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Validation and application of the MEWA code to analysis of debris bed coolability
KTH, School of Engineering Sciences (SCI), Physics, Nuclear Power Safety.ORCID iD: 0000-0002-1179-2256
KTH, School of Engineering Sciences (SCI), Physics, Nuclear Power Safety.
2018 (English)In: Nuclear Engineering and Design, ISSN 0029-5493, E-ISSN 1872-759X, Vol. 327, p. 22-37Article in journal (Refereed) Published
Abstract [en]

This paper was first aimed at validating the MEWA code against experiments for two-phase flow and dryout in particulate beds, and then investigating the coolability of ex-vessel debris beds with cylindrical, conical and truncated conical shapes assumed to form under severe accident scenarios of a boiling water reactor. The validation was mainly performed against the POMECO-FL and POMECO-HT experiments carried out at KTH for investigating frictional laws and coolability limit (dryout) of particulate beds, respectively. The comparison of the experimental and numerical results shows that the MEWA code is capable of predicting both the pressure drop of two-phase flow through porous media and the dryout condition of various stratified beds. While the coolability of a one-dimensional homogeneous debris bed is bounded by counter-current flow limit (CCFL), the coolability of a heap-like debris bed can be improved due to lateral ingression of coolant in a multi-dimensional geometry. The simulations showed that the dryout power density of a prototypical debris bed was roughly inversely proportional to the bed's height regardless of the bed's shape. The impacts of a debris bed's features on coolability are manifested in three aspects: multidimensionality and contour surface area of the bed, as well as the uniformity of its shape. The contour surface area is defined as the interface between debris bed and water pool, and its effective value depends on the surface orientation that determines the amount of water ingress and vapor escape. The perfect uniformity in bed's shape as cylindrical bed results in even distributions of temperature and void fraction. The dryout power density was also predicted to be strongly correlated to the uniformity of bed's shape. The MEWA simulation also predicted that coolability was improved by an downcomer embedded in the center of debris bed. The efficiency of such enhancement was largely determined by the downcomer's length, whose optimal value was obtained in simulation.

Place, publisher, year, edition, pages
Elsevier, 2018. Vol. 327, p. 22-37
National Category
Physical Sciences
Identifiers
URN: urn:nbn:se:kth:diva-220367DOI: 10.1016/j.nucengdes.2017.11.038ISI: 000425302400003Scopus ID: 2-s2.0-85037543831OAI: oai:DiVA.org:kth-220367DiVA, id: diva2:1167922
Note

QC 20171219

Available from: 2017-12-19 Created: 2017-12-19 Last updated: 2019-05-03Bibliographically approved
In thesis
1. Numerical Investigations on Debris Bed Coolability and Mitigation Measures in Nordic Boiling Water Reactors
Open this publication in new window or tab >>Numerical Investigations on Debris Bed Coolability and Mitigation Measures in Nordic Boiling Water Reactors
2019 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

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.

Abstract [sv]

Syftet med denna avhandling är att bedöma kylbarhet av en grusbädd (”debris bed”) av fragmenterad härdsmälta under ett hypotetiskt svårt haveri i Nordiska kokvattenreaktor där strategin för haverihantering kan antingen vara att flöda nedre drywell med vatten eller att kyla härdsmältan i reaktortankens nedre plenum med hjälp av kylflöde genom styrstavarnas drivdon. Med hjälp av koden MEWA har torrkokning (”dryout”) av en grusbädd, som bildas till följd av en ångexplosion, studerats numeriskt.

Avhandlingen består av fyra delar:

(1)    Validering av MEWA-koden med hjälp av senaste resultat av:

       a)   POMECO-FL och Tutu experimentella undersökningar av friktionstryckfallet

       b)  POMECO-HT dryout tester

       c)   PEARL tester av grusbäddens kylning

(2)   Undersökning av grusbäddens kylbarhet i långtidsperspektiv, som funktion av grusbäddens höjd, form, etc.

