High Energy Density Research Center of the Institute for High
Temperatures (IVTAN) Russian Academy of Science
Izhorskaya, 13/19, Moscow, 127412, Russia
Consideration a data from an accidental explosion of a chemical extraction reactor with analysis of the explosion debris and area surrounding show a realized explosive energy to 100 kg TNT explosion. Nature of an explosion is shown to be connected to the formation of an auto-ignition explosive substance when no mixing of the reactor contents.
The mechanism for the formation of a gas and liquid systems fuel-oxidizer is presented. This mechanism and process has much in common with gas release events in high-level waste tanks(Norton G. McDuffie, Westinghouse Hanford Company) and in light water nuclear reactors, in particular in nuclear submarines.
Accidental explosion occurred at 9:00 a.m. April 6, 1993 on plant 15 of the Siberian Chemical Complex in Tomsk-7(1,2). Usually this radio-chemical plant refines a fuel elements of a nuclear-power plants(3). In this case plant refines similar but other material(4)/.Event was occurred before pause for the time-limit work(3) when the chemical extraction reactor exploded.
The reactor had a volume of 34 m3 (5) or 34.1 m3 (6). The reactor body was made from stainless steel(6). The reactor contained 20 m3 (7) or 25 m3 (6) uranium solution/nitric acid (8) where 310 g (6) or 500 g (7,8) Pu and 8773 kg U(6) were dissolved.
The explosive reactor was in the end technology line(9). Isotope 137 Cs was separated before this operation(9).The uranium solution was prepared for an extraction(6). It was into contact with an organic substance before (6). The explosion was occurred after 6 min later than a concentrates nitric acid was supplied in reactor(5). The cause of an explosive discharge of gas was a chemical reaction of nitric acid with an organic substance in result a weak motion of components (6,9,11). A concentration of the nitric acid was above the true value (8). In addition a operator of reactor did not open the safety valve of reactor(8). The control device of a reactor air barbotage had not been working for quite 18 hours(8).The reactor body had working pressure 12 atm(12) and margin of a safety equaled to 2(8). The destruction of reactor body was occurred near 17 atm(12) or 36 atm(8). The explosion led to a partial destruction of an industrial building(souther wall(8) and fire(6,13).
The radionuclides total activity in reactor was 559.3 Ci, including alpha-total activity - 22.4 Ci(Pu alpha activity - 19.3 Ci) and beta-activity - 536.9 Ci(6). 5% of a total activity left in environment(6). The locus of a destructive reactor had gamma-power dose 10-15 R/h after 20 days cleansing work(13).The explosive reactor will be covered by sarcophaqus(11,14).The maps of a radioactive situation in a trace are shown in Fig. 1(13) and Fig.2(17). The figures in a maps are a power doses/mR/h/ on 1 m height at April 12, 1993. The trace was extended to the 210 degree NE from Tomsk-7(5). An original area of a radioactive contamination was 100 km2. The wasting had time-interval about 3 hours(3). The total radionuclides activity on the contamination territory was valued about 40 Ci(6). A chemical analysis show existence isotopes: 103Ru, 106Ru, 95Zr, 94Nb, 95Nb, 125Sb, 137Cs(slight)(9,11,14,16). Content 239Pu on trace was 0,008 Ci/km2(6,14) and 0,02 Ci/km2 on plant territory(6). The trace have a same ``hot`` particles. Hot particles has diameter several ten micrometers and radio-activity several ten 10 R(9). The ``usual`` a power dose for the industry waters in Tom` river is 500 mkR/h(15). The trace water radioactive is about 100 km long(15).
Fig. 1. The map of the radioactive situation near Tomsk-7 on April 12, 1993. The figures are power of dose in unit µR/h.
Fig. 2. The map of the radioactive situation and plans Tomsk and Tomsk-7 on April 12, 1993. Surface measuring. Air measuring. The figures are power of dose in unit µR/h.
RUIN ANALYSIS. TNT EQUIVALENT EXPLOSION
The explosive reactor was housed in a concrete canyon. The industrial building scheme is shown in Fig. 3. The basic(building # 201) and the neighbor industrial structures were partly destroyed(13). Air shock wave was generated hence. An air shock wave was generated and propagated through the corridor in a neighboring building and destroyed a glass-block wall(13). The view of a destroyed basic building is shown in Fig. 4. The explosion coated the fire roofing of the basic building area 3 m2 (6) of ruberoid(18) and vessels with waste(industrial?)(5). The explosion destroyed a concrete floor plate of building # 201(13). The practice show that this ruin is are the result of 100 kg TNT internal explosion. Taking the results of a special experiment into consideration and assuming that length of corridor was about 10 m we also calculated the value of the resultant TNT explosion (19) at 100 kg.
