Patent Number: 046718974
Section: description

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS EXAMPLE 1 A simulated liquid waste for the concentrated liquid waste occurring in a pressurized water reactor (PWR) nuclear power station was incorporated with an additive in a given amount, and the mixture was dried into powder and solidified with a hydraulic solidifier. The simulated liquid waste had the same composition as the real liquid waste, and an aqueous solution of Na.sub.2 B.sub.4 O.sub.7 was prepared by dissolving H.sub.3 BO.sub.3 with NaOH. The simulated liquid waste contained 10 .mu.Ci of .sup.137 Cs (typical nuclide of nuclear fission products). In an additive tank 9 was placed an aqueous calcium hydroxide solution (0.1 wt %) as the additive, which was maintained at 40.degree. C. by a heater and stirred continuously. Then, a given amount (50 kg/batch) of the simulated liquid waste was introduced into an adjusting and weighing tank 10. The aqueous calcium hydroxide solution was subsequently transferred from the additive tank 9 to the adjusting and weighing tank 10 in such an amount that its calcium content be in equivalent moles to the boric acid present in the simulated liquid waste, and the liquid mixture in the tank was stirred at 40.degree. C. for about one hour. As a result, the sodium borate in the liquid waste reacted with the calcium hydroxide solution to give a hardly water-soluble salt (calcium borate). Subsequently, the simulated liquid waste was introduced into an evaporator 11 and dried into powder. The steam generated by the evaporator 11 was condensed by a condenser 15 and recovered as condensed water, which was stored in a condensed water tank 16 and treated later in a separate system. The exhaust gas passing through the condenser 15 was discharged in the air via a filter 22. The dry powder formed in the evaporator 11 was transferred to a drier 12 provided between the evaporator 11 and a mixer 13, so that the powder is prevented from absorbing water and increasing its water content in the course of its being introduced into the mixer 13. The drier 12 had such a structure that the dry powder could be stored therein for feed to the mixer 13 in a certain rate. Meanwhile, a powdery solidifier (an alkali silicate composition) was introduced into a solidifier tank 17, where it was stored temporarily, and then introduced into a solidifier weighing tank 19 via a rotary feeder 18. The tank 19 was provided with a load cell for controlling the amount of the solidifier introduced. Additional water for solidification was introduced from a water feed system into an additional water weighing tank 20 and weighed. The solidifier comprising the alkali silicate composition and the additional water, after being weighed, were introduced into a solidifier kneading tank 21, where they were kneaded, and then introduced into the mixer 13 containing the dry powder of the simulated radioactive waste. In the mixer 13, the dry powder and the alkali silicate composition in amounts adjusted to 50 wt % each were kneaded and then introduced into a 200-l vessel 14 for solidification. The solidified body obtained in this Example 1 was cut, so that its inside structure was observed. As a result, it was confirmed to be a consistent solid body, with no pores occurring due to the exudation of sodium borate. In the course of the solidification procedure, any exothermic reaction such as the conventional one occurring in the solidification with powdery sodium borate did not occur, either. Since the solidification with powdery sodium borate in prior art had been attended by an exothermic reaction as described above, its content in the solidified product had been limited to at most 30 wt %, and the volume reduction ratio had accordingly been low. In contrast, the present process made it possible to add the solidifier up to at least 50 wt % to thereby raise the volume reduction ratio outstandingly. The solidified product prepared in this Example 1 was further observed for changes in its leaching characteristics and crushing strength with time, and the values obtained thereby were found to be satisfactory. FIG. 2 is a diagram showing changes in relative leaching rate with time, and FIG. 3 showing changes in relative crushing strength with time. The figures shown are relative values assuming the value observed on a solidified body prepared by a process using intact sodium borate to be 1. It was confirmed from these figures that the leaching characteristics were improved on the order of 10.sup.2 and the crushing strength increased 1- to 1.5-fold when the solidification treatment