Patent Number: 054955114
Section: summary

TECHNICAL FIELD The present invention relates to a device for preventing the formation of a flammable mixture of hydrogen and oxygen in the reactor containment of a nuclear power plant in the event of an accident involving the release of hydrogen with a simultaneous rise in temperature. BACKGROUND ART During the build-up leading to a serious accident in a nuclear power plant chemical processes of various natures cause hydrogen to be produced. This can lead to the formation of flammable gas mixtures in the reactor containment. If hydrogen is released and concentrated over a longer period of time, mixtures capable of detonation can be formed. This means that the integrity of the reactor containment, the last barrier for the retention of fission products, will be jeopardized. (The term "reactor containment" is used here as a generic term for all compartments in which the problem described may arise and must thus be solved). Known in the art are measures for the prevention of the danger arising from such a flammable gas mixture that are aimed at eliminating the hydrogen in the compartments of a reactor containment. These measures include the use of igniters, as well as the catalytic recombination into water of the hydrogen with the oxygen present in the reactor containment (e.g., EP-A-0 303 144). Especially promising is the use of catalytic recombiners, which meanwhile have become known in the art in various designs (EP-A-0416 143, DE-A-36 04 416, EP-A-0 303 144, DE-A-40 03 833), although these are not fully capable of eliminating the danger of a detonation or even a deflagration, for reasons that will be explained in the following. Depending on the steam content of the atmosphere within the compartments of a reactor containment, the deflagration limit may be reached even with a local concentration of hydrogen of as little as 4%. It is a known fact that steam has an inerting property, which is to say that with a higher steam content the deflagration limit is not reached until higher concentrations of hydrogen are generated. (The term "inerting," which is translated from the German word "Inertisierung," is used herein to mean decreasing the danger of explosion of an explosive gas mixture by reducing the concentration of explosive components in the gas mixture.) From model tests it is known that at the beginning of a nuclear core melt-down accident steam is released first while hydrogen is not released until after a certain delay. The composition of the gas mixtures in the different compartments of a reactor containment can, however, vary from one another very extensively and can change continuously during the further progression of the accident. The reaction speed of the catalytic recombiners (catalysts) increases exponentially with the temperature. The catalysts heat up until an equilibrium is reached between the heat that is produced and the heat that is carried off. It is only after higher catalyst temperatures have been reached that the reduction of hydrogen will accelerate and the convection resulting from the increase in temperature will cause mixing of the surrounding atmosphere. If the supply of hydrogen within a given compartment proceeds faster than it is eliminated, an increased hydrogen concentration will result within the gas mixture. The steam content, which at first will not necessarily be equal in all compartments of the reactor containment, will be reduced during the continued course of the process by condensation at the cold walls, thereby reducing its inerting effect. The so-called detonation cell size constitutes a measure for the propagation of a detonation as well as for the sensitivity of a gas mixture to detonation. The smaller the cell size, the greater will be the susceptibility of the gas mixture to detonation. It is known that dilution of the gas mixture containing hydrogen by the use of steam and even more by CO.sub.2 causes an increase in the detonation cell size. This is true for both lower and higher temperatures. In a gas mixture at 100.degree. C. with a stoichiometric composition, the detonation cell size will be increased fivefold or 34-fold by the addition of 10% or 20% by volume of CO.sub.2, respectively (fourfold or sixfold in the case of steam), compared to that without the addition of CO.sub.2 (or steam). Nothing has been demonstrated so far about what the effect would be of diluting the gas mixture simultaneously with steam and CO.sub.2. It may be assumed, however, that the effect would be at least additive. The detonation cell size of a gas mixture of like composition will be reduced through an increase in temperature and pressure. During an accident situation a temperature of around 100.degree. C. will prevail in the compartments of the reactor containment. Opposing this, a significantly lower temperature in the gas mixture can result in the immediate vicinity of a cold concrete wall. This will cause an increase in the detonation cell size. However, the detonation cell size