XA9744131 AECL's EXPERIENCE IN MOX FUEL FABRICATION AND IRRADIATION F.C. DIMAYUGA Chalk River Laboratories, Atomic Energy of Canada Limited, Chalk River, Ontario, Canada Abstract Atomic Energy of Canada Limited's mixed-oxide (MOX) fuel fabrication activities are conducted in the Recycle Fuel Fabrication Laboratories (RFFL) at the Chalk River Laboratories. The RFFL facility is designed to produce experimental quantities of CANDU* MOX fuel for reactor physics tests or demonstration irradiations. From 1979 to 1987, several MOX fuel fabrication campaigns were run in the RFFL, producing various quantities of fuel with different compositions. About 150 bundles, containing over three tonnes of MOX, were fabricated in the RFFL before operations in the facility were suspended. In late 1987, the RFFL was placed in a state of active standby, a condition where no fuel fabrication activities are conducted, but the monitoring and ventilation systems in the facility are maintained. Currently, a project to rehabilitate the RFFL and resume MOX fuel fabrication is underway. The initial campaign will consist of the production of thirty-eight 37-element (U, Pu)O2 bundles containing 0.3 wt. % Pu in Heavy Element (H.E.) destined for physics tests in the zero-power ZED-2 reactor. An overview of AECL's MOX fuel irradiation program will be given. Post-irradiation examination results of (U,Pu)O2 bundles irradiated to burnups ranging from 18 to 49 GWd/te H.E. in the Nuclear Power Demonstration reactor are highlighted. The results demonstrate the excellent performance of CANDU MOX fuel to high burnup, at power ratings up to 45 kW/m. The paper also outlines the status of current MOX fuel irradiation tests, including the irradiation of various (U, Pu)O2 and (Th, Pu)O2 bundles. 1. INTRODUCTION AECL's MOX fuel program was started more than thirty years ago. The program consisted of two components: • irradiation testing and post-irradiation examination (PIE) of various types of MOX fuel, with the objective of studying the general behaviour of these types of fuel in comparison with natural UO2 fuel; and, • development of MOX fuel fabrication technology. Irradiation testing and PIE of MOX fuel is still continuing at AECL; it has gone from multi- element to multi-bundle demonstration testing. This paper discusses recent PIE results of (U,Pu)O2 bundles irradiated in the Nuclear Power Demonstration (NPD) reactor, and outlines the status of current MOX fuel irradiation tests, including the irradiation of various (U,Pu)O2 and (Th,Pu)O2 bundles in the National Research Universal (NRU) reactor. AECL's MOX fuel fabrication experience is summarized, including an update on the current fabrication campaign. *CANDU: CANada Deuterium Uranium; registered trademark. 373 2. MOX FUEL FABRICATION AT AECL 2.1 Recycle Fuel Fabrication Laboratories Various forms of fuel containing plutonium have been handled by AECL at its Chalk River Laboratories since 1960. Research activities were carried out in glove boxes between 1960 and 1970, including the development of MOX fuel fabrication technology, measurement of physical properties, production of fuel samples for experimental irradiation, etc. In 1970, a decision was made to re-model the plutonium laboratory and install new facilities, to focus on MOX fuel fabrication technology. Installation of the new facilities was complete by 1975 [1], The facility, collectively referred to as the Recycle Fuel Fabrication Laboratories (RFFL), is designed to produce experimental quantities of MOX fuel for reactor physics tests or demonstration irradiations. Subject to special precautions due to the presence of Pu (e.g., essentially all operations are done inside glove boxes), the processes employed in the RFFL follow conventional natural UO2 practice. The fabrication line was designed for the production of sealed individual fuel elements, starting from UO2 or ThO2 powders as the major component and PuO: or ^UO; as the minor component. The fabrication process adopted in the RFFL is outlined in Fig. 1. Weighed amounts of the starting oxide powders are mixed either in single-stage or double-stage blending, the latter being used for more dilute mixtures to achieve better homogeneity. After blending, the MOX powder is pre-pressed using an isostatic press, to convert the mixed powder into compacts, which are, in turn, fed into a granulator. The resulting free-flowing granules are then suitable for final pressing into green pellets using a single-cavity hydraulic press. The green pellets are then loaded into a batch furnace, where sintering is done in a dilute hydrogen cover gas. Sintered pellets are then centreless ground to a specified diameter and surface finish. The pellets are washed and then dried in warm air. Acceptable pellets are loaded into empty sheaths that already have one endcap welded and all appendages brazed in place (these sub-assemblies are supplied by commercial fabricators). The second endcap is welded to the loaded sheath using a tungsten inert gas (TIG) welding system. The sealed elements are then helium leak-tested, scanned for surface alpha contamination, and assayed by neutron interrogation. Following assay, the elements are ready for bundle assembly. 