# Power Distribution and Possible Influence on  Fuel Failure  in WWER-1000

Authors: Ján Mikuš
Journal: Science and Technology of Nuclear Installations
 (2008)

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## Abstract

The work is focused on the influence of investigation of some core heterogeneities and construction materials on the space power (fission rate) distribution in WWER-1000-type cores, especially from viewpoint of the values and gradient occurrence that could result in static loads with some consequences, for example, fuel pin (FP) or fuel assembly (FA) bowing and possible contribution to the FP failure root causes. For this purpose, experimental data and their analysis from two earlier performed measurements on light water, zero-power reactor LR-0 were used, concerning the relative radial power distribution determined by measurements in a WWER-1000-type core containing single FPs with homogeneous gadolinium admixture (Gd2O3) and the relative radial power distribution determined by measurements in FA situated on the periphery of a WWER-1000-type core neighbouring the baffle (thermal shielding).


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## Body

## 1. Introduction

The LR-0 reactor in  the Nuclear
Research Institute Řež plc is  an experimental facility for determination  of the neutron-physical characteristics
of the WWER- and PWR-type lattices and shielding with UO2 or MOX fuel.
Exploitation of this facility is determined by maximum power of 5 kW and
maximum thermal neutron flux density of 1013m−2s−1,
atmospheric pressure, and room temperature (or heating up to 70°C). The fuel consists of the shortened
WWER-1000- and WWER-440-type fuel assemblies (FAs) containing the fuel pins
(FPs) with UO2 pellets  (O.D.
7.53 mm, internal central hole 1.4 mm) having a Zr+1%Nb cladding (O.D.
9.15×0.72mm) with the active length of 1250 mm and enrichment of 1.6–4.4 wt.% in U235. The FAs can be arranged in a reactor vessel of aluminium
(diameter 3.5 m, height 6.5 m);  the criticality is controlled by moderator
level (boron acid with concentration up to 12 g/L) and  control clusters (B4C
pellets).The most important applications on
LR-0 reactor concern the nuclear safety and economy of the WWER-type nuclear
power plants (NPPs) operation. In the frame of their modernization, a set of
experiments has been performed on the LR-0 reactor, for example, with new type
FAs with various burnable absorbers arranged in appropriate configurations and
cores of the both WWER-440 and WWER-1000 types including a number of
experiments with FAs containing single FPs with Gd (Gd FPs) burnable absorber
(Gd2O3) integrated into fuel (e.g., [1, 2]). Next
experiments concerned the reactor pressure vessel dosimetry and they included
measurements of the power (fission rate) distribution in periphery FAs neighbouring the baffle in WWER-1000-type
cores (e.g., [3]). Results of power (fission rate) distribution measurements presented in [1–3] are used and analysed in this work.It is well known that presence of
single Gd FPs in reactor core results in a local depression of thermal neutrons
in these FPs and corresponding gradients in neighbouring FPs. As for the baffle
influence, thermal neutron gradients can be expected in neighbouring FAs, too.
Similar situation can also be stated as for the power release and corresponding
temperature.It is also well known (e.g., [4, 5]) that neutron flux nonuniformity, gradients of the temperature, and neutron
current can represent root causes of the FP/FA growth and bowing leading to
local limitation of coolant flow, reduction of heat transfer, magnifying
cladding corrosion, and pellet/cladding interaction (PCI). Therefore detailed
information about power distribution in FA containing single Gd FPs and in
periphery FA neighbouring the baffle in WWER-1000-type core can be useful for
above phenomenon investigation. Since such data cannot be obtained in the NPPs,
some experiments on research reactors are provided.As for the fuel performance reliability
and fuel failure rates and root causes concerning WWER fuel, it was stated [6]
that an overview on WWER fuel failure root causes identified during 1992–2002 shows that
(i)
besides a large number of unidentified
causes, the most frequent cause is damage by debris;(ii)
different from Western
PWR fuel experience, fuel-rod-to-spacer-fretting does not play a
significant role;(iii)
at the same time there is a very high number of
defected fuel where no  failure cause is known.In case of the PWR fuel the EPRI evaluation of
2004 on US PWR fuel failure rates shows [6] that
(i)
after a continuous decrease from 1980
to 2001, there is some increase observed in 2002  and 2003;(ii)
the major contributor to fuel failure
rates in PWRs remains grid-to-rod fretting;(iii)
there is also an increase of fuel
failures with unknown root causes that primarily affects optimised fuel designs
with a thinner rod diameter.The above information is in
accordance with results concerning examination of 5 WWER-440 and 7 WWER-1000
FAs presented in [7]: causes of failure are debris fretting (54%), local
overheating (15%), grid-rod fretting (8%), and “the cause is not determined”
(23%).


