XA0201785

OUT-OF-PILE INVESTIGATION OF CLADDING BEHAVIOUR FOR WWER-TYPE FUEL ELEMENTS rN ATMOSPHERE SIMULATING FISSION PRODUCTS (I, Cs) DAYS IN REACTOR I 12 3*5 6. J? 8 9 10 || 12 75 - I I I 1 I I II I 1 I I Yu.K. BIBILASHVILI, V.Yu. TONKOV, A.S. S0TN1KOV, 60 LA. KULIKOV, V.V. NOVIKOV, V.I. SOLYANY 45 30 All-Union Scientific and Research Institute of 15 inorganic Materials, 0 Moscow, DIAMETER vs ELEMENT POWER Union of Soviet Socialist Republics ELEMENT ACK OENSITY IQ8 Mg/m* BURNOP 9BMW-h/hgU PELLET INTERFACE LOCATIONS 1. Introduction

At present the reliability of fuel elementa of WWER-typo reactors that uro base loud operated is ao high that fuol element failurea that take place in exceptional cases do not appreciably affect the coolant activity of the primary circuit. The further Soviet developments of WWER-type reactoro envisage ohifting from baae loud to loud following operation. The preliminary analyais showed tliut fuol elements designed for baoo louilo will havo reduced margins of utrenght. Therefore, for tho improvement of the available fuel design to provide it with high reliability under transient conditions a great number 10 20 30 10 50 60 70 80 of studies are required that are being carried on in Soviet ELEMENT POWER (kW/m) Union. Preliminary, tho studios ura concerned with the determinat- of the reliable operation criteria for fuel under transient FICUKE 7 Element power and averuge percent change in dlauecer ot pellet Interface locations measured during IHDMK irradiation of liigh conditions. In this respect it is very important to study the density element ACK. Note die difference tn vcrclcal scale from Klgureu 2, 5 and 6. ultimate condition of WWEJ1 fuel cluds in an atmosphere simulating fission products (I, Co). 'Xhia puper considers some roaulto oC out-of—pile utudies. 113 b) to study the ultimate clad condition on internal presauri- 174 2. Experimental zation with containing atmosphere. The test in (a) were performed under iaotarraal conditions 2.1. test and Specimens and a temperature gradient. The specimens wore filled v/ith helium The following teats were employed Tor experimental studies: and . The specimens were irradiated with a gamma- — tension of annular specimens in a controlled atmosphere oourco of Co in a unit that is schematically shown in fig.4. (air, air+iodine, , argon+iodine); To determine the amount of elemental iodine and caesium - internal preoaurization of tubular specimens with a resulting from the Irradiation induced decomposition of the aalt controllod atmosphere (hellum+Cal, heliumtl). (caesium iodide) the gamma-counting of 1-131 and 1-137 lootopoa The shape and geometrical dimensions of annular specimens was measured (see fig.5). When controlling I—131 account was are shown in fig.1. taken of the reduction in the gamma-counting duo to the 1-131 The shape and geometrical dimenaiona of tubular specimens decomposition ( ^"Q C=9 days). The results obtained were compared are shown in fig.2. to the results of studies of unirradiated specimens. All the specimens were prepared from commercial tubes - . In tha studies (b) two methods of registration of the moment WWER fuel clada 0 9.15x0.65 mm of fully rocryatalllzed of tubular specimen leakage were employed baaed on: Zr+1iS wt Mb alloy. - a decrease in vacuum; 2.2. Testing; Procedure - un avalancho-like Increase of tho total acoustic emission. 2.2.1. Testing of Annular Specimens The specimens wero heated to the tost temperature and then held Annular specimens ware tensile tested in a controlled until they failed. The specimen temperature waa measured with a atmosphere using "Instron TT-DM" at a temperature of 300°G and chromel-aluiael thermocouple. To find out the point of leakage the 1 straining rates of 2.1x10""-' - 10"' a" . clads were Inspected using an optical microscope. They were fixed with special clamps in a testing chamber to

which tension rods ore attached through adapters (see fig.3). 3. Results and Discussion The testing chamber waa placed into a thennooryochomber of a "cabinet" type of the testing machine "In3tron TT-DH" and after 3.1. Annular Specimens the testing temperature was roachod it waa held for no less than Annular specimons wore tested at a temperature of 300°C and 30 min before load wxa applied. straining rutes of 2.1x10"^ - 1x10"' a"1 in the following controll- 2.2.2. Te3tinft of Tubular Specimens ed atmospheres: Tubulur specimens were tested with the following aim in view: - air; a) to study the gamma-irradiation effected redistribution - air+iodiae (iodine concentration of~1 aig/ciir); of Csl compound and its radiolyzis products; - argon; The results of studies into the gamma-induced redistribution - argon+iodine (iodine concentration of -- 1 mg/cia ). of Csl in a non-umformly heated specimen ("cold spots") ahem The comparison of the reaulta of the tests showed that iodine that: present in the touting atmosphere ("• 1 uig/cm ) practically does - the preferential accumulation of iodine resulting from not affect tho atrenght properties ( <5*B, G' ^i and total olongat- the radiolytic decomposition of Csl is obaervod at areaa of lower ion of annular specimens and leads to a ~ 2054 and -» 50JS decrease temperaturoa (aeo fig.8); in uniform elongation in air+iodino mid argoa+iodine, respectively, - up to the doae — 3*10 Gr tho accumulation of I and Cal with the reduced in tho deformation rates (see fig.6). ia proportional with tho further inoreaae of the dose the observed deviation from tha proportionality is due to a decreasing amount

