Comparative Nuclear Safety Analysis of Regular and Compact Spent Fuel Storage at Chornobyl NPP

Comparative nuclear safety analysis of regular and compact spent fuel storage AT Chornobyl NPP

Yu.Kovbasenko, Y.Bilodid, V.Khalimonchuk,

State Scientific and Technical Center for Nuclear and Radiation Safety

A.Novikov, E.Lebedev, D.Cherkas

State SpecializedEnterpriseChornobyl NPP

ABSTRACT

Spent fuel wet storage facility ISF-1 is currently used for intermediate storage of spent nuclear fuel removed from Chornobyl-1, 2 and 3.

Since commissioning of the ISF-2 dry spent fuel storage facility issignificantly delayed, ISF-1 is going to be used as the main spent fuel storage facility for the Chornobyl NPP in the next few years. As ISF-1is not capable of accommodating all SFA from ChNPPwith use of the regular (design) storage scheme, compactstorage of nuclear fuel in ISF-1is under consideration.

The paper presents a comparative criticality safety analysis of regular and compact spent fuel storage inISF-1 inoperational and emergency conditions.

ABBREVIATIONS

ChNPP / Chornobyl Nuclear Power Plant
FA / fuel assembly
Hcur/H0 / relative water level,
current to nominal level ratio
ISF / interim spent fuel storage facility
RFA / regenerated fuel assembly
RP / reactor pool
SFA / spent fuel assembly

1SHORT DESCRIPTION OF RBMK-1000 spent NUCLEAR FUEL STORAGE AT CHNPP ISF-1

ISF-1 is currently the main SFA storage facility at the ChNPP. It is intended for reception and long monitored storage of SNF.

The designed capacity of the ISFis 17,520 fuel assemblies, which can be placed in five compartments of the RP (one of them is onstandby), each for 4,380 SFA. SFA are stored in vertical canisters filled with water that are cooled by reactor pool water. Hence, the canisters isolate the FA from the RP water; i.e.,waters are not mixed.

The canisters are hanged vertically on consoles inISF-1 RP compartments with a nominal pitch of 230110mm(Fig.1) and a minimum possible pitch of 230102 mm (as assumed in calculations).

At present, compact storage of nuclear fuel in RP compartments and ISF-1 canyon with a pitch of SFA placement equal to 115146 mm(the distance between the canisters in one pair is 115 mm, the distance between pairs in a row is 146 mm, seeFig. 2) is under consideration.

In ISF-1 there are:

• Reactor pool for SFA storage consisting of five compartments, among which four compartments are in operation (rooms 134/14) and one is on standby (room 134/5);
• Compartment for storage of transport covers (room 135);
• Reactor pool canyon (room 137).

Compact SFA storage only in the reactor pool and canyon was considered.

• The regular SFA pitch in RP compartments is 230110 mm (minimum possible pitch in use of the existing canisters is 230102 mm, as assumed for calculations), Fig.1.

110 (102) mm

Fig.1 –Regular placement of canisters in RP compartments and ISF-1 canyon

• Compact storage of canisters in RP compartments and ISF-1 canyon (Figs. 2 and 3):

- SFA placement pitch- 115 146 mm

- Distance between canisters in one pair - 115 mm,

- Distance between pairs in a row - 146 mm,

- Height displacement of canisterpairs relative to each other - 45 mm.

146 mm

146 mm

Fig.2 –Compactplacement of canisters in RP compartments and ISF-1 canyon

Fig.3 –Section of Canister in ISF-1RP

2DESCRIPTION OF PROGRAM AND COMPUTER MODEL

The calculations were performed with SCALE software package that used the Monte-Carlo method for computing the neutron multiplication factor.

The programs included in the SCALE software package are described in detailed in [1].

SCALE (Standardized Computer Analyses for Licensing Evaluation) is a modular code system that was originally developed by Oak Ridge National Laboratory (ORNL) at the request of the U.S. Nuclear Regulatory Commission (NRC). The system was developed for problem dependent cross-section processing and analysis of criticality safety, shielding, depletion/decay, and heat transfer problems. Since the initial release of SCALE in 1980, the code system has been widely used for evaluating nuclear fuel facility and package designs.

In SCALE it is possible to use nine libraries of neutron-physical constants based on estimated data files ENDF/B-IV and V. Eight of these libraries can automatically be requested by the program in calculations. Six of these libraries have beendeveloped specially for criticality analysis.