Fig. 4. The view of the destroyed building.
Fig. 3. The industral building 201 scheme. 1) explosive reactor, 2) cavity, 3) control unit, 4) design of preparation and reactants supply.
According to data from Part 1 of the present paper, the explosion occurred in one of the last stages of the process of extracting the Pu and U as shown by the remaining isotopes(20). The organic substance was a mixture of tributil phosphate((C4H9O)*3PO4 with TBP compound ether and kerosene(30%)), which was usually used as the extraction agent. Pu and U were extracted from an aqueous solution of nitric acid by means of addition F-ion(20,27). Probably the necessity of dissolving residual U required the addition of more nitric acid (apparently hot) into reactor(20). Explosion occurred after this operation. Off-normal operation lies in the fact that the normal operation procedures for the reactor were not followed. Possibly a reactor was disconnected from power. The simultaneous disconnecting of the three devices(mixer - mechanical blade or/and hydrodynamical generator(20), safety valve, air barbotage) lend support to the validity of this supposition. It is noted(20) that a special step must be used with the purpose of decreasing radiolysis in the extraction agent.
Explosive gases and liquids are produced by a combination of radiolitic and chemical processes in the reactor. To follow N.G. McDuffie(21) let us consider radio-chemical processes in a non-convecting extraction reactor. This products and processes are listed in Table I. Part of the this products are fuel and another part is oxidizer under normal conditions. This system is given in Table II. Practically any combinations of these fuels with oxidizer are a typical propellants(22-24). The liquid mixtures of fuels(for example, kerosene) with nitric acid(HNO3) or N2O4 , F2 , H2 are auto-ignited after a time induction(22-26). The decomposition of mixtures: H2O2, H2O2 + admixtures of a metals and oxides and a salts of heavy metals, H2O2 + organic materials will produce an explosion(22). In this case as with nuclear power plants(NPP) if ammonia is used we shall have a simple auto-ignition of gases(propellants): NH3 + O2, NH3 + F2, NH3 + H2O2(24). Tri-butyl phosphate, TBP, is hydrolyzed to a mono-or di-butyl phosphate and butyl alcohols(36).
Table I Radio-Chemical Processes Products in Extraction Reactor
Table II Fuels and Oxidizers in Extraction Reactor
The ignition of gaseous and liquid propellants can be realized also by cavitation of bubbles with gaseous propellants or vapor liquid propellants and also by electrostatic discharges. For example, the flash-points are for a gas mixtures: hydrogen(30%) - air - steam was taken place 510 C(0% H2O) - 550 C(30% H2O), tributil phosphate - 145 C(27), butyl alcohols - 10-34 C (36) and kerosene - 28-70 C. The auto-ignition temperatures for TBP is 145 C(27), for butyl alcohols - 345-480 C(36).The liquid propellents: H2O2 + hydrazine, H2O2 + HNO3, H2O2 + salts of heavy metals, hydrazine + oxidizer of metals(porous) are auto-ignited under normal conditions(37). Normally various obstacles for smooth current promote a transition of combustion to a detonation process.
The energy of 1 kg of this propellants(liquid explosives) is about 1 kg TNT explosive. According to (19) an explosion of 10 kg hydrogen in air is equivalent to explosion of 100 kg TNT. That is why the TNT equivalent about 100 kg is not unusual.
The special role of nitrogen in formation of explosive mixture was noted by Soddy(28) and was discovered in experiments of B.S. Svetlov with colleagues(29). It will be observed that in reality components of a broken extraction reactor must correspond to components of high-level waste in the tank type 101-SY(Hanford). Both designs have non- convecting zones. The physical and chemical models of flammable gas generation, retention, and release in a high-level waste tanks was studied detail by Sinev, et al.(21).
The tank 101-SY gas release components are given in Table III. Similar mixture of gases(% v.: H2 - 58.6, CH4 - 5.3, NH3 - 9.6, N2- 22.2, Ar - 4.3) was detonated on the chemical plant "Azot"(Rovno, former Soviet Union) under accident 19 February 1990. Analysis of ruinis of industrial structures under explosion of 350 m3 this mixture show this ruinis was a result of 100 kg TNT explosion.
