INERT MATRIX — CERMET FUELS: TOWARD OPTIMIZED UTILISATION OF Pu IN A PWR NEUTRONIC STUDIES — THE TANOX EXPERIMENT

J. PORTA, J.Y. DORIATH, S. BALDL B. GUIGON Centre d etudes nucleaires de Cadarache, XA9953271 Saint-Paul-lez-Durance Ph. DEHAUDT, A. MOCELLIN Centre d'etudes nucleaires de Grenoble, Grenoble

C. AILLAUD Centre d'etudes nucleaires de Cadarache, Saint-Paul-lez-Durance

France

Abstract

In order to address the new issues linked to the checking of the nuclear programme and the abandonment of the fast reactor system in France, the Nuclear Reactor Division (DRN) of the CEA launched a study programme with the aim of increasing the plutonium load in pressurized water reactors.

INTRODUCTION

The nuclear industry landscape has considerably changed since the 1960s: the development of the fast breeder reactor system was first curtailed, then progressively left in abeyance, which resulted in the accumulation of large amounts of plutonium. A temporary solution for these stocks of plutonium consisted in using them in pressurized water reactors (PWRs). However, for safety reasons, essentially related to the great sensitivity of the cores of these reactors - due to a very negative moderating coefficient that makes cold hazards particularly difficult to control -, the safety authorities decided to limit core loading of Pu in its conventional, MOX (Mixed Oxide) form to only 30 %.

On the other hand, for reasons linked to proliferation resistance and economics, increased burnup of these fuels is highly desirable.

Although 20 reactors of the French nuclear power plants are authorized to operate with a load of 30 % MOX, only 17 currently use this fuel, while eight units are soon to receive authorization to operate using this fuel.

100 % MOX core loading could obviously be envisaged [1], [2]; however, this would mean evolving towards higher moderation ratios of nearly 4 or 5 to enable efficiently controlling the reactor core, and this would entail making considerable changes to both the core array and more generally to the whole architecture of the core. Another way of considering the problem of increasing plutonium consumption and eliminating the conversion of U 238 to Pu 239 would be to use plutonium not in MOX form, but on a neutronically inert support, otherwise known as an inert matrix.

331 Very rapidly, analyses revealed the importance of micro structures in the nature of the fuel. Theoretical development, supported by experiments performed in the SELOE reactor at Grenoble, France, resulted in a better determination of the role played by the size of the grains and their interlinks in the retention or release of fission gases (3, 4, 5, 6)

Obtaining high burnups also poses numerous problems with respect to the resistance of the Zircalloy alloys used for fuel cladding: corrosion, thermomechanics, aging under irradiation, etc.

An experimental tool that would permit rapidly achieving high burnups was required to enable testing various microstructures and different compounds. This is why the TANOX device was designed and installed in the SILOE reactor at Grenoble.

I. CHOICE OF MATRICES

Studies were oriented towards two types of composite fuels, CERCER in which the ceramic fuel (UO2, MOX, PUO2) is embedded in a ceramic matrix (spinel, magnesia), or CERMET, in which the fuel is dispersed in a metallic matrix.

From the neutronic point of view, the ceramics implemented are transparent to neutrons and do not require particular analysis. This, however, does not hold true of the metals and, as can be observed in Figure 1 and Table 1, their responses to reactivity can vary widely.

—H— 25,7%PuO2 + 74,3%Mo — 14,3%PuO2 + 85,7%lnconel 6,9%PuO2 + 93,1 %2ircaloy

0 5000 10000 15000 20000 25000 30000 35000 40000 45000 50000 Mwj/t* (PuO2 volumic content)

