Annals of Nuclear Energy 40 (2012) 215–220

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Annals of Nuclear Energy

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Review

Fuel assembly with inert matrix fuel rods as reload options for Laguna Verde NPP

Hector Hernandez Lopez ⇑Instituto Nacional de Investigaciones Nucleares, Carr. Mexico-Toluca s/n (km 36.5), La Marquesa, Ocoyoacac, Mexico 52750, Mexico

a r t i c l e i n f o a b s t r a c t

Article history:Received 15 August 2011Received in revised form 28 October 2011Accepted 1 November 2011Available online 26 November 2011

Keywords:BWRInert matrix fuelFuel reload

0306-4549/$ - see front matter � 2011 Elsevier Ltd. Adoi:10.1016/j.anucene.2011.11.001

⇑ Tel.: +52 55 53297200x2461; fax: +52 55 532973E-mail address: hector.hernandez@inin.gob.mx

Four different fuel assembly reload options were studied for Unit 1 at the Laguna Verde Nuclear PowerPlant (LVNPP) in Mexico. The purpose was to design a fuel assembly as a mixture of UO2 and weaponsgrade plutonium in an inert matrix. This assembly design has a 10 � 10 geometry array, containing 42PuO2–CeO2 fuel rods, 34 UO2 fuel rods and 16 UO2–Gd2O3 fuel rods. This assembly design is equivalentto those not containing plutonium and already designed for LVNPP-1.

The neutronic design and nuclear data bank generation of the two types of fuel assemblies, with andwithout plutonium, were performed with CASMO code. The steady state reactor core simulation was per-formed with SIMULATE code, part of the Core Management System (CMS). Fuel rod cladding material wasconsidered the same for both the UO2 and inert matrix fuel rod. The rod to rod pitch was kept the same inthe fuel assembly design, but the pin diameter is smaller for the inert matrix fuel rod design.

To determine the possible use of the proposed mixed fuel assembly for a BWR fuel reload, three pos-sible reload schemes were considered. Four reload schemes proposed in this study. In the first case, thebase reload pattern, 116 new UO2 fuel assemblies are exchanged for the full core reload. The second pat-tern replaced 36 IM and 80 UO2 fuel assemblies. The third case introduced 80 IM and 36 UO2 new fuelassemblies. In the last reload scheme, 116 IM fuel assemblies were placed instead of UO2 fuel assemblies.

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Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2152. Fuel assembly design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2163. Simulation of operation of reactor core . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2174. Results. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2175. Conclusions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 219

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 220

1. Introduction

The availability of large amounts of reactor and weapons gradeplutonium in the world shows the necessity of anticipating situa-tions for the use and disposition of plutonium. Because light waterreactors (LWRs) prevail on the stage of electric energy generationby nuclear power, it is important to take into account the potentialof these reactors to reduce the plutonium inventory. Several stud-ies performed in Pressurized Water Reactors (PWRs) show thatreactor and weapons grade plutonium can effectively be burnedin these reactors, in assemblies with fertile-free fuel, and maintain-

ll rights reserved.

01.

ing reactivity control and other safety issues at least comparable tothose related to the standard fuel normally used.

Full cores of mixed oxide (MOX) fuels have been proposed forPWRs. In these studies, degradation of some nuclear parametershas been found, for example delayed neutron fraction. This prob-lem leads to issues related to operation and control. However,other studies (Tochihara et al., 1995; Aniel-Buchheit, 1995) showthat some modifications in control design and operation made pos-sible the use of 100% MOX cores. Highly-moderated reactors con-sume more plutonium, while producing less actinides. Since thetwo main actinides, plutonium and americium, present significantresonances at thermal energies, the moderation ratio (water vol-ume to fuel volume) becomes and important parameter for thecore design of PWRs with 100% MOX fuel (Prunier et al., 1995;Barbrault, 1996). By increasing the moderation ratio, more

216 H.H. Lopez / Annals of Nuclear Energy 40 (2012) 215–220

neutrons have the chance of reaching thermal energies andincreasing the probability of escaping the actinides resonance re-gion. Some kinetic parameters also show favorable performancein high moderation ratio reactors.

