ISSN 0040�6015, Thermal Engineering, 2014, Vol. 61, No. 4, pp. 291–294. © Pleiades Publishing, Inc., 2014.Original Russian Text © A.N. Prytkov, A.B. Tereshchenko, Yu.N. Kravchenko, N.V. Boldyrev, I.V. Pozychaniuk, D.I. Lisitsyn, E.I. Golubev, 2014, published in Teploenergetika.

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Unit no. 5 of the Novovoronezh nuclear powerplant is the first power unit in our country with theVVER�1000 reactor installation (RI), which was putinto operation in 1980. After finishing the designed30�year service life, unit no. 5 was upgraded globally.As a result, the service life of upgraded unit no. 5 isprolonged and its safety is substantially improved.

The reactor core of the unit no. 5 VVER�1000 RIhas several distinctions from the serial power units,which substantially affect its neutron�physical charac�teristics:

(i) the reactor�core volume is substantially smallerthan in serial units; it contains 151 fuel assembliesinstead of 163 fuel assemblies in the serial reactor;

(ii) the number of regulating units in the reactorcore is 109 (instead of 61 in the serial reactor);

(iii) the number of groups of regulating units is 14(instead of 10 in the serial reactor);

(iv) the fuel assembly has a zirconium casing1.5 mm thick with 3% perforation (this casing is notavailable in the serial reactor);

(v) the number of fuel elements is 317 (instead of312 in the serial reactor);

(vi) the compact arrangement of 12 directing chan�nels for absorbing elements and the rods of the burn�ing�up absorber (there are 18 channels in the serialreactor);

(vii) the presence of the channel for the assembly ofin�core detectors, which is arranged separatelybetween two directing channels for absorbing elementsin the external row (in the serial reactor, this role isplayed by a central channel);

(viii) the arrangement of the thermocouple at theoutput of heat carrier from the fuel assembly symmet�rically to the directing channel under the assembly ofin�core detectors relative to the central channel(eccentrically over the tube for the absorbing elementin the internal row).

The changeover to the improved uranium–gado�linium nuclear fuel (UGF) had become an importantlandmark preceding the prolongation of the servicelife of the power unit. Operational limits over localparameters, which limit the admissible linear powerdensity and the fuel element relative power, wereestablished in the course of upgrading the unit no. 5reactor core of the Novovoronezh nuclear powerplant.

Allowing for distinctions of the neutron�physicalcharacteristics of the unit no. 5 reactor cores of theNovovoronezh nuclear power plant and the reactor ofthe serial power unit, the UGF�containing fuel assem�blies with the average initial concentration of 3.9 and4.3% were specially developed. The features of thesefuel assemblies is the gadolinium content in gadolin�

Development and Implementation of Monitoringfor the Reactor Core of Unit No. 5 of the Novovoronezh

Nuclear Plant by Local ParametersA. N. Prytkov, A. B. Tereshchenko, Yu. N. Kravchenko, N. V. Boldyrev, I. V. Pozychaniuk,

D. I. Lisitsyn, and E. I. GolubevNovovoronezh Nuclear Power Plant, Branch of OAO Rosenergoatom Concern, Industrial’naya zona Yuzhnaya 1,

Novovoronezh, Voronezh oblast, 396072 Russiae�mail: golubevEI@nvnopp1.rosenergoatom.ru

Abstract—In the course of upgrading the unit no. 5 reactor core of the Novovoronezh nuclear power plant,operational limits by local parameters, which limit the admissible linear power density and the relative powerof fuel elements, were established. Due to applying modern computer technologies in systems of the in�coremonitoring, the calculation of power density for all fuel elements in the real�time mode is implemented. Tomonitor the power density of fuel elements, the algorithm for determining the limiting linear power densityis developed depending on the reactor core height and on the average nuclear fuel burnup. The admissiblerelative power of fuel elements is determined. In the course of the performed work, the excessive conservativelimitations on nonuniformity of the reactor power density are excluded. The monitoring of power density bylocal parameters instead of indirect Kq (fuel�assembly relative power) and Kv (relative power of the fuel assem�bly section) made it possible to increase the fuel efficiency and to improve the economic parameters of fuelcycles of the unit no. 5 reactor core of the Novovoronezh nuclear power plant.

Keywords: monitoring by local parameters, fuel�element linear power density, fuel�element relative power

DOI: 10.1134/S0040601514020104

AUTOMATION AND HEAT CONTROL IN POWER ENGINEERING

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PRYTKOV et al.

ium�containing fuel elements (3.35%) compared withthe one previously adopted for the fuel assemblies ofthe serial VVER, which made it possible to accept theadditional linear power density in them comparedwith fuel elements.

The linear power density and fuel�element powerin the unit no. 5 reactor of the Novovoronezh nuclearpower plant are monitored using the in�core instrumen�tation system (ICIS). Monitoring is performed over theentire core volume including the fuel assemblies withoutestablished assemblies of in�core detectors. Monitoringof the power density in 16 layers over the core height iscaused by a model of unit no. 5 accepted in the ICIS,which represents the core as the totality of 151 ×16 prisms (the number of fuel assemblies with divisionof each of them into 16 layers over the height). Thethree�dimensional fields of linear power density aresynthesized from power densities of monitored bulklayers, into which the core is separated, and calculateddistributions of relative power density in the layers foreach fuel element. Volume power densities arerestored by the data of measurements of assemblies ofin�core detectors and thermocouples.

