MOX Recycling in PWR

MOX Recycling in PWR

Giovanni B. BrunaIRSN – DSR dir

Zone Vidangée

3.7% UOX

Summary

• MOX (Mixed Oxide) Fuel Recycling in PWRs

2.2. Pu Recycling in France Pu Recycling in France

3.3. Design & safety featuresDesign & safety features

4.4. Void Effect in PWR Plutonium fueled Void Effect in PWR Plutonium fueled corescores

Pu Recycling in France: Year-Lasting Experience

• In 1976 France adopted a « partially closed » cycle in 900MWe PWRs aiming at

• Improving the fossil fuel utilization• Limit Pu build-up• Use the huge amount of depleted

Uranium,• Reduce the amount of wastes (and their

activity

• Concentrate Pu in reactors:

Pu Rec.

With FBR

Open UOX Cycle

• MOX loading in 900 MWe PWR cores:

a. Three-zoned assembly, b. At equilibrium, 1/3 of the core assemblies

contain MOX fuel,c. Average Pu enrichment of the fuel : 7,0%,d. Objective burn-up : 50000 MWd/ton heavy

metal

Pu Recycling in France: a Year-Lasting Experience

Gd-poisoned Assembly

Water tubes eau

CYCLADES L.S. – 12 Gd2O3 pin/ass.

8 % C Gd2O3 pins

Current MOX Assembly

Low-enrichment pins

Intermediate-enrichment pins

High-enrichment pins

Water tubes

Pu Recycling in France: a Year-Lasting Experience

• MOX fuel in PWRs 1/4: • A grain-structured fuel

• Pin power distribution,• Pin thermo-mechanical behavior,• Volatile F.P. release,

• A lower number of fission per MWth •Fission energy release•Pu : 210 Mev / fission, vs. U : 200 Mev / fission

• P.F Build-up• Short-term Residual power

Physics of MOX Recycling in PWR

• MOX fuel in PWRs 2/4:• A Fission efficiency (per gram)

•~ U235 for WG Pu,•< U235 for RG Pu

• A roughly equivalent Doppler Coefficient,• A slightly higher Moderator Coefficient,• A reduced absorber worth (up to 60 – 70 % for

the assembly):•Soluble boron, •Control clusters, •Poisons (burnable and not-burnable).


• MOX fuel in PWRs 3/4 :

- An increased competition among fuel, structural materials and moderator, and a slightly increase of leakage.

Shorter prompt neutron lifetime,

- An increased epi-thermal efficiency, A reduced capacity to escape traps.

- A lowered thermal fission,- An increased epi-thermal and fast fission,

Improved fast neutron utilization.



1.MOX fuel in PWRs 4/4 :

2. A smaller Delayed-neutron Fraction (eff),

3. An almost absent Xenon poisoning,

4. A smaller reactivity swing vs. Burn-up (higher Internal Conversion ratio ~0.75 vs. 0.60)

Contribution frommain Isotope Families to reactivity swing vs. Fuel Burn-up 0

20

40

60

FissionProducts

HeavyIsotopes

StructuralMaterials

MinorActinides

• Pin-wise Power Control• Compensation of physical effects through the

assembly design

FISSION REACTION RATES vs. LETHARGY(Infinite medium calculations)


• Pin-wise Power Control• Compensation of physical effects through the

assembly design


Original assembly design

• Pin-wise Power Control• Compensation of physical effects through the core

loading strategy

OUT-IN


• Fuel Burn-up / Breeding Process• Actinide build-up chain

Possiblesimplification Real process

n - 2n

n

Fission products and energy production by fusion

242Cm 243Cm 244Cm

243Am

240Pu 241Pu 242Pu239Pu238Pu

237Np

236U 237U 238U235U

242Am241Am

- 25 minutes32 years

16 hours 18,1 years163 days

13 years~ 5 hours

2,10 days 2,35 days 33 minutes

5,57 days 23,5 minutes

239U


• Fuel Burn-up / Breeding Process •Contribution of Actinide families to the reactivity swing vs. Fuel burn-up [MOX] UO2 MOX RCVS

