Core Disruptive Accident in Heavy Liquid Metal Fast Reactor - Case Study: MYRRHA

Advanced fast nuclear reactors are being studied to complement the current-day nuclear reactor fleet due to the variety of important advantages they
offer [1, 2].
Unlike reactors that rely on thermalized neutrons, the core of a fast reactor is not operating in its most reactive geometry/arrangement. It is therefore
theoretically possible that a change of geometry and/or rearrangement of fuel material might result in an increase of reactivity (i.e. measure of deviation
from a critical state) and an explosive energy release inside the reactor vessel [2, 3]. This could expose the reactor vessel to high mechanical forces, challenging
its integrity and hence its confinement function.
These events, the so-called Core Disruptive Accidents (CDAs), are intensively studied in the safety assessment of Sodium-cooled Fast Reactors
(SFRs), and more recently also in the case of other reactor concepts. Although these accidents were always classified as ‘highly unlikely’, many designs
of SFR containment structures were designed according to the outcomes of these studies [2].
A CDA scenario hypothesized to occur in an SFR core was initially proposed by Hans Bethe and John Tait in 1956 for the safety studies of
Dounreay Fast Reactor [4] in the United Kingdom. This scenario is illustrated in Figure 1. Upon loss of coolant flow (1 - A), an imbalance between the heat production and the heat removal (due to reduced convection and Sodium boiling) develops in the core. This imbalance results in fuel melting
(1 - B) and its gravitational collapse (1 - C), leading to the fuel material rearrangement. Fuel material rearrangement further results in increase
of the reactivity beyond promptcritical level and an exponential increase of power. Upon reaching the temperatures high enough to vaporize the fuel, the system pressure rises rapidly and the hydrodynamic disassembly of the core occurs (1 - D). This inevitably results in the reduction of the reactivity and the termination of the energy excursion. Bethe and Tait made a simplified,
conservative model to estimate the energy released in such a case, before the core would ultimately be disassembled by the pressure of the fuel vapor [4].
More recently, the probability of occurrence of a CDA, together with the assessment of the potential consequences of such an event, were
comprehensively studied and discussed in the framework of safety studies of the Schneller Natriumgekühlter Reactor (SNR) 300 [5]. Among the variety of conceivable initiating events and sequences leading to a CDA, the Unprotected
Loss Of Flow (ULOF) accident was chosen for a particularly detailed study. The ULOF accident occurs if all primary coolant pumps fail simultaneously as a result of a loss of power supply, and if subsequently all of the reactor protection systems fail. The probability of occurrence of an ULOF accident was determined to be 1.2×10-6/year.
ULOF accident would lead to Sodium boiling and fuel melting, after which a Bethe-Tait type of a scenario would unfold. Study [5] indicates that the
energy release in such accident would be greater than the design values of the SNR-300 reactor vessel (which amounts to 370 MJ) in 0.3 % of cases
following the ULOF accident. This result provides an order of magnitude estimate of probability of occurrence of a CDA that will challenge vessel’s
integrity, i.e. ∼ 4×10-9/year.