Fast reactors accidents

In considering the operational safety and accident analyses of sodium-cooled fast reactors, similar information on the release of fission products from sodium is needed. Although the extent of vaporization can often be calculated from thermodynamic considerations (3, 4), appropriate transport models are required to describe the rate phenomena. In this chapter the results of an analytical and experimental investigation of cesium transport from sodium into flowing inert gases are presented. The limiting case of maximum release is also considered. 

The Integral Fast Reactor would also be capable of breeding plutonium which could be used as nuclear fuel. This type of reactor was seen as the key to a nuclear future. Liquid sodium is a volatile substance that can burst into flames if it comes into contact with either air or water. An early liquid sodium-cooled breeder reactor, the Fermi I, had a melting accident when 2% of the core melted after a few days of operation. Four years later when the reactor was about to be put into operation again a small liquid sodium explosion occurred in the piping. 

At the present state of the art, a corner of an article about evaluation of population hazards is hardly an appropriate place in which to attempt an exposition of reactor safety. Nevertheless, we may contrive a brief description of these types of reactor accidents which, it is thought, could lead to fission product release. The intention is to illustrate ways in which fuel could be damaged and then release fission products ultimately to the atmosphere. Though gas-cooled reactors, water-cooled reactors, and sodium-cooled fast reactors will be discussed, no comparisons, invidious or otherwise, are intended between the safety of these systems. 

Fast Reactor Meltdown Accidents Using Bethe-Tait Analysis... 

An under-sodium leak occurred in PFR superheater 2, one of the original units made from austenitic steel, in February 1987. It provided valuable information on the behaviour of sodium-water reactions in an operating steam generator and led to a complete re-assessment of the design-basis steam generator accident for subsequent fast reactors. 

This type of incident is specific to fast reactors and justifies them being placed in a separate category. It will be observed that the risk involved in sodium fires motivated the regulating authority to demand, specifically after the Almeria accident, that a sprayed sodium fire resulting from a complete and sudden rupture of a main secondary pipe be taken account of. 

INTERNATIONAL ATOMIC ENERGY AGENCY, Fast Reactor Fuel Failures and Steam Generator Leaks Transient And Accident Analysis Approaches, IAEA-TECDOC-908, Vienna, 1996. 

One of the attractive features of the fast reactor is its hard neutron spectrum. To expand this feature, a metallic fuel core is employed in the 4S. However, it is more difficult to reduce void reactivity for a core with a harder spectrum. It is very important to design the void reactivity to be negative in order to prevent a severe nuclear accident in the event of sudden loss of coolant, sudden loss of coolant flow or a large gas bubble entrainment in the core. 

Detroit (Fermi), USA Sodium cooled fast reactor accident... 

The RCCAs of a reactor control protection system (CPS) are the most important elements of the reactor maintenance safe operation, which ensure control of reactor core power level and fast reactor core transfer from initial condition to sub critical condition during the accident. Share of RCCA in fuel reloading cost is 1-1,2%. However for CR materials choice it is necessary to take into account a possibility of inexpensive RCCA recovery or disposal. 

Kazakhstan has a nuclear scientific-industrial complex which was set up as a part of a nuclear infrastructure of the former USSR. More than 50% of the uranium resources of the former Soviet Union are in Kazakhstan, with seven uranium mines. Two UO2 plants produced up to 35% of the total uranium in the USSR in 1990. There are extensive facilities for producing UO2 pellets for VVER fuel elements from Russian enriched uranium. Kazakhstan has several research reactors and one operating nuclear power plant, the BN-350 fast reactor, which started operation in 1973 with a design life of 20 years. Work on its lifetime extension has the intention of bringing it into compliance with current safety standards. 1995 and 1996 were devoted to this work. In October 1996. experimental investigation on accident-proofdecay heat removal by natural circulation was carried out. The reactor BN-350 was restarted in February 4, 1997 at a power level of 420 MW(th). 

Accident studies with the European Accident Code (EAC-2) were continued, but at a reduced pace, for the sodium-cooled 800 MWe fast reactor design used in the European WAC benchmark calculation of 1989. 

The investigation of safety and more particularly of severe accident conditions is important for accelerator driven systems (ADS). Subcritical ADS could be of particular interest for the actinide transmutation from the safety point of view, because fast reactors with Neptunium, Americium and Curium have a much smaller fraction of delayed neutron emitters (compared to the common fuels and U), a small Doppler effect and possibly a positive coolant void coefficient. This poses a particular problem of control since the fraction of delayed neutrons is essential for the operation of a nuclear reactor in the critical state. In addition, the IRC presented in the past a review of accelerator-driven sub-critical systems with emphasis on safety related power transients followed by a survey of thorium specific problems of chemistry, metallurgy, fuel fabrication and proliferation resistance. 

Computations have been performed for an ADS consisting of an accelerator coupled to a subcritical fast reactor with thorium/ U fuel and natural convection lead cooling. After introducing a source into the point kinetics module of the EAC2 code, Loss-of-Flow accident calculations for an ADS were performed. 

There is the prospect that loss of coolant and hence core meltdown can be made incredible and that eventually designs will be released from these serious limitations. However, in the current work, the aim has been to determine how seriously the design is handicapped even if you allow for the unbelievable. Public acceptability of the first large, fast reactors may well require designs that meet the consequences of the meltdown accident. In this context, then, we must strive to understand the reactivity implications of voiding in sodium-cooled reactors. The first reactor of this class will establish failure statistics of control drives, demonstrate independence of different sources of failure, and thereby permit subsequent assertion that failure to scram is impossible. 

FAST REACTOR MELTDOWN ACCIDENTS USING BETHE-TAIT ANALYSIS... 

In a meltdown accident, the reactor is put on a fast period, and the energy density of the core material rises until the point is reached where there is a significant pressure buildup, which tends to cause fuel movement and ultimately causes core disassembly and shutdown of the reactor. In the calculations performed, it was assumed that the clad is molten so that pressure tending toward fuel movement starts at the normal boiling point of the fuel. Therefore, the course of the accident is completely determined if one gives the initial conditions at the time the boiling point of the fuel in the core is first reached. Prior to this time, the reactor behavior is described by the standard kinetics equations. 

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