WO1991016714A1

WO1991016714A1 - Method for cleaning contaminated gas

Info

Publication number: WO1991016714A1
Application number: PCT/SE1991/000284
Authority: WO
Inventors: Ann-Charlotte Larsson; Leif Lindau
Original assignee: ABB Fläkt AB
Priority date: 1990-04-24
Filing date: 1991-04-23
Publication date: 1991-10-31
Also published as: AU7775091A; SE9001457D0

Abstract

In a method for cleaning a gas contaminated by radioactive methyl iodide and/or radioactive ethyl iodide, radioactive iodine isotopes are separated from the contaminated gas by contacting the latter with a washing liquid to which has been added an iodised salt which is readily soluble therein and contains non-radioactive iodine.

Description

METHOD FOR CLEANING CONTAMINATED GAS

The present invention relates to a method for clean¬ ing a gas contaminated by radioactive methyl iodide and/or radioactive ethyl iodide, by contacting the gas with a washing liquid.

The inventive method is especially intended for use in safety systems in nuclear power stations employing so- called filtered venting in the event of an accident. Fil- tered venting reduces the effects of a nuclear accident in which radioactive material from the reactor vessel leaks into the containment. The pressure in the containment increases as a result of a steep rise in temperature, and to prevent the containment from cracking and thereby leaking radioactive material to the surroundings, the con¬ tainment is vented in a controlled manner, the vented gas being cleaned in a filter. Thus, uncontrolled radioactive discharge can be avoided, and the effects of the accident be considerably reduced. As many as possible of the radio- active components, which may be in the form of dust and gaseous impurities, should of course be separated in the filter.

Swedish Patent Specification 8603914-6 discloses a so-called multiventuri scrubber plant in which the dust separation is ensured by a filter in the form of a venturi scrubber immersed in washing liquid in a container. Also gaseous impurities can be absorbed in this and similar constructions. To the washing liquid are, for this pur¬ pose, added chemicals of such properties that chemical reactions between the gaseous impurities and the chemicals result in the formation of non-volatile chemical com¬ pounds. The gaseous impurities encountered in nuclear accidents include iodine gas and inorganic iodine com¬ pounds, as well as organic volatile iodine compounds. In the case of iodine gas, for example, conventional tech¬ niques can be used in which soda and thiosulphate ions are added to the washing liquid, which, owing to chemical reactions with a much-displaced balance, results in that the iodine gas is absorbed and oxidised to a non-volatile iodised salt in solution. It is much more difficult to solve the problem of how to absorb organic iodine com- pounds, which in nuclear accidents may consist of radio¬ active methyl iodide and radioactive ethyl iodide, inter alia because their solubility in a water-base system is low at high temperatures (100°C and above).

The object of the present invention is, therefore, to provide an efficient method for cleaning a gas conta¬ minated by radioactive methyl iodide and/or radioactive ethyl iodide, by contacting the gas with a washing liquid.

According to the invention, this object is achieved by a method which is characterised in that radioactive iodine isotopes are separated from the contaminated gas by adding to the washing liquid an iodised salt which is readily soluble therein and contains non-radioactive iodine.

Preferably, water is employed as washing liquid, and an alkali iodide is employed as iodised salt.

The above-mentioned multiventuri scrubber plant is advantageously used for implementing the inventive method. The method according to the invention is based on an unusual combination of physical and chemical phenomena, and the fundamental idea of the invention will be describ¬ ed in more detail below.

Suppose that a substantially chemically inert carrier gas contains gaseous impurities of the element A which is present in the form of a radioactive isotope, here designated A*. The gaseous impurities are chemical com¬ pounds of the type A*B, A*C, A*D etc. When the contami¬ nated carrier gas is contacted with the washing liquid, e.g. water, the radioactive impurities A*B, A*C, A*D etc. will to some extent be dissolved therein. At this point, there is a dynamic balance in that individual radioactive molecules are continuously transported between the two phases, liquid and gas. The net absorption corresponds to the amount dissolved, which is much less than the amount which, during the contact time, passes to and fro across the phase boundaries. These are well-known chemical phe¬ nomena described in elementary textbooks on chemical kinetics, and are independent of whether the isotopes of the compounds are radioactive or not.

If a certain amount of non-radioactive compounds con taining the element A in non-radioactive form is initiall added to the washing liquid, also the non-radioactive atoms will take part in the chemical process, since this process is not noticeably affected by these atoms having different number of neutrons than do the radioactive ones. Thus, the non-radioactive atoms will also participate in the chemical exchange between the liquid phase and the ga phase, the net result being that a certain proportion of the gaseous impurities A*B, A*C, A*D etc. in the carrier gas are, by interaction with the washing liquid, replaced with the chemically identical but non-radioactive com¬ pounds AB, AC, AD etc. , the radioactive atoms A* accumu- lating in the washing liquid. Thus, the carrier gas is cleaned of radioactive isotopes, the total amount of che¬ mical impurities remaining, however, unchanged.

To illustrate this line of thought and substantiate the effect, the following test was carried out. Methyl iodide containing a radioactive iodine isotope was vaporised in a streaming carrier gas, in this case air. The contaminated carrier gas was conducted to a bubble bottle containing an aqueous solution with a basic iodide solution of an iodised salt readily soluble in water, the concentration being 1 mol/1 aqueous solution. In this test, potassium iodide was employed as iodised salt, but it would also have been possible to use another alkali iodide, such as sodium iodide. The amount of methyl iodide entering the gas phase was 20 ppm. The radioacti- vity was measured in the bubble bottle and in the carrier gas leaving the bottle. Thus, 75% of the radioactivity supplied to the bubble bottle stayed therein, while 25% remained in the carrier gas.

During this test, the temperature of the carrier gas was 20°C and that of the aqueous solution was 60°C. When a corresponding test was carried out under the same conditions as to methyl iodide concentration and tem¬ perature but without iodide ions in the aqueous solution, 10% of the radioactivity stayed in the bubble bottle.

Further, tests were carried out at three different temperatures of the aqueous solution, namely 40°C, 60°C and 80°C. All the tests gave the same degree of separation of radioactivity in the bubble bottle.

Claims

1. A method for cleaning a gas contaminated by radio- active methyl iodide and/or radioactive ethyl iodide, by contacting the gas with a washing liquid, c h a r a c ¬ t e r i s e d in that radioactive iodine isotopes are separated from the contaminated gas by adding to the wash¬ ing liquid an iodised salt which is readily soluble there- in and contains non-radioactive iodine.

2. A method as claimed in claim 1, c h a r a c ¬ t e r i s e d in that water is employed as washing liquid, and that an alkali iodide is employed as iodised salt.