Uranium 80 5 1

Plutonium - 20 41 26

Minor Actinides 3 7 16

Fission Products 33 47 57

4

TOTAL 100 100 100

Typical Reactivity swing

(Annual cycle 10 Gwd/ t - pcm -)

8500 4300 3500

*Lower than 0.5


*Lower than 0.5

Xenon-poisoning Effect at equilibrium 1500 pcm

Soluble Boron Worth ( per ppm) 7 pcm

Black Control Rod Worth (per Rod) 600 pcm

Gray Control Rod Worth (per Rod) 450 pcm

Doppler Coefficient 3 pcm/K°


Moderator Coefficient > UOX

1.1. Sensitivity of PWR core to the Sensitivity of PWR core to the Plutonium contentPlutonium content::

a. Reactivity Quite Low ( 600 pcm / % Pu)*b. Void Effect Very High (5 000 pcm / %

Pu)*c. Control Rod Worth Mediumd. Soluble Boron Worth Mediume. Burnable Poison Worth Mediumf. Power and Temperature Effects Low

*1% increase of Plutonium content (RG Pu)

Physics of MOX Recycling in PWRPhysics of MOX Recycling in PWR

1.Transient sensitiveness to Plutonium content

-LOCA -RIA-Main Steam Line Break (RTV)

2.Additional Control Rods,

3.Constraints on the Loading Strategy,

4. System Modification


• Design constraintsDesign constraints:

Limit the Plutonium enrichment in the fuel and its core content to guarantee the safe operation against:

- The Soluble Boron and Control Rod Worth decrease,

- The Modified et more sensitive Operating conditions,

- The Increased Uncertainty.

Physics of MOX Recycling in PWRPhysics of MOX Recycling in PWR

• Neutronics behavior of PWR cores in case of LOCA is sensitive to the Plutonium content because:

- The MOX Moderator Coefficient is slightly different compared to UOX

- The Void Effect depends on the core

◊ Overall Plutonium content,

◊ Plutonium isotope composition,

◊ Heterogeneity.

Void effect in MOX fueled coresVoid effect in MOX fueled coresVoid effect in MOX fueled coresVoid effect in MOX fueled cores

• Reactivity swing in a Voided core:Reactivity swing in a Voided core:The reactivity swing in a Voided core results from The reactivity swing in a Voided core results from

compensations among a large number of huge compensations among a large number of huge individual isotope and reaction-rate individual isotope and reaction-rate contributions having opposite sign: contributions having opposite sign: - Every isotope contributes through Every isotope contributes through

several rates (absorption, fission, several rates (absorption, fission, slowing-down …)slowing-down …)

- Every individual component worth can be far bigger than the whole Void Worth,

- Big Uncertainty- Very large Sensitiveness of Void Worth

to the base data and the computation methodology.


1.Moderator vs. Void Effect in UOX & MOX Fuel

MOXUOX

Reactivity

Void Fraction

0 100


Full Void Reactivity depending on Plutonium content

Moderator Effect

Void Effect

• X.S. Behavior vs. Energy

0.2 Log E60 100

Résonances

Zone 1/v

1.0

Fission à seuil

0.3

1.8 8E56

Pu240

U238, Pu240,

…U238

U235,Pu239


Studies on Heterogeneous Void

1.Homogeneous Void : Progressive et uniform void of the sample,

2.Heterogeneous Void : Non-uniform, spotted Void of the sample; some regions are privileged,

3.The void fraction is the same but the reactivity swing is far different.

Studies on Heterogeneous VoidStudies on Heterogeneous Void

1.Accounting for leakage effect reduces the reactivity swing significantly

2.For sake of conservatism, the design calculations are always performed in an infinite medium, no leakage modeling approximation.


1.Coupling Effect

a. The reactivity of each region changes with the void fraction,

b. The neutronics importance of the region (i.e., the asymptotic contribution of the region to the reactivity) changes too, in the meantime.

2.The actual reactivity of the sample depends on region-wise importance (as a weighting function).


