems to occupy a middle position between cemented and vitrified waste forms in terms of radionuclide retention ability. For certain samples of the bitumen waste form, the testing covers a period of more then a quarter of a century that is of great importance for prediction of the waste form behavior over the required time period of several hundred years.
INTRODUCTION The waste matrix is considered the first and the most important barrier to radionuclide release in a multi-barrier waste disposal concept. For over two decades, SIA 'Radon' has been investigating three main types of matrices for low and intermediate level waste (LILW) conditioning [1, 2]. Cemented, bituminized and vitrified waste blocks were produced in industrial and pilot scale facilities and disposed of at the testing area consisting of an open testing site and experimental shallow-ground repositories. Bitumen has been used as a matrix material for encapsulating liquid radwaste (sludges from evaporation or precipitation, etc.) since 1968. Two industrial bituminization plants started operation in 1974 and 1978: a continuously operating, two-stage facility of high capacity and one more based on the usage of the rotary thin film evaporator/ mixer. Three types of bitumen were used in the laboratory and field experiments (Table I). Table I. Fractional compositions and standard physical properties of bitumens.
Type of bitumen
BN-I1
BN-III
BN-IV
Fractions,
oils (aliphatic hydrocarbons) 55.5
54.5
50.0
wt %
resins
17.5
15.0
11.0
asphaltenes
27.0
30.5
39.0
81-120
41-80
21-40
Softening temperature, C
40
45
70
Ignition temperature (in an open crucible), °C
200
200
230
Depth of the hardening zone at 25°C, mm 0
This paper presents results from laboratory and long-term field tests performed on samples/blocks of bituminized waste. Institutional and real NPP-operational wastes were used. EXPERIMENT The scope of laboratory work involved research of interactions in the system waste form - water (the impact of waste content, bitumen type and bitumen admixtures on radionuclide retention
571 Mat. Res. Soc. Symp. Proc. Vol. 608 0 2000 Materials Research Society
properties of bituminized waste) and in the water - soil system (the influence of waste form on backfill and soil parameters), as well as estimation of waste product radiation stability. Leach resistance was measured according to E. D. Hespe [3] on samples with waste loadings of 30, 35, 40, 50, and 60 %. Two leach stages were identified. At the initial stage of high leach rates (on the order of 1.10- to 1.10-3 cm/day), the dissolution of salts from the outer layer of the waste product occurs, for 15-50 days.
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