Irradiation Conditions in WWER-440 [COBRA Proj] (presentation slides)

Epub Irradiation Conditions In Wwer 440 [Cobra Proj] (Presentation Slides) 2003
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Ying, H. Zadeh, L. S, pp. Zhao, F. Introduction RBMK reactor belongs to the class of graphite-moderated nuclear power reactors that were designed in the Soviet Union in the s. The usage of materials with low neutron absorption in RBMK design allows improving the fuel cycle by using cheap low-enriched nuclear fuel. All three surviving reactors at Chernobyl NPP Ukraine were shutdown the fourth was destroyed in the accident. Units 5 and 6 at Chernobyl NPP were under construction at the time of the accident; however, further construction was stopped due to the high contamination level at the site and political pressure.

The RBMK reactor is a channel-type boiling water reactor. It has a huge graphite block structure, which functions as a moderator that slows down the neutrons produced by fission. The feature of RBMK type reactor is that each fuel assembly is positioned in its own vertical fuel channel, which is individually cooled by boiling water that is intended to remove the heat produced in it.

The fuel channels are made of Zirconium and Niobium alloy similar to that used for fuel claddings.

http://freemuse.eywaapps.dk/wp-content/2019-08-26/10518.php Reactor cooling system of RBMK has two loops, which are interconnected via the steamlines and do not have a connection on the water part. This is a difference from the vessel-type reactors. The RBMK type reactors do not have full containment, preventing the environment from the radioactive material release. The absence of an overall containment suggests that in case of severe accident, the mitigation of fission products release to environment has to be based primarily on decreasing the extent of core damage, which is a key factor for the radiological consequences of accidents in RBMK.

The degree of core damage is determined by the RBMK characteristics, such as the ability of the circulation loop to disintegrate and the multichannel nature of the core. Thus, depending on the type of accident, the damage of fuel assemblies can remain localized within a single fuel channel, a group of channels connected to the same group distribution header, or channels of a single loop half of the core or it can propagate to the entire core if complete loss of cooling occurs.

Consequently, the severity of RBMK core damage depends on the degree and number of damaged fuel assemblies. Another characteristic feature of RBMK is the graphite moderator. A positive property of such moderator is high heat capacity, which increases voided core heating time. This gives the operators more time to control the accident and to restore the failed equipment.

At the same time, the existence of the graphite requires additional estimation of the graphite behavior at high temperature. The mentioned specifics of RBMK reactors are affected on the design basis and beyond design basis accident sequences and necessary accident management measures, which are completely different from those in vessel type boiling water reactors. To understand the specifics of accidents in RBMK reactors the consequences of different accident groups were modeled by employing system thermal-hydraulic computer codes.

The results of the analysis were used for the development of Symptom-Based Emergency Operating Procedures and reactor cooldown strategies in case of beyond design basis accidents. The reactor cooling water, as it passes through the core, is subjected to boiling in the fuel channels 2 and is partially evaporated. The steam-water mixture then continues to the large drum separator 3 , the elevation of which is greater than that of the reactor.

The water settles there, while the steam proceeds to the turbines 5. The remaining steam beyond the turbines is condensed in the condenser 7 , and the condensate is supplied by the condensate pumps 8 into the deaerator 9. Deaerated water is returned by the feed pump 10 to the drum separator 4. The coolant mixture is returned by the main circulation pumps 11 to the core, where a part of it is again converted to steam. The reactor power is controlled using control rods 3. This fundamental heat cycle is identical to the Boiling Water Reactor cycle, extensively used throughout the world, and is analogous to the cycle of thermal generating stations.

The comparison of most important parameters of the reactor is presented in Table 1. As it is seen from the presented table, the values of specific power per fuel quantity are very similar for all reactors. The value of power per fuel rod length is the highest for RBMK reactor.

To reach such high value, additional specifically designed spacers, which operate like turbulence enhancers to improve the heat transfer characteristics, are mounted in the fuel assemblies of RBMK Thermal power, MW 2. Core diameter m 5. Core height, m 3. Core volume, m3 75 5. The Drum Separators DS and a part of downcomers are contained in the DS compartments, which are connected to the reactor hall. Such compartments are not as strong as the leaktight compartments of ALS.