Homogeneous Void Heterogeneous Void

Computation sample : the central region can contain a MOX assembly


OCDE Benchmark sample

UO2

MOX


1. OCDE Benchmark

2. 3*3 assembly sample with 10*10 pins/ass.; (1.26 cm pitch): Inf. Medium Calc. with a variable Pu enrichment central MOX assembly:

a.HMOX 14.40b.MMOX 9.70c.LMOX 5.40

d.(UO2 3.35)


1.In the MMOX sample with water, typical parameter values are, respectively:

2.Zone Kinf*Imp*.

3.UO2 1.3697 0.88

4.MOX 1.1447 0.12

5.Sample 1.3427

a.*Rounded-off values


1. In the central-void MMOX sample, typical parameter values are, respectively:

2. Zone Kinf *Imp*.

3. UO2 1.3697 0.96

4. MOX 0.7738 0.04

5. Sample 1.3458

*Rounded-off values


K Inf Water K Inf Void

1. UO2 Inf. M. 1.3697* 0*

2. MOX Inf. M. 1.1447* 0.7738* -41900*

3. Sample 1.3427* 1.3458* + 170*

a.*Rounded-off values

Homogenous Void

Heterogeneous Void

« Envelop »


• Main calculation challenges:Main calculation challenges:

a.Space and Energy Heterogeneity;b. Streaming inn the voided regions;c. Self-shielding and dependence on the

temperature of epi – thermal resonances:- Pu39, Pu41 0,3 eV,- Pu40 1,0 eV,- Pu 42 1.8 eV;

d. Mutual resonance self-shielding.


MOX

3.7% UOX

Low and High Enrich.

UOX-MOX EPICURE

Qualification Qualification of Void of Void

calculations: calculations: MOX fueled MOX fueled

corescores

• Pin-power distribution measurement technique 1/2:

• A very careful characterization of the fuel is to be performed (to avoid effect of fabrication uncertainties);

• Activity is measured pin by pin through gamma spectrometry (relative values);

•But U and Pu R.R. are different (due to X.S. );•Thus gamma-scanning activities in U and Pu regions

are inhomogeneous: absolute values are necessary•Activities of some F.P. the Yields of which (both U

and Pu) are very well known (with equivalent uncertainty level) are measured independently as tracers,

•Y-scanning activity distribution are re-normalized to obtain absolute distributions;

•To obtain the power distribution from the activity, a suitable normalization is performed via a “ P/A ” conversion factor experimentally measured in reference mock-ups.

Qualification of Void calculations: Qualification of Void calculations: MOX fueled coresMOX fueled cores


• Pin-power distribution measurement technique 2/2:

• The process of measurement is very hazardous and complex,

• It is not fully independent from data and computation,

• The quality of the pin-wise experimental distribution depends on:

•The fuel fabrication process (homogeneity of composition and density),

•The representativeness of the experimental mock-ups The experimental techniques,

•The base-data used (Yields);•The robustness of the overall reconstruction process.

Qualification of Void calculations:Qualification of Void calculations:MOX fueled coresMOX fueled cores

Qualification of Void calculations:Qualification of Void calculations:MOX fueled coresMOX fueled cores

• Analysis of results:

• Despite

•The same experimental techniques are used a for all measurements

•The same schemes and options are adopted for computations,

• The discrepancies C/ E increase significantly with the sample Pu enrichment.


,

K Inf

• Possible explanation 1/2:

• Differences in the C/ E results can be explained by the effect of :

•Measurement uncertainties •Computation precision,

•Which both are sensitive to the spectrum hardiness (Pu enrichment).


• Possible explanation 2/2 :•Measurement are less precise with increasing enrichment, because:

•R.R. decrease,•Yield uncertainty increases;

•Computation precision is reduced with increasing enrichment because:

•The worth of the non-resolved resonance region increases;

•This region is generally far less well described in the libraries;

•Improvements to be made both in measurement techniques and computation.


• CONCLUSION CONCLUSION

The complexity of physical problems and the difficulty in the modeling increase with MOX fueling, which demands:

- A huge effort to improve the base-data and the computation tools,

- New qualification needs,- A conservative approach at the design

stage,- Several modification in the design and

operation- A wide integration of the operational

experience feed-back:- That’s current practice, now ….That’s current practice, now ….


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MOX Recycling in PWR