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JP0450145 Analysis of Reactivity Effects in Reactors with Cavities V.F.Kolesov, V.Kh.Khoruzhi (Federal Nuclear Center of Russia

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JP0450145 Analysis of Reactivity Effects in Reactors with Cavities V.F.Kolesov, V.Kh.Khoruzhi (Federal Nuclear Center of Russia JAERI-Conf 2003-019 JP0450145 Analysis of Reactivity Effects in Reactors with Cavities V.F.Kolesov, V.Kh.Khoruzhi (Federal Nuclear Center of Russia - All-Russia Research Institute of Experimental Physics) E-mail: otd4(d)expd.vnHefru Evaluations of ultimate energy releases in these events as well as information on the absence of the dangerous autocatalysis effect display in the systems mentioned are of importance when analyzing consequences of postulated incidents on reactors and critical assemblies. In the report there are given results of calculation of fission bursts in super-critical assemblies with large central cavities implemented at VNIIEF using multi-group neutron and gas dynamic programs. From calculations it follows that: 1. In the assemblies with empty cavities the effect of auto-catalysis is not observed. 2. The same assemblies, if one places some matter in their cavities, exhibit a very strong auto-catalysis as a multi-time (up to 40 thousand times) increase of the nuclear burst energy [1,2]. he effect is observed in the assemblies with neutron absorber on the periphery or in the cavity volume, and it is increased at additional introduction of neutron moderator into the cavity. The effect is most explicit in the event of fast systems and such fillers as boron IO and hydrogen. KEYWORDS: reactor incidents, super-critical assembly, central cavity, neutron absorber, boron-10, fission burst, autocatalysis 1. Introductory notes from earlier calculations it was traced in VNIIEF activities that the effect to energy output in bursts of When analyzing outcomes of postulated incidents not only central cavity itself but different fillers as at the reactors and critical assemblies the estimations well. of ultimate energy outputs in these events is of major At carrying out these activities there were importance. In particular, of importance are calculated assemblies highly super-critical at the conclusions on the availability of lack in abnormal initial time moment (shape of assemblies is as a rule processes of positive reactivity effects often called spherically symmetric). In calculation model the auto-catalytic effects. The works in this field have assemblies underwent nuclear explosion accompanied been being published since the 50-s till today.'-6) In by melting and evaporation of the core material, terms of autocatalysis there are investigated the scattering of material outside and its implosion to the phenomena related both to geometric reconstruction of cavity. The reactivity feedback in these processes was materials distribution in the reactor and to purely characterized almost wholly by the phenomena of nuclear effects changing interaction cross-sections of assembly materials space re-distribution while the fissile materials and neutrons. effects of nuclear cross-sections change with In the papers on autocatalysis the problem of the temperature growth were not essential. The final break effect to energy output in abnormal bursts of the of chain reactions in the assemblies was realized as a internal cavity in the core is also tackled. The data result of the core material scattering. available for that time moment testified to the fact that The initial super-criticality of assemblies Akeff(O) under real conditions of fissile material shifts the kff(O) -1 was established within the range from 0.005 autocatalysis related to the availability of cavities in to 0.05 (keff(O - initial value of neutron multiplication the core was most probably not revealed. The coefficient). It was assumed that the initial reactivity developed and all-round investigation of this problem Ake 0) is introduced instantaneously. is carried out in VNIIEF .7-9) The corresponding ff(In the calculations there were compared the calculations have been being performed in VNIIEF characteristics of fission bursts at one and the same since 1994 with the aid of multi-group 26 groups) initial supercriticality in the systems with no cavity, neutron-gas-dynamics programs ARCTUR and with empty cavity and with the cavity containing NEPTUN and their predecessors. As differentiated unfissile material. The comparison of energy of bursts - 773 - JAERI-Conf 2003-019 confirmed reasonability of prior notes concerning the cavity. These results point directly to the lack at probable lack of autocatalysis in the assemblies with least in assemblies with enriched metal fuel of empty cavities. Moreover, it turned out to be that manifestations of autocatalysis associated with the some cavity fillers lead to the occurrence of a cavity in the core, - the core that is not filled or momentary positive component in reactivity feedback contain material that is not a strong absorber of and abrupt growth of the burst energy, i.e. to strong neutrons. reveal of autocatalysis effect. The effect was observed in the assemblies with neutron absorber in the cavity 3. Assemblies on fast neutrons with a cavity and amplified when neutron moderator is