Main Results Of Nuclear Danger Examination Of The "Shelter" Object

The Shelter of the 4 th Unit of the Chernobyl Nuclear Power Plant object belongs to the category of nuclear dangerous objects. The main source of danger in the Shelter object (SO) is considerable concentration of fuel-containing materials (FCM) which appeared as a result of the nuclear accident of 26 April, 1986. Nuclear danger of this concentration is caused by the existence of a potential possibility (risk) of the beginning of the self-supporting chain reaction of fission (SCR). It is known that the reaction of the 235 U and 239 Pu fission has no lower limit and occurs during an interaction with any energy neutrons. There are always free neutrons in the fuel-containing medium. Their natural sources are cosmic radiation, spontaneous fission, as well as photo-neutron and (a, n) reactions. In the reaction of forced neutron-induced fission, instead of one neutron used for the fission, 2-3 new neutrons are released (the average output for 235 U-~2.43 neutron/fis., for 239 Pu-~2.89 neutron/fis.). It entails multiplication of the initial quantity of neutrons which can continue the chain of fission. Up to now the process of forced fission has been of a damping character, the multiplication property of the system can be characterized by the multiplication coefficient M: M=S/S source , where: S source (neutron/s) is a power of an independent neutron source; S (neutron/s) is a stabilized neutron power of the multiplication system, taking into consideration an additional inflow of neutrons due to the reaction of forced fission of nuclear fuel. In contrast to spontaneous nuclear transformations, the intensity of which is stable and determined by the only law of radioactive decay, intensity of the reaction of forced neutron-induced fission changes, depending on the multiplication properties of a fuel-containing composition. The index of the multiplication properties of the medium is the multiplication coefficient K ¥ characterizing the balance of neutrons in the reaction of forced fission at unlimited sizes of systems. If K ¥ ³1-the criticality becomes possible, i.e. reaching the level of neutrons reproducibility in the system by means of the reaction itself, sufficient for continuing forced fission of nuclear fuel without an inflow of neutrons from any other sources. In a critical state the multiplication coefficient M=¥ and neutron power may increase unlimitedly by means of the self-supporting chain reaction of fission.