(3)   Undersökning av kylning av prototypiska grusbäddar

(4)   Effektiviteten av strategier för att förbättra kylbarheten av grusbäddar i Nordiska kokvattenreaktorer

Omfattande validering har visat att MEWA-koden är ett pålitligt och beräkningseffektivt verktyg för simulering av termohydrauliska förlopp hos två-fasflöde i grusbäddar vid termisk jämvikt och termisk ojämvikt. Jämförelse med experimentalla data har visat tillfredställande överenstämmelse och att viktigaste fenomen har beaktats.

Numerisk simulering av prototypiska grusbäddar med cylindrisk, konisk och stympad konisk form har visat att grusbäddens form har betydande påverkan på grusbäddens kylbarhet, och att effekten vid vilken dryout inträffar är omvänt proportionell mot bäddens höjd oavsett dess form. Baserad på experimentalla resultat har en korrelation tagits fram som kan användas vid utformning och genomförande av relevanta experiment.

Påverkan av grusbäddens form på kylbarhet kan karakteriseras med hjälp av tre faktorer, nämligen: multidimensionalitet, grusbäddens konturyta och likformighet av bäddens form. En ökad likformighet kan förbättra bäddens kylbarhet eftersom den leder till likformig fördelning av temperaturen och voidhalten i bädden.

Simuleringar av ex-vessel grusbäddar i en typisk Nordisk kokvattenreaktor har visat att kylfrontens propagering genom bädden har multi-dimensionell karaktär. Risken för återsmältning är högre i bäddens övre region, nära bäddens centrumlinje. Man har också funnit att oxidering av Zr har en stor påverkan på bäddens kylbarhet genom värmeutveckling och vätgasbildning. Det är därför mycket viktigt att sänka hela bäddens temperatur för att undvika eskalering av Zr-oxidation.

Baserad på insikter från tidigare forskning, har flera metoder för att öka grusbäddens kylbarhet föreslagits, som downcomers, tillförsel av vatten till bädden underifrån och kylning av grusbädden i reaktortankens nedre plenum med hjälp av kylflöde genom styrstavarnas drivdon (”CRGT cooling”). Beräkningar har demonstrerat att alla dessa metoder kan förbättra kylbarheten. En downcomer ökar kylbarheten genom att den dels bidrar till ökat vattenflöde genom bädden och dels ökar ångans flöde ut ur bädden. Tillförsel av vatten till bädden underifrån är mer effektiv eftersom vattentillförseln inte hindras i detta fall av uppåt strömmande ånga och vatten kan snabbt fylla hela bädden.

Analyserna har visat att det är möjligt att kyla en grusbädd i reaktortankens nedre plenum med hjälp av vattenflöde genom styrstavarnas drivdon. En faktor som dock måste beaktas här är negativa effekter av Zr-oxidation, som kan resultera i försämrat kylningsförlopp och inte kylbar bädd på grund av värmetveckling och vätgasbildning vid Zr-oxidation. Detta innebär att in-vessel grusbädd bör vara tillräckligt kyld under dess bildningsfas för att förhindra omfattande Zr-oxidation när kylning via styrstavarnas drivdon initieras.

Place, publisher, year, edition, pages
KTH Royal Institute of Technology, 2019. p. 87
Series
TRITA-SCI-FOU ; 2019:27
Keywords
Severe accident; Debris bed; Coolability; Quench; Oxidation; MEWA code; Coupled analysis, Svåra haverier; Grusbädd; Kylbarhet; Kylning; Oxidering; MEWA program; Kopplad analys
National Category
Engineering and Technology
Research subject
Physics
Identifiers
urn:nbn:se:kth:diva-250711 (URN)978-91-7873-214-2 (ISBN)
Public defence
2019-05-28, FD41, AlbaNova universitetscentrum, Roslagstullsbacken 21, Stockholm, Stockholm, 09:30 (English)
Opponent
Supervisors
Note

QC 20190503

Available from: 2019-05-03 Created: 2019-05-03 Last updated: 2019-05-09Bibliographically approved

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