Table III Tank 101-SY Gas Release Components, Dry Basis
It has to be noted that N2O can explosive decompose in catalytic (oxide of Ni?) reactor. This effect was used in the Germany reactive aerial bomb type HS-296. According(21) a liquid waste has metals Na, Al, Fe, K, Ni, Zn. The formation of a big quantity(10%) of N2, H2, NH3, and oxides nitrogen in a liquid waste(products extraction?) and metals in an alkali liquor can generate a hydrazoid acid and metal azides, azides of organic and hydrazine(N2H4) or a hydrazine organic(38). Khlopin V.G. gave an example of producing an sodium azide(starting substance for synthesis of a hydrazoic acid and a many auto-- explosive azides) by scheme: N2H4*H2O + NaOH + C2H5ONO(or C5H11ONO) = NaN3(38).
Azides and a liquid or vapor hydrozine and hydrazoid acid(NH3) are detonators for explosives by action of thermal or a mechanical energy(heat and mechanical sensitive explosives - initiating explosives(38)). The interaction on falling drop of HN3 with floor lead to HN3 explosion. The fluoro-azide(FN3) is explosive at evaporation(38). Possible local formation of sodium azide NaN3 is process with very dangerous results. Recently work(39) showed that the ignition temperature of NaN3 was 450 C and the type of atmosphere has no effect on the decomposition. Existence of ammonia and nitric acid or nitrous oxides in the rector can lead to formation of the solid ammonium nitrite NH4NO2 and ammonium nitrate NH4NO3. Mixtures of this salts are unstable and can explode. For example this process was realized in the Turkmen plant of a nitrogen fertilizer at June 1989. Probably high-level radioactive particles form uranium and plutonium are pyrophoric slurries. These slurries can ignite ruberoid roofing such a that of building # 201. Notice that a mixtures of water with weightless particles of metals and fuel are a good explosive(it is called ``Slurry``(30)). The fire-danger of a compounds: TBP + solvents was discussed Flanary(27), and Benedict, Pigford(36). In general a certain mechanisms examined could cause explosions can take place in nuclear water-water reactors of NPP`s and nuclear submarines. The explosions of a non-working reactors of Russian atomic submarine were reported in Feb. 1965(The Northern Fleet)(31) and Aug.10 1985(Shkotovo-22, Pacific Fleet)(32).
IS THE TOMSK-7 EVENT THE FIRST AND LAST EXPLOSIONS IN THE RUSSIAN ATOMIC-CHEMICAL DEVICES?
According to (6) an explosion Tomsk-7 on Apr.6,1993 was the first of that kind. On the other hand(14) such explosions as Tomsk-7 numbered 23. It is significant that a similar technology used in the other Siberian Chemical Complex in Krasnojrsk-26(11).
During a Tomsk-7 explosion in 1978 the lid of a vessel was stripped loose. In this case the building was not destroyed(8) but the lid of non-working reactor atomic submarine was propelled to about 100 m in Shkotovo-22.
The last explosion occurred in a nuclear plant Majak at July17, 1993. The sorbing vessel with a capacity 20 l solution 238Pu exploded (33-35).
Hence accidental explosions in an atomic plants(Sept. 29, 1957, July 1993, Chelybinsk-40), at NPP in Chernobyl, in nuclear submarine are not unusual for non-working(non-convecting) atomic-chemical devices.
Although the certain circumstances of an explosion Tomsk-7 on Apr.6, 1993 tally with circumstances of explosion Chernobyl on Apr.26, 1986: i.e.,- Absence containment capable of bearing a dynamic loading,
But they are not a determined physics and conditions of the event.Possibly the process of extraction is inherently dangerous. State of immobility realize possibility of explosion - hap-hazard reality.
This investigation shows some inability to understand a number of major phenomena of a radio-chemical processes in the non-convecting nuclear-chemical devices. Future work should be to concentrated on the basic research leading to fundamental understanding:
This information is necessary for the technology safety of design basis and off-normal accidents of extraction in processing plants for radioactive and chemical systems.
It should not to be excluded that explosions in an extraction reactor and explosions in light water nuclear reactor during the planned work in an active zone have the same nature.
The authors are grateful to Professor Morton E. Wacks from the University of Arizona and Doctor Norton G.McDuffie from Westinghouse Hanford Company for help in work and support.