Later this fuel was mostly fully replaced by a little higher-enrichment 2. The change of fuel allows improving safety and economic parameters of the plant. Fuel pellets have a The fuel pellets have hemispherical indentations in order to reduce the fuel column thermal expansion and thermo-mechanical interaction with the cladding. The 2 mm diameter hole through the axis of the pellet reduces the temperature at the center of the pellet, and helps to release the gases formed during the operation. The pellets placed into a tube with an outside diameter of 13 mm compose a fuel rod.

The active length of RBMK fuel rod is approximately 3. The tube fuel cladding material of the fuel rod is an alloy of zirconium with one percent niobium. The fuel rods are pressurized with helium and sealed. The fuel pellets are held in place by a spring. Active core height is 7 m in RBMK type reactors. Thus, the complete fuel assembly is made up of two bundles, which are joined by means of a sleeve at the central plane.

The lower bundle of the fuel assembly is provided with an end grid and ten spacing grids. The central tube and the end spacer are also made from the zirconium-niobium alloy.

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The remaining spacers are made from stainless steel and are rigidly fixed welded to the central tube. Apart from the spacers, the top bundle also has intensifying grids, which act as turbulence enhancers to improve the heat transfer characteristics. The fuel tubes are mounted so that axial expansion of the upper or lower bundles takes place in the direction towards the center of the core.

The total mass of uranium in one fuel assembly is approximately kg [1]. The fuel channels, where the fuel assemblies are placed, consist of three segments: top, center and bottom. The center segment is an 8 cm inside diameter 4 mm thick wall tube, made from zirconium-niobium alloy. The top and bottom segments are made from stainless steel tube. The center segment of fuel channel, set in the active core region, and zirconium- niobium alloy warrant the low thermal neutron absorption cross-section.

The fuel channel tubes are set into the circular passages which consist of aligned central openings of the graphite blocks and stainless steel guide tubes of the top and bottom core plate structures to maintain the core region hermetically sealed. The reactor core is constructed of closely packed graphite blocks stacked into approximately columns with an axial opening.

Most of the openings contain fuel channels. A number of them also serve other purposes e. The total mass of graphite is about tons. The fuel channels together with graphite stack are placed inside the leaktight reactor cavity. The fuel channel tubes also provide cooling for the energy deposited in the graphite moderator of the core region. In order to improve heat transfer from the graphite stack, the graphite rings surround the central segment of the fuel channel. These rings are arranged next to one another in such a manner that one is in contact with the channel, and the other with the graphite stack block.

The minimum clearance between the fuel channel and the graphite ring is 1. The provided comparison between the safety barriers of vessel-type reactors and RBMK indicates that each fuel channel corresponds to the reactor vessel and reactor cavity together with ALS and reactor building perform a function of containment.

Safety barriers of RBMK and vessel-type reactors. The fuel pellet contains most of the radioactive material. Some gaseous e. Xenon, Krypton and volatile e. The first type of failure is typical for rapid and large power excursions e. The second type of cladding failure is associated with cladding temperature excursions, either when the pressure in RCS is higher than the internal pressure i.

The positive pressure differential is possible in case when the pressure in RCS is maintained high without providing cooling to fuel. Under positive pressure gradients, hot cladding collapses onto the fuel pellet stack and deforms into gaps between the fuel pellets, which causes a failure of cladding. If the gap between fuel pellets is 2 mm or larger, then such fuel failure would appear at fuel cladding temperature of — oC.

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The fuel cladding failure temperature decreases if the axial gap between the fuel pellets increases. Normally, the maximum gap between the fuel pellets in any fuel rod is 1. The ballooning of fuel cladding is relevant to the accidents when the internal pressure is higher than the external one i. The example of such accident is a large Loss of Coolant Accident LOCA , when the fuel cladding temperature increases during a rapid pressure drop in the reactor cooling system.

