How to manage a core meltdown in a large nuclear reactor
Tid: Fr 2020-02-21 kl 13.00
Medverkande: Walter Villanueva
In 2011, a powerful earthquake followed by a major tsunami initiated the Fukushima Daiichi nuclear accident that led to reactor core meltdowns in three units. The accident directly affected about half a million people to be evacuated and reignited concerns about safety of nuclear reactors worldwide, in particular, the radioactive releases during severe accidents. The accident also highlighted the need to improve nuclear safety approaches and safety culture, and prompted the enforcement of ongoing safety improvements both to prevent severe accident and to mitigate its consequences. A series of stress tests have been carried out at operating Nuclear Power Plants (NPPs) and severe accident (SA) research has been intensified worldwide, for instance with a focus on in-vessel melt retention (IVMR) as a possible concept of severe accident management (SAM).
In light water reactors (LWRs), there are 3 barriers preventing environmental radioactive releases from the heat-generating fuel. First, the fuel pellets are enclosed with cladding material forming fuel rods which constitute fuel assemblies and the core. Second, the core is housed in a reactor pressure vessel (RPV) that is filled with water. The third and the last barrier, is the reactor containment isolating its inventory but providing evacuation of heat. During the in-vessel phase of SA progression, i.e. after failure of the 1st barrier, the core melt can relocate into the lower head of the RPV and sets thermo-mechanical loads on the vessel affecting RPV integrity. If the second barrier (RPV) is broken, SAM is focused on protection of the last barrier which can be affected by ex-vessel SA phenomena.
One of the most promising SAM concepts is IVMR with external RPV cooling by water. Significant safety margins has been demonstrated for RPV integrity during IVMR in several low- and medium- power reactors, allowing successful licensing and implementation of the concept in various designs. For larger reactors, however, it is much more difficult to prove the effectiveness of IVMR strategy. This is mainly due to the higher loads on the RPV associated with higher reactor power. The lecture will focus on our research on SA phenomena involved in IVMR and innovative approaches to limit the loads and to improve RPV cooling.