Numerical Investigations on Debris Bed Coolability and Mitigation Measures in Nordic Boiling Water Reactors

Abstract: This thesis is aiming at coolability assessment of particulate debris beds formed in hypothetical severe accidents of Nordic boiling water reactors (BWRs) which may employ either lower drywell flooding or control rod guide tubes (CRGT) cooling as severe accident management strategies. For this purpose, quench and cooling limit (dryout) of debris beds after their formation from fuel coolant interactions were investigated by numerical simulations using the MEWA code. The thesis works consist of four following tasks: (i) validations of the computational tool (the MEWA code) against latest experiments: POMECO-FL and Tutu tests for friction laws, POMECO-HT tests for dryout, and PEARL tests for quench; (ii) Assessment of the long-term coolability of prototypical debris beds with varied characteristics (e.g. height, shape); (iii) studies on quench of prototypical debris beds; (iv) effectiveness analysis of mitigation measures dedicated to Nordic BWRs in term of debris coolability enhancement.The comprehensive validations indicate that the MEWA code is a credible and computationally-efficient tool to simulate the two-phase thermal hydraulics of particulate debris beds under both thermal equilibrium and non-thermal equilibrium conditions. Comparisons of the predicted results with experimental data showed a satisfactory agreement, and key phenomena were reproduced.Simulation for the prototypical debris beds of the cylindrical, conical and truncated conical shapes showed that the beds’ heights were significantly affecting their coolability, and the values of their dryout power density were roughly inversely proportional to their heights regardless of shapes. Such a relationship was correlated based on the simulation results, which can be employed to guide design and operation of relevant experiments. The impacts of bed’s shape on coolability can be characterized by three factors: multidimensionality and contour surface area of debris bed, as well as uniformity of bed’s shape. An increase in uniformity can improve coolability, since it promotes uniform distributions of temperature and void fraction.Simulations for an ex-vessel heap-like debris bed for a reference Nordic BWR showed that the quench front propagated in a multi-dimensional manner. It was found that the upper region adjacent to the centerline of the bed was subject to a higher risk of remelting. It is also found that the oxidation of the residual Zr in the corium had a great impact on coolability of the debris bed due to the release of reaction heat and H2. Therefore, it is crucial to lower the temperature of the whole bed to avoid escalation of oxidation.Based on insights from previous studies, several coolability enhancement concepts were proposed as mitigation measures, including downcomer, bottom injection and CRGT cooling. Simulations demonstrated that all the three measures were effective to improve the debris bed coolability. An embedded downcomer increases the cooling capacity by inducing an extra downward flow of water and providing a preferential exit path for steam. Compared with top-flooding, water injection from bottom was predicted to be more efficient to quench a debris bed, since the water inflow was not hindered by the upward flow of steam and therefore could infiltrate the whole bed quickly. The CRGT cooling strategy, applying coolant injection to a particulate debris bed in the lower head of PRV, was proved to be practically feasible to quench the in-vessel debris bed. However, a special attention should be paid to the side effect of Zr oxidation, since it may deteriorate the quenching process and lead to an uncoolable state as a result of release of considerable heat and H2. Therefore, it is essential that the in-vessel debris bed is sufficiently cooled to such an extent during its formation that substantial oxidation would not occur when the CRGT cooling is applied.