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ADS versus Uranium Enrichment in a Thorium Loaded PWR

Ahmed,a Hanaa H. Abou-Gabal,bAlyaBadawib and El Kafasa

aAtomic Energy Authority, ETRR-2, Cairo, Egypt

b Alexandria University, Faculty of Engineering, Department of Nuclear and Radiation Engineering,Alexandria, Egypt


In this study, 40% of the nuclear fuel in the first core of a typical four-loop Westinghouse AP-1000 reactor was replaced by thorium in two different configurations, namely the Whole Assembly Seed and Blanket (WASB) and the Seed-Blanket Unit (SBU). The aim was to investigate and compare the neutronic parameters due to the increase in the uranium enrichment and the use of Accelerator-Driven Systems (ADS) to make up for the sub-criticality that results from the insertion of thorium.The Monte Carlo code MCNPX 2.7.0 was adopted to simulate the neutronic aspects of the fuel. The variations of the multiplication factor (keff) and the power peaking factor (PPF) at beginning of cycle and over the first cycle, and the evolution of the nuclear fuel during the burn-up were evaluated.The results indicated that the combined use of 232ThO2 and low enriched uranium fuel (UO2) with ADS allows233U production without the initial requirement of 233U enrichment and reduces the amount of high radiotoxicity isotopes.The main advantage of the ADS is the capability of such system to produce energy and to sustain nuclear chain reaction in a subcritical system.

1. Introduction

The thoriumis three to fourtimes more abundant in naturethan the uranium.However, being afertile material, it can only be used in breeding mode by producingthe fissile uranium isotope 233U, in a neutron capture and decay chain analogous to theproductionofplutonium (239Pu)from the uranium238Uisotope.The conversion of 232Th to 233U is driven by other fissile isotopes (so called “seeds”, such as 235U or 239Pu), or by an external neutron source.

In recent year’s great interest has been given to Accelerator Driven Systems (ADSs). This is mainly because of their inherent safety features, their capability to breed the required 233U when the thorium fuel is used and their waste transmutation potential. In such a subcritical system,ADSs are very important for energy production and sustainment of nuclear chain reaction. A great number of works on the ADS and the relevant neutronics have been reported in the scientific literature [1–10]. In an accelerator-driven system, an accelerator is coupled to a subcritical core loaded with nuclear fuel. The particles accelerated are injected into a spallation target that produces neutrons, which are used in the subcritical core for the fission chain maintenance.

Pazirandehet al. simulated an ADS core with mixed uranium thorium fuel. The core contained two types of fuel assemblies: (85% ThO2 + 15% UO2) and MOX (U-Pu). In the first step, only the thorium-contained fuel assemblies were loaded into the core. Criticality calculations using MCNPX showed that keffis so low that the fuel assemblies cannot run the subcritical core. That implied that MOX (U-Pu) assemblies must be loaded as well. Neutronic parameters of the thorium fueled Accelerator Driven Subcritical core were then calculated as well as other parameters related to the accelerator coupled with the core. [9]

   ADS systems of this project produce less nuclear wastes, eliminating the complicated problem confronted in the nuclear waste management. The new generation of nuclear fuel (e.g., Minor Actinides (MA) and MOX (Th-U) or MOX (Th-Pu)) needs no uranium enrichment and no plotuinum, therefore, is much more economic. The accelerator that drives the system plays a very important role in technological and safety aspects. [9]

GANEX and UREX+ reprocessed fuels in ADS.The aim of this study was to compare the neutronic behavior of the Accelerator Driven Systems (ADS) core using reprocessed fuel and thorium. The fuel used in some rods was232ThO2 for 233U production. In the other rods, two different reprocessed fuels were used.One of the studied fuels was a mixture based upon Pu-MA, removed from PWR-spent fuel, theoretically reprocessed by GANEX reprocessing and spiked with 50% of thorium. Theother fuel was a reprocessed fuel obtained theoretically from UREX+ (Uranium Extraction) process and spiked with 50% of thorium. Monteburns 2.0 (MCNP5/ORIGEN 2.1) code wasused to simulate the neutronic aspects of the fuels. The results indicated that the use ofGANEX or UREX+ fuel spiked with thorium allowed theproduction of 233U and the reduction of highradiotoxicity isotopes. [10]