in this Example 1 was conducted after sodium borate was converted into calcium borate. EXAMPLE 2 In the preceding Example 1, the simulated liquid waste incorporated with calcium hydroxide was powdered and the powder was directly solidified. In the present Example 2, however, the powder was solidified after it was further granulated by a granulator, whereby a consistent solid product with good leaching characteristics was likewise obtained. The solidification procedures employed herein are shown in FIG. 4. The concentrated liquid waste occurring in a pressurized water reactor was subjected to the same process of adding calcium hydroxide as in Example 1 and then dried into powder, which was then pelletized by a granulator 23, and about 160 kg of the pellets were packed in the 200-l vessel 14. Subsequently, 160 kg of a solidifier comprising an alkali silicate composition kneaded with water was poured from above into the vessel to effect the solidification. The solidified body prepared in this Example 2 had the same characteristics and effects as the one prepared in Example 1. EXAMPLE 3 The Example 3 used a simulated liquid waste for a concentrated liquid waste consisting chiefly of Na.sub.2 SO.sub.4 occurring in a boiling water reactor, unlike Example 1 and Example 2 for a concentrated liquid waste occurring in a pressurized water reactor. In Example 3, the same procedures as in Example 1 were employed, except that the simulated liquid waste was composed of Na.sub.2 SO.sub.4. It was confirmed that the solidified product prepared in Example 3 had the same characteristics and effects as in Example 1. In this Example 3, the powder was solidified directly. EXAMPLE 4 In Example 4, a powder was solidified after it was pelletized as in Example 2. It was confirmed that the solidified product prepared in the Example 4 had same characteristics and effects as in Example 2. EXAMPLE 5 In the Example 5, as shown in FIG. 5, a concentrated liquid waste occurring in a pressurized water reactor was powdered and granulated, and the granules were micro-encapsulated with a water-insoluble coating and then solidified. A simulated liquid waste used herein had the same composition as in Example 1. The simulated liquid waste was transferred to a storage tank 24, and a given amount (50 kg/batch) of it was transferred from the tank 24 to an evaporator 11, where it was dried into powder. The exhaust gas generated in this case was treated in the same manner as in Example 1. The powder was subsequently shaped into granules, about 0.5 mm in size, by a granulator 25 and then introduced into a reaction tank 27. Separately, a dichloromethane solution of ethylcellulose (9 wt %) and n-hexane as microencapsulation solvents were placed in additive tanks 26 and 29, respectively. In the first step, about 200l of the ethylcellulose solution was introduced into the reaction tank 27 containing the granulated radioactive waste, and the mixture was stirred at 25.degree. C. for 5 minutes to disperse the granules. In the second step, 500l of n-hexane was introduced into the same reaction tank 27, and the mixture was stirred at 25.degree. C. for about one hour. Subsequently, the mixture was cooled rapidly to 4.degree. C. and allowed to stand for 24 hours, after which the supernatant was removed and the capsules formed were separated. In the third step, the capsules were cleaned, and their wall membrane hardened, by 1 m.sup.3 of cold n-hexane, and then transferred into a vacuum drier 28. The organic solvent occurring in this step was stored temporarily in a storage tank 30 and then disposed by burning with a burner 31, while the exhaust gas was passed through a filter 32 and discharged in the air. Subsequently, the capsules were dried completely in the vacuum drier 28 maintained at a temperature of about 60.degree. C., and a given amount (about 160 kg) of the dried capsules were introduced into a mixer 13. A solidifier feed system was arranged in the same manner as in Example 1. About 160 kg of a paste of an alkali silicate composition with water was introduced into the mixer 13 and kneaded with the capsules therein, and the mixture was poured into a 200-l vessel 14 to effect the solidification. The solidified body prepared in this example exhibited the same leaching characteristics and crushing strength as the one prepared in Example 1. EXAMPLE 6 In the Example 6, Na.sub.2 SO.sub.4 solution simulating a concentrated liquid waste occurring in a boiling water reactor was used. It was confirmed that the solidified product prepared in the Example 6 had the same time characteristics and effects as in Example 5.