[en] The causes of the accident at Chernobyl Unit 4 on April 26, 1986 are discussed. The IAEA's International Nuclear Safety Advisory Group has reconsidered the root cause of the accident in the report INSAG-7. The shortcomings in the neutron physics characteristics of RBMK-type reactors were the positive void reactivity coefficient and the possibility of positive scram. The operating reactivity margin concept is seen as inappropriate for complete assurance that the control rod configuration secures fully adequate reactor protection. Two earlier accidents at RBMK reactors, a fuel channel failure at Leningrad 1 in 1975 and a fuel failure at Chernobyl 1 in 1982, had indicated weaknesses in the characteristics and operation of RBMK units. IAEA organized a consultants' meeting on void reactivity effects in RBMK reactors in Moscow 1995. In the meeting the phenomena influencing the void reactivity effects in RBMK reactors in Moscow 1995. In the meeting the phenomena influencing the void reactivity coefficient, the calculational tools and the status of experimental determination of the void reactivity coefficient in operating RBMKs were discussed. (orig.) (3 figs.)

[en] The effects of ATWS and ATWC-events with control rods failing to enter the core has been evaluated in this project. To understand the uncertainties in using modern 3D-calculation methods two different codes were used in the project. The outputs from the two code packages were compared. Within the project the used code were first evaluated against a real event, pancake core at Forsmark 3. The results give important knowledge of the core responses for such events and on how to use different code to perform such calculations. The NKS report is only one minor part of the total project. The project was sponsored by TVO, Forsmark, OKG, Ringhals, SKI besides the NKS-funding. The results could be used for PSA-studies and for deterministically safety analysis. (au)

[en] A fast databuffer system was constructed, in which cosmic ray events in the Turku hadron spectrometer, including particle arrival times, are recorded with time resolution of 100 ns. The databuffer can be read continuously by a microprocessor, which preanalyzes the data and transfers it to the main computer. The time span that can be analyzed in every detail, is a few seconds. The high time resolution enables a study of time correlated groups of high energy particles. In addition the operational characteristics of the spectrometer can be monitored in detail

[en] The maximum entropy method has been applied in the spectral analysis of high-energy cosmic-ray intensity during the large Forbush event of July 13, 1982. An oscillation with period of about 2 hours and amplitude of 1 to 3% was found to be present during the decrease phase. This oscillation can be related to a similar periodicity in the magnetospheric field. However, the variation was not observed at all neutron monitor stations. In the beginning of the recovery phase, the intensity oscillated with a period of about 10 hours and amplitude of 3%

[en] Particle fluxes and pitch angle distributions of relativistic solar protons at 1 AU were determined by Monte Carlo calculations. The analysis covers two hours after the release of the particles from the Sun and total of eight 100000 particle trajectories were simulated. The pitch angle scattering was assumed to be isotropic and the scattering mean free path was varied from 0.1 to 4 AU. As an application, the solar injection time and interplanetary scattering mean free path of particles that gave rise to the GLE on May, 1978 were determined. Assuming exponential form, the injection decay time was found to be about 11 minutes. The m.f.p. of pitch angle scattering during the event was about 1 AU

[en] An experimental arrangement for studying multiple hadrons produced in high-energy hadron-nucleus interactions is under construction at the university of Turku. The method of investigation is based on the detection of hadrons arriving simultaneously at sea level over an area of a few square metres. The apparatus consists of a hadron spectrometer with position-sensitive detectors in connection with a small air shower array. The position resolution using streamer tube detectors will be about 10 mm. Energy spectra of hadrons or groups of simultaneous hadrons produced at primary energies below 10 to the 16th power eV can be measured in the energy range 1 to 2000 GeV

[en] Modulation of the cosmic-ray energy spectrum was studied by using the Turku double neutron monitor. The multiplicity region of detected neutrons produced by cosmic ray hadrons in the monitor was divided into seven categories corresponding to mean energies 0.1, 0.3, 1.0, 3.2, 8.6, 21, and 94 GeV of hadrons at sea level. Based on 24-hour frequencies, a statistical analysis showed that modulation of the intensity in all categories occurred during several periods in the fall 1984. The magnitude of the variation was a few per cent

[en] The FINIX fuel behaviour module has been under development at VTT for six years for modelling traditional western light water reactor and Russian WWER fuel. The module is designed to be implemented in various other reactor safety analysis tools, such as neutronics and reactor dynamics codes. At VTT, it has been internally coupled with Serpent 2 to provide fuel temperatures for neutronics, and to thermal hydraulics and reactor dynamics codes TRAB1D, TRAB3D and HEXTRAN. FINIX will also be a part of the new VTT reactor analysis framework Kraken, the development of which is currently underway. In such couplings, the fuel temperature distribution is one of the most important parameters passed on to the host code. From the start, FINIX has been able to model the fuel temperature distribution in reactivity insertion accidents (RIA) comparably to established codes such as FRAPTRAN. The irradiated state was typically taken into account with a restart file from FRAPCON. Over the years, additional models have been implemented in the code, and long irradiation periods can now also be modelled with FINIX. The average error in temperature predictions across several Halden irradiations is 6.6%, and FINIX predicts typically 50 to 100 K higher temperatures at the highest temperatures compared to FRAPTRAN. Recently, the ability of FINIX to model loss-of-coolant accidents (LOCA) has been investigated. The main limitation of the current FINIX version is in mechanical modelling, as finite strain deformation cannot be accurately modelled, and the currently implemented failure models are rudimentary. Temperature predictions of FINIX were compared to FRAPTRAN and found to be close to FRAPTRAN until the blowdown phase, at which FINIX overpredicts the temperatures. FINIX LOCA predictions have also been compared to results calculated by FRAPTRAN for the Halden IFA-650.5 LOCA test. (author)

[en] The energy spectra of cosmic-ray neutrons, protons, and pions were determined at sea level in the range 0.1 - 1000 GeV. The results are based on the measurements of the neutron-multiplicity distributions produced by cosmic hadrons in the Turku spectrometer. Below 0.5 GeV, the values gamma 1.47 + or - 0.04 and 0.8 + or - 0.1 were obtained for the slopes of the neutron and proton spectra. At 1000 GeV, the nucleon spectra are much steeper with gamma 2.79 + or 0.06. The pion spectrum was found to obey the power law with gamma 1.41 + or - 0.05 in the low-energy region up to 30 GeV. Above 500 GeV, the spectrum of pions, containing an admixture of accompanied particles, was found to reach the high energy limit with gamma 2.92 + or - 0.06. 7 references

[en] PCI related phenomena have been investigated at VTT and novel models have been developed for their study. As standard creep models do not accurately model stress reversals and reductions, new transient creep model with the anelastic creep component has been developed. The model accurately predicts creep behaviour during stress reversals. Accurate modelling of creep is required for the determination of cladding stresses during pellet-cladding interaction. The chemical behaviour of the fuel pellet-cladding gap has also been investigated with a new thermochemical model including radiolysis. As caesium iodide is unreactive towards the cladding, its radiolysis may contribute to the amount of corrosive iodine on the cladding inner surface, and cause stress-corrosion cracking. In addition, missing pellet surface defects have been investigated with the BISON fuel performance code with a simplified model, and capacity to model such phenomena at VTT has increased. (author)