Mitigation of core disruptive accident energetics in burner cores

Progress in Nuclear Energy. Vol. 32, No. 314, ZIP.639-646, 1998 Q 1997 Published by Elsevier Science Ltd Printed in Great Britain 0149-1970198 $19.00 + 0.00

PII: SO149-1970(97)00068-l

MITIGATION OF CORE DISRUPTIVE ACCIDENT ENERGETICS IN BURNER CORES

W. MASCHEK, M. FLAD*, G. ARNECKE, P. LO PINTO”

Forschungszentrum Karlsruhe Institut fir Neutronenphysik und Reaktortechnik Postfach 3640, D-7602 1 Karlsruhe, Germany * D.T.I. Ingenieurgesellschatl mbH Postfach 6326, D-76043 Karlsruhe ’ Commissariat a 1’EnergieAtomique Centre d’Etudes de Cadarache F-13 108 Saint Paul lez Durance, France

ABSTRACT As part of the CAPRA Program (Consommation Accrue de Plutonium dans les RApides) the feasibility of fast reactors is investigated to bum plutonium and also to destruct minor actinides. The design of CAPRA cores shows significant differences compared to conventional cores. Especially the high Pu-enrichment increases the recriticality risk and the associated energetics levels of secondary excursions. Other features of the core have the potential to mitigate this risk again. Of special importance are the numerous diluents in the core which might both prevent coherent liquid fuel compactive motions and can also be used as dedicated fuel discharge paths. The early release of fuel could prevent the escalation to large whole core pools with their energetics potentials. 0 1997 Published by Elsevier Science L td

INTRODUCTION Within the CAPRA program the feasibility of fast reactors has been investigated to burn plutonium and also to destruct minor actinides (MAs) /I/. The main research effort focuses on an oxide fuel core, but also uranium-free core concepts are investigated. The design of CAPRA cores shows significant differences compared to conventional cores. To mention are the high Pu-enrichment, the MAs in the fuel, the absence of axial blankets, the new pin design and the introduction of diluent material into the core. Especially the high Pu-enrichment and the introduction of diluents have an influence on the core disruptive accident behavior and the associated recriticality risk.

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Analyses

have been performed to assess the recriticality and core melt-down energetics potentials for the CAPRA core designs 121with the assumption that the accident progression ends into a large whole core pool, These large pool studies can serve as indicators for accident energetics. Analyses show that compared to a conventional core (as EFR /3/) the risk of a recriticality is increased because of the twice as high Pu-enrichment (Tab.1). In a more realistic assessment one can show that the special design of the CAPRA cores possesses inherent features to mitigate energetics and also allows the introduction of specific measures to enhance the fuel discharge. The mitigation is achieved by the heterogeneous core structure itself(diluents + absorbers). The diiuents can also be utilized for an enhanced fire1discharge from the core under core melt-down conditions. By an early and massive fuel relocation from the core the escalation into a massive core disruption and to a neutronically active whole-core-melt with its associated energetics levels might thus be prevented. The general safety strategy for CAPRA cores goes along the lines for risk minimization developed for the EFR /3/. In addition to the two ordinary shutdown systems within the design basis a so-called third shutdown level is introduced ( e.g. with control rod enhanced expansion devices (CREED)), which should exclude a core-melt at all (Fig. 1). For CAPRA cores, in the beyond design basis area, additional mitigative measures (besides containment measures), might be foreseen and an additional line of defense might be introduced against recriticalities (Fig.1). The introduction and design of specific safety measures/devices for risk reduction in the beyond design basis area must fulfill several requirements e.g. the non-interference with normal reactor operation or the safety systems. A feasibility study is initiated to investigate possibilities for recriticality risk reduction. REDUCTION OF ENERGETICS OF RECRITICALlTlES In spite of the safety measures foreseen for the CAPRA cores a core disruptive accident is postulated in our investigations. The high Pu-enrichment and the introduction of h4A.s into the CAPRA cores change important safety parameters as the Doppler and the kinetics data. The reactivity ramp rates which might be caused by a fuel compaction are significantly increased. The neutron generation time which determines the excursion behavior is also strongly influenced (Tab.1). The impact of these changes on the energetics potentials of a CDA have been investigated and data are given in /2/. In these investigations the behavior of large connected “whole core pools” have been analyzed. The results show that for large scale compactive liquid tie1 sloshing motions the energetics levels of these CAPRA cores could increase by -, 50% compared to a conventional (low enriched) core. Also, the CAPIU cores contain twice as many critical fuel masses as a conventional core. In a reactor core with high enrichment the prime measure to prevent/mitigate recriticalities is the dilution of the fUe1.Another important measure is to prevent the fuel to become highly mobile and move coherently.