Six European Commission projects considered the applications of the ADSs in different areas: PDS-XADS – Preliminary Design Studies of an Experimental Accelerator-driven System (2001 - 2004) [11], EUROTRANS – European Research Program for the Transmutation of High Level Nuclear Waste in an Accelerator Driven System (2005 - 2009), PUMA – Plutonium and Minor Actinides Management in Thermal High Temperature Reactors (2006 - 2009), ELSY – European Lead-cooled System (2006 - 2009), LEADER – Lead-cooled European Advanced Demonstration Reactor (2010 -  ongoing), FREYA – Fast Reactor Experiments for Hybrid Applications (2011 - ongoing).[11]

In all cases, the Monte Carlo N-Particle Transport Code (MCNP) was the main software used for neutron transport and burn-up calculations. [12]

In this work, the first cycle of a typical PWR core (AP1000)in which some fuel assemblies (rods)are232ThO2 (about 40% from total fuel) for 233U production while the restof the fuel assemblies(rods) uses lowenriched uranium fuel UO2 is simulated.This paper compares betweenthe effects of the use of ADS andthe increase in uranium enrichment to compensate for the sub-criticality produced by the addition of thorium fuel in this core.

2. Reactor Core Modeling

The simulated core is a typical four-loop Westinghouse AP-1000 reactor core [13]. It was considered to be on operation mode with a power of 3411 MWth.

Monte Carlo method is a statistical process that tracks individual particles throughout its life to its death. The MCNPX 2.7.0 code (Los Alamos National Laboratory) [12] was used to model the nuclear fuel behavior with the purpose of investigating the effect of thorium on the reactor cycle length and the nuclear fuel burn-up with and without the use of the ADS.The multiplication factor (keff) evolution, the power peaking factor (PPF at the beginningof cycle (BOC) and over the core cycle length), and the nuclearfuel evolution during the burn-up were evaluated.

2.1. First core loading with thorium

The initial loading of 232ThO2is 40% from the total fuel mass,the remaining 60% of the fuel is fresh low enriched uranium with three batches distribution.

The two configurations examined under the Nuclear Energy Research Initiative (NERI) funded by the United States Department of Energy [14] were considered. They are:

1) TheWhole Assembly Seed and Blanket (WASB), where the seed and blanketunits each occupy one full-size PWR assembly. The WASB core of choice consists of 117 seed assemblieswith 3.1% (40 - FA), 2.6% (40 - FA)and 2.1% (37 - FA) enrichmentarranged in a checker board pattern and 76 blanket assemblies distributed inside and at periphery of the reactor core as shown in figure 1. Both the seed and blanket assemblies use the standard Westinghouse 17×17 lattice design.

2) The Seed-Blanket Unit (SBU, also known as the Radkowsky Thorium Fuel (RTF)) concept, which employs a seed-blanket unit that is a one-for-one replacement for a conventional PWR fuel assembly. The SBU core of choice consists of 104 blanket fuel rods (ThO2) and 160 seed fuel rods (UO2) in each fuel assembly. The SBU configuration adopted in this work is illustrated in Figure 2.

Figure 1:WASB full core configuration.

Figure 2:SBU full core configuration.

Figure 3 shows The effective neutron multiplication factor (keff) for both cases. Although the amount of the thorium inserted in both cases is the same, the WASB configuration achieves a value of Keff higher than 1 during the first 4 months (120 days) while the SBU configuration indicates a deep sub-criticality from the BOC.This can be explained by the fact that the ThO2is placedmainly in the peripheryassemblies where the neutron flux is relatively low.

Figure 3:Keff versus time for the WASB and SBU configurations.

In order to counteract the reduction in criticality due to the addition of thorium, two methods are studied and compared: increase the seed fuel enrichment and use the ADS as an external neutron source.