Figure 1 : PuO2-based CERMET fuels - Reactivity swing for different metals

332 Table 1 : Kinetic coefficients for several fuels

Fuel BOL EOL Doppler pcm/°c / Drain, coefpcm Doppler pcm/°c / Drain, coefpcm

UO2 -2.00 -61429 -3.39 -94613 UO2+INCONEL -1.99 -33029 -3.29 -63650 UO2+MOLYBDEN -1.68 -45168 -2.59 -75969 UO2+Zry -1.72 -53536 -3.55 -181167 MOX+INCONEL -2.7 +4953 -2.98 +3281 MOX+MOLYBDEN -1.93 +11618 -1.99 +13434 MOX+Zry -2.97 -13471 -3.42 -28877

PuO2+INCONEL -1.04 +4183 -1.10 -3841 PuO2+MOLYBDEN -0.71 +9481 -0.57 -8922 PuO2+Zry -1.09 -9873 -1.80 -142821

PuO2+ThO2 -3.30 -16690 -3.47 -31704

Table 2 : metal volumic fraction=0.5, cycle lenght=18 months, fissile material content(%)

Matrix UO2 CERMET CERMET CERMET CERCER MOX PuO Pu/Th uo2 2 3.7% **uo2** Inconel 14.1% 32.5% 14.3% Molyb. 18.7% 50.5% 25.7% Zry 7.5% 18% 6.9% **Th** 14%

Table 2, in particular, gives an overview of the contents in fissile material required by the various composites to ensure a burnup (BU) of 120 Gwd/t up to unloading.

The logical choice should be directed towards a metal that is transparent to neutrons, such as Zircaloy, which is able to assure the desired cycle with the lowest plutonium content and the highest potential reactivity value (Figure 1).

However, for reasons of availability and know-how, molybdenum was selected for the first experiments in the TANOX device on CERMETs and the spinel MgAl2O4 for those on CERCERs.

n. THE TANOX DEVICE

The TANOX device (Figure 2) is a rotating cylinder that has six positions. The fuel rods are about six millimeters in diameter and are pierced by a central hole to allow the passage of the thermocouples that enable controlling the experiment through the core temperature of the fuel. The device turns regularly to ensure that all the rods have the same combustion rate.

333 Table 3 : Release fraction of 85Kr for the TANOX pins

Fuel BU GWd/t 85Kr release 85Kr release 1580°C 30 mn 1700°C 30 mn UO2 19.58 0.1 0.4 CERCER 40.3 0.25 0.51 CERMET 55.4 [ 6.12 0.17

The device is contained in a box that can be moved from actual contact with the core of the SILOE reactor to a distance of little less than twenty centimeters away from it (Figure 2). The device permits achieving burnups in the composite pellets of about 0.5 Gwd/t per day of operation at full power of the SILOE reactor.

Eg. FIRST TANOX EXPERIMENTS USING CERCER AND CERMET RODS

The TANOX 2 experiment was performed on spinel rods (64 % MgAl2O4 - 36 % UO2) (% in volume) with 19.6 % U235 enrichment.

The CERMET used was 64 % Mo, 36 % UO2, also with 19.6 % U235 enrichment.

Fuel Pin Location

EDITH MOX DENSIMOX MAGPIE IRIS 3

QUADRJREME 2

iRCAS 2

Figure 2 : SILOE core and TANOX location

334 Table 4 Some characteristic of mass balance

Initial Reactivity

Month UO2 MOX CERMET CERMET CERMET Pu 40% UO2 40% MOX 12 1 0.54 1.44 0.81 1.17 18 1.19 0.62 1059 0.87 1.17 24 1.31 0.69 1.67 0.93 1.19

Enrichment

Month UO2 MOX CERMET CERMET CERMET Pu 40% UO2 40% MOX 12 3.19 8.26 9.49 17.65 6.74 18 4.49 11.28 14.15 27.38 10.39 24 5.88 14.14 19.02 37.21 14.22

Initial Fissile content (kg)

Month UO2 MOX CERMET CERMET CERMET Pu 40% UO2 40% MOX 12 750 1938 732 1262 1063 18 1056 2647 1091 1958 1661 24 1382 3318 1466 2660 2304