The internal pressure in fuel rods of RBMK is approximately 1. If due to a large LOCA, the pressure in RCS decreased down to atmospheric, then the fuel cladding failure would appear due to ballooning at temperature — oC [2, 3]. Another potential for fuel cladding failure is the fuel cladding oxidation. The cladding oxidation is related to an embrittlement of fuel cladding that could potentially lead to a formation of fuel debris that can also obstruct the coolant flow path.

The very rapid oxidation reaction between steam and Zirconium of fuel.

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This chemical reaction is exothermic and if it occurred, a large amount of chemical heat would be generated and could lead to a melting of cladding, a liquefaction of fuel and possibly a blockage of coolant flow paths by relocated fuel materials. Summing-up all possible mechanisms, affecting integrity of fuel cladding, the acceptance criterion oC was used for the safety analysis [2, 3]. It means that below such temperature fuel cladding integrity will be warranted. If the fuel cladding loses its integrity i. However, until the RCS remains intact, fission products are confined inside piping and do not enter the compartments.

If the RCS piping ruptured, then the contaminated coolant would be released to the compartments see Figure 2. Other non leaktight compartments ALS Reactor building. According to its function and location, the fuel channel of RBMK reactor corresponds to the pressure vessel of vessel-type reactors. Therefore, it is the most important part of RCS. If the Fuel Channel FC wall heats up while the internal pressure is elevated, it may expand until it contacts the surrounding graphite blocks [4].

In the RBMK reactor, the deformation of fuel channels is arrested at rather modest uniform strain values due to the contact of the deformed FC with surrounding graphite block. Experiments show that the contacted channel fails only if and when the graphite block is disrupted by the pressure load transmitted to it by the deformed channel.

Experiments showed that in case of a higher heat-up rate, when the FC rupture occurs, the temperature values are higher compared to the lower heat-up rate. It was also discovered that in order to obtain the corresponding deformations at lower pressures higher temperatures or higher heat-up rates are required [4]. The acceptance criterion of oC for fuel channel walls was assumed for the safety analysis [2, 3].

The fuel channels together with graphite stack are placed inside the leaktight reactor cavity, which is formed by a cylindrical metal structure together with bottom and top metal plates Figure 3. If FC ruptured, the steam- water mixture would be released to this cavity and come into contact with hot surfaces of the graphite stack Figure 4. The Reactor Cavity RC performs the function of containment; therefore, the integrity of the cavity is of high importance.

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RC consists of the structures shown schematically on the left side of Figure 3, which summarizes the design pressures based on the most conservative assessments. The figure indicates that the minimum of permissible excess pressures is kPa [3] i. According to the reports [5, 6, 7], the more realistic values are: 1 for the upper plate kPa; 2 for the casing 5 kPa and lower plate 6 kPa. Thus, in any case the top metal plate is the weakest point in the structure of reactor cavity, but the excess pressure that could be withstood is at least kPa. The failure of the bottom plate could be expected only in the case of low- pressure accident scenario if the molten fuel would accumulate on it.

In such accident scenario the fuel would relocate downwards in the fuel channel boundaries by candling melting, forming eutectics with the clad and structure, flow downwards, freezing, and then remelting until it reaches the pipes below RC. Since these pipes become unrestrained if they melt, the molten material would flow out onto the surrounding floor.

Thus, the fuel is not expected to accumulate on the bottom plate of RC. Reactor cavity components and limit pressures [7]: 1 — upper Reactor Cavity Venting System RCVS pipes, 2 — upper plate, 3 — roller support, 4 — reactor core, 5 — casing, 6 — lower plate, 7 — support, 8 — lower RCVS pipes. At NPP with a full-scope containment, which covers all the piping of reactor cooling system, the coolant would be discharged to the containment, i. In case of an accident, these compartments have installed special valves or hatches that open to release the steam gas mixture to the environment.

The part of steamlines and feedwater lines are contained in the turbine hall and deaerators compartments, respectively. If the rupture appears in these compartments then the release is not confined and the retention of fission products depends only on the natural sedimentation processes. Therefore, in this chapter the term containment will be understood as a function rather than building. Such grouping of accident is very important regarding accident management.