SCALE software package has been developed and validated first of all for calculation of PWR and BWR fuel systems. In recent years, this package has been widely used for modelingVVER and RBMK fuel management systems. The applicability of the SCALE package and its libraries of neutron-physical constants for modelingVVER and RBMK fuel management systems is considered in detail in [2].

All nuclear safety calculations have been performed with up-to-date SCALE-4.4a and SCALE-5 versions. According to [2], the calculations were performed using the 44GROUPNDF5 library which, with good correlation between calculational and experimental data,somewhat overestimates the calculated neutron multiplication factor as compared with experimental data. The results of [2] provide a basis for using the SCALE code without additional justification of its acceptability.

All FA were modeled pin-by-pin. The arrangement, geometry and material structure of the FA elementscorrespond to the description in [3]. The effective density of the cylindrical uranium pin used for modeling the fuel part of the pin was determined from the total fuel weight in the FA taking into account technological tolerances.

The spacing grids, top and bottom tailpieces of the FA were not modeled; they were replaced with the moderator (water), which is obviously a conservative assumption.

The canister model used in calculations represents a cylindrical vessel with the RBMK-1000 FA placed in the center (see Fig.4).

3ACCEPTANCE CRITERIA, INITIAL AND BOUNDARY CONDITIONS

3.1Acceptance Criteria

The basic criterion of nuclear safety assessment is the requirement of Ukrainian regulatory documents [4, 5] according to which the effective neutron multiplication factor should be Keff0.95 in normal operation, operational events and design basis accidents taking into account conservative initial and boundary conditions, technological and operational tolerances.

MATERIAL 1 - fuel; MATERIAL 2 - water in canister; MATERIAL 4 - zirconium alloy; MATERIAL 6 - canister; MATERIAL 7 - central channel; MATERIAL 9 - water in reactor pool

Fig.4 – Model of regular FA and RFA in canister

3.2Initial Conditions

According to current requirements for nuclear safety analysis [5],the following initial conditions have been accepted:

• Fuel with maximum enrichment should be considered if there is nuclear fuel with different enrichments.

• Errors of calculational methods and manufacture tolerances should be considered.

• The following possible situations should be considered: penetration of water or steam-water mixture into a canister, cover, container; water boiling, formation of a steam-water mixture.

• It is necessary to consider the amount, distribution and density of the moderator (in particular, water) in the system as a result of initiating events which lead to the maximum effective neutron multiplication factor.

• Spent nuclear fuel should be considered as fresh if the neutron multiplication factor decreases in burning, except for cases when burnup is used as the nuclear safety parameter and should be monitored with special installations.

• It is necessary to assume the presence of reflector.

• It is necessary to consider possible FA regrouping in covers, racks, or containers leading to an increase in the effective neutron multiplication factor.

Tolerances in manufacturing a RBMK fuel pin are 0.05 % for the enrichment of fuel in a pin [3]. This value was used in initial conditions for calculations. For other characteristics of fuel pins, design values were used.

Fuel and moderator (water) temperature in most cases, if not specially stated, was assumed to be equal to the average ambient temperature of 20 оС. In addition to these calculations,the increase in water temperature to 50 оC (upper limit of normal operation conditions) and 80 оС (failure of the pumping and heat exchange facility) for all most critical cases. According to current regulatory requirements, conditions of water boiling at 100 оС are considered also.

3.3Boundary Conditions

As boundary conditions for calculations, if not otherwise stated, mirror reflectionwas used in the horizontal plane, and reflection on an infinite layer of water was accepted in the vertical direction above and below the FA canisters.

This choice is necessitated by mirror reflectionthat isthe most conservative condition because there is no neutron leak through the calculational cell.

4COMPARATIVE ANALYSIS OF multiplication PROPERTIES OF REGULAR FA AND RFA

The ISF-1 reactor pool, canyon and reception pool can store regular FA and RFA.

The geometrical parameters of RFA are identical to those of the regular FA; the only difference is that with the same fuel weight (114.7 ± 1.6 kg) it additionally contains 500 g U236 and 100 g U235. Hence, the content of U238 in the RFA is 600 g less than in the regular FA.

Therefore, the first analysis stage dealt with comparison of multiplication properties of regular FA enriched to 2.4 % and RFA under ISF storage conditions. Figure 4shows across-section of the FA model.

Fuel enrichment and uranium weight are providedtaking account of conservatism of technological tolerances in manufacturing the assemblies.

