[en] The IFA-533.2 experiment represents the first in-reactor test of pre-irradiated fuel rods with fuel thermocouples. It has been irradiated in the HBWR since March 1992. The rig contains BWR-type fuel rods, pre-irradiated in IFA-409 to an average burnup of about 44 MWd/kgUO₂, and then re-instrumented with fresh fuel thermocouples. This report presents an evaluation of the results in steady-state and transient operation to a burn-up of 70 MWd/kgUO₂. The data can be interpreted in terms of fuel conductivity degradation, gap conductance, and fission gas release. The results from the steady-state fuel temperature studies are summarized as follows: 1. For burnups from 43 to 50 MWd/kgUO₂, the temperatures at constant power showed an increasing trend. This is interpreted as fission gas release into the still open gap as the dominant factor. 2. For burnups > 50 MWd/kgUO₂, the isopower temperatures remained essentially constant, indicating that the effects of conductivity degradation, gap closure and continued fission gas release appear to balance each other. The transient temperature behaviour was investigated by measuring fuel centre temperature changes following reactor scrams. The trend in fuel rod dynamic response showed a continuous increase in the time constant until a burnup of 63 MWd/kgUO₂, and thereafter a slight decrease. (author)

[en] The comparative WWER/PWR test in IFA-503.1 was commenced in July 1995 and successfully finished at the end of November 1998. The main objective of the test was generation of representative and comparative data of standard WWER-440 fuel fabricated at the 'MSZ' Electrostal (Russia) and PWR type fuel manufactured at IFE Kjeller (Norway). The test assembly comprised two clusters, each with 3 WWER rods and 3 PWR type rods. Eight rods with two types of fuel were instrumented with expansion thermometers, four rods were equipped with both fuel stack elongation detectors and pressure transducers. All sensors worked satisfactorily during the test. The average burnups achieved in the lower and upper clusters were around 25 and 20 MWd/kgUO₂, respectively. Some difference in densification of the two types of fuel was revealed during the first irradiation period. However, the fuel temperatures and commencement of fuel stack swelling were similar despite this fact. At the end of the test the rig was moved to a higher flux position in the HBWR core with the aim of promoting FGR and to compare the behaviour of the two types of fuel under higher power. Pressure measurements indicated a comparable low FGR (around 1 percent) in both types of rods. The centreline temperatures measured in the PWR rods were very close to the Halden FGR threshold whilst the WWER fuel temperatures were slightly lower. Despite the differences found in the behaviour of the two types of fuel during the test, the analysis of the in-pile data showed that these differences would not affect the fuel efficiency, at least, up to the burnup achieved in the test. It is supposed that these differences can be related to the fuel microstructure, in particular to the fuel grain and pore sizes (author) (ml)

[en] IFA-503 is the base irradiation experiment with the main objective to generate representative and comparative information about standard WWER-440 fuel with that of PWR rods. The rig carries two clusters with six rods in each. Each cluster holds three rods of each origin. The rod instrumentation includes 8 expansion thermometers, 4 fuel stack elongation detectors and 4 pressure sensors. Fuel pellets are hollow with a bore of 1.8 mm, except the two PWR rods with PFIEF instruments. Two pellet-clad gaps are used, .210 and .270 mm As of April '96 the test has reached a burnup of 6 MWd/kg UO₂. The preliminary results from the test indicate that the WWER fuel densities somewhat more than the PWR fuel. The centreline temperature measurements during the initial operation were in compliance with FTEMP2 code calculations. A slight steady increase in temperature with burnup is observed at isopower conditions for all rods. The rod internal pressure measurements show stable pressure readings with burnup since power levels is far below the threshold for fission gas release. The WWER rods show in general some higher pressure than the PWR rods. It is believed that this is due to the hollow fuel in WWER causing a higher temperature of the filler gas than in the PWR fuel rods which contain solid pellets. (author)

[en] IFA-503.1 has been irradiated in the HBWR since July 1995 with the main objective being to generate representative and comparative data of standard WWER-440 fuel fabricated at the Elemash fuel plant (Russia) and conventional PWR type fuel produced at IFE Kjeller (Norway). The cladding tubes used for all test rods are a standard WWER alloy Zr-1%Nb and have been manufactured by Russia (Glazov). Fuel pellets are hollow with a bore of 1.8 mm except for the PWR rods with PFIEF instrumentations. The test rods were manufactured with two pellet-clad gaps, 0.21 and 0.27 mm. The test rig carries two clusters, each with 3 WWER rods and 3 PWR type rods. The fuel rods are instrumented with expansion thermometers (8), fuel stack elongation detectors (4) and pressure transducers (4). All se sensors have been working satisfactory. The maximal level of burnup is about 18 MWd/kgUO₂. The average burnup in the lower and upper clusters are 17.1 and 12.3 MWd/kgUO₂, respectively. The latest results from the test indicate that both fuel types have approximately the same fuel stack swelling rate. The fuel temperatures are similar despite that the WWER fuel has densified somewhat more than the PWR fuel. The gas pressure measurements show that there has been no fission gas release so far in all fuel rods. The WWER rods have slightly higher pressures than the PWR type rods at power and lower pressure at hot standby condition, obviously related to the higher gas average temperature in the hollow WWER fuel stack during operation. (author)

[en] The extension of fuel burn-up beyond previously accepted levels is currently being applied in varying degrees throughout the nuclear industry, with the aim of improving fuel economics and reducing the spent fuel volume. So it is necessary that the current fuel knowledge base should be extended. Modifications of fuel rod/assembly concepts, together with fuel management schemes, should be gradually implemented so that the operation of power reactors becomes even more reliable and flexible than it is today. Extrapolation to extended burn-up levels does not cause concern but will have to be made in steps, in order to demonstrate expected performance trends. The fuel testing programmes at the OECD Halden Reactor Project have over the years significantly contributed to the understanding of LWR fuel behaviour in the high burn-up range. A broad range of versatile and integrated in-reactor test rigs and high pressure loops have been developed which allow simulations of LWR irradiation conditions, comparative testing of alternative fuel rod designs and use of test segments pre-irradiated in power reactors. A number of in-core instruments and experimental techniques have been developed for detailed investigations of various aspects related to the thermal behaviour, fission product release and mechanical response of high burn-up LWR fuel rods, under a variety of operating conditions. The paper reviews recent measurements in the area of burnup-dependent steady-state and transient thermal behaviour of fuel rods, intermixing of fission and helium filler gases in the pellet cladding gap, fission gas release kinetics under changing heat loads and power excursions (burst release) and dimensional changes of fuel rods subjected to cyclic load changes. (author). 14 refs, 12 figs