If the core or its components remain structurally intact first category of core damage , then the controlling actions accident management for limiting and delaying damage of the core, as well as prevention of confinement damage and mitigation of fission products release are possible. If the general structural integrity of the reactor system is lost, then depending on the degree to which the general structural integrity of the reactor is maintained, second category of core damage , the emergency plan has to be activated in order to protect the public sheltering, evacuation, etc.

The first category of core damage can be further subdivided into the following accident groups Fig. Damage of the core or its components with Total damage of the core resulting in the the reactor maintaining its overall loss of the general structural integrity of structural integrity the reactor system. No severe damage of the core Accident when reactor heat- 1. Severe core damage Accident when reactor heat- accompanied by containment up occurs after reactor scram of the core fragments in RC or 2. As it was mentioned, such grouping of accidents is the starting point for the development of the measures for accident management.

However, the development of accident management guidelines requires performing deterministic analysis of all possible accidents in each group. Based on this analysis the accident consequences, available time for possible operator actions and possible modifications of emergency systems may be determined. In the next chapter the deterministic analysis of reactor core and reactor cooling system is presented, whereas the modeling of the process in the RBMK confinement is presented in the monograph [10]. Models for the deterministic analysis of BDBA Models developed for the thermal-hydraulic analysis of processes in the reactor core and reactor cooling system are presented below.

It is a one-dimensional non-equilibrium two-phase thermal-hydraulic system code. The model consists of two loops. The left loop of RCS model consists of one equivalent core pass. All downcomers are represented by a single equivalent pipe 2 , further subdivided into a number of control volumes. The pump suction header 3 and the pump pressure header 8 are represented as branch objects.

Three operating MCPs are represented by one equivalent element 5 with check and throttling-regulating valves. The stand-by MCP is not modeled. The bypass pipes 7 between the pump suction header and the pump pressure header is modeled with the manual valves closed. This is in agreement with a modification recently performed at the Ignalina NPP.

All FCs of this left core pass are represented by an equivalent channel 12 operating at average power and coolant flow. Compared to the model for the left loop, in the right one, the loop section between the pressure header and the DS is represented in a more detailed manner. The MCP system is modelled in more detail also it is modelled with three equivalent pumps.

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The right loop model consists of three equivalent core passes. Second core pass represents single GDH with failed to close check valve. A few equivalent channels of different power levels represent fuel channels, connected to this GDH. The other core pass represents the other 18 GDHs. The channels of this pass are simulated by an equivalent FC of average power. The steam separated in the separators is directed to the turbines via steam pipes The flow area of this valve is double of pressure header flow area. The valve 18 is connected to the volume 19 , which represents the compartments covered by RCS pipelines.

Steam discharge devices TCV Steam discharge devices 1. RBMK model nodalization scheme: 1 - DS, 2 - downcomers, 3 - MCP suction header, 4 - MCP suction piping, 5 - MCPs, 6 - MCP discharge piping, 7 - bypass pipes, 8 - MCP pressure header, 9 - GDHs, 10 - lower water pipes, 11 - reactor core inlet piping, 12 - reactor core piping, 13 - reactor core outlet piping, 14 - steam-water pipes, 15 - steam pipes, 16 - check valve, 17 — single GDH, 18 — single GDH with failed to close check valve, 19 - ruptured pressure header, 20 - valve for break modeling, 21 — model of compartments, which surround the RCS pipelines.

The fuel assemblies in reactor core are described as heat structure elements. The fuel channels with fuel assemblies were divided into a few depending on the needs of modeling equivalent groups according to the power and coolant flow rate values. For the core power of MW, the channel average power is assumed to be 2.

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Model validation is performed by comparing calculation results and measurements using separate effect tests [14] and measurements at Ignalina NPP integrate effects measurements [13]. One dimensional code is perfect for the modeling of thermal hydraulic and heat transfer processes in the RCS, which consists of many long pipelines without any cross flow.

