Zur selbsttätig sicheren Begrenzung von nuklearer Leistung und Brennstofftemperatur in innovativen Kernreaktoren

Nuclear energy probably will not contribute significantly to the future worldwide energysupply until it can be made catastrophefree. Therefore it has to be shown, that theconsequences of even largest accidents will have no major impact to the environmentof a power plant.In this paper one of the basic conditions for such a nuclear technology is discussed.Using mainly the modular pebble-bed high-temperature reactor as an example, thedesign principles, analytical methods and the level of knowledge as given today incontrolling reactivity accidents by inherent safety features of innovative nuclear reactorsare described. Complementary possibilities are shown to reach this goal with systems ofdifferent types of construction . Questions open today and resulting requirements forfuture activities are discussed .Today's knowledge credibly supports the possibility of a catastrophefree nucleartechnology with respect to reactivity event

ACFA - a versatile activation code for coolant and structural materials

The ACFA code calculates the neutron-induced activation, afterheat, transmutation, gas production, biological hazard potential, and activation gamma ray spectra in the components of a nuclear system. The quantities of interest may be computed by spatial interval and zone or only by zone of the system considered. To calculate the transmutation coefficients for the neutron-induced reactions the code uses multigroup activation cross sections and space-dependent multigroup neutron fluxes in one- or two-dimensional geometry. The neutron reaction types incorporated in the code are : (n,n'), (n,2n), (n,$\gamma$), (n,p), (n,$\alpha$), (n,n'p), (n,n'$\alpha$), (n,t), (n,3n), (n,He-3), (n,d), and (n,n'd) considering both reactions to the ground state and to isomeric states. The code uses a variable dimensioning technique to adapt the core data storage requirements to the particular problem considered and uses the FIDO inputsystem to read the input data. The numerical methods for establishing and solving the decay chain equations are taken from theORIGEN code. To test the ACFA code and the nuclear data libraries used, the activation, composition change, and gas production inthe first wall of the UWMAK-I fusion reactor are calculated. The results of the activation calculation are compared with earlier results of the University of Wisconsin Fusion Study Group

Dose rate calculations for a fusion ignition experiment

In the next generation of fusion experiments, which are planned to reach thermonuclear ignition conditions, a deuterium-tritium mixture will be used as fuel. If significant D-T burning is achieved in these devices a large number of 14.1 MeV neutrons will be produced which will activate the components used in the experiments. The resulting activation radiation will create additional problems of access to the machine for maintenance and repair, which should be taken into account in the design and construction phase of these experiments. To give an idea of the dose rates to be expected, some activation calculations for a typical large Tokamak ignition experiment are presented. It is assumed that the machine is operated in a pulsed mode at an average of 100 plasma discharges within one week and that 10$^{20}$ neutrons per pulse are produced. The investigation shows that 30 days after shutdown of the machine the total absorbed dose rate in the vicinity of the device is about 0.2 rem/h which limits the working time in this area to about 10 h per quarter. For repairs inside the vacuum vessel or dismantling of the machine it is assumed that the machine can be divided into eight identical segments. The dose rates from an isolated segment and from the remaining part of the machine after removal of one segment are calculated. The results for both cases are of the same order of magnitude and are in the range of 2 to 4 rem/h for decay times between 5 and 30 days. Hence, all repairs inside the vacuum vessel or dismantling of the machine must be done by remote control