2.2. Increase the seed fuel enrichment

The seed fuel batches enrichment is increased as following:

- Case (1): The WASB coreconsisting of 76 blanketfuel assembliesdistributed inside and at periphery of the reactor core and 117 seed fuel assemblies as shown inFigure 4. The seed fuel assemblies enriched with 4.95% (40 - FA), 4.5% (40 - FA)and 3% (37 - FA)are arranged in a checkerboardpattern.

- Case (2): The SBU core consisting of 104 blanket fuel rods and 160 seed fuel rodsin each fuel assembly.The PWR common uranium batches configuration with enrichment equal to4.95%, 4.5% and 3% of 235U is followed as shown in Figure 5.

Figure 4:WASB full core configuration with increased uranium enrichment.

Figure 5:SBU full core configuration with increased uranium enrichment.

2.3. Use of ADS

An accelerator driven system consisting of a high-power proton accelerator, a heavy metal spallation target (such as lead) that produces neutrons when bombarded by the high-power protons beam of 1 GeVis placed at the center of the core.Two more cases are examined:

- Case (3): The original WASB core with the low seed fuel assemblies’enrichment, namely3.1% (40 - FA), 2.6% (40 - FA)and 2.1% (36 - FA). The ADS is founded at the center of the core surrounded by ThO2 assemblies as shown inFigure 6.

- Case (4): The original SBU core with the low seed fuel assemblies’enrichment, namely3.1%, 2.6% and 2.1%.The core is coupled with the ADS at the center as shown inFigure 7.

Figure 6:WASB full core configuration with ADS.

Figure 7: SBU full core configuration with ADS.

3. Results

3.1. Effective Multiplication Factor (Keff)

The differences in the rate of decrease of Keff depend on the balance betweenconsumption of fissile material through fission and production of fissile material throughthe neutron captures in the fertile material. Efficient conversion gives a continuous buildupof new fissile isotopes, sustaining Keff at a higher level.

Figure 8 illustrates the changing of Keffwith time for the four mixed thorium -uranium reactor core cases. It can be noticed that the increase of the uranium enrichment in the WASB configuration realizes the longest core cycle length reaching almost 480 full power days. The use of ADS with the WASB core also extends the cycle length up to 180 full power days. Although the value of Keff drops below one beyond these 180 days decreasing steadily to about 0.92 at the end of cycle, it can be accepted with ADS.

It can also be seen from the curve of case 2 corresponding to the increase in the uranium enrichment in the SBU core that Keff exceeds the value of one at the BOC, but drops rapidly below one then increases again to one in the period between 110 and 250 full power days. This fluctuation can be attributed to theconversion of relatively large amount of 233U inside the reactor core. Comparing the curve of case 4 in Figure 8 to the SBU curve in Figure 3 shows that the use of ADS does not affect significantly the value of the Keff. A higher accelerator current will probably be needed to compensate for the relatively deeper sub-criticality associated with the SBU configuration.

Figure 8: Keffversus time for the four mixed thorium uranium cases.

3.2. Power Peaking Factor (PPF)

The values of the PPF should be kept as close to unity as possible to ensure an even radial fuel burn-up and to decrease cladding stresses due to elevated fuel temperatures resulting in an increase of the fuel cycle length.

The PPFvaries throughout the burning fuel lifetime. Figure 9 shows the time variation of the radialPPF during the core cycle length for the four cases under consideration.The values of the PPF for both SBU cores (cases 2 and 4) are approximately equal at the BOC ( 1.35) then fluctuateinsignificantly within acceptable values. This can be attributed to the relatively homogeneous distribution of the thorium inside the reactor core.

In the WASB core, the increase in the uranium enrichment (case 1) leads to a very high value of the PPF, around 3.7, at the BOC but dropping to about 1.6 after 50 days, then decreasing to 1.1 at approximately 100 full power days. The use of ADS in the WASB core (case 3) also results in a relatively high value of PPF, around 2.7, that drops steadily to about 1.6 at the end of cycle. This behavior is expected since in the WASB configuration the thorium is concentrated in specific assemblies in the core resulting in a considerable variation of power through the core at the BOC. Then as the power distribution flattens because of the reduction in power in the center of the core, mainly due to the consumption of 235U, accompanied by an increase in the power in the core periphery assemblies due to the production of 233U, the PPF tends to decrease. The insertion of some kinds of burnable poisons can be a solution to achieve an appropriate radial power distribution and consequently decrease the PPF below the safety limits.

