This thesis is done in cooperation with SWECO Energuide in Växjö. They have been assigned by SKB in Oskarshamn to renew their electrical power supply and the ancillary processing systems in their Central provisional storage of spent nuclear fuel, CLAB. Today in the CLAB facility, the spent fuel is cooled down with water pumps, driven by asynchronous induction motors, where the water is regulated by mechanical valves to the desired flow. After the upgrade, the engines will be powered by AC drives that can control the pumps by changing the rotational speed and thus change the water's cooling effect on the nuclear fuel. In this study, I investigated the interference AC drives causes the feeding network and how to go about averting them. I show what profits there are between the valve- and speed control of the pumps/motors and the reason for this. I then describe the three main systems, which form the core of CLAB's operation, how they work individually and how they interact with eachother. In the end I want to show how much energy you can save on the planned upgrade and it shows the approximated running costs over time. The results are based on calculations made by using Matlab, simulations in FluidFlow and graphs in Excel, which provides an indication of how much costs differ between the valve- and frequency controls of the three systems. / Detta examensarbete är gjort i ett sammarbete med SWECO Energuide i Växjö. De har fått i uppdrag av SKB i Oskarshamn att förnya deras elkraftsförsörjning och tillhörande processystem på deras Centrala mellanlager av använt kärnbränsle, CLAB. På CLAB kyls det använda kärnbränslet i dag med vattenpumpar, drivna av asynkronmotorer, där vattenflödet stryps av mekaniska ventiler till önskat flöde. Efter uppgraderingen kommer motorerna istället drivas med frekvensomriktare vilka kan varvtals styra pumparna, och på så sätt ändra vattnets kylande effekt på kärnbränslet. Med denna studie har jag undersökt vilka störningar frekvensomriktare orsakar på matande nät och hur man kan gå till väga för att avvärja dem. Jag visar vilka vinster det finns mellanventilstyrning och varvtalsstyrning av pumpar/motorer samt orsaken till detta. Därefter beskrivs de tre huvudsystemen, vilka utgör kärnan i CLAB:s verksamhet, som tar upp hur de fungerar var för sig och hur de interagerar med varandra. I slutändan vill jag visa hur mycket energi man kan spara på den planerade uppgraderingen och det görs i form av en kostnadsberäkning över tid. Resultatet baseras på beräkningar gjorda med hjälp av Matlab, simuleringar i FluidFlow och diagram i Excel vilka ger en fingervisning på hur mycket kostnaderna skiljer sig mellan ventilstyrning och frekvensstyrning av systemen.

To safely encase and store spent fuel assemblies in its final geological storage, accurate measurements of their heat output must be performed. To this end, information about the geometry and sensor set-up of the calorimeter at the interim storage facility CLAB was gathered. The data from the temperature sensors where compared to each other and an analytical expression was proposed to fit the data. A methodology for calculating calibration curves was formulated and its uncertainties were evaluated. Differences were found between the fits of measurements from differing measurement campaigns. The measurement campaigns resulted in calibration curves with notable offset from each other. The computer code SERPENT2 was used to construct a 3d geometry model of the calorimeter and the ratio of photons entering and escaping the calorimeter wall was calculated for photon energies which dominate the gamma spectrum from spent fuel assemblies of about 5 to 40 years cooling time.

Swedish Nuclear Fuel and Waste Management Company- SKB has conducted measurement campaigns at the Swedish central interim storage facility for spent nuclear fuel- CLAB over the years, extending from year 2003 to 2019 where the gamma energy escape was acquired. At CLAB the spent nuclear fuel assembly is inserted into the calorimeter; device intended to measure temperature increase due to decay heat from the fuel assembly. The calorimetric construction is surrounded by water the medium in which the temperature deviations occur and thus are also measured by the calorimeter. However, there is some leakage of gamma energy from the calorimetric construction and does not contribute to the heating of the water. Therefore, only considering the calorimetric measurements is not enough to estimate the total decay heat in the fuel assembly since these measurements fail to account for the gamma escape. Measurements of gamma energy escape acquired over the years at CLAB were observed to have some tendencies that where questionable, mainly some stochastic behavior indicating that their uncertainty was profound. In the scope of the thesis a computational method was developed to calculate the gamma energy escape and thus assist in determining which measurements to discard. Combination of two programs were used one being Spent Nuclear fuel- SNF and the second Monte Carlo N particle Simulator- MCNP, to obtain the computational gamma energy escape for different fuel assemblies and cooling times- CTs. It was established that the escape had a range between 1-3,5% and that it had a dependency on CT, fuel assembly type and operational history. Calculated radial exponential decay coefficient for fuel assemblies of the medium; water had also a clear dependency on CT where values of the coefficient increased over CT. Normalized gamma energy distribution over a rotation around the fuel assembly was calculated and it showed that the assembly tended to have the highest radiation coming from its corner rods. The verification of the computational gamma escape results with corresponding measurements yielded that the agreement was quite good for the earlier measurement campaigns. However, deviation became evident after the 2007 campaign where the calculated values were underestimated compared to the measured.

It is important to know the heat generated due to nuclear decay in the final repository for spent nuclear fuel. In Sweden, the heating powers generated in spent nuclear fuels are currently measured in the calorimeter System 251 at the Clab facility, Oskarshamn. In order to better measure, and increase understanding, of the temperature measurements in the calorimeter, a simple thermodynamic model of its temperature evolution was developed. The model was described as a system of ordinary differential equations, which were solved, and the solution was applied to calibration measurements of the calorimeter. How precise the model is, how its parameters affect the model, et cetera, are addressed. How the temperature evolution of the system changes as the values of parameters in the model are changed is addressed. The mass correction of the calorimeter could be estimated from this model, which validated the established mass correction of the calorimeter. How the measurement results from the calorimeter would be affected if the volume of the calorimeter was changed was also considered. Additionally, gamma radiation escape from the calorimeter without being detected as heat in the calorimeter. The gamma escape energy fraction was estimated by SERPENT simulations of the calorimeter, as a function of the initial photon energy. The gamma escape was also estimated for different values of the radius of System 251.