Transport and Deposition of Metals in Sodium-Stainless Steel Systems, (VI) Diffusion of Deposited Radioisotopes into Stainless Steel

Abstract

The diffusion of radioactive manganese, zinc and cesium into stainless steel tubes exposed during 700~1,500 hr to liquid sodium containing the radioisotopes was studied by measuring the residual radioactivity on samples with their surface layers successively removed by electropolishing.

Plots of the logarithm of the residual radioactivity in the tube wall drawn against the square of the thickness polished off from the surface were found to fall along straight lines, which changed their slopes in three steps. That of outermost step was most steep, with the residual radioactivity indicating a drop of about one order of magnitude within a layer of a few microns from the surface. The ensuing step presented a much gentler slope, from which the diffusion coefficients were determined. The ratio obtained between the diffusion coefficient and the approximate jump frequency of the diffusing radioisotope was correlated with the reciprocal of absolute temperature, which indicated that the diffusion mechanism of radioactive manganese and zinc differs from that of radioactive cesium.     

Cold Trapping of Fission Products in a Stainless Steel Sodium Loop

The behavior of fission products in sodium system was studied using a cold trap installed in the Toshiba Fission Product Loop, which is a stainless steel inpile sodium loop. The fission products were generated within the liquid sodium by irradiating UO² fuel under sodium convection. The cold trapping behavior of ⁹⁵Zr, ⁹⁵Nb, ¹³²Te, ¹³¹I ¹³⁷Cs and ¹⁴⁰La (¹⁴⁰Ba) was examined by measuring the γ-rays in the cold trap and in the sodium dump tank by means of Ge(Li) detectors. Such fission product nuclides as ^{131 132}Te and ¹³⁷Cs were found to be reversively removed by the cold trap at lower temperatures, with negligible hysteresis. The distribution coefficient K(cm) for the cold trap surface was found tobe expressible by log K=(3.08±0.17)-(0.0112±0.0006)×T(°C) for ¹³¹I, and by log K:=-(4.55±0.29) + (1830±1/140) × 1/T(K) for ¹³⁷Cs. The behavior of ¹³¹I and ¹³⁷Cs during cold trapping can be explained clearly by adsorption model using these K values. While ⁹⁵Zr, ⁹⁵Nb, ¹⁰³Ru and ¹⁴⁰La(¹⁴⁰;Ba) also were detected in the cold trap, the efficacy of cold trapping is far lower for these nuclides than for ¹³¹I and ¹³⁷Cs, and changing the cold trap temperature did not appear to have any appreciable effect on the cold trapping. Presence of ¹⁴¹Ce and ¹⁴⁴Ce was not detectable in the cold trap, although small amounts of these nuclides were detected in the sodium dump tank.     

Measurement of Gamma Photon Distribution from Narrow Slit

Deposition profiles of corrosion products on fuel surfaces and products chemical compositions were determined at Shimane Nuclear Power Plant, a low-crud BWR. The deposited amounts were expressed as functions of crud concentration in the reactor water, fuel burnup and flow velocity in a fuel channel. The results could be predicted satisfactorily by the crud deposition model based on microlayer evaporation and drying out phenomena. The amount depositing at high flow velocity gave saturated values which were inversely proportional to the flow velocity (∝ 1/v), while those at low velocity did not reach saturated values, but rather were proportional to 1/v ³. The depositing corrosion products could be divided into two layers, inner and outer, with more Co ions being taken into the former layer.     

Measurement of Diffusion and Sorption of Radionuclides in Rocks

The diffusion of Cs, Sr, Am and Np into granite tablets has been examined. Sorption of these nuclides on the five kinds of rock: granite, granodiorite, basalt, andesite and rhyolite was measured and expressed as a function of interstitial water of the rock. The interstitial water was measured by weighing the penetrated water and also by counting the penetrated ^99mTc. Technetium counting represents that the sorption is affected by the rocks and does not correspond linearly to the interstitial water content.

The ratio K_p , amount of sorbed nuclide per unit of interstitial water divided by amount of nuclide per unit of solution, is introduced to make clear the relationship between nuclide sorption and interstitial water. Results indicate that there is a correlation between the sorption of nuclides and the amount of interstitial water. However, mechanisms that retard the movement of the nuclide are different from each other. Americium shows nearly equal K_p values with different rocks. In contrast, Np varies greatly.     

Statistical Analyses of Soil-to-Plant Transfer Factors: Strontium and Cesium

The soil-to-plant transfer factors (T _f s) of Sr and Cs summarized in the IUR report were statistically analyzed for three leafy plants: cabbage, spinach and grass. According to their statistical characteristics, suitabilities of default T _f s reported by IAEA were examined. The T _f s in the IUR report showed log-normal distributions for the three leafy plants. The geometric mean was found to be significantly different between plant species and between two soil types: loam and sand. In comparison to the IUR values, the default T _f s of Sr were considered to be overestimated for cabbage and underestimated for spinach, while the default T _f s of Cs were likely to be overestimated for cabbage and underestimated for spinach and grass.     

