COMPARATIVE NUCLEAR SAFETY ANALYSIS OF REGULAR AND COMPACT SPENT FUEL STORAGE AT CHORNOBYL NPP

1. 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 Specialized Enterprise Chornobyl NPP 17th Symposium of AER on VVER Reactor Physics and Reactor Safety Yalta, Crimea, Ukraine September 24-29, 2007

2. INTRODUCTION 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 is significantly 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-1 is not capable of accommodating all SFA from ChNPP with use of the regular (design) storage scheme, compact storage of nuclear fuel in ISF-1 is under consideration. 17th Symposium of AER on VVER Reactor Physics and Reactor Safety

3. SHORT 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 ISF is 17,520 fuel assemblies, which can be placed in five compartments of the RP (one of them is on standby), 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. 17th Symposium of AER on VVER Reactor Physics and Reactor Safety

4. The canisters are hanged vertically on consoles in ISF-1 RP compartments with a nominal pitch of 230110 mm 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) is under consideration. 17th Symposium of AER on VVER Reactor Physics and Reactor Safety

5. 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. 17th Symposium of AER on VVER Reactor Physics and Reactor Safety

6. 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). 110(102) mm 230mm Fig. 1 – Regular placement of canisters in RP compartments and ISF-1 canyon 17th Symposium of AER on VVER Reactor Physics and Reactor Safety

7. 146 mm 115 mm 146 mm 115 mm 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 canister pairs relative to each other 45 mm. Compact storage of canisters in RP compartments and ISF-1 canyon Fig. 2 – Compact placement of canisters in RP compartments and ISF-1 canyon 17th Symposium of AER on VVER Reactor Physics and Reactor Safety

8. Regular storage scheme 11.30 Compact storage scheme 50 50 - 450 RP water level 10695 - 10800 7300 7300 0.0 Fig. 3 – Section of Canister in ISF-1 RP 17th Symposium of AER on VVER Reactor Physics and Reactor Safety

9. DESCRIPTION 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. 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. 17th Symposium of AER on VVER Reactor Physics and Reactor Safety

10. 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 17th Symposium of AER on VVER Reactor Physics and Reactor Safety

11. Tolerances in manufacturing a RBMK fuel pin are 0.05% for the enrichment of fuel in a pin. 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 °С. In addition to these calculations, the increase in water temperature to 50 °C (upper limit of normal operation conditions) and 80 °С (failure of the pumping and heat exchange facility) for all most critical cases. According to current regulatory requirements, conditions of water boiling at 100 °С are considered also. 17th Symposium of AER on VVER Reactor Physics and Reactor Safety

12. COMPARATIVE 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 500g U236 and 100g U235. Hence, the content of U238 in the RFA is 600g less than in the regular FA. 17th Symposium of AER on VVER Reactor Physics and Reactor Safety

13. Fuel enrichment and uranium weight are provided taking 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. 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. 17th Symposium of AER on VVER Reactor Physics and Reactor Safety

14. NUCLEAR SAFETY ANALYSIS OF ISF-1 CANYON FOR PLACING FA AND RFA The reactor pool canyon (room 137) is an ISF-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. 17th Symposium of AER on VVER Reactor Physics and Reactor Safety

15. With the regular scheme, the nominal pitch of holding the canisters on consoles is equal to 230110 mm, minimally possible is 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. 17th Symposium of AER on VVER Reactor Physics and Reactor Safety

16. Normal and emergency operating conditions were analyzed 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 with drying the canisters in the presence of water in the canyon). For the worst case (Hcur/H0 = 0.0, ρcanister = 1.0 g/cm3), it is considered how changes in water and fuel temperature affect the multiplication properties of the system modeled. 17th Symposium of AER on VVER Reactor Physics and Reactor Safety

17. The calculations show that the neutron multiplication factor with the compact scheme increases by ∆keff=0.14 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 drained 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. Nevertheless, even in this case the neutron multiplication factor remains lower than the safety limit of 0.95. 17th Symposium of AER on VVER Reactor Physics and Reactor Safety

18. Fig. 5 – Calculational scheme of canyon, top view 17th Symposium of AER on VVER Reactor Physics and Reactor Safety

19. Fig. 6 – Calculational scheme of canyon, side view. Case of water level decrease in pool to upper fuel boundary (Hcur/H0 = 1) 17th Symposium of AER on VVER Reactor Physics and Reactor Safety

20. NUCLEAR 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. 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. 17th Symposium of AER on VVER Reactor Physics and Reactor Safety

21. Fig. 7 – Calculational scheme of ISF-1 RP, top view Fig. 8 – Calculational scheme of ISF-1 RP, side view. A case of water level decrease in RP by 3/4 height of fuel column (Hcur/H0 = 0.25) 17th Symposium of AER on VVER Reactor Physics and Reactor Safety

22. 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 storage will be ∆keff=0.12 greater than in SFA regular arrangement but remains lower than 0.95. 17th Symposium of AER on VVER Reactor Physics and Reactor Safety

23. However, if water density in the canisters and RP decreases below 0.2 g/cm3 in regular storage and 0.3 g/cm3 in compact arrangement, the permissible 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 canisters with SFA were filled with water with the nominal density of 1.0 g/cm3. If the water level remains unchanged in canisters and 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. 17th Symposium of AER on VVER Reactor Physics and Reactor Safety

24. CONCLUSIONS Under normal operation conditions for compact SFA 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 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 compact SFA storage increases insignificantly and exceeds the similar value for regular storage by ∆keff=0.018. 17th Symposium of AER on VVER Reactor Physics and Reactor Safety

25. 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 in the 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 compact canister 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% of the fuel column height for regular storage and full fuel column height for compact storage. 17th Symposium of AER on VVER Reactor Physics and Reactor Safety

26. 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 compact storage. In general, compact SFA storage does 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. 17th Symposium of AER on VVER Reactor Physics and Reactor Safety