Reflection was assumed at boundaries of the calculational cells.This is equivalent to modeling an infinite lattice of canisters filled with FA. Dependence of multiplication properties on moderation conditions - density of the water-air mixture - was investigated. The lattice pitch was chosen to be minimally possible based on sizes of the canisters: 230102 mm.

Based on the results of calculations, the multiplication properties of the FA types considered under ISF-1 storage conditions differ insignificantly taking into account optimum moderation of neutrons (water density in a canister and between canisters changes at the same time). Note that the multiplication properties of the RFA are 0.1 to 2.6% greater than multiplication properties of other fuel types considered.

5NUCLEAR SAFETY ANALYSIS OF ISF-1 CANYON for PLACING FA AND RFA

The reactor pool canyon (room 137) is anISF-1 component. It is intended for intermediate storage of canisters with SFA before placing into the ISF RP or reloading from one RP compartment to another. Assemblies are placed in single canisters which are held on consoles. The canisters can be held on consoles in a regular or compact way.

With the regular scheme, the nominal pitch of holding the canisters on consoles is equal to 230110 mm, minimally possible 230102 mm.

With the compact scheme, the pitch of holding the canisters on consoles is equal to 115×146 mm.

In nuclear safety analysis of the canyon,the maximum possible filling with SFA was assumed at the minimum possible pitch of canisters.

The calculational model of the canyon is shown in Figs. 5 and 6.

Normal and emergency operating conditions wereanalyzed under optimum moderation conditions of neutrons determined by independent change in the moderator density (water) inside and outside the canisters. As emergency operating conditions, change in the water level in the canyon was considered. It was assumed that the canisters remain completely filled with water of the nominal density 1.0 g/cm3. This choice was based on calculations of optimum moderation of neutrons because this scenario is most conservative (as compared withdrying the canisters in the presence of water in the canyon). For the worst case (Hcur/H0 = 0.0, ρcase = 1.0 g/cm3),it is considered how changes in water and fuel temperature affect the multiplication properties of the system modeled.

The calculations show that the neutron multiplication factor withthe compact scheme increases by ∆Keff=0.15 in normal and emergency operating conditions, and thus remains below the admissible value of 0.95. The maximum neutron multiplication factor is observed if the canyon is dehydrated in the presence of water in canisters heated to 100 °С for the system filled with RFA.

The effect from changes in the canister arrangement pitch was analyzed for the most conservative initiating conditions associated with simultaneous overlapping of some emergencies, such as full emptying of the canyon, rise in water temperature in canisters. Such conditions should be related to the category of beyond design-basis as they are connected with two and more independent initiating eventsoccurring at the same time. Besides, according to [5], the accident caused by full emptying of the canyon is a beyond design-basisone.

Nevertheless, even in this case the neutron multiplication factor remains lower than the safety limit of 0.95.

The increase in the canister arrangement pitchon consoles in the canyon leads to some increase in the neutron multiplication factor both for the regularand compact scheme. This is due to the more optimal uranium-water ratio resulting from the increase in the arrangement pitch, hence the neutron multiplication factorincreases also.

Fig.5 –Calculational scheme of canyon, top view

Fig.6 –Calculational scheme of canyon, side view. Case of water level decrease in pool to upper fuel boundary (Hcur/H0 = 1)

6NUCLEAR SAFETY ANALYSIS OF ISF-1 RP in PLACING FA AND RFA

The reactor pool is the basic component of ISF-1. The RP consists of five identical compartments (rooms 134/1-5). SFA are stored here prior to their reprocessing or disposal. Assemblies are placed in single canisters which are held on consoles. The canisters can be held on consoles in a regular or compact way.

With the regular scheme, the nominal canister arrangement pitch on consoles is equal to 230110 mm, minimally possible 230102 mm.

With the compact scheme, the canister arrangement pitch on consoles is equal to 115×146 mm.

The calculational scheme for the RP differs from that of the canyon in the quantity of canisters placed on consoles. In the canyon, there are three canisters on a console in regular storage and 3×2 canisters in compact storage. In the RP,there are 24 canisters on the console in regular storage or 2×15 canisters in compact storage.

The nuclear safety analysis of the RP assumed the greatest possible filling of SFA using the minimally possible pitch of canister arrangement.

The calculational model of the RP is shown in Figures 7 and 8.