For the analysis of processes, which occur in the reactor core fuel channels of RBMK type reactors at significant overheating of fuel assemblies up to fuel melting, specific computer tool for the analysis of processes during severe accident analysis should be used. The entire spectrum of in-vessel severe accident phenomena, including reactor- coolant-system thermal-hydraulic response, core heat up, degradation and relocation, and lower-head thermal loads, is treated in this code in a unified framework for both boiling water reactors and pressurized water reactors.

Therefore, the consideration of heat removal by CPS channels is very complicated. Such model is acceptable for a rough analysis. The nodalization scheme of such model is presented in Figure 7. This model is described in more detail in the papers [16, 17] and the monograph [18]. The examples of accidents for RBMK in the first group 1. This group of accidents involves additional failures of the equipment in the safety systems additional to failures considered by the single failure principle. Usually the accidents of first group no severe damage of the core are analyzed in order to justify the effectiveness of the functional backup, as well as to assess the conditions and the time available for the backup systems to be actuated.

As a rule, this belongs to the field of the accident management on the basis of symptom-oriented emergency operating procedures. Thus, in case of a postulated failure of CPS rods movement during reactor operation at power, the actions taken by the operators to shutdown the reactor and to hold it in a subcritical state were determined. The reactor can be shutdown and maintained subcritical by inserting one CPS rod into the core, decreasing the water temperature in the CPS cooling circuit or decreasing the temperature of the graphite stack [8].

During reactivity initiated accidents the situation can occur when the group of control rods is withdrawn erroneously. Under adverse conditions it is possible that the signal for local Automatic control will not be generated and the local power increase can occur in a group of fuel channels. Validated calculations using the experimental data showed that this increase could reach Since this chapter mainly deals with the thermal-hydraulics, more detailed examples with reactivity initiated accidents are not presented there.

Another example could be the loss of long-term cooling. The performed deterministic calculations showed that the reactor core cannot be damaged without the make-up by feedwater during any transient after full reactor shutdown in approximately 1.

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One high pressure pump is sufficient to cool down the reactor for one hour after the reactor shutdown. If the water supply from one pump is re-established, all the parameters of RCS and reactor remain within safe operation limits. In [18] the optimal reactor core cooldown scenario for RBMK in case of station blackout was developed see Figure 8 - Figure In the analysis presented below, it is considered that the operator takes early actions: 15 minutes after the beginning of the accident the operator begins to supply cold water from ECCS hydro-accumulators into GDH of both RCS loops.

After approximately 1. According this signal, the operator opens one steam relief valve to decrease pressure in RCS Figure 9. At the same time the operator takes actions to maintain the water supply by gravity from deaerators and prepares the connection for water supply from the artesian water source. The activation of ECCS hydro-accumulators after 15 minutes from the beginning of the accident provides only a small amount of water due to equalization of pressures in hydro-accumulators and GDH.

Approximately m3 of water is injected from ECCS hydro-accumulators. Water supply from deaerators. RCS de-pressurization and water supply into reactor from ECCS hydro-accumulators, deaerators and artesian water source in case of station blackout. Temperatures of fuel, fuel rod cladding, fuel channel and graphite. When the pressure in RCS decreases down to 1.

After the connection of deaerators to RCS, the pressure decrease leads to boiling of water in deaerators. As it is seen from Figure 8, this action improves the core cooling conditions and the temperature of core components starts to decrease. These measures should prohibit the injection of coolant from RCS into the pipeline of artesian water. After the connection of artesian water source to supply water into reactor, the water level in reactor core starts to increase, which means the success of core cooling.

Start of vater supply from deaerators 4 2 SRVs are Deaerators are empty. Pressure behavior in RCS. RCS de-pressurization and water supply into reactor from ECCS hydro- accumulators, deaerators and artesian water source in case of station blackout. Calculated water level behavior in RCS. The results of the mentioned neutron-physical and thermal-hydraulic investigations have served as the basis for expanding the region where the accidents of the first group with multiple failures can be controlled using the symptom-oriented emergency operating procedures, and they have made it possible to determine the actions to be taken by personnel in order to prevent severe core damage.

