On the other hand, the restart from a frozen coolant state seems questionable if voids created during freezing in orbit (following reactor shutdown early in the life of the system) are inappropriately distributed. On the contrary, start and restart of gas cooled reactors do not rise specific problems. Only special care should be taken to rise the reactor power during the preheating phase when gas does not circulate. 3.9 Safety Aspects All examined reactor design concepts proved able to passively withstand a loss of coolant accident at full power - 110 kWth. Differences with regard to safety therefore mainly relate to the behaviour of the system in case of compaction or immersion accidents following launch abort. Presently, all systems are equipped with safety rods designed to ensure a subcriticality margin of 5% during such accidents. However, assumptions for the mechanical behaviour of the reactor during compaction have to be experimentally confirmed. Furthermore, possible explosive behaviours of immersed liquid metal cooled reactors, due to the liquid metal water chemical reaction might lead to critical configurations not yet taken into account. Due to the uncertainties mentioned above, safety considerations are not discriminants at this stage of the study. 3.10 Reliability From a theoretical viewpoint, direct cycle systems, which do not need E.M. pumps, primary heat exchangers and thawing devices should prove more reliable. However, a preliminary reliability survey gave indications that a dual Brayton conversion might be required to meet the aimed system overall reliability of 0.90. In such a case, either helium leak tight valves or an intermediate heat exchanger would also be necessary for the HTGR derivative system, thus mitigating its reliability advantage. 3.11 Development Cost and Lead Time A preliminary evaluation of development costs, recurrent costs, and lead times was made on the basis of the following development scenario: - the decision of the first nuclear SPS powered mission is taken after having performed ground demonstration tests of a nuclear reactor prototype and of an electrically heated prototype of the whole system for 2 and 4 years respectively. - three flight prototypes are fabricated, one for ground tests, one to be launched, and one spare. Ilie flight prototype is launched once 4 years of ground qualification and reliability tests of a prototype identical with the flight prototype have been accumulated.
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