Failure Propensities of Pressurized Zircaloy Tube Containing Iodine and Other Chemical Species

Abstract

Failure propensities of Zircaloy-4 cladding tube internally pressurized with Ar gas containing iodine and iodine plus each of other chemical species were examined at 360°C, to study the effect of corrosive fission products (FPs) on the integrity of spent nuclear fuel rods during dry storage, and also to assess the capability of preventing the spent fuel rod degradation.

The iodine stress corrosion cracking (SCC) of Zircaloy tube occurred in the long time/low stress exposure tests at stresses much lower than the conventional “threshold stress”, with considerably large strains at failure. The addition of cesium to iodine perfectly suppressed the SCC. It is inferred from these results that the degradation of spent fuel rods induced by corrosive fission products is unlikely during dry storage. Even if iodine alone should take effect, a proper strain limit could prevent spent fuel rods from incurring iodine induced effects because of considerably large strains necessary for iodine SCC of Zircaloy tube at low stresses.     

Surface Alteration of Sintered Product of Cesium Aluminum Silicate under Hydrothermal Conditions

Abstract

A sintered product of cesium aluminum silicate, CsAlSi₅O₁₂, formed an outer precipitated layer and a reaction layer when exposed to 3.0 M (= mol/dm³) NaCl under hydrothermal conditions above 250°C to 300°C. The precipitated layer consisted of crystallized phases of analcime and montmorillonite, and the reaction layer exhibited a concentration gradient for Cs. These layers grew with temperature and reaction time. When the solid sample was contacted with NaCl solutions below 0.1 M, the crystallized phase of pollucite was formed in the precipitated layer. Successive accumulation of pollucite layer contributed to the depression of leaching of Cs.     

Local Gamma-Ray Counting Efficiency of Halogen Counter with Thick Anode

A substance for solidifying waste containing ¹²⁹I is sought that effectively sorbs iodine to inhibit its release from repository into the environment. Three candidate media—commercial alumina cement mixed with calcium sulfate and/or calcium hydroxide—were investigated. The criterion applied for evaluating iodine sorption performance was the distribution coefficient Kd of iodine between solid and solution in an emulsion of sodium iodide solution and powder prepared from the alumina cement/calcium compound mixture, which was molded, cured and ground. Batch sorption experiments were performed on different combinations of the above-mentioned calcium compounds added to alumina cement. The solidified substance was also examined for mechanical strength. The highest iodine sorption performance was obtained with calcium sulfate added to alumina cement to a SC₄/Ca mole ratio of 0.16, which ensured a Kd value raised to a level of not lower than 0.2 m³/kg from the (1.8–3.2)x10^-3 m³/kg obtainable with alumina cement before calcium compound addition. The enhancement of iodine sorption with addition of calcium sulfate is attributed to formation in the substance of monosulfate (3CaO·Al₂O₃·CaSO₄12H₂O) or tetracalcium aluminate hydrate (4CaO·Al₂O3·XH₂O (X=13 to 19)).     

Variation of Intensity of Environmental Gamma-Rays Measured with Ge(Li) Spectrometer

The variation of the intensity of environmental γ-rays has been studied with a Ge(Li) spectrometer. The 12 γ-rays were selected to be examined and typical characteristics of variations were obtained for each γ-rays. The intensity of γ-rays of ²¹⁴Pb and ²¹¹Bi which are ²²²Rn daughter nuclei increases on rainy days to the severalfold value of the average in the case of no precipitation. These variations were observed to be roughly proportional to rainfall rate in a sequence of precipitation. The γ-rays intensity of nuclides of the thorium series and ¹³⁷Cs is nearly constant within about 3% regardless of the weather. Its variation is about equal to the statistical fluctuation of the present measurements. The mean diurnal variations of f-ray intensity were obtained on days without precipitation. The intensity of ²¹⁴Pb and ²¹⁴Bi j-rays fluctuates nearly sinusoidally with an amplitude of about 6% of the average value and the maximum in the early morning.     

Corrosion of Austenitic Stainless Steel in Steam Containing Cesium Hydroxide

Corrosion behavior of SUS-304L SS, which is used as a typical material for reactor structures has been studied under the steam containing CsOH at temperatures of 450~750°C. The steam was supplied by heating the aqueous solution of CsOH with concentrations 0~320 mM at 110°C. The mass gain by oxygen uptake increases parabolically with reaction time. Since the activation energy of reaction rate constant is consistent with that of self diffusion of Fe in Fe₃O₄, it can be considered that the corrosion reaction is determined by the diffusion rate of Fe atoms in the outer oxide layer. The effect of CsOH on the corrosion reaction under the steam is large in the initial stage of the reaction, but small in the final stage. The mechanism of corrosion reaction has been discussed on the basis of microstructure and composition of the reaction layers.