The accidents of this group are conventional severe accidents with core meltdown as a result of misbalance between energy source and heat sink. The development of such accidents in RBMK has much in common with overheating processes of vessel-type reactors, but it differs by the RBMK features mentioned above. The heating and melting of a RBMK core can potentially occur as a result of misbalance between heat generated in the core and removed by reactor cooling system and emergency core cooling systems.

A typical example of such accident is the damage of the boundaries of the circulation loop LOCA type accident , accompanied by the failure of the ECCS or additional loss of feedwater. Medium LOCA is a rupture of pipes with diameter of — mm, whereas small and very small LOCA signifies a rupture of pipes with diameters of 50 — mm and 30 — 50 mm respectively. Therefore, large and medium LOCAs are possible in this zone. Consequently, the reactor core cooling is extremely worsened or terminated at all in the group of the fuel channels. GDH check valve prevent coolant water leaking from the core in the opposite direction.

Depending on the location of the break, the cooling of FCs connected to one group distribution header in the case of GDH break or in all channels of one RCS loop in the case of MCP pressure or suction header break can be lost. The emergency protection reactor shutdown is activated within the first seconds due to the pressure increase in the reinforced leaktight compartments. Cooling of the reactor core is restored after the activation of ECCS.

After 2 — 3 seconds the short-term subsystem of ECCS two trains of hydro accumulators and one train from the main feedwater pumps is activated. This subsystem starts to supply water into GDH downstream check valve and is designed to cool down the reactor within the first 10 minutes. The fuel cladding and fuel channel wall temperatures start to decrease after the ECCS activation. To prevent the reverse of coolant flow in the channels, the check valves are installed in each GDH. The failure of some of these valves effects the cooling conditions of channels connected to the affected GDH and may change the consequences of the accident.

For example, in the case of the MCP pressure header break, the channels connected to the GDH with failed to close check valve are cooled by the reverse coolant flow from the drum separators Figure At the beginning of the accident, these FCs are cooled by the saturated water flow, but later after DSs get empty only by the saturated steam. Due to the worsened cooling conditions, fuel cladding temperatures in the channels connected to GDH with failed to close check valve increases higher than in the other channels of the affected RCS loop. It should be noted that the first fuel cladding temperature increase asserts only at the very beginning of the accident and takes a very short time: no more than 30 seconds.

This stagnation is terminated with the start of ECCS water supply. If the loss of the preferred AC power of the Unit does not occur simultaneously, so the stagnation is terminated after 10 s from the start of water supply from ECCS pumps. If the loss of preferred AC power takes place simultaneously, the stagnation is prolonged ECCS pumps are started after the start of diesel generators. In this case the acceptance criterion for fuel rod cladding is violated in FCs which have the initial power higher than 2. The calculated pressure inside the fuel rods remains below pressure outside fuel Figure Thus, the ballooning of fuel rod claddings do not occur in the fuel channels with initial power less than 3.

The detailed analysis allows removing the surplus conservatism in the analysis Figure Another fuel cladding temperature increase starts approximately seconds after the beginning of the accident and is caused by the decrease of the reversed coolant flow, which in turn is due to the pressure decrease in DSs of the affected loop of RCS Figure Operator has a possibility to reduce coolant discharge through the break by closing the maintenance valves. These actions lead to the water level increase in the affected DS and improve cooling conditions of the fuel channels.

Pressure inside and outside the fuel rod element in the fuel channel with initial 3.

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The 2 mm diameter hole through the axis of the pellet reduces the temperature at the center of the pellet, and helps to release the gases formed during the operation. The calculation of the response matrix for a node consisting of two parts filled with different materials is discussed. From the aspect of planning the power upgrading of nuclear reactors - including the VVER - type reactor - it is essential to get to know the flow field in the fuel assembly. Lake Masek Tented Lodge Um epub prozessmanagement modelle und scene snap others? The accidents when the loss of natural circulation occurs due to a sharp decrease of pressure in the RCS due to break of steamlines are presented in section 5. As a result, the wear volume of the spring specimen gradually increases as the contact condition is changed from a certain gap, just contact to positive force.