Regarding Boiling Water Reactors (BWRs), new fuel assemblydesigns present a decrease in the geometric dimensions of fuelrods. As a consequence, an increase in the moderation ratio isachieved. Some of these new designs also consider the possibilityof using plutonium oxide as fuel material. Studies are then per-formed to determine the amount of power generated and if allsafety constraints are satisfied.

In addition to existing MOX fuel designs for plutonium stockpilereduction, alternative fuel designs are being and studied for possi-ble use in power reactors. In particular, for LWRs, Inert Matrix Fuel(IMF) and Rock-like Oxide (ROX) fuel (Yamashita et al., 2002; Deg-ueldre et al., 1995, 1997) concepts have been shown as a viablealternative for plutonium consumption in the future.

The homogeneous solid solution of PuO2 in stabilized ZrO2, withErbium as burnable poison, is the most studied concept for pluto-nium burning in LWRs. This IMF concept has important physicaland economical advantages. However, the PuO2–ZrO2 IMF has alower thermal conductivity than the UO2 matrix.

Thus, other types of IMFs have been proposed because of theirhigher thermal conductivity, as MgAl2O4, CeO2, Y3Al5O12 and SiCfor cercer fuels, and Zr and SS for cermet fuels (Degueldre et al.,1997). Although all these material compounds still present physi-cal and chemical issues, the fuel assembly design and simulationof neutronic behavior during a fuel cycle can help identify potentialproblems for power reactor operation.

A more realistic option for use of IMFs in power reactors re-quires that fuel assemblies contain both UO2 fuel rods and otherfuel rods with some of the IMFs mentioned above. For use in PWRs,design concepts of a fuel assembly have been proposed. Both full(Paratte et al., 1995) and partially loaded cores have been studied.Heterogeneous fuel assembly designs contain UO2 and PuO2–MgA-l2O4 (Lombardi and Mazzola, 1996) or PuO2–CeO2 fuel rods (Aikeet al., 1994).

Computations of the dynamics of a reactor core loaded with tra-ditional UO2 or MOX fuel can be performed with high confidencewith the current analysis techniques. On the contrary, simulationof power reactor operation is still not fully acceptable when IMFs

Fig. 1. Uranium fuel rod (left) and inert

are introduced to the reactor core. This is so because of the quitedifferent neutronic characteristics of the IMF in a LWR, when com-pared to those of UO2 or MOX fuels.

The Instituto Nacional de Investigaciones Nucleares (ININ, Na-tional Institute for Nuclear Research) of Mexico, at its Departamen-to de Sistemas Nucleares (DSN, Department of Nuclear Systems),currently carries out research on diverse alternatives to use IMFas an option to fuel reloads for the Laguna Verde Nuclear PowerPlant (LVNPP). This work presents first the neutronic analysis ofa fuel assembly design, which contains a combination of plutoniumoxide (in an inert matrix) fuel rods, uranium oxide fuel rods, anduranium oxide with gadolinia fuel rods. Also, simulations for threedifferent fuel assembly reload options were performed.

2. Fuel assembly design

Some studies have shown that one of the best candidates to be-come the IM, in a uranium free fuel, is the oxide of cerium. One rea-son is that the CeO2 crystalline structure is similar to that of thePuO2. Other reason is its very low microscopic cross section of neu-tron capture at thermal energies (0.13 barns at 0.625 eV). Anotherimportant advantage of a PuO2–CeO2 fuel matrix constitutes a solidsolution no soluble in hot water. This type of IMF was thereforechosen to design fuel assemblies for different options for a BWRfuel reload.

Geometrically, the IM fuel rods have smaller dimensions thanthose of UO2, but the rod to rod pitch was kept the same. In thisway, the moderation ratio was increased for the IM fuel rods.Fig. 1 shows the dimensions of fuel pellet and cladding for boththe standard uranium oxide pin and the IM pin.

