The objective of this M. Eng. Diploma work was to validate the new version, CASMO-5M, and compare the results with CASMO-4E, by using the same input in both programs. The tasks that were included in this work was to compare TIP-measures, k-effective curves (warm), k-effective curves (cold), the effect of void on k-effective, the isothermal temperature coefficient (ITC), the moderator temperature coefficient (MTC), damping ratio calculations, internal effect in a fuel bundle of a BA-rod, the speed of BA-out burn, impact of xenon, falling control rod, margins and isotopes. The TIP-measures showed that the differences were small for all reactors, at most, 6% nodal improvement by C5M (Casmo-5M) for Oskarshamn 3. The warm measurements of k-effective showed that C5M had a higher value for all cycles of Oskarshamn 1, 2 and 3 in comparison to C4E (Casmo-4E). Important improvements were noted for C5M, which solved the “tub behaviour” that used to occur in Oskarshamn 2 when using C4E. For all three reactors the the differences of the cold k-effective measurements between C5M and C4E were decreasing by growing reactor cycle, especially after changing fuel rods from SVEA64 to SVEA 96 Optima/Optima2. The new fuel (Optima/Optima2) contains part length fuel rods and contains 10x10 fuel rod positions to compare with the old 8x8/9x9 previously used in all three reactors. The effects of void on k-effective were consistently better in C5M than in C4E for all three reactors. The isothermal coefficient was lower in C5M in comparison to C4E for all three reactors, except in the middle of cycle 34 for O2 where C5M was higher than C4E. The average of MTK is lower in C5M then in C4E for all three reactors. The uniform Doppler coefficients are consistently lower in C5M than in C4E, for all three reactors. In the damping ratio calculations C5M is marginally higher than C4E. For all three reactors C5M consistently calculates a higher effect in the studied fuel bundle of a BA-rod in comparison to C4E, at 0% and 80% void. The speed differences of the BA burnout are marginal, but notably exhibits a consistent behaviour. C5M has a higher power development in the beginning of the cycle’s for all voids (0-80%) and lower in the end of the cycles compare to C4E. The xenon impact has the same trend for all three reactors. C4E has a consistently higher value than C5M in the beginning of the cycles and contrary in the end of cycles. The test of the falling control rod has proved that in C5M we maintain a worse fuel temperature- and moderator coefficient which results in a higher reactivity maximum value, compared to C4E. In C5M we find that the margins are consistently higher than C4E for all three reactors, which is to be preferred. Regarding the isotopes, the only difference observed in the test was that according to C5M there is approximately 50% less U-237. This does not have a big effect on the reactor because this amounts to only about 1/1000 of the total amount of the fuel in the core.

This is a M. Eng. degree project at Uppsala University carried out at Vattenfall NuclearFuel AB. The goal of it is to present a best estimate method based on the code package CASMO/SIMULATE for the purpose of calculating the isotopic concentrations of a specified number of isotopes in a fuel sample. The calculations done with the method shall produce small deviations from reliable measured values, which characterize the accuracy of CASMO/SIMULATE, but also simplicity based onthe computing time and handling of the amount of data is an important factor in the development of the method. The development of the method has been based on a sensitivity calculation with CASMO/SIMULATE on a number of relevant parameters affecting the isotope concentrations. The proposed method has then been applied on three samples irradiated in Ringhals 4 and Ringhals 3. At last the calculated isotopic concentrations have been benchmarked against measured data from Studsvik Laboratory. The sensitivity analyzes has shown that the parameters affecting the neutron moderation are very important for calculating the isotopic concentrations. The core axial resolution is also an important factor for the samples taken from top of the rod,where the power gradient is large. The comparison of the calculated and measured values has shown that SIMULATE, in the analysed cases, simulates a lower finalburnup. This has created a need to correct the final burnup in order to get better results in terms of lower relative deviations between the measured and calculated data.

In recent years, the enthalpy raise hot channel factor limit has decreased significantly due to the power upgrade of Ringhals 4 and the use of shielding fuel assemblies. The shielding fuel assemblies task are to reduce the neutron leakage to the reactor vessel and in that way extend the reactor lifetime. This is achieved by replacing a few fuel rods with steel rods. Experiences from the last fuel cycles show that the core design procedure has been hampered because of this and that it’s hard to stay under the design limit. A way to overcome this problem and to improve the fuel economy is to introduce the use of radial enrichment distribution in the fuel assembly. This master thesis shows, through a case study of three fuel cycles at Ringhals 4,  that the internal peaking factor can be improved by roughly 2–3 % and that the maximum enthalpy raise hot channel factor can be improved by about 2.0–2.5 % if the fuel assemblies contain three different levels of enrichments instead of currently one. This can be achieved without any noticeable decrease in cycle length. / Genom en fallstudie av tre driftcykler för Ringhals 4 visar den här rapporten att max FΔH under cykeln kan sänkas med 2,0–2,5 % om bränsleknippena radiellt anrikningsoptimeras med minst tre delanrikningar. Totalt under cykeln kan FΔH sänkas med upp till 4 %. Om radiell anrikningsoptimering införs för Vattenfalls PWR:er skulle arbetet med att designa härdarna förenklas och utrymme ges för att ladda reaktorerna på ett mer ekonomiskt sätt.