Nuclear Behaviour Under RIA Conditions


The design basis RIA in a PWR is the Control Rod Ejection (CRE), while in a BWR, it is the Control Rod Drop Accident (CRDA). The CRE is based on the assumption of a mechanical failure of the control rod drive mechanism located on the reactor vessel top, followed by the ejection of the mechanism and the control rod by the internal reactor pressure. The resulting, significant power surge is limited partly by Doppler feedback and finally terminated by the reactor trip. The BWR CRDA is assumed to occur if a control rod is detached from its drive mechanism in the core bottom, stays stuck while inserted in the core, then if loosened, drops out of the core by gravity, without involvement of a change in reactor pressure as in the CRE. Partly as a result of these differences, the BWR power pulses are slower than for a PWR. The pulse widths for PWRs are in the range of 10–30 ms and for BWRs in the range of 20–60 ms.
The reactivity transient during an RIA results in a rapid increase in fuel rod power leading to a nearly adiabatic heating of the fuel pellets.
The RIA-simulation experiments conducted in the 1960’s and 1970’s using zero or low burnup test rods showed that cladding failure occurred primarily by either:

  • Post-Departure from Nucleate Boiling (DNB)
  • Cladding contact with molten fuel

This Special Topic Report (STR) will give insight and understanding of the parameters impacting the fuel RIA performance and reviews the applicability of the data to high burnup fuel cladding. The STR also provides the latest RIA regulatory acceptance criteria.