The fuel assembly base design is a 10 � 10 geometric array,which contains 42 PuO2–CeO2 fuel rods with weapons-grade pluto-nium, 34 fuel rods of UO2, and 16 fuel rods contain fuel and burn-able poison, UO2–Gd2O3. The isotopic concentrations for each typeof fuel pin used in the cell calculations are shown in Table 1, andthe distribution of the pins in the assembly is shown in Fig. 2.The fuel lattice neutronic design and nuclear data bank generationwere carried out with CASMO code (Dave and Forssén, 2005).

To pursue the three-dimensional simulation of the core, threeaxial zones were considered in the design: two of them onlycontain natural uranium and are located at the ends of the fuel

matrix fuel rod (right) dimensions.

Table 1Fuel rod isotopic composition of inert matrix fuel assembly.

Pintype

# ofpins

235U(w/0)

238U(w/0)

239Pu(w/0)

Gd2O3

(w/0)CeO2

(w/0)

1 3 2.00 98.00 – – –2 1 2.40 97.60 – – –3 2 2.80 97.20 – – –4 4 3.20 96.80 – – –5 6 3.60 96.40 – – –6 18 3.95 96.05 – – –IMF 42 – – 32.60 – 67.40G 16 3.95 96.05 – 4.00 –

H.H. Lopez / Annals of Nuclear Energy 40 (2012) 215–220 217

assembly, while the central zone was considered as a homoge-neous region of either uranium oxide, with or without gadolinia,or IMF. Dimensions for the natural uranium regions and the centralregion are shown in Fig. 2.

3. Simulation of operation of reactor core

To determine the possible use of the proposed mixed fuelassembly for a BWR fuel reload, three possible reload schemeswere considered. The base case was a previously chosen reloadpattern for LVNPP. This base case satisfies both the energy genera-tion required for the full reload and the safety requirements (ther-mal limits). A comparison can then be performed between keyparameters to determine if some gain can be obtained when usingthe IMF containing weapons-grade plutonium.

The simulation of the three-dimensional, steady state, neutroniccore behavior was performed with the simulation codes that com-pose the Core Management System (CMS). CASMO-4 was used togenerate the nuclear data bank, and the simulation of the reactoroperation in steady state was performed with the SIMULATE code(Dean, 2005). For the simulation, the same plant operational condi-tions are used for the UO2-only and Pu2–CeO2 fuel assemblies.

Fig. 3 shows the four reload schemes proposed in this study. Inthe first case, the base reload pattern, 116 new UO2 fuel assemblies(29 assemblies per quarter of core, as shown in Fig. 3a) are ex-changed for the full core reload. The second pattern replaced 116UO2 spent fuel assemblies with 36 IM and 80 UO2 fuel assemblies

Fig. 2. Inert matrix fuel lattice

(Fig. 3b). The third case introduced 80 IM and 36 UO2 new fuelassemblies (Fig. 3c). In the last reload scheme, 116 IM fuel assem-blies were placed instead of UO2 fuel assemblies (Fig. 3d).

4. Results

The moderator–fuel ratio in a reactor affects two importantcompeting neutronic phenomena. The first of these is the neutronabsorption fraction in the fast and resonance neutron energy re-gion. As the moderator fuel ratio increases, the fraction of epither-mal neutrons absorbed in the fuel material decreases, and thefraction of neutrons reaching thermal region energies increases,thus increasing the infinite multiplication factor, as shown in Fig. 4.

In the other hand, in a partially enriched system, the greaterpart of the epithermal absorptions are captures in the fertile mate-rial, for example, 238U, that do not contribute to neutron multipli-cation. Since the fertile material in the fuel is the main thermalneutron absorber in the system, increased neutron thermalizationincreases the neutron total fraction that causes thermal fissions,increasing the thermal power and directly affecting the local peak-ing factor, as shown in Fig. 5.

During the simulation of four reload schemes, it shows that inthe bottom core, especially with reloads achieved with schemes1 and 2, there is a greater power at the beginning of the cycle(BOC) as a result of the spectral shift strategy, as shown in Fig. 6.In the other hand, at the end of the cycle (EOC), approximately ina burnup of 12.2 GWD/MT or 480 days of operation at full power,all reload schemes show a similar behavior and it is also observedthat the maximum values at top is due to the operation reactorstrategy.