Figure 9: Radial PowerPeaking Factorversus time for the four mixed thorium uranium cases.

3.3. Core Actinides

Figures 10 and 11 present the evolution of the 239Pu and 233U during the core cycle length respectively. As shown in Figure 10, the trends of plutonium accumulation in the reactor core are approximately the same for cases 2, 3 and 4 while case 1 produces higher amount of plutonium. Thus cases 2, 3 and 4 can be considered to be more attractive from the safeguards point of view. In addition, the use of ADS plays a very important role in fuel actinides burn-up [9] leading to less amount of high radiotoxicity isotopes in the dischargedspent fuel; This gives more advantage to cases 3 and 4.

Neutron capture by232Th produces233U which is extremely significant for the core performances and the next fuel cycles. From Figure 11, it can be seen that the breeding of 233U isotope is larger in the SBU cores than in the WASB cores. This is expected as in the SBU configuration the thorium is distributed almost uniformly over the core and is subject to relatively high neutron flux compared to the WASB case. In addition, in both configurations the presence of ADS enhances the 233U breeding.

Figure 10:Build-up of 239Pu over the first cycle length for the four mixed thorium uranium cases.

Figure 11:Build-up of 233U over the first cycle length for the four mixed thorium uranium cases.

3.4. Fission product poisons

Two fission products, Xenon-135 and Samarium-149 have substantial impact on reactor design, operation and control.These two nuclei have very large neutron absorption cross sections and are produced in large quantities directly from fission as well as from the decay of fission products. Their destructions are driven by burnout in neutron flux because of high absorption cross section.In addition, 135Xe decays to a stable isotope.

Figures 12 and 13 representthe accumulation rates of 135Xe and 149Sm in the core for thefourstudied cases. The figures show that in all the cases, the buildup of both isotopes increases and reaches equilibrium in the early stages of the core cycle. They also show that the presence of the ADS plays an important role in burning up these fission product poisons in agreement with reference [9].

Figure 12:Accumulation of 135Xe over the first cycle length for the four mixed thorium uranium cases.

Figure13:Accumulation of 149Sm over the first cycle length for the four mixed thorium uranium cases.

4. Conclusion

MCNPX 2.7.0 code was used to simulate the first core of a Westinghouse AP1000reactor with about 40% of its total fuel replaced by 232ThO2considering two configurations, mainly the WASB and the SBU. A comparison between the effects of increasing uranium enrichment and using of ADS for the compensation for the consequent sub-criticality is presented. The effective multiplication factor, the power peaking factor and the fuel burnupwere evaluated and compared for four different core configurations studied in this work.

Using the WASB configuration, the first core cycle length of the AP1000 based only on uranium fuel could be recovered by increasing the enrichment of the uranium in the uranium-thorium fuel mixture. With the ADS, the cycle length reached 180 days but the reactor can still operateunder the condition of accelerators (sub-critical mode). The SBU core with the ADS can also operate under this condition; but in this case the reactor core is deeply sub-critical so higher enriched uranium fuel is needed to avoid using higher proton source intensity.Although adrawback arises with both WASB cases because of the relatively high PPF, this can be remedied by the insertion of appropriate burnable poisons.  

An important target for using thorium fuel is to breed as muchfissile isotope 233U as possible in addition to decrease the amount of 239Pu produced for safeguards concerns. The use of ADS either in the WASB or the SBU cores achieves both goals. In addition, it reduces the production of the fission product poisons especially the Xenon-135 and Samarium-149.

Therefore, the ADS can be used with thorium for energy production while enhancing the fertile-to-fissile conversion as well as the transmutation of long-lived radioisotopes. However, it is suggested that the use of ADS is associated with an increase inthe 235U enrichment in order to compensate for the sub-criticality of the core.

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