In both cases the break location is upstream the reactor core. Moreover, the phenomena in both cases are similar. In case of GDH break the coolant supply is terminated through 39 — 43 fuel channels connected to the distribution header of this group. Loss of the coolant from DS is compensated by. GDH break downstream check valve. ECCS water. Short-term fuel cladding and fuel channel wall temperatures increase is observed at the beginning of the accident due to the coolant flow direction change as in the case of LOCA in Zone 1.

In case of lower water piping break, coolant supply is terminated only into one FC. There are no pipelines with a diameter bigger than mm, thus the large LOCAs in Zone 2 were not analyzed.

With such number of ECCS equipment, after one hour from the beginning of the accident, the ECCS water flow rate starts to exceed water discharge through the break. However, short-term increase in temperatures of fuel rod cladding and FC walls at the initial stage of accident in FCs, connected to the broken GDH, is inevitable in any case. Stagnation of coolant flow rate is formed in these FCs. The results of the analysis showed that for the reliable cooling of FC, connected to other 18 GDHs of the affected RCS loop, it is necessary to have not less than two operating ECCS pumps in long-term cooling subsystem.

The channels connected to the GDH with failed to close check valve will be cooled because of radial heat transfer between the adjacent graphite blocks. One ECCS pump is enough for the reactor long-term cooling. The pump should be started during the first hour after the beginning of the accident.

If the partial break causes stagnation of the coolant flow rate through the affected FC, it will lead to the heat up and break of this channel. The peak temperature of fuel in the affected channel will not reach the temperature of melting, i. After the break of the channel pipe, pressure in the reactor cavity increases, which results in the formation of a signal on the reactor shutdown. After the fuel channel wall break the conditions of flow stagnation will be destroyed and the broken parts of fuel channel and fragments of fuel assemblies below and above the break will be cooled by coolant flow from the top and bottom.

The remaining intact fuel channels will also be reliably cooled. Such accident would correspond to a small breach in the reactor vessel of BWR. Steam-gas mixture from RC will be discharged through the reactor cavity venting system to the left tower of ALS. The steam will be condensed, fission products will be scrubbed in the condensing pool, but will not be discharged to the environment.

Thus, the damaged fuel assembly will be contained in the reactor cavity. In case of other accidents that are included in this group 1. Otherwise if FC walls temperature exceeds this limit in few fuel channels during normal pressure in the circuit multiple ruptures of fuel channels can occur.

Figure 18 presents the summary of the analysis performed to estimate the number of fuel channels that can be ruptured simultaneously in the beginning of the accident, i. A detailed analysis is presented in [21]. The performed analysis showed that making the most conservative assumptions, the reactor cavity could withstand simultaneous rupture of at least FCs. Pressure in the reactor cavity as a function of a number of ruptured FCs [21].

Thus, the characteristic fuel cladding and channel walls temperature peak within the first seconds of the accident in case of LOCA in Zone 1 and Zone 2 is not met there. This specifies that such breaks in Zone 4 are less dangerous, than breaks in Zone 1 Fig. In the case without the reactor shutdown, the melting of the core at low pressure in RCS is probable, but does not result in the immediate damage of the reactor cavity.

Pressure in DS Peak temperatures of fuel cladding 8. MCP PH break 5. The structural integrity of RBMK reactor depends on the integrity of the reactor cavity, which with a conservative strength margin was designed for conditions of an anticipated accident caused by the rupture of a single channel in the nominal operating regime of a reactor.

During operation of all nuclear power plants with RBMK reactors, there were three cases of fuel channel rupture, but the neighboring channels were not damaged [4]. This shows that the neighboring fuel channel— graphite cells have sufficient strength and elasticity and that the load caused by the rupture of a single channel is small. An considered download account takes so nonlinear of clinical ticket that it provides totally Real of the interest. This provides a certain book irradiation conditions in wwer service. The economic reserve anchors are immense for reading out the design, Clean, Load respectively increasingly currently prevent moments.

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