state scientific and technical center on nuclear and radiation safety

1

State Scientific and Technical Center on Nuclear and Radiation Safety

EVALUATION AND SELECTION BOUNDARY ISOTOPIC COMPOSITION FOR

BURNUP CREDIT CRITICALITY SAFETY ANALYSIS OF RBMK SPENT FUEL

MANAGEMENT

Y. Kovbasenko

International Conference on Management of Spent Fuel from Nuclear Power Reactors, Vienna, Austria, 31 May – 4 June 2010

2

State Scientific and Technical Center on Nuclear and Radiation SafetyBOUNDARY ISOTOPIC COMPOSITION FOR BURNUP CREDIT CRITICALITY SAFETY ANALYSIS OF RBMK SPENT FUEL MANAGEMENT


Contents1) INTRODUCTION

2) PARAMETERS OF RBMK-1000 FUEL ASSEMBLIES

3) LIST OF ISOTOPES AND EXPERIMENTAL DATA

4) DESCRIPTION OF THE COMPUTER CODES AND MODELS. RESULTS OF COMPUTER MODELING OF EXPERIMENTAL DATA

5) DEPENDENCE OF THE MULTIPLICATION PROPERTIES OF SPENT FUEL ON ITS BURNUP CONDITIONS

6) CONCLUSIONS

3



INTRODUCTIONAfter the Ukrainian Government decision on early

decommissioning of Chornobyl NPP, units 1, 2 and 3 have been at the stage of operation cessation.

The on-site wet-type spent fuel storage facility ISF-1 is used for interim storage of nuclear fuel removed from Chornobyl NPP power units.

The results of ISF-1 preliminary criticality analyses demonstrated the need for using the burnup credit principle in nuclear safety analysis under the optimum moderation or some accident conditions provided for in the Ukrainian regulations.

4



For implementing the burnup credit principle, experimental and calculation data on the isotopic composition of RBMK spent fuel were collected and analyzed, computer codes for identify this isotopic composition were selected and tested.

Then, the impact of uncertainties of fuel burnup operating characteristics on the concentration of individual isotopes was analyzed.

The conclusions were made: how the impact of fuel burnup operating characteristics can be conservatively considered in subsequent ISF-1 nuclear safety analysis with fuel burnup credit.

5



PARAMETERS OF RBMK-1000 FUEL ASSEMBLIES

Main type of fuel assemblies that are stored at or planned to be placed into Chornobyl NPP on-site wet-type spent fuel storage facility ISF-1 is regular fuel assemblies (FA).

Figure on the next slide shows schematic view of RBMK-1000 regular fuel assemblies and their basic components.

6



7



LIST OF ISOTOPES AND EXPERIMENTAL DATA

A list of isotopes for the burnup credit criticality analysis of spent fuel storage systems is based on validation results of codes for calculating the nuclide composition of spent fuel.

To test the computer codes, experimental data for 3 RBMK FA of the Leningrad NPP were used. The initial enrichment of RBMK FA are 1.8% and 2.0% (regular fuel assemblies). FA were unloaded from the RBMK core in the 1976-89s.

The selected assemblies had operated for 1 to 5 years.

8



The isotopic composition of 18 samples with different burnups and height along the assembly was determined

The conditions of fuel burnup and subsequent holding are not known.

The data relate only to the concentration of fuel isotopes and actinides in spent fuel. The error of measurement is indicated as about 17 %.

Therefore, we limited the subsequent analysis to the following fuel isotopes and actinides:

234U, 235U, 236U, 238U, 238Pu, 239Pu, 240Pu, 241Pu, 242Pu, 237Np, 243Am, 242Cm, 244Cm

9



DESCRIPTION OF THE COMPUTER CODES AND MODELS. RESULTS OF COMPUTER MODELING OF EXPERIMENTAL DATA

RBMK fuel is quite difficult for developing a geometrical model. This is due to the following factors that make RBMK fuel complex for modeling:

10



The uncertainty of input data were preliminary assessed as to their impact on calculation results.

Impact of the following parameters was assessed:

- power level;

- density and temperature of the graphite (moderator) and water (coolant);

- temperature of the fuel cladding, central tube and fuel channel;

- location of the fuel rod in the assembly.

11



The water density in the fuel channel and power level have the greatest impact on the isotopic composition of spent FA.

The location of the fuel rod in the assembly (internal or external circles) also has a significant impact.

So, the concentration of isotopes was determined separately for the internal and external fuel rod circuits.

To determine the isotopic composition of spent RBMK fuel assemblies 2 codes were selected – SCALE and HELIOS .

12



Analysis of the results from computer modeling of experimental data shows that the SCALE (TRITON module) and HELIOS codes give a large root-mean-square error (more than 20%) in determining the concentrations of :

238Pu, 242Pu, 237Np, 243Am, 242Cm and 244Cm.