The presence of a greater amount of plutonium in the core ismanifested in the increase of effective multiplication factor values,Fig. 7. Since, as mentioned above, there is an increase in the infinitemultiplication factor and a decrease in the thermal non-leakageprobability due to softening of the neutron spectrum.

The excess reactivity taken into account for reloading schemes 3and 4 would result in a cycle length increase. With respect to theaverage fuel burned in the core, it is very similar and it only man-ifests a displacement produced by uranium decrease in reactorcore. Fig. 8.

design and axial regions.

Fig. 3. Fuel reload schemes proposed for Unit 1 of LVNPP.

Fig. 4. Infinite multiplication factor comparison from the IMF and UO2 lattices atthree void fractions.

Fig. 5. Local peaking factor comparison from the IMF and UO2 lattices at three voidfractions.

218 H.H. Lopez / Annals of Nuclear Energy 40 (2012) 215–220

Fig. 6. Axial Relative power average for four fuel reload schemes analyzed at bothBOC and EOC.

Fig. 7. Effective multiplication factor during the operating cycle for the four reloadschemes.

Fig. 8. Average core exposure during the operating cycle for the four reloadschemes.

Fig. 9. Fast neutron flux for UO2 and inert matrix fuel assemblies at three points inthe cycle of operation.

Fig. 10. Thermal neutron flux for UO2 and inert matrix fuel assemblies at threepoints in the cycle of operation.

H.H. Lopez / Annals of Nuclear Energy 40 (2012) 215–220 219

Fig. 9 shows how the fast neutron flux behaves in both fuelassemblies UO2 and in the inert matrix; thus, is it possible to seethat the UO2 fuel assembly has more hardened flux in the lowerpart, up to three points that were taken into account during theoperating cycle (beginning of cycle, middle of cycle and end of cy-cle), while the inert matrix fuel spectrum hardening occurs in thetop of the fuel at the end of the cycle.

For thermal flux, Fig. 10, which is only at the beginning of thecycle in the bottom of UO2 fuel assembly, a difference can be ob-served in the neutron flux, compared to the inert matrix fuel.Meanwhile, the top of the inert matrix fuel has a higher neutronflux for the three points raised during the cycle, emphasizing itmainly in the middle and at the end of the cycle, which directly af-fects the multiplication factor due to the greater thermalization, ta-ken in the inert matrix fuel.

5. Conclusions

As can be seen in the results, the proposed fuel design based in-ert matrix plutonium used as a total or partial reload shows a

220 H.H. Lopez / Annals of Nuclear Energy 40 (2012) 215–220

behavior similar to a reload of UO2. However, some details mayhave influence in the planning of operation cycle length as resultfrom reactivity of plutonium. Furthermore, certain considerationsmust be taken respect to performance of each elements of inertmatrix fuel assembly due to that is exposed to neutron spectrummore hardened. Also, could contain high peak linear heatgeneration.

The applications of these studies are mostly focused on PWRreactors, however, one of the objectives of this study is to conductstudies aimed at BWR such as Laguna Verde to discuss the impactthat this represents both the power generation as an alternative toconsider for reducing the inventory of plutonium weapons-gradein the world.

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Tochihara, H., Komano, Y., Ishida, M., Mukai, H., 1995. Full MOX core design foradvanced PWR. In: Proc. Int. Conf. Evaluation of Fuel Cycles for Future NuclearSystems, Global ’95, Versailles, France, September 11–14, 1995, p. 1401.

Yamashita, Toshiyuki, Kuramoto, Ken-ichi, Akie, Hiroshi, Nakano, Yoshihiro, Nitani,Noriko, Nakamura, Takehiko, Kusagaya, Kazuyuki, Ohmichi, Toshihiko, 2002.Rock-like oxide fuels and their burning in LWRs. J. Nucl. Sci. Technol. 39 (8),865–871.