## 2. Aim of Work

The aim of this work is providing
some information about below heterogeneities and construction material
influence on radial power (fission rate) distribution in WWER-1000-type cores, namely, concerning the(i)
single Gd FPs by means of power release
in an FA containing these FPs,(ii)
baffle by means of power release in
selected FPs in neighbouring FA.


## 3. Influence of Single Fuel Pins with Gadolinium on Power Distribution in a WWER-1000-Type Core

### 3.1. Experimental Arrangement and Conditions

The materials published in [1, 2] were used for this work preparation. Experiment was realised on reactor
LR-0 with critical height of the H2O moderator level being 266.10 mm (“0”
level is at the lower end of FP active part),
at atmospheric pressure and room  temperature in a WWER-1000-type core shown in
Figure 1(a), consisting of 7 FAs, each containing 312 FPs with 4.4%
enrichment, whereas 18 FPs in central FA (CFA) were replaced by FPs with 3.6%
enrichment containing 2.0% (by weight) of Gd2O3 admixture  (Gd FP pellets with O.D. 7.50 mm, internal central hole 1.5 mm), arranged with FP pitch of 12.75 mm; the
30-degree symmetry sector of CFA with 3 Gd FPs is shown in Figure 1(b). More
information concerning experimental arrangement and conditions can be found in
[1, 2].(a) Schematic arrangement of the LR-0 reactor core containing 7 WWER-1000 type-FAs with
central FA (CFA) for Gd FP influence investigation and (b) the 30-degree
symmetry sector of the CFA with 3 Gd FPs.
(a)(b)


### 3.2. Results

The power (fission rate)
distribution was determined by means of gamma scanning of irradiated FPs, measuring
their gamma radiation in the energy range of 600 to 900 keV. The radial
distribution (in a plane perpendicular to the core axis) was determined measuring
FPs activity of 50 mm region of FP surface at the central irradiated part along rotating FP using corresponding collimator. Measurements were performed in CFA at 74 FPs with positions shown in
Figure2(a); detailed results were presented in [2] and used for evaluation
of power distribution in symmetry sector of CFA shown in Figure 2(b). As
for this evaluation, absolute value of the difference of power release
concerning two FPs in symmetry positions (Figure 2(a)) was calculated and
related to the average of these two values and this was done for all above
symmetry positions to check reproducibility of the results presented in [2].
The mean and maximum above relative differences are 1.1% and 3.8%,
respectively.(a) Positions of measured FPs in CFA and (b) power distribution in the 30-degree symmetry
sector of the CFA with 3 Gd FPs.
(a)(b)Further experimental results were compared with
calculations based on a code treats the two-dimensional, four-group diffusion
equations using the so-called fine mesh method (each FP cell or absorbing rod
cell is considered as one point) and presented in [2]. Results of calculations
and measurements are in fairly good agreement: the mean value of the relative
power distribution difference absolute values is 2.4%; maximum positive and
negative relative differences are 6.2% and −12.7%, respectively.Finally the shape of the power distribution in the
neighbourhood of a Gd FP can be characterised by the following ratio:(1)Q=(average  energy  release  at  ring  of6FPs  around  a  Gd FP)(energy  release  in  that  Gd FP). As for Gd FPs (Figure 1(b)) in  positions nos. 1, 2, and 3, the
following experimental QE/calculated QC values were
determined: 3.718/3.553, 3.669/3.501, and 3.694/3.523 with relative
difference d=45%, 4.7%, and 4.7%, respectively, where(2)d=(QE−QC)[(QE+QC)/2].A fairly good
agreement between measured  and calculated power distributions can be stated,
given by above mean value of 2.4%,  as well as between experimentalQE and calculated QC quantities given by above difference d being about 4.7% for all three Gd FP positions. The power release depression in Gd FPs can be characterised by the
value of 1/Q with corresponding values in positions 1, 2, and 3 being   26.9%, 27.3%, and 27.1%, respectively.


