The Use of Additives for Reducing Hydrogen Yield in Mortar Containing Slag and Chloride Salts
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THE USE OF ADDITIVES FOR REDUCING HYDROGEN YIELD IN MORTAR CONTAINING SLAG AND CHLORIDE SALTS*
MICHELE A. LEWIS AND DAVID W. WARREN Argonne National Laboratory, 9700 S. Cass Avenue, Argonne, IL
60439
ABSTRACT Cementitious waste forms are being considered for immobilizing nuclear waste before disposal. In earlier work, it was found that irradiation of a mortar formulation consisting of slag, portland cement, fly ash, water, and up to 10 wt % KCI-LiCl salt resulted in the generation of hydrogen. Yields were relatively high and the rates of generation were constant for the irradiation period investigated. The addition of small amounts of oxygenrich electron scavengers (NaN0 3 , NalO 4 , KMnO 4 , or Ag2 0) to the mortar was investigated as a means for reducing hydrogen yields. The addition of NaNO 3 reduced the hydrogen yield; changed the radiolytic products from hydrogen to a mixture of hydrogen, nitrogen, and N2 0; and reduced the pressurization rate after exposure to 400 Mrads. The addition of NaIO4 and KMnO 4 reduced hydrogen yields slightly while the addition of Ag2 0 increased the yield. Moreover, the addition of FeS to a non-slag mortar changed the radiolysis mechanism but the addition of FeO did not. The results of these experiments provided an insight into the nature of the radiolytic reactions occurring in the mortar formulations and indicated that the radiolytic generation of gases might be controlled with the proper choice of additive. INTRODUCTION Much work has been done to demonstrate the viability of using cementitlous waste forms to immobilize nuclear waste. Although these waste forms are stable to radiation exposure and have good leach resistance, hydrogen gas is generated from the radiolysis of the water used in making the mortar. In cements such as portland cement or high alumina cement, this is not a serious disadvantage because hydrogen generation rates decrease with time and equilibrium conditions are attained when relatively low amounts of hydrogen are produced [1, 2]. On the basis of these results, the use of cementitious materials was investigated as a possible waste form for immobilizing chloride salts generated as a waste stream in the fuel recovery process for the Integral fast reactor (IFR), an advanced nuclear reactor concept being developed at Argonne National Laboratory [3]. The IFR waste salt will not contain transuranic elements but will contain the IFR process salt, the chlorides of the fission product alkali metals, alkaline earth metals, and some of the rare earth metals and iodides. The anticipated initial self-irradiation dose rate for a waste form with a 10% salt loading will be about 0.07 Mrad/h and the internal temperature of the waste form may be as high as 120°C. The total dose absorbed over 300 years will be on the order of 1010 rad. Mortar developmental work showed that a mortar consisting of slag, fly ash, and portland cement is capable of immobilizing chloride salts [4]. The effects of radiation were subsequently investigated [5]. The conclusions from this work were th
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