MONITORING OF LINEAR POWER DENSITY OF FUEL ELEMENTS

To determine the reserve over the linear power den�sity, the extremely admissible value of this magnitudeshould be calculated:

(1)

where i is the layer number; j is the fuel assembly num�

ber; k is the fuel element number; is thelimiting linear power density of the kth fuel element inthe jth fuel assembly of the ith layer; is the coreheight (from its bottom); is fuel burnup in the kthfuel element of the jth fuel assembly; Kndef is the coef�ficient of account for the indefiniteness of finding the

heat power of the core, which equals 1.02; and

( )adm adm operdef eng( , ) ,q

ijk i jk nQl Ql H B K K=

adm( , )i jkQl H B

iH

jkB

operengqK

is the operational engineering reserve coefficient forlinear power density.

To determine the limiting linear power density

, its dependences on both the core heightand on the average burnup of the fuel in fuel elementsare considered. In the RI design, the maximum limit�ing value of this parameter of 448 W/cm is accepted.The dependences of limiting admissible fuel�elementlinear power density on the core height for four oper�ating reactor coolant pumps (RCPs) are presented inFig. 1a. The dependence of the limiting admissible lin�ear power density on the average fuel burnup in fuelelements is also taken into account (Fig. 1b). Curves inFigs. 1a and 1b are substantiated by the entire totalityof heat�hydraulic safety analyses, which were per�formed in a conservative statement. As these limita�tions are exceeded, the fulfillment of safety criteria isprovided in all design modes.

Limiting linear power density isdetermined as the minimal one of two values takenfrom Fig. 1a at corresponding height H of the ith axiallayer and from Fig. 1b for average fuel burnup in thekth fuel element of the jth fuel assembly.

Operational engineering coefficient isdetermined as

Using this coefficient, the contributions to the lin�ear power density of fuel elements are statisticallysummed allowing for the following characteristics:

(i) tolerances for the concentration, the fuel den�sity, the outer diameter of the fuel element shell, thestep of the grid of fuel elements in the fuel assembly,Ytech = 2.2%;

(ii) axial gaps between the fuel pellets, Ypel = 1.92%;(iii) the error in determining the power peaking

factor in the ICIS over core volume in GOST(State Standard) 26635�85 “Nuclear Power�Generat�

adm( , )i jkQl H B

adm( , )i jkQl H B

B

operengqK

( ) ( )oper

eng tech pel oper oper

1 22 2 2 21 .qK KkK Y Y Y Y⎡ ⎤= + + + +⎣ ⎦v

;Kv

100

100 20 30 40 50 60 70 80 90

200

300

400

Height from the core bottom, %100

Qladm

, W/сm (а) (b)

100

100 20 30 40 50 60 70

200

300

400

Burnup, МW day/kg U

Qladm

, W/сm

Fig. 1. Dependence of the maximal admissible linear power density (a) on the core height and (b) average fuel burnup in fuel ele�ments.

THERMAL ENGINEERING Vol. 61 No. 4 2014

DEVELOPMENT AND IMPLEMENTATION OF MONITORING 293

ing Vessel�Type Pressurized�Water Reactors; GeneralRequirements for the In�Core Instrumentation Sys�tem,” the error of no higher than 5% with the con�fidence probability level no lower than 95% is estab�lished; the fulfillment of this requirement is confirmedby the tests for power in stationary and transient oper�ational modes without imitation and with the imita�tion of sensor failures; allowing for this, =5.0%;

(iv) errors in the ICIS calculation of the fuel�ele�ment relative power Kk, which is determined in auton�omous tests via comparing Kk from the ICIS with thecalculations by the PERMAK�A design program forthe same core states; the condition of the criterion oftest success is fulfilled since the deviation of the ICIScalculation of Kk and the design calculation is in thelimits of 0.5%; from here, allowing for the attestederror for the PERMAK�A program, which is 4.0%,YKk(oper) = 4.5%.

In the general form, Ytech and Ypel appear in with definite weights. However, for conservativeness,the weight of all contributions to the linear power den�sity is taken equal to unity. Starting from the presented

values of components, = 1.073.

Settings for 16 layers over the core height are calcu�lated by formula (1) (Fig. 2).

The calculation of the linear fuel element powerdensity reduced using the ICIS by the data from self�powered detectors (SPD) and thermocouples is per�formed by the formula

where is the average linear power density, whichis determined as the ratio of the core heat power N tothe total length of the fuel column of all fuel elementsin the core; is the relative power of the ith layer inthe jth fuel assembly reduced by the indications fromSPDs and thermocouples; and Kkijk is the relativepower of the kth fuel element in the ith layer of the jthfuel assembly calculated in the ICIS.