2.2 Fabrication Campaigns in the RFFL Extensive commissioning and test production programs using natural UO2 were implemented in the RFFL. This enabled several modifications to, and optimization of, various process and safety-related equipment prior to the introduction of Pu in the fabrication line in 1979. From 1979 to 1987, several MOX fuel fabrication campaigns were run in the RFFL, producing various quantities of fuel with different compositions [1]. As listed in Table I, the first campaign consisted of (U, Pu)O2 fuel with 0.5 wt.% Pu in H.E. Later, about 1.3 Mg of (Th,Pu)O fuel elements were produced, containing up to 2.3 wt.% Pu in H.E. The last campaign was particularly challenging, since it involved the fabrication of 1350 elements containing 1.4 wt.% ^U in H.E. (due to the presence of232!!, which has a gamma-active daughter) [2]. The fuel elements and bundles produced in the RFFL were used for test irradiations in NRU, and for physics tests in the zero power ZED-2 reactor. About 150 bundles, containing over three tonnes of MOX were fabricated in the RFFL before operations in the facility were suspended. In late 1987, the RFFL was placed in a state of active standby, a condition where no fuel fabrication activities are conducted, but the monitoring and ventilation systems in the facility are maintained. Currently, a project to rehabilitate the RFFL and resume MOX fuel fabrication is underway. The pre-operational testing (using inactive material) is scheduled for mid-1995, with regulatory approval to 374 resume MOX operations anticipated by late 1995. The initial campaign will consist of the production of thirty-eight 37-element (U,Pu)O: bundles containing 0.3 wt.% Pu in H.E., destined for physics tests in ZED-2. Product Analysis MOX Fuel Production and QA/QC Sample to Analysis Return Repackaged PuO2 to Storage Sample to Analysis Granulate & Sieve Optional Operations Master-Mix Pre-weigh Sample to Analysis Waste Management & Recycle Sample for Recover Metallography and Grinding Fines Auto radiography Sample for Density Measurement Ultrasonic Wash & Dry 100% visual H Vacuum Degas Inspection 100% Diameter ft Sample to visual Inspection Analysis Weld Second End-Cap Immobilize >,,. Liquids 100% He Leak Test, -4 a Contamination Store Finished "\ Transfer to Waste Check, Dimensions, Elements Treatment Centre Assay FIGURE 1. RFFL Fabrication Process Flowsheet. 375 TABLE I Fuel Produced in the RFFL. Experiment DATE FUEL TYPE QUANTITY DP-12 1977-78 Natural UO2 50, 19-element bundles BDL-419 1979-80 (U, Pu)[email protected]%Pu 15, 36-element bundles BDL-422 1981-83 (Th, Pu)O,@ 1.75% Pu 6, 36-element bundles BDL-430 1982 Natural ThO2 1, 36-element bundle WR1-1012 1982 (Th, U)[email protected]%Pu 2, 21 -element bundles 1982 (Th, Pu)O2 @ 2.3% Pu 2, 21-element bundles WRl-1010 1982-85 (Th, Pu)O2 @ 2.3% Pu 1332 elements BDL-432 1986-87 (Th, U)O2@ 1.4%U-233 1350 elements 3. AECL's RECENT IRRADIATION EXPERIENCE (NPD-40) 3.1 The NPD-40 Experiment The NPD-40 experiment was a demonstration-scale irradiation test involving six bundles containing (U,Pu)O2 fuel in NPD. The objective of the experiment was to show that MOX fuel could operate successfully to extended burnup, under typical CANDU operating conditions. Subsequent to the NPD irradiation, several bundles were power-ramped in NRU. 3.2 Fuel Design and Fabrication The six bundles were of the standard 19-element design, but contained two different types of non- standard fuel pellets from two different suppliers, and two sheath wall thicknesses. As listed in Tables II and HI, three bundles had normal-(CANDU)-density hollow pellets (supplied by Aktiebolaget Atomenergi, Sweden) contained in thin-wall, collapsible Zircaloy-4 sheaths, while the other three had low-density solid pellets (supplied by BelgoNucleaire, Belgium) contained in thick-wall, free-standing Zircaloy-4 sheaths. Both pellet designs considered the effects of in-reactor operation: • the low-density pellets had more internal voidage to accommodate pellet swelling during operation. In-service densification was accounted for by using free-standing sheaths; and • the hollow design of the normal-density pellets served to reduce pellet thermal swelling, due to its lower average operating temperature. The pellets were produced by both suppliers using normal fabrication methods; i.e., mixing and blending, pre-pressing, granulation, final pressing, sintering and grinding. In addition, the low-density pellets were degassed immediately prior to loading into the sheaths. Zircaloy-4 sub-assemblies (i.e., appendaged sheaths with one endcap welded in place, and with graphite CANLUB coating on the sheath inside surface) were obtained from General Electric (GE) Canada, a commercial fuel supplier.