operation conditions
Y. Kovbasenko
State Scientific and Technical Center on Nuclear and Radiation Safety, Kiev, Ukraine
Abstract. Spent nuclear fuel with the same burnup value can have different isotope composition, and, therefore, different multiplicating properties. This work has analyzed the impact on WWER-1000 spent fuel multiplicating properties of different operation conditions, such as the presence or absence of absorbers-rods in an assembly, changes in the concentration of the boric acid dissolved in the moderator (water) during the campaign, oscillations of fuel and/or moderator temperature during the campaign in different areas of the core, and changes in water amount at the periphery of an assembly due to its location in the central or periphery part of the core during the fuel campaign and/or due to changes in inter-assembly gaps.
1. INTRODUCTION
The use of the burnup as a nuclear safety parameter in assessment safety of spent fuel management systems (burnup credit principle) can be divided into three main stages. There are:
• determination of fuel burnup;
• determination of fuel isotope composition depending on its burnup;
• determination of multiplicating properties of spent fuel depending on its isotope composition.
The isotope composition of spent fuel depending on its burnup is determined, as a rule, by means of reactor cell programs, such as CASMO, HELIOS, WIMS, NESSEL, KASSETA, etc., or by means of specialized calculation programs for spent fuel isotope composition such as ORIGEN, NUKO, etc.
It is evident that the isotope composition of spent fuel is determined not only by its burnup value, but also by burnup conditions, in other words by that neutron spectrum in which this burnup took place.
Let us separate the main factors, which can affect changes in the neutron spectra during the campaign, and along with it changes in spent fuel isotope composition.
1) The presence of absorbers in an assembly, such as rods of burnable absorbers or control rods (CR) clusters.
2) The change in the concentration of the boric acid dissolved in the moderator (water) during the campaign.
3) Oscillations of fuel and/or moderator temperature in different areas of the core during the campaign.
4) Changes in water amount at the assembly periphery due to its location in the central or periphery part of the core during the fuel campaign and/or due to changes in inter-assembly gaps.
FIG. 1. WWER-1000 reactor cell.
The calculations have been conducted on an example of the FA with the maximum multiplicating properties. Taking into account the manufactory tolerances, WWER-1000 reactor FA with 4.45-% enrichment and fuel (UO2) mass 460.02 kg/FA was selected as such FA, figure 1.
The isotope composition of spent fuel has been determined for rated (or normal) operation parameters of WWER-1000 reactor.
All calculations have been performed by the program NESSEL developed by a firm for calculating WWER reactor cell.
Figure 2 presents the calculation results for multiplicating properties of WWER-1000 reactor cell mesh under the rated operation parameters of the reactor for several possible conditions:
• when there are no additional absorbers in the FA (f445h2o curve);
• when burnable absorber rods with different boron concentration are located in Control and Protection System (CPS) guide tubes (f445ba20, f445ba36, and f445ba65 curves);
• when CR clusters are located in CPS guide tubes (f445cl curve).
The behavior of the curves is completely explainable and logical. The impact of the burnable absorbers on FA multiplicating properties becomes negligibly small at the level in 20 GW*day/t approximately, CR clusters work in the whole burnup range, at that, their effectiveness slowly decreases due to boron burnup.
In following section multiplicating properties of spent FA have been determined for infinite layout of FA located with 23.6-cm pitch in unborated water with temperature 293 K (without any absorber rods in CPS guide tubes). Fuel temperature was also supposed 293 K, Figure 1.
This conditions was denoted in following as storage conditions.
0.7 0.8 0.9 1.0 1.1 1.2 1.3
0 20 40 60
f445h2o f445ba20 f445ba36 f445ba65 f445cl
BURNUP, MW*d/kg
kinf(NESSEL)
FIG. 2. Multiplicating Properties of WWER-1000 Reactor Cell under Rated Reactor Operation Parameter.
2. THE PRESENCE OF ABSORBERS, SUCH AS BURNABLE ABSORBER RODS OR CR CLUSTERS, IN THE FUEL ASSEMBLY
Based on the calculations of WWER-1000 reactor cell burnup under the rated reactor operation parameters, the isotope composition of spent fuel has been determined in the cases when water, burnable absorber, and CR clusters are in CPS guide tubes. Based on this isotope composition, multiplicating properties of the reactor mesh have been calculated in storage conditions (Tf = Tmod = 293 K, unborated water, no removable absorbers, such as burnable absorber rods or CR cluster). The results of these calculations are given in Figure 3, three lower curves (nes445h2o, nes445h2o(ba65), nes445h2o(cl)).
It is evident that multiplicating properties of spent fuel as applied to storage conditions should be calculated without credit of short-lived isotopes. Therefore, in the next series of these calculations we did not credit the impact of the isotopes associated with lines Xe135 on multiplicating properties of the reactor cell, three middle curves in Figure 3 (nes445h2os, nes445h2os(ba65), nes445h2os(cl) ).
The last series of the calculations, three upper curves in Figure 3 (nes445h2os(10is), nes445h2os(ba65)(10is), nes445h2os(cl)(10is)), presents the results of spent fuel multiplicating property calculations which have been performed with the credit of changing during burnup in the concentration of 10 isotopes that are used in the “burnup credit”
methodology the most frequently – there are basic fuel isotopes U235, U236, U238, Pu239, Pu240, and Pu241, some actinides and fission products such as Pu242, Am241, Sm149, and Sm151. In all cases the spent assemblies whose burnup took place with the presence of CR clusters have the maximum multiplicating properties.
0.9 1.0 1.1 1.2 1.3 1.4
0 10 20 30 40 50 60
nes445h2o(cl)(10is) nes445h2o(ba65)(10is) nes445h2o(10is) nes445h2os(cl) nes445h2os(ba65) nes445h2os nes445h2o(cl) nes445h2o(ba65) nes445h2o
Burnup
kinf
FIG. 3. Multiplicating Properties of WWER-1000 Reactor Cell under Storage Conditions.
3. COMPARISON OF THE NESSEL CODE WITH THE CASMO-4 AND SCALE-4.3 CODES
The possibility of comparing the results obtained with the similar ones obtained applying other codes is emerged already at this stage of calculations.
Figure 4 demonstrates the comparison of the results of the calculations presented in Figure 2 by f445h2o curve with the similar results obtained by means of the well-known code CASMO-4 (performed within the framework of BMU-project SR 2331, German).
The close coincidence of the results by neutron multiplication factor can be considered as the integral characteristic, which demonstrates the close coincidence in these calculations of the changes during burnup in concentrations of the basic isotopes affecting assembly multiplicating properties.
Figure 5 demonstrates the comparison of the results of the calculations presented in Figure 3 by three upper curves (nes445h2os(10is), nes445h2os(ba65)(10is), nes445h2os(cl)(10is)) with the similar results obtained by means of the well-known Monte-Carlo SCALE-4.3 (KENO-VI) code.
It is evident that the multiplication factor result discrepancies have the systematic nature, and is 0.015 under all burnup values (from 0 to 60 GW*day/t), and for different possible types of neutron spectra (the presence of burnable absorbers, CR clusters, or their absence), in the range of the multiplication factor from 1.0 to 1.5. These results completely confirm the applicability of the code NESSEL for such studies, at least as regards the comparative analysis of different possible variants of changes in WWER-1000 assembly multiplicating properties during fuel burnup.