added to the containing strong absorber of neutrons cavity. The relative energy increase of the burst grows with the increase of the cavity radius and when the There were fulfilled presented the calculation assembly is equipped with a reflector of heavy results of fission bursts in 19 critical assemblies on material. In any system with a reflector or without it fast neutrons with the core of metal highly enriched the increase of the burst energy is most expressed in uranium, with a reflector of tungsten, iron, beryllium case of 1013 and hydrogen fillers. or with no reflector. Many calculation versions refers The mentioned above ARCTUR and NEPTUN to the systems with filler in the form of 13. programs are the programs of one-dimensional The comparison of burst energies in the assemblies calculation of neutron-gas-dynamics processes as well with the cavities of 12cm radius, wolfram reflector as of (neutron multiplication constant) and kff and kff(O) =1.05 demonstrated that the positioning of parameters of nuclear systems. 8) The fst program the layer of 1013 in the cavity of assembly leads to put into operation in 1996 is mainly oriented to the approximately 1.5-fold growth of uranium mass in the calculation of systems possessing high energy release. core and 37-fold increase of energy release in the burst The NEPTUN program was created in 1995 especially at the same supercriticality. The introduction to the to calculate the systems operating in the mode of non- cavity (in addition to boron) of a moderator of explosive chain reaction. Nowadays one can calculate neutrons in the form of gaseous hydrogen or solid with the aid of this program the systems possessing beryllium considerably raises energy release in burst. low energy release as well. Both programs created as As a means of energy release increase hydrogen is independent ones were combined in 1998 into a much more efficient than beryllium. The energy unique complex and now. they differ only in modules release in assembly containing hydrogen and 1013 is of gas dynamics calculation. The task of neutrons 230 times higher than the energy release in assembly multiplication and distribution in both programs is that does not contain neutron moderator and absorber. solved in a kinetic multi-group approximation in terms It was demonstrated a considerably higher of scattering anisotropy and with the aid of DS,,- efficiency of tungsten as neutron reflecting material as method in Lagrange system of coordinates. compared to iron and beryllium. The manifestations of In the report there are presented: autocatalysis effects are essential also in assemblies - the results of the carried out in VNIIEF without reflector, although the effect of the core calculations of energy releases in different-type expansion competing the effect of autocatalysis turns systems with no cavities, with empty cavities and out to be in this case more considerable. As a result of cavities containing neutron absorber; placing moderator and boron to the cavity the increase - the based on calculations conclusions on the lack of energy release in burst is in this case approximately or presence of autocatalysis manifestation in 16-fold. hollow assemblies on fast neutrons with the cores The calculation data testify also to the considerable of metal uranium, in the assemblies of composite decrease of autocatalysis to energy release per burst material with a moderated neutron spectrum as with the reduction of the cavity radius and to the well as in pulse reactor models BR I and BR-K 1. equally essential increase of this effect at the reduction of super-criticality level of the assembly A keff(O in 2. Assemblies with empty cavities or those filled the second case the relative growth of energy output is with low-absorbing material meant). So in assembly with A kff(O) =0.05, tungsten Such data were obtained for five couples of reflector and 12-cm cavity radius the autocatalysis assemblies on fast neutrons with a core of metal effect leads as mentioned above to the 230-times uranium of 90-% or 36-% enrichment by 135U, and growth of energy output while in the similar assembly without cavity or with cavities different (from 915 to with the 5-cm cavity radius - only to 8-times growth. 18,1 cm) radii. In assembly differing from first system only in initial From the calculation it follows that the energies of super-criticality Akfl(O) equal to 0.005 the energy bursts in the assemblies with empty cavity and in the release per burst increases due to the autocatalysis corresponding assemblies with no cavity do not effect 40 thousand times approximately. practically differ. The other data show that the burst It should be mentioned that in the overwhelming energy in the assembly with a cavity containing low- majority of the performed calculations relating to fast absorbing inert material (for example mixture of systems at no moment of fission burst the value keff(t) fluoride of natrium and zirconium) is even lower than does not exceed its initial value kfr(O).Thus, the in equivalent assemblies with no cavity or with empty - 774 - JAERI-Conf 2003-019 increase of energy release in this case occurs not due to the system super-criticality rise but because of the reduction of kff fall in burst, i.e.
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