Reactivity accidents

The Chernobyl accident also has been a terrible reactivity accident. It is more completely described in Appendix 1. 

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Examples of such events are reactivity accidents and steamline breaks which are regularly analysed at zero reactor power. The quality of the safety analysis report directly influences the quality of NPP documentation, technical specifications and EOPs in particular. 

Destructive reactivity accidents due to accidental expulsion of control rods or to control rod melting before fuel melting during a severe accident. 

A structural cage around the vessel resistant to the burst of the vessel itself (destructive steam explosion, destructive reactivity accident) or to jet force caused by its perforation in conditions of high pressure in the primary system (the energies involved are illustrated in Fig. 5-3). 

The probability of destructive reactivity accidents is considered negligible, but an uncertainty remains for up to one hour between the melting of the control rods and the fuel melting in the core. The situation might be more critical for a BWR where the... 

Minimum primary pressure for which DCH is possible Maximum thermal energy released in a very serious reactivity accident (AP600) ... 

At the start, the research was mainly concentrated on reactivity accidents (SPERT and BORAX experiments in the USA). Subsequently, in the 1960s, the most studied issue was a large LOCA. After Three Mile Island, the attention moved to small LOCAs because that event, together with the results of the Rasmussen Report, highlighted their danger, and to severe accidents. 

AR141 Accident management programmes in nuclear power plants A guidebook. No. 368, 13 July 1994. AR142 Reactivity accidents, No. 354, 23 April 1993. 

Criterion 28 - Reactivity limits. The reactivity control systems shall be designed with appropriate limits on the potential amount and rate of reactivity increase to assure that the effects of postulated reactivity accidents can neither (1) result in damage to the reactor coolant pressure boundary greater than limited local 5uelding nor (2) sufficiently disturb the core, its support structures or other reactor pressure vessel internals to impair significantly the capability to cool the core. These postulated reactivity accidents shall include consideration of rod ejection (imless prevented by positive means), rod dropout, steam line rupture, changes in reactor coolant temperatme and pressure, and cold water addition. 

Stability and reactivity Accident risks (incompatibilities, forbidden conditions, etc.)... 

During 1996 it was agreed by CEA-EDF and PNC to conduct the RAFT programme (Reactivity Accident Fuel Tests). 

The calculated Loss-of-Flow and reactivity accidents were described and estimates of the behaviour of a lead-cooled fast system and that of thermal ADSs with a circulating fuel/salt mixture were described in the chapter on ADS safety in the IAEA State of the Art (SOAR) report on accelerator-driven systems which will soon be published. 

Safety considerations of such new approaches and in particular the investigation of severe accident conditions are important. An investigation of a severe reactivity accident in an accelerator-driven fast reactor was earlier undertaken. 

Design and neutronic characteristics of the reactor core ensure negative power, temperature, and void reactivity coefficients. As a result, self-limitation of the reactor power at the reactivity accidents and transients without scram takes place. Reactor self-control properties enable to change reactor power in the range of 20 - 100% of rated power (No) at a rate up to 0.5%No/s without control rods displacement and just for automatic control of feed water flow rate. 

Traditional control rod drives require a lot of space either above or below the core. There are possibilities to use liquid absorber materials, which do not require the space for rod drives and for in vessel storage when withdrawn. There have also been designs for in vessel mechanical drives (PSR, MRX, HR 200). These eliminate the need to consider control rod ejection which is one of the main, but unlikely, reactivity accident initiators. A more radical solution is in the JAERI SPWR design where liquid filled tubes are used instead of control rods. 

In the event of reactivity accidents, even in the case of postulated sticking of all CGs following on emergency protection signal, core damage is excluded by virtue of the inherent self-protective properties of the reactor. 

III-l. The main source of radiation in a nuclear power plant under accident conditions for which precautionary design measures are adopted consists of radioactive fission products. These are released either from the fuel elements or from the various systems and equipment in which they are normally retained. Examples of accidents in which there may be a release of fission products from the fuel elements are loss of coolant accidents and reactivity accidents in which the fuel cladding may fail due to overpressurization or overheating of the cladding material. Another example of an accident in which fission products may be released from the fuel rods is a accident in handling spent fuel, which may result in a mechanical failure of the fuel cladding from the impact of a fuel element that is dropped. The most volatile radionuclides usually dominate the accident source term (the release to or from the reactor containment). Recommendations and guidance on the assessment of accidents are presented in Section 4 of Ref. [III-l]. 

This shutdown was preceded by two safety tests in September which allowed, on the one hand, to confirm the ability of Superphenix to evacuate decay heat by natural convection of the primary circuit and the secondary loops after reactor shutdown, and on the other hand, to demonstrate the absence of risk of a reactivity accident, caused by a passage of argon gas in the core following depressurization of an intermediate heat exchanger argon-filled bell. 

In contrast to the absence of reactivity accidents there have been several instances of sodium fires. None of these has caused injury to personnel, but the event at the MONJU plant caused a serious interruption to reactor operation. For this reason there will undoubtedly be important efforts to reduce the incidence of sodium leaks and to improve the protection against the consequences of fires. 

Understanding Chemical Reactivity Accidents. Household Chemistry Use and Misuse. 

Reactivity events are a specific category due to their specific issues and consequences. Reactivity accidents can lead to a local or a full core criticality. Examples like boron dilution, unintentional withdrawal of control rods or refuelling errors may be considered in the SLP PSA. International experience has shown that many such events occurred at NPPs, and their frequencies is high, though the consequences are low (recoveries are possible in many of those events). Some phenomena, like unborated slug of water entering the core and its consequences, are still being analysed. 

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