BU in at%

Month UO2 MOX CERMET CERMET CERMET Pu 40% UO2 40% MOX 12 67.4 34.8 88.9 71.2 97.5 18 76.1 41.8 95.4 74.8 97.9 24 80.8 46.7 98 77 97.4

BU in weight (kg)

Month UO2 MOX CERMET CERMET CERMET Pu 40% UO2 40% MOX 12 505.3 378(Pu9) 650 542 653(Pu9) 18 803.5 620.5 1041 885 1018 24 1117 869.5 1438 1238 1399

335 50%MOX + 50%Zr Evolution of the isotopic composition of Plutonium

• OEFPD ® 112,5 EFPD

8337,5 EFPD • 450 EFPD

Pu-238 Pu-242

Figure 3 : Plutonium consumption in a MOX CERMET

Table 3 shows the excellent behavior of the CERMET as regards the release of fission gases.

This was confirmed when the cladding was opened (Figures 4 and 5). The behavior of the CERCER was disappointing: the composite evidenced considerable swelling and generalized pellet restructuring, whereas the integrity of the CERMET was preserved. This is why a second series of irradiation tests was launched, using a CERMET composite: the aim of TANOX CCE, (a composite fuel comprising erbium) was to reach, or even exceed, a burnup of 100 Gwd/t, to examine the behavior of the materials, the fuel, the matrix, and the absorbers, and to validate the consumption kinetics of erbium. The composite was formed of 80 % Mo, 20 % UO2, with 40 % U235 enrichment; the ceramic also included 2 % Er2O3 in weight.

The test series ended in December 1997 and the results obtained at 136 Gwd/t confirmed the following points:

The excellent dimensional stability of the CERMET under irradiation, The very slight degassing revealed in the TANOX 2 experiment, The excellent thermal conductivity of the composite which permits core temperatures under PWR conditions of about 420° C (the TANOX CCE irradiation test was performed at 350 W/cm, i.e., a temperature of about 530° C), The good agreement of the theoretical models provided by calculation of the composites' thermal conductivity (Maxwell-Euken) with the experimental results.

IV. EVALUATION OF THE CYCLE

Using the simplified model of a PWR-type assembly, we determined various parameters, such as enrichment, initial fissile material balance, etc. (Table 4) for different CERMET (Zircaloy) composites for cycles of 12, 18, and 24 months.

This clearly showed that Pu CERMETs, as well as MOX CERMETs, are excellent plutonium consumers and that, in addition (Figure 3), good cc umption of Pu 239 is achieved without too high an increase of the Pu 240 concentration

336 Figure 4 : CERCER fuel after irradiation a - aspect of the pellets after opening of the pin b - macrography of two stuck pellets c - fuel microstructure: transversal section of a pellet d - presence of a strip surrounding the UO2 nodules at the pellet edge (after chemical attack) e - absence of strips in the pellet center (after chemical attack)

337 ^s*-^ • - - - - ij -*, < v*y. &

Figure 5 : CERMET fuel after irradiation a - aspect of the pellets after opening of the pin b - fuel microstructure along a radius c & d - fuel microstructure with UO2 nodules in the Molybdenum matrix

338 CONCLUSION

CERMET composites have numerous advantages not only as regards their behavior under irradiation and their behavior as cold fuels, but also as Pu consumers. However, their low (3eff will make these cores very sensitive to accidents such as rod ejection; in addition, their very hard spectrum reduces the efficiency of soluble boron and their low Doppler coefficients are liable to be of little effect in countering power excursions.

As these 100 % Pu CERMET assemblies are too delicate to be used from the standpoints of reactivity control and safety coefficients, studies should be oriented towards heterogeneous assemblies, in which conventional UO2 fuels would provide the necessary margins for the (3efF and safety coefficients. It is why the APA (Advanced Pu fuel Assembly) which is presented in a companion paper (7, 8, 9, 10) was defined

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