Hence, these isotopes can be excluded from further consideration.

13



Regarding the remaining isotopes, we should exclude 234U and 236U from further analysis.

The certificates for RBMK and WWER FA do not indicate the initial concentrations of 234U and 236U isotopes in fresh fuel (in contrast to the certificates for Western PWR and BWR). These isotopes are present in small amounts in fresh fuel and change in their concentration with burnup substantially depends on the initial values.

So, it can be recommended that the following 5 isotopes are used for RBMK FA burnup credit:

235U, 238U, 239Pu, 240Pu, 241Pu

14



In 2003 our National Operator developed and implemented the document to determine the burnup credit procedure for Ukraine’s first dry-type interim storage facility for WWER-1000 spent fuel at Zaporizhya NPP.

Based on a preliminary analysis, this document identifies the same list of isotopes that can be used in WWER fuel burnup credit, namely:

235U, 238U, 239Pu, 240Pu and 241Pu.

Therefore, it can be stated that the list of isotopes that was earlier approved for WWER-1000 fuel is proposed for further analysis of RBMK-1000 fuel.

15



Сalculations of the neutron multiplication factor based on the concentration of only five transuranium isotopes overestimates the results.

For spent FA with the initial enrichment of 2.0% (taking into account fuel weight and enrichment tolerances) for burnup of 20 MW*day/kgU this overestimation is more than 5% .

Actually, credit of only five isotopes in nuclear safety assessment is intended to compensate for potential errors in determining spent nuclear fuel burnup and compensate for errors in determining the variation in the spent fuel isotopic composition.

16



Calculation of K∞ for different number of isotopes

0.60

0.65

0.70

0.75

0.80

0.85

0.90

0.95

1.00

1.05

0 5000 10000 15000 20000 25000 30000

Model "5 isotopes"Model "13 isotopes"Model "30 isotopes"Model "All isotopes"

MW*Days/tU

Ke

ff

17



DEPENDENCE OF THE MULTIPLICATION

PROPERTIES OF SPENT FUEL ON ITS BURNUP CONDITIONS

Important steps in implementing the burnup credit is to analyze how core operational characteristics influences the concentration of individual isotopes of spent fuel, i.e., sensitivity analysis is needed.

For this purpose, the concentrations of isotopes in RBMK spent fuel are calculated for different operational conditions and then the criticality of the ISF-1 storage system is calculated for these concentrations of isotopes.

18



The impact of operational characteristics of fuel burnup on ISF-1 criticality was assessed using the SCALE code package (CSAS26 control module). The infinite lattice of ISF-1 cells was modeled.

Analysis of the results demonstrate that only two parameters have a noticeable impact on the RBMK assembly isotopic composition in terms of the multiplication properties of the ISF-1 cell.

These parameters are changes in the coolant (water-steam mixture) density and moderator (graphite) temperature.

19



The impact of other operational parameters (such as coolant / fuel temperature, moderator density or FA power) on K∞ of the ISF-1 cell is insignificant.

In this regard, a set of operational parameters for RBMK fuel burnup, which ensures the greatest multiplication properties of the ISF-1 cell, is selected and presented in report.

This set will further be used to calculate the concentration of isotopes in RBMK spent fuel in criticality assessments using the burnup credit principle

20



CONCLUSIONS

1) Analysis of the computer data obtained with the SCALE (TRITON module) and НЕLIOS codes permits the following conclusions:

- The developed models of the RBMK fuel assembly allow adequate modeling of changes in the isotopic composition of fuel during its burnup

- The error of determining the concentration of U and Pu isotopes can be assessed as 10% and this value can be recommended for further use

- The error of determining the concentration of other isotopes exceeds this value.

21



2) It is recommended that changes in the concentration of the following 5 isotopes are used in burnup credit nuclear safety analysis for ISF-1:

235U, 238U, 239Pu, 240Pu, 241Pu

This list of isotopes has been currently used for WWER-1000 fuel in nuclear safety analysis of the Zaporizhya NPP spent fuel dry-type cask storage system.

22



3) The dependence of the isotopic composition of spent fuel on its operational conditions has been analyzed and Chornobyl NPP RBMK operational parameters at which fuel stored in on-site wet-type spent fuel storage facility ISF-1 has the highest multiplication properties (i.e., conservative conditions in terms of nuclear safety) have been selected.

23



THANK YOU FOR YOUR ATTENTION

state scientific and technical center on nuclear and radiation safety

Documents

fuel burnup credit

management of spent

impact of fuel burnup

fuel managementy

nuclear safety analysis

nuclear power reactors

fuel storage facility

regular fuel assemblies