Pu -~2.89 neutron/fis.).It entails multiplication of the initial quantity of neutrons which can continue the chain of fission.Up to now the process of forced fission has been of a damping character, the multiplication property of the system can be characterized by the multiplication coefficient M: M=S/S source , where: S source (neutron/s) is a power of an independent neutron source; S (neutron/s) is a stabilized neutron power of the multiplication system, taking into consideration an additional inflow of neutrons due to the reaction of forced fission of nuclear fuel.
In contrast to spontaneous nuclear transformations, the intensity of which is stable and determined by the only law of radioactive decay, intensity of the reaction of forced neutron-induced fission changes, depending on the multiplication properties of a fuel-containing composition.
The index of the multiplication properties of the medium is the multiplication coefficient K ¥ characterizing the balance of neutrons in the reaction of forced fission at unlimited sizes of systems.If K ¥ ³1 -the criticality becomes possible, i.e. reaching the level of neutrons reproducibility in the system by means of the reaction itself, sufficient for continuing forced fission of nuclear fuel without an inflow of neutrons from any other sources.
In a critical state the multiplication coefficient M=¥ and neutron power may increase unlimitedly by means of the self-supporting chain reaction of fission.
V.I.Kupnyi, E.L.Belousov, A.S.Tovstogan Subcriticality reserve is generally evaluated by the value that is inverse to the multiplication coefficient -subcriticality level dK ef =1/M.
Subcriticality dK ef =1-K ef (K ef <1), where K ef -effective coefficient of neutron multiplication in the system of limited sizes.
K ef is used to characterize both subcritical (K ef <1), and supercritical systems (K ef >1).If K ef =1, then the system is in a critical state, i.e. the process of forced neutron-induced fission becomes undamping.The level of deviation of the multiplication system state from the critical state is characterized by the reactivity value r: At the positive reactivity r > 0,5% the period of the neutron power doubling is reduced to the fractions of a second which makes the chain reaction of fission  As for the FCM within the range K ¥ >1 (picture 1), the calculation of the critical masses parameters were performed.After the end of the active phase of the accident, a set of diagnostic measurements has been pointing out at subcriticality of all the fuel-containing materials in the Shelter object.
Quantitative indices of the multiplication coefficient obtained experimentally on the basis of special passive neutron methods, for all the FCM are less than 0.4, and on the basis of active methods -less than the sensitivity limit (0.7).
The calculations confirmed that all the FCM are in a deep subcritical state.
Various natural calamity events which cause the shifting of construction fragments of the destroyed unit (without water flooding), do not entail the appearance of critical systems.
Water can be the main factor causing the reduction of subcriticality of the FCM in the Shelter in the course of time.2) Examination of the fuel-containing lava samples from the premises 305/2 (under the control room), carried out in 1992 -1993, showed that some samples contained AZ (active zone) fragments in a non-melted form.
Besides, visual observations showed zone fragments which were directly adjoining the lava.Thus, for calculations and assessment of the nuclear safety it is necessary to take into consideration the new composition Lava + AZF + water which is more dangerous than the Lava + water composition.
Preliminary calculations of critical parameters for such mixtures were performed in the Kurchatov Institute [2].Water content: chosen under condition K ¥ -max.
The following data were obtained for the sphere geometry (table 3): At fuel enrichment 2% for Uranium-235 (fuel with low burning-out) the radius of critical spheres reduces two times.
Assessment calculations for analogous compositions were also performed in SSC RF PEI [6].Previously verified complex REDUT was used as a software.When there is no water, the system remains deeply subcritical under any conditions.But, as it has been pointed out, in practice there are no working barriers for water inflow.
Taking the following facts into consideration: • The considerable lowering of the barriers which used to present the selfsupporting chain reaction; • Detection of new, potentially dangerous compositions of FCM; • Classification (based on a conservative approach) of the Shelter premises according to the level of nuclear danger and a number of other reasonswe can arrive at the conclusion that: at present, in some premises of the Shelter object and with respect to those FCM concentrations about which there is no sufficient information, under certain initial conditions there exists a possibility for the appearance of FCM criticality.
Such premises include the Central Room, where dozens of tons of nuclear fuel may be under the layer of thrown-off materials, the reactor pit, the premises under the control room where the most part of the fuel lava is located.
It should be noted that until recently some abnormal phenomena were registered related to the increase in the density of a neutron flux which may have been caused by both the change in the multiplication properties of fuel massesdue to water inflow, and the increase in the instrumental error -due to the same reason.
4. Assessment of the self-supporting chain reaction consequences on the premises of the Shelter object [5,6] At present, the most dangerous scenario of the development of the local selfsupporting chain reaction (SCR) is connected with a fast increase of radioactivity when flooding the fuel-containing masses with water.Without protection barriers, the consequences of such an event will be irradiation of the Shelter object personnel with a powerful penetrating neutron g-radiation.
Duration of the neutron burst during the SCR -0.3 s, energy release -10 As long as several hours will take to decrease the dose rate of g-radiation from fission products (formed during the SCR) from 300 R/s down to the level commensurable to background values at the FCM surface (800 ¸ 80 R/h).
Assessment of the explosion energy that may happen in case of the SCR shows that with the existing geometry of FCM, the kinetic energy of scattering the critical mass fragments will not exceed 0.5 MJ.The trinitrotoluene equivalent of this value is no more than 0.5 kg of the explosive.Such an explosion is dangerous not because of the possibility of destroying building constructions, but because of the possibility of releasing radioactive dust and aerosol from inside the Shelter object into the environment.Water evaporation during the SCR may also cause formation of aerosols and result in the additional release of radioactivity into the atmosphere.

First priority measures for the Shelter object water management
Water is not only a factor of the potential nuclear danger of FCM, but it also determines the radio-and-ecological danger of the Shelter object in the longterm perspective.Different projects, elaborated at present for the object modernization, are called upon, first of all, to prevent the contact of the FCM with water.
In parallel, in order to avoid radioactive leakage into the soil, it is necessary to solve the problem of collection, removal and treatment of the water accumulated in the northern and central parts of the Shelter object.The first stage of the work will be completed in 1998, after the final approval of the conceptual design and principal consent of the regulatory body.

