Accelerator Driven Systems - Some Safety and Fuel Cycle Considerations

A fast spectrum accelerator-driven system (ADS) may become a very versatile tool in the nuclear industry and allow a number of new applications. Such a system may serve as a means of transmuting large quantities of actinides and long lived fission products into short lived waste. Other possibilities, include the cross-progeny fuel cycle burning Pu in a thorium matrix and generating 233U which can be used to refuel light water reactors. In addition, use of a thorium cycle could result in reduced actinide production and a much lower radio-toxicity of the waste. In this paper we focus on the use of thorium hosted weapon and reactor grade plutonium in a fast ADS from the viewpoint of neutron economy and Pu annihilation rate. We consider a sub-critical system (keff = 0.98 / 0.99) such that super-prompt critical excursions, which may result from coolant voiding, molten cladding and fuel movement, or positive temperature coefficients due to a large actinide inventory, etc. are not possible. Such a system would combine the intrinsic safety of an ADS with the advantages of a reactor with a large delayed neutron fraction (imitated by the spallation neutrons) and a rather flat power distribution for nearly uniform burn-up across the whole fuel region. In a system of this kind, a proton beam power of 10-20 MW would produce enough spallation neutrons to obtain a 1 GWe power plant. In this case, a much less expensive and a rather compact multistage cyclotron arrangement might be sufficient to produce the required proton beam power. As safety is the main concern, the JRC embarked in a series of safety studies of near critical systems. Initially we used a simplified kinetics code, which accounted only for negative Doppler feedback in an adiabatic system, to calculate the effect of fast transients. Later, after some modifications, the JRC's sophisticated European Accident Code, (EAC-2) has been used. This includes additional feedback due to axial fuel expansion, sodium voiding and fuel motion. The analysis showed that for fast, or medium to fast reactivity ramps, the ADS has a major advantage in coping with serious reactivity insertions when compared to conventional reactors. On the other hand, "slow" core melting may occur, if in a loss of heat sink or coolant flow accident the accelerator beam is not switched off. Therefore passive means for shutting-off the proton beam are of primary importance. 1 NATO Advanced Research Workshop on Advanced Nuclear Systems Consuming Excess Plutonium, 13 16 October 1996, Moscow, Russia