## 3.1. Experimental Arrangement and Conditions

The materials published in [1, 2] were used for this work preparation. Experiment was realised on reactor
LR-0 with critical height of the H2O moderator level being 266.10 mm (“0”
level is at the lower end of FP active part),
at atmospheric pressure and room  temperature in a WWER-1000-type core shown in
Figure 1(a), consisting of 7 FAs, each containing 312 FPs with 4.4%
enrichment, whereas 18 FPs in central FA (CFA) were replaced by FPs with 3.6%
enrichment containing 2.0% (by weight) of Gd2O3 admixture  (Gd FP pellets with O.D. 7.50 mm, internal central hole 1.5 mm), arranged with FP pitch of 12.75 mm; the
30-degree symmetry sector of CFA with 3 Gd FPs is shown in Figure 1(b). More
information concerning experimental arrangement and conditions can be found in
[1, 2].(a) Schematic arrangement of the LR-0 reactor core containing 7 WWER-1000 type-FAs with
central FA (CFA) for Gd FP influence investigation and (b) the 30-degree
symmetry sector of the CFA with 3 Gd FPs.
(a)(b)


## 3.2. Results

The power (fission rate)
distribution was determined by means of gamma scanning of irradiated FPs, measuring
their gamma radiation in the energy range of 600 to 900 keV. The radial
distribution (in a plane perpendicular to the core axis) was determined measuring
FPs activity of 50 mm region of FP surface at the central irradiated part along rotating FP using corresponding collimator. Measurements were performed in CFA at 74 FPs with positions shown in
Figure2(a); detailed results were presented in [2] and used for evaluation
of power distribution in symmetry sector of CFA shown in Figure 2(b). As
for this evaluation, absolute value of the difference of power release
concerning two FPs in symmetry positions (Figure 2(a)) was calculated and
related to the average of these two values and this was done for all above
symmetry positions to check reproducibility of the results presented in [2].
The mean and maximum above relative differences are 1.1% and 3.8%,
respectively.(a) Positions of measured FPs in CFA and (b) power distribution in the 30-degree symmetry
sector of the CFA with 3 Gd FPs.
(a)(b)Further experimental results were compared with
calculations based on a code treats the two-dimensional, four-group diffusion
equations using the so-called fine mesh method (each FP cell or absorbing rod
cell is considered as one point) and presented in [2]. Results of calculations
and measurements are in fairly good agreement: the mean value of the relative
power distribution difference absolute values is 2.4%; maximum positive and
negative relative differences are 6.2% and −12.7%, respectively.Finally the shape of the power distribution in the
neighbourhood of a Gd FP can be characterised by the following ratio:(1)Q=(average  energy  release  at  ring  of6FPs  around  a  Gd FP)(energy  release  in  that  Gd FP). As for Gd FPs (Figure 1(b)) in  positions nos. 1, 2, and 3, the
following experimental QE/calculated QC values were
determined: 3.718/3.553, 3.669/3.501, and 3.694/3.523 with relative
difference d=45%, 4.7%, and 4.7%, respectively, where(2)d=(QE−QC)[(QE+QC)/2].A fairly good
agreement between measured  and calculated power distributions can be stated,
given by above mean value of 2.4%,  as well as between experimentalQE and calculated QC quantities given by above difference d being about 4.7% for all three Gd FP positions. The power release depression in Gd FPs can be characterised by the
value of 1/Q with corresponding values in positions 1, 2, and 3 being   26.9%, 27.3%, and 27.1%, respectively.


## 4. Influence of Baffle in WWER-1000-Type Core on Power Distribution in Neighbouring FA

### 4.1. Experimental Arrangement and Conditions

A 60-degree symmetry sector of WWER-1000
mockup in radial direction was realised in the LR-0 reactor at4.6±0.1g/l boron acid
concentration in moderator with critical height of 1500 mm  (“0” level is at the lower end of FP active part), at atmospheric pressure and room temperature—Figures 3 and 4.Figure 3
The section of the
mockup by the XY plane.Figure 4
WWER-1000 mockup general view on
reactor LR-0.This
mockup represents the core periphery and radial shielding heterogeneities of
the WWER-1000. The core loading was chosen to imitate neutron source inR−θ
geometry with following FAs no./enrichment: 2 FAs nos. 9, 17/3.3%, 6 FAs nos.
18, 24, 25, 29, 30, 32/3.0% and remaining 24 FAs with 2.0% enrichment (more
information in [3]). The materials published in [3] were used for this work
preparation.