of the gas mixture will potentially tend to decrease at the same time because of a reduction in the steam content owing to condensation. Consideration has been given to the possibility of making use of the inerting effect of CO.sub.2 to prevent the danger of detonation during an accident situation in a reactor containment. In conjunction with this, a distinction has been made of a so-called pre-inerting and a so-called post-inerting. In pre-inerting the compartments of the reactor containment of the nuclear power plant are filled with nitrogen (N.sub.2) so that when an accident begins to occur, no oxygen would be available to form a flammable gas mixture with the hydrogen that would then be produced. But such a type of pre-inerting involves such practical problems that no actual significance attaches to it. It is sufficient to merely mention that problems would arise with accessing a reactor containment containing a pure nitrogen atmosphere during normal operation. By post-inerting is meant an injection of liquid CO.sub.2 into the reactor containment that is triggered only at the onset of an accident. This post-inerting represents an active safety measure and for this reason in itself is not very realistic. The word "active" means that some sort of device has to be present which senses the fact that an accident has occurred and which activates the introduction of CO.sub.2. Every type of active measure suffers from the fact that it cannot be relied on one hundred per cent to function properly in an emergency. In addition, serious problems arise from feeding in cold CO.sub.2 of -78.degree. C. Feeding in this cold gas would cause a drastically increased condensation of the steam present in the reactor containment and cancel its inerting effect. In addition, this injection would of necessity lead to a subsequent increase in pressure, which, as explained above, would reduce the detonation cell size. Finally, it is very uncertain what the effect would be of the low temperatures involved in supplying cold gas on the relevant safety devices in the reactor containment. The catalytic recombiners, which are passive safety devices, represent mechanisms contributing considerably to reduce the risks involved in an accident situation as described, but they do not eliminate the danger. The possibilities of both pre-inerting and post-inerting do not appear to be practicable. SUMMARY OF THE INVENTION The object of the present invention is to create a device of the type indicated at the beginning which in case of an accident would have the effect of making the atmosphere in the reactor containment inert without being encumbered with the problems described for pre-inerting and post-inerting. This object is achieved in accordance with the present invention by a device as described in claim 1. Preferred embodiments of the invention are set forth in the dependent claims. The present invention offers the possibility for a passive inerting that does not require any auxiliary energy. To achieve this passive inerting, chemical substances are employed as inerting materials, which triggered by the rise in temperature that sets in when an accident occurs, release an inerting gas or gas mixture, e.g., CO.sub.2 and/or steam, into the reactor containment. This can involve two different types of chemical substance, those which are allowed to react with one another when a certain temperature is reached (temperature of reaction) or materials which disintegrate, releasing the inerting gas or gas mixture when a certain temperature of reaction is reached. The first of the above types includes substances which, when coming into contact with one another, will also react with one another even at much lower temperatures than the temperatures considered as temperatures of reaction in the instance under discussion here. These substances must obviously not come into contact with one another until the onset of an accident. But this can be accomplished in a passive manner by using, say, a membrane to keep the substances separated from one another, which membrane liquefies when a target temperature is reached or otherwise releases the chemical substances into direct contact to react with one another. Examples of materials which can be made to react when a certain temperature is reached to release an inerting gas or gas mixture (here CO.sub.2 and steam) are calcium bicarbonate in conjunction with hydrochloric acid and potassium permanganate in conjunction with a mixture of oxalic acid and some sulfuric acid. The following compounds are examples of inerting materials which, when a certain temperature is reached, will disintegrate, releasing CO.sub.2 and/or steam and can be used for purposes of the present invention: Smithsonite (ZnCO.sub.3) exists in the form of a white powder with a density of approximately 4 g/cm.sup.3. This compound has a disintegration temperature of 300.degree. C., at which gaseous CO.sub.2 is released. The portion of CO.sub.2 in the molecular weight of this compound is 49%. Zinc