Nuclear safety during the treatment of the Shelter water
In order to facilitate the designing of installations and selection of equipment, meeting the requirements of nuclear safety, scientific and research work was carried out in 1996 for the purpose of nuclear and radioactive safety during the management of water (LRW) containing fissionable materials [7].Below is given an assessment of minimum critical and safe parameters for the most dangerous case -a mixture of Uranium dioxide of 2% enrichment with water (table 5).Specified values of parameters will be regarded as outdated.The assessment of nuclear safety of individual equipment requires data and information on the processes parameters.
Due to the possible formation of critical masses out of new formations the calculations were carried out for the assessment of critical parameters of water composition systems on the basis of salt Na 4 UO 2 (CO 3 ) 3 .These assessments, as well as the known experimental data on criticality of other Uranium compounds, show that minimum critical parameters of the systems with low-enrichment

3 . 1 )
Results of the 1991-1996 examinations It should be noted that conclusions in the Feasibility Study of Nuclear Safety (FSNS) were based on the examinations carried out on the surface of fuel lava concentrations, since there was no technology for the extraction of highly active cores to penetrate into the lava.During the last few years (after the publication of FSNS) numerous experimental and computing examinations were performed for the determination of nuclear-physical, mechanical physico-chemical characteristics of the FCM related directly to the nuclear safety of the Shelter object.Below we list the most essential findings which resulted in a need for a substantial addition to the FSNS conclusions.While earlier the penetration of water into lava-like FCM was impeded by its high temperature and water resistance of the matter, now the conditions have changed.Calculations and experiments reveal considerable cooling of the lava, its cracking and transformation into water-tight structure.

. 6 R
At this number of fissions, about 4 kKi of radioactive inert gases (RIG) and radionuclides of iodine are generated.The consequences of their release into the atmosphere will be irradiation of the personnel nearby the Shelter object with up to 1 rem doses[2].As to the personnel working inside the Shelter object, criticality of the FCM may result in irradiation with greater doses.Density of the prompt neutron flux in the zone of SCR development is assessed expected dose of neutron irradiation in the zone of SCR development is assessed to be at the level of ~10 7 rem, and at the distance of 20 m -10 4 rem.The exposed dose rate of a prompt g-radiation in the zone of SCR development is assessed as the value equal to ~10 /s, and at the distance of 20 m ~ 300 R/s, at the same time g-doses during the SCR (0.3 s) at these points (due to prompt g-quantum) may be equal to ~10 6 rem and 100 rem, correspondingly.
will be conducted in compliance with the Procurement Policies and Rules for the Projects Financed by the EBRD and any special instructions issued by the EBRD for implementing the SIP.The total cost of first priority measures within the 13 th Task (WBS 1.3.02.03 -1.3.02.30) is estimated in the amount of $1 833 000.To reduce the impact of the aqueous medium of the Shelter object on its safety and environment it is envisaged to determine the sources, water migration routes, quantity of radioactive and fissionable materials contained in the water of the Shelter object.The plan of water management should be developed and implemented, which would include specification of the water collection points, determination of the strategy of the control and the monitoring of water, as well as technological schemes of pumping-out and the purification of water.

Table 1 .
The values of burning-out and the UO 2 share at which K ¥ virtually uncontrollable and entails a nuclear accident with grave radioactive consequences.In 1986 the chain nuclear reaction inside the 4 th reactor of the ChNPP was stopped by the total self-destruction of the active zone.But a large quantity of the nuclear fuel that remained inside the 4 th unit (almost 200 tons with the effective enrichment >1% for 235 U) still keeps the threat of a new formation of local critical masses and appearance of a self-supporting chain reaction (i.e. the secondary nuclear accident).It is not excluded that such critical compositions could have spontaneously appeared during the active phase of the accident and have been one of the causes of the intensification of accidental ejection of 2 in the mixture, which makes possible the existence of critical mass for optimum humidified LFCM (table1).The value of dry mixture density (SiO 2 +UO 2 ) was accepted to be equal to 2.5 gr/cm 3 .

Table 2 .
Critical parameters of homogeneous multiplication compositions

Table 3 .
Critical parameters for the mixture lava + active zone fragments + water with a concrete reflector.

Table 4
shows results of calculations of critical parameters for an equally sized cylinder (H=D) with a 50 cm.thick concrete reflector.

Table 5 .
Values of minimum critical and safe parameters for the homogeneous mixture of UO 2 (2% enrichment) with water.Reflector -water.