Fast Reactors and Accelerator Driven Systems Knowledge Base

IAEA-TECDOC--1139: Transient and accident analysis of a BN-800 type LMFR with near zero void effect.

International Atomic Energy Agency, Vienna (Austria)

Abstract

Large conventional liquid metal cooled fast reactor (LMFR) cores show a significant reactivity increase if a coolant loss occurs by boiling or gas intrusion. Since this positive reactivity effect is very important for the overall behaviour of LMFRs from a safety point of view, a lot of attempts have been undertaken worldwide to reduce the sodium void reactivity effect (SVRE). One proposal has been made by the Institute of Physics and Power Engineering (IPPE), Obninsk, Russian Federation, in which the core upper axial blanket is replaced by a sodium plenum consisting of sodium filled wrapper tubes. In this case the enhanced axial neutron leakage would result in a strong negative reactivity effect in case of sodium voiding which would compensate a large fraction of the positive SVRE in the core region. The International Atomic Energy Agency (IAEA) and the European Commission (EC) Joint Benchmark Programme have assessed the capability of reducing the SVRE of such innovative core design. The analysis (IAEA-TECDOC-731, 1994) showed that overall SVRE for the reference 2100 MW(th) MOX fuel core might be close to zero. This method of reducing the SVRE has been adopted in the BN-800 reactor design in the Russian Federation. However, investigations were needed to determine differences in severe accident responses in order to estimate the feedback to overall safety that could be achieved by a reduction in the SVRE value for mixed-oxide (MOX) fuel reactor core. Therefore, recognizing the importance of such an innovative LMFR core design, a comparative exercise of severe accidents for BN-800 type reactors with reduced sodium void coefficient was jointly initiated by the IAEA and the EC in 1994. The Russian specialists took over the task to prepare the benchmark input data, a revised draft of which was distributed by the IAEA to the participants at the end of June 1995. The specifications of the benchmark were finally fixed in a meeting at the IAEA in Vienna on 11- 13 December 1995. The main findings resulted from intensive discussions through eight IAEA/EC joint meetings held in turn in Vienna and Brussels. The final meeting was held at IPPE, Obninsk, Russian Federation, on 2-6 June 1998. The benchmark programme resulted in an effective information exchange among the Member States sharing requirements as well as experience in advanced reactor design and computer codes for transient calculations.

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