oxide (ZnO) remains as a reaction product following the reaction and has a high melting point of 1260.degree. C. Iron(II) oxalate (FeC.sub.2 O.sub.4.2H.sub.2 O) exists in the form of yellowish crystals with a density of some 2 g/cm.sup.3. This compound disintegrates at a temperature of 190.degree. C. into FeO+CO.sub.2 +CO+2H.sub.2 O. The portion of CO.sub.2 and water of crystallization in this compounds is 25% and 20%, respectively. This means that, given 100 g of this compound at 190.degree. C., 25 g of CO.sub.2 and 20 g of steam can be released for inerting. The FeO remaining after disintegration reacts with the oxygen present in the air and transforms into a higher form of oxide. This brings the additional advantage of lowering the partial pressure of oxygen inside the reactor containment, thus also contributing to the inerting process. Iron(II) carbonate (FeCO.sub.3), which is found in nature and is known as siderite, disintegrates into iron oxide (FeO) and carbon dioxide (CO.sub.2) at approximately 300.degree. C. The portion of CO.sub.2 in the molecular weight of this compound is 38 g. In this case the occurrence of iron oxide resulting from disintegration also reacts with the oxygen present in the atmosphere of the reactor containment. Borax (Na.sub.2 B.sub.4 O.sub.7.10H.sub.2 O) is found in nature under the name of tincal. In its pure state, borax is formed of large, colorless, transparent crystals superficially efflorescent in dry air, which when heated to 350.degree. to 400.degree. C. transform into anhydrous Na.sub.2 B.sub.4 O.sub.7 with a melting point of 878.degree. C. The portion of water of crystallization in the molecular weight is 47%. Potassium aluminum sulfate (KAl(SO.sub.4).sub.2.12H.sub.2 O) is found in nature. Of the 12 molecules of water, six are in a loose bonding with the potassium and the other six in tight bonding with the aluminum. This means that when a temperature of 100.degree. C. is reached, first, half the water of crystallization is released in the form of steam and later, at a higher temperature, the remaining half. The portion of the water of crystallization in the molecular weight is 45.5%. The compound [Mg(MgCO.sub.3).sub.4 ](OH).sub.2 5H.sub.2 O exists in the form of a white powder and is known in the industry as "magnesia alba" or "magnesium carbonate". The portion of CO.sub.2 and H.sub.2 O in the molecular weight of this compound amounts to 51% and 23%, respectively. The use of inerting elements according to the present invention offers, among others, the following substantial advantages: inerting is completely passive, i.e., no auxiliary source of energy is required that might fail to function in case of an accident; inerting does not take place until it is needed, i.e., during normal operation of the reactor the compartments protected in accordance with the present invention can be accessed without any added hindrance whatsoever; by the selection of the quantity and type of the inerting materials with their respective temperatures of reaction, the degree of inerting can be achieved that, depending on the expected quantity of hydrogen that will be released, is necessary for avoiding a deflagration or detonation; the inerting process controlled to meet the need does not cause any excessive increase in pressure; after the initial inerting process, catalytic recombiners also present can operate at higher temperatures and thus be more effective, without this higher temperature posing a risk. The passive inerting according to the present invention as a rule is employed in addition to the use of catalytic recombiners. According to a preferred embodiment, additional advantages can be achieved by exploiting the synergistic effects of the two measures. As already mentioned at the beginning, the catalyst structures used as catalytic recombiners heat up because of an exothermic reaction. If the chemical substances used in accordance with the present invention are placed in the vicinity of a catalyst structure, then the heat developed from the latter can be exploited to start the reaction or disintegration desired, which means that the choice of chemical substances is not restricted to those that have a temperature of reaction for reaction or disintegration in the range of 100.degree. C. It is also possible, especially, to employ various substances with different temperatures of reaction to achieve a passive inerting effect staggered in time according to increases in temperature. The arrangement of the substances in the vicinity of the catalysts offers the added advantage that the released CO.sub.2 and/or steam can rapidly mix in with the surrounding atmosphere due to the convection currents caused by the heated-up catalyst structure. The heat resulting from the catalysts can thus be exploited to great advantage for purposes of passive inerting. On the other hand, the withdrawal of heat that this involves will prevent the temperature in the catalysts rising too high, which otherwise could cause the gas mixture to ignite.