### 4.2. Results

Influence of the baffle was investigated
by means of power (fission rate) distribution in neighbouring FA no. 4, using gamma scanning of irradiated FPs, measuring their gamma radiation of theLa140 with energy 1596.5 keV. The radial distribution (in a
plane perpendicular to the core axis) was determined by FPs activity of 50 mm
region of FP surface at the central part along rotating
FP using corresponding collimator.Results of the power distribution
measurements carried out in 107 FP positions in FA no. 4 are presented in Figure5(a). Power distribution values were determined
with mean and maximum relative errors being 3.2% and 5%, respectively.(a) Experimental and (b) calculation power distribution values in
selected FP positions in FA no. 4.
(a)(b)Obtained experimental values
were completed by calculations carried out in all 312 FA positions [3]. They
are illustrated by values in selected FP positions in Figure 5(b).Presented results demonstrate depression
of the power distribution in FA no. 4 neighbouring baffle that can be characterised
by the ratio of the power distribution mean values in two opposite FP rows in
FA no. 4 to- and outwards the baffle. The depression, for example, the above
ratio of about 19%, can be stated.


## 4.1. Experimental Arrangement and Conditions

A 60-degree symmetry sector of WWER-1000
mockup in radial direction was realised in the LR-0 reactor at4.6±0.1g/l boron acid
concentration in moderator with critical height of 1500 mm  (“0” level is at the lower end of FP active part), at atmospheric pressure and room temperature—Figures 3 and 4.Figure 3
The section of the
mockup by the XY plane.Figure 4
WWER-1000 mockup general view on
reactor LR-0.This
mockup represents the core periphery and radial shielding heterogeneities of
the WWER-1000. The core loading was chosen to imitate neutron source inR−θ
geometry with following FAs no./enrichment: 2 FAs nos. 9, 17/3.3%, 6 FAs nos.
18, 24, 25, 29, 30, 32/3.0% and remaining 24 FAs with 2.0% enrichment (more
information in [3]). The materials published in [3] were used for this work
preparation.


## 4.2. Results

Influence of the baffle was investigated
by means of power (fission rate) distribution in neighbouring FA no. 4, using gamma scanning of irradiated FPs, measuring their gamma radiation of theLa140 with energy 1596.5 keV. The radial distribution (in a
plane perpendicular to the core axis) was determined by FPs activity of 50 mm
region of FP surface at the central part along rotating
FP using corresponding collimator.Results of the power distribution
measurements carried out in 107 FP positions in FA no. 4 are presented in Figure5(a). Power distribution values were determined
with mean and maximum relative errors being 3.2% and 5%, respectively.(a) Experimental and (b) calculation power distribution values in
selected FP positions in FA no. 4.
(a)(b)Obtained experimental values
were completed by calculations carried out in all 312 FA positions [3]. They
are illustrated by values in selected FP positions in Figure 5(b).Presented results demonstrate depression
of the power distribution in FA no. 4 neighbouring baffle that can be characterised
by the ratio of the power distribution mean values in two opposite FP rows in
FA no. 4 to- and outwards the baffle. The depression, for example, the above
ratio of about 19%, can be stated.


## 5. Discussion

As for gadolinium influence, the
obtained results have limited information relevance since the experiment was realised
in 1990 at special conditions as mentioned above and therefore these results
can differ from ones corresponding to the real NPP cores because of dependence,
for example, on enrichment and dimensions of the (Gd) FPs,Gd2O3 contents, FP pitch, and Gd FP positions in FA, boron acid concentration in
moderator, temperature, pressure, and so forth.Similar situation can also be stated concerning the baffle influence. Further, a less influence of baffle
on power distribution (depression) can be expected in an NPP core (with real
WWER-1000 dimension) because of less dimension of LR-0 core used for
investigation.


## 6. Conclusions

On reactor LR-0 in WWER-1000-type cores the influence of
single Gd FPs and the baffle on the space power (fission rate) distribution was
investigated. A strong power release depression of about 27% in a single Gd FP
(3.6% enrichment, 2.0 wt.%Gd2O3)
was determined, given by the ratio of energy release in that Gd FP and average
energy release at ring of 6 FPs (4.4% enrichment) around this Gd FP. As for
baffle influence in neighbouring FA, the power release depression of about 19%
was estimated, given by the ratio of the power distribution mean values in two
opposite FP rows in this FA to- and outwards the baffle.The above results can be utilised for obtaining some
information for the temperature gradients and resulting loads estimation in FPs
neighbouring a Gd FP and in FA neighbouring the baffle to consider possible
contributions of these loads to the FP failure root causes.



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*Source: 753091-2008-10-14.xml*