3.2. Tubular Speoimen3 of the decomposing salt (seo fig.9). 3.2.1. Cal Redistribution m 3.2.2. Failure in Iodine Atmosphere Aa compared to the results of out—of-pile studies the study Fig.10 illustrates the relationship at a temperature of 3BO°C between the time-to-failure and the hoop stress of tabular speci- into the effect of the gamma-irradiation doae

4.1. In the teata of annular Zr-1# wt Mb alloy apecimena 0 9.15x0.65 nun at a temperature or 300°C aa the rate of strain- ing ia reduced from 2.1x10"^ 3~1 to 1x10"*^ a"1 the relative uni- clad p 9.15x0.65 mm form elongation ia observed to decrease in "air+iodino (~ 1 mg/cur and "argon+iodine ( •» 1 ng/cm )" aa compared to atmoapherea wit- Fig.2. Tubular specimen hout iodine.

4.2. The study into the gamma-irradiation induced redistri- top tenaion-rgd adapter bution of Cal in the Y/WER-typo clad showa that: — if temperature ia non-unifonn over a specimen lenght Cal ia redistributed into "cold" specimen apots; - under isothermal conditions Cal accumulates at defect

points in a clad.

4.3. It ia shown experimentally that on tensile streaaaa close to the yield atreaa at a temperature of 380°C and the concentration of iodine — 0.2 mg/cin Zr-1{» wt lib alloy clada of the WWER-type fuel aica prone to corrosion cracking.

atand for iodine

bottom tension-rod adapter

Pig.1. Annular specimen made of tube jjj 9.15x0.65 Pig.3. Testing chamber temperature monitor

specimen resistance furnace furnace power supply

thermocouples

Co-60 radiation - lead shield "•aenaor measuring source gamma-count

Pig.4- Schematic of gamma-irradiation plant for tubular Schematic of unit for measuring gamma-counting specimens

177 Initial specimen, 25°C dose of 2x105 Gr, 38O°C Relative variation in uniform elongation dO3e of 3x10-'Gr, 380°C

air / air + iodine (~ 1 mg/cm3)

Specimen length

Pig.7. Results of measuring gamma-count over specimen Deformation rate, a length containing 20 rag CaI-131 under lsothetaa conditions

Rolative variation in uniform elongation

Initial specimen do3e of 1.5x1O5 Gr argon / axgon + iodine (~1 doae of 3.5x1Ov Gr

Defonnotion lato, a

Pig. 6. Effect of iodino present in different atiitoapliereo on unifGHfl elongation of annular specimens under Fig.8. Effect of gamma-^irradiation on iodine accumulation at tension loner temperature spot under radiation-thermal decomposition of Csl XA0201786

10, A PROPOSED MODEL FOR THE CORRELATION OF POWER RAMP TEST RESULTS

M.G. ANDREWS, H.R- FREEBURN, J.C. LaVAKE 5 Combustion Engineering, Incorporated, Windsor, Connecticut, United States of America

I. INTRODUCTION AMD BACKGROUND .0 The trend toward extended burnup operation in the world's light: water gamma-irradiation doac, 10 Gr reactors (LWRs) has recently received more attention than considerations for reliability at conventional burnups. Although the performance level of LWR Pig.9. Iodine accumulation at a cold upot on gamma-irradiation, fuel rods has been racher high, the few remaining threats to fuel rod 20 ing Cal integrity should not be overlooked. Pellet cladding interaction (PCI) or stress corrosion cracking (SCC) Is one of the remaining threats.

The use of operating guidelines that precondition the fuel rods and/or reduce stresses during a power escalation has been beneficial in reducing the incidents of fuel failure. The guidelines for boiling water reactors (BHEs) 200 I have been more restrictive than those for pressurized water reactors (PWRs). The incentive to reduce or eliminate operacing restriccions is, therefore, higher for BWRs. But the PCI/SCC issue has not disappeared completely from the PUR. 175 PCI models that have been offered over the last five-to-ten years Include, for example, the Canadian Fuelograos where a minimum power level and a minimum power change are necessary conditions for PCI/SCC Q). Another PCI model, recently modified, was called FOS1I0 for power shock. With the early 150 version of this model, Scandpouer predicted PCI solely on the basis of power change (2^3)- Another approach to predicting PCI evolved from the simplified treatments in publications where data from test reactor experiments are plotted OD graphs of power vs.. butiiup and thresholds are drawn on the basis of these results (*...5). A threshold, for example, has been treated as a power level below which failures by PCI or PCI/SCC do not occur. If the ramp experiments that lead to a conclusion on thresholds for a given design all start from a common power level, however, a peak power and a change in power 100 cannot be distinguished.^ 10" 1 3 I0 Ctma oC Cullura, h.IO Host Investigators now recognize that chemistry plays a role In the failure of fuel rods by PCI. Thus stress corrosion cracking Is being treated explicitly in more recent models. Although the actual nature of the chemical Pig.10. Relationship between time-to-failure and hoop stresses of attack is not known, the chemical influence needs to be accounted for in Che Zr-1/S Nb alloy specimens effected by internal pressure of predictions of PCI/SCC. Prediction!! based on mechanical considerations alone iodine containing atmosphere (concentration ~ *O.2 wg/etu ); tiavu not been universally successful (2). 179