Thus, monitoring of the linear power density is per�formed by constant verification of the ratio between

and

MONITORING OF THE FUEL�ELEMENT RELATIVE POWER

The design requirement on the fuel�element powerin normal operational conditions of the RI with theoperation of four RCPs is fulfilled with the provisionthat the fuel�element power determined from ICISindications does not exceed the limiting admissibleone

Kv

( )operKYv

operengqK

operengqK

( )oper ,ijk ij ijkQl Ql N K Kk= v

( )Ql N

ijKv

operijkQl adm.ijkQl

oper admfu.el fu.el.jkQ Q≤

The limiting admissible power of the kth fuel ele�ment in the jth fuel assembly is determined by the for�mula

where is the design fuel�element power of 110 kW

and is the operational engineering reserve coef�ficient for the fuel�element power.

Coefficient is determined as

Using this coefficient, contributions to the fuel�element power density are summed statistically, andthe following factors are taken into account in thiscase:

(i) tolerances for enrichment, the fuel density, theouter diameter of the fuel element shell, the array stepof fuel elements in the fuel assembly, Ytech = 2.2%;

(ii) the error in measuring the fuel�assembly power,YKq(oper) = 5.0%;

(iii) the error in the ICIS calculation of the relativefuel�element power Kk in the YKk(oper) = 4.5%.

Proceeding from the presented values, which con�

stitute the engineering coefficient, = 1.071.Allowing for these coefficients, the admissible fuel�

element power = 100.7 kW.The fuel�element power is calculated according to

the ICIS by the formula

where hi is the height of the ith calculated layer and nzis the number of monitored layers over the core height.

The maximal relative fuel�element power is deter�

mined as Kr = where is the averagefuel�element power at a nominal reactor power. Con�sequently, at a nominal reactor power (102%), allow�ing for the error in its determining of 2%, we find themaximal relative fuel�element power

The reserve coefficient on holding and determining thereactor power was accepted to be equal 1.02 × 1.02 = 1.04,while the maximal relative fuel�element power Kr =1.58, i.e., with the proviso of accounting only thereserve coefficient over the exactness of the power cal�culation, the maximal admissible relative fuel�elementpower at the nominal power = 1.58 × 1.02 =1.61.

A part of the reserve coefficient, which is responsiblefor holding the power (1.02), is carried out into the for�mula for the calculation of the admissible value of Kr:

(2)

adm design oper fu.el fu.el det eng( ),r

nQ Q K K=

designfu.elQ

oper engrK

oper engrK

( ) ( )oper

eng tech oper oper

1 22 2 21 .rKq KkK Y Y Y⎡ ⎤= + + +⎣ ⎦

oper engrK

admfu.el.Q

oper operfu.el.

1...

,jk ijk i

i nz

Q Ql h=

= ∑

adm avfu.el. fu.el. ,Q Q av

fu.el.Q

( )( )

adm avfu.el. fu.el. 1.02

100.7 62.67 1.02 1.58.

Kr Q Q= ×

= × =

admKr

( ) ( )cur cur adm1.61 0.83 0.17 ,Kr N N N= +

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PRYTKOV et al.

where Ncur is the current reactor power and Nadm is theadmissible reactor power depending on the number ofoperating circulation loops of the reactor installation.In order so that the admissible Kr would not tend toinfinity at low power, the dependence through coeffi�cients 0.83 and 0.17 is introduced rather than theinversely proportional dependence on the power.

According to formula (2), at the nominal reactorpower, admissible Kr will be 1.61, and upon reachingthe reactor power of 102%, admissible Kr will be 1.58,which corresponds to the starting data accepted inthermal�hydraulic calculations.

CONCLUSIONS

(1) The development of computer technologies andtheir ICIS application made it possible to perform thereal�time calculation of power density of all fuel ele�ments and limiting admissible values for monitoredparameters such as linear power density and relativefuel�element power Kr.

(2) Monitoring of power density by local parame�ters instead of indirect Kq (the relative fuel�assemblypower) and Kv (the relative power of the fuel�assemblysection) provides more effective and safe operation ofa nuclear reactor.

(3) Sufficient reliability of determination of andKr made it possible to put the core into operation with�out exceeding the design limiting values of the param�eters. The possibilities for the formation of the loadssubstantially increased, and the average burnup of thefuel assembly elevated.

(4) The incorporation of the monitoring system ofthe in�core power density by local parameters made itpossible to exclude excessively conservative (which arereferred to as the fuel assemblies and fuel�assemblysections rather than for the fuel elements) limitationson nonuniformity of the reactor heat power, toincrease the efficiency of the use of the fuel, and toimprove the economic characteristics of fuel cycles ofthe unit no. 5 core of the Novovoronezh nuclear powerplant.

Translated by N. KorovinQl

Ql

220100 20 30 40 50 60 70

240

260

280

300

320

340

360

380

4001–8

910

1112

13

1415

16

Qladm

, W/сm

Burnup depth, МW day/kg U

Fig. 2. Dependence of the maximally admissible values of fuel�element linear power density for 16 layers over the core height onthe fuel burnup.