Normal and emergency operating conditions have been analyzed. Conditions of optimum neutron moderation are considered by the example of simultaneous change in the water density in the canisters and RP. As emergency operating conditions, the initiating event associated with the decrease in water level in the RP has been considered and it was assumed that canisters were completely filled with water.

As the calculations show, under normal operation conditions when the canisters and RP are completely filled with water of nominal density, the neutron multiplication factor in compact storagewill be ∆Keff=0.12 greater than in SFA regular arrangement but remainslower than 0.95.

However, if water density in the canisters and RP decreases below 0.2 g/cm3in regular storageand 0.3 g/cm3 incompactarrangement, thepermissible value of the neutron multiplication factor is exceeded.

The same situation is observed for the initiating event connected with decrease in the water level in the RP. It was assumed that the RP and canisterswith SFA were filled with water with the nominal density of 1.0 g/cm3. If the water level remains unchangedin canistersand decreases to 50% of the fuel column height in the RP, the requirement that subcriticality of the system should be no less than 5% is not met, and further level decrease to 25% leads to the neutron multiplication factor exceeding 1.0, which becomes ∆Keff=0.02 greater in compact storage in the RP than in regular fuel arrangement.

Fig.7 –Calculational scheme ofISF-1 RP, top view

Fig.8 –Calculational scheme ofISF-1 RP, side view. A case of water level decrease in RP by 3/4 height of fuel column (Hcur/H0 = 0.25)

CONCLUSIONS

The calculational analysis has shown that:

1) Under normal operation conditions for compactSFA storage,the maximum neutron multiplication factor is ∆Keff=0.14 greater than the similar value for regular storage for the canyon and ∆Keff=0.12 greater for the ISF-1 RP.

Under optimum neutron moderation conditions (canyon drainage in the presence of water with nominal density 1.0 g/cm3 in canisters heated to 100 °С, the system is filled with RFA), the maximum neutron multiplication factor for the canyon in compact SFA storage is ∆Keff=0.15 greater than that in regular storage. In the RP compartments, the maximum neutron multiplication factor in compactSFA storage increases insignificantly and exceeds the similar value for regular storage by ∆Keff=0.018.

2.1) In regular and compact storage of spent nuclear fuel in theISF-1 canyon (room 137), the effective multiplication factor does not exceed 0.95 for normal operation and design-basis accidents. If some emergencies, such as full emptying of the canyon, water temperature increase in canisters, maximum increase of the canister arrangement pitch, occur at the same time in compact storage, the permissible value of the neutron multiplication factor can be insignificantly exceeded. Nevertheless, first, the neutron multiplication factor remains lower than 0.96 and, second, this situation should be treated as a beyond design-basis one.

2.2) In regular and compact storage of spent nuclear fuel in the ISF-1 RP compartments, the effective multiplication factor does not exceed 0.95 for normal operation. However, in cases of simultaneous change in the water density inside and outside the canisters and also decrease inthe water level in the RP while the nominal water level in canisters remains unchanged the permissible value of the neutron multiplication factor is exceeded both for regular and compactcanister arrangement. The neutron multiplication factor exceeds 0.95 if the water density decreases inside and outside the canisters below 0.3 g/cm3 for regular storage and 0.4 g/cm3 for compact storage and also if the water level in the RP decreases below 75 % ofthe fuel column height for regular storage and full fuel column height for compact storage.

3) The increase in the canister arrangement pitch on consoles in the RP and canyon leads to some increase in the neutron multiplication factor both for regular and compactstorage.

4) In general, compact SFA storagedoes not lead to noticeable deterioration of nuclear safety of the storage system considered. The greatest increase in the neutron multiplication factor is equal to ∆Keff=0.15.

REFERENCEs

1. SCALE User’s Manual. NUREG/CR-0200 Revision 6. RNL/NUREG/CSD-2/V2/R6.
2. Yu.Kovbasenko, V.Khalimonchuk, A.Kuchin, Y.Bilodid, M.Yeremenko, O.Dudka NUREG/CR-6736, PNNL-13694 “Validation of SCALE Sequence CSAS26 for Criticality Safety Analysis of VVER and RBMK Fuel Designs”, Washington, U.S. NRC, 2002.
3. RBMK-1000 FA Specifications, TU 95.804-81(in Russian).
4. PBYa RU AS–89,Rules for Nuclear Safety of Nuclear Power Plants, PNAE G–1–024–90 (in Russian).
5. PNAE G -14-029-91, Safety Rules for Storage and Transportation of Nuclear Fuel at Nuclear Power Facilities (in Russian).