The dilution of fuel under accident conditions is achieved via a discharge of a significant part of the inventory from the core and the reduction of the criticality of the system. In addition, absorbing material can be introduced into the fuel. This ‘dilution principle’ is already applied under normal operating conditions in the highly enriched CAPRA cores. The CAPRA cores contain significant “empty” space to increase the neutron leakage and to cope with the high Pu-enrichment of 40 - 45%. This ‘dilution” manifests on the core level by the introduction of numerous diluent-subassemblies (52 S/As, see Fig.2), on the subassembly level by dummy pins (133 per S/A) and on the pin level by a large central hole of more than 2 mm. The strategy for the increased fuel discharge might rely on three levels which come into action sequentially with the extending core destruction:

Burner cores

Quantity

Dim

CAPRA

EFR

MW,h % kg HM

3645 44 21025

3717 21 41904

Number of fissile S/As Number of diluent S/As Number of pins per S/A Number of fissile pins per S/A External clad diameter Internal Clad diameter Fuel pellets diameter Fuel central hole diameter

mm mm mm mm

366 52 469 336 6.35 5.45 5.27 2.16

387 I (central position) 331 331 8.2 7.16 6.94 2

Doppler constant at End of Cycle Sodium void worth at End of Cycle

pcm pcm

-455 1560

-650 (core) 2100

P CG.ay constant (h) Prompt neutron lifetime

pcm

324 8.5*10-’ 8.4*10-7

362 9.1*1o-2 4.01*10-;

Total reactor power Fuel Pu content Fuel inventory

S-l

s

Tab 1: Comparison of characteristic data of a CAPRA oxide core and the EFR

. Sbtdmm

I

Fig.1

Safety strategy for CAPRA cores

+BSlTkr

kvel

A

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Pin level:

With the large central hole in CAPRA pins the ‘fuel squirting’ effect observed in conventional pins (see CAIN experiment LT2) should be enhanced /4/.

Subassemblv level:

The diluent positions within the subassembly could provide a gap for fuel relocation in the case of pin disruption and melting.

Core level:

In the case of an extended core destruction the fuel could melt into the diluent subassemblies. The large hydraulic diameter of the diluents could provide sufficient relocation space. Additionally, absorber material could be introduced into the core.

Additionally to these ‘dedicated’fuel release paths, the subchannel system, the hexcan gaps of the axial and radial reflector and also the control rod guide tubes could tirther contribute to the fuel discharge. The important difference of the ‘dedicated’ fuel release paths is that they are well defined and should be available for controlled material relocation (CMR). The path of fuel released from the core must be further monitored. It must be guaranteed that the relocated fuel cannot form a critical configuration in the lower subassembly structures or further down in available fuel catch trays (core-catcher). The impact of the hot fire1 on the integrity of the lower core support structure must be analyzed and a safe post accident heat removal (PAIR) must be guaranteed.

Taking into account the heterogeneous structure of the CAPRA cores the coherent me1 sloshing motion assumption as applied in 121 could be doubted. As can be seen from Fig.2 the CAPRA core is strongly heterogeneous with at least one diluent element among 6 fuel subassemblies. This leads to the following reasoning which is backed by experiments /3/ and experience from other disciplines (marine and spacecraft engineering) /S/. . .

A coherent sloshing motion (outward slosh) cannot be triggered in a CAPRA core with the existing heterogeneous structure. The ‘undisturbed’ acceleration distances are at the most around 0.3 m. Any back-slosh and compaction (inward slosh) is strongly mitigated by the heterogeneous structures and instabilities of converging waves.

In the CAPRA cores the axial fuel motion pattern dominates - a fuel slumping type motion. Due to the high Pu-enrichment the reactivity gradients and the related ramp rates in oxide cores could be enhanced by 50 % compared to conventional cores. This transfers into increased energetics. However, these CAPRA cores have a much reduced fuel mass which mitigates the ramp rate effect,

The diluent structures thus provide a two-level barrier against the build-up of a neutronically active whole core melt with its energetics. . As long as the diluents are mostly intact they prevent or mitigate any extensive fuel sloshing motion. . After failure they can provide for the necessary paths for extensive fuel removal. As has been observed in our analyses, the hexcan failure and fuel injection into the diluent takes place under wet conditions and can trigger local fuel / coolant interactions. The still available surrounding diluents are of importance to mitigate any sloshing motion under these conditions,

Burner cores

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Without taking into account the potential of the diluent system to curb any extensive sloshing motion, the necessary fuel masses which must be discharged to mitigate energetics are higher in a CAPRA core than in a conventional core. Transient analyses of core melt accidents have shown that for conventional cores with e.g. 20% Pu-enrichment a loss of 20% of fuel already strongly mitigates energetics potentials and a loss of 40% makes a power burst very improbable /6/. In the CAPIU core, more tie1 must be discharged with the above percentages increased to 40% and 70% respectively. In CAPRA cores critical configurations might theoretically be conceivable even with fuel masses below 70%, but as transient calculations show the fuel is cooled down and distributed in the lower part of the core without sign&ant neutronic activity. The final goal when exploiting these CMR devices/measures might be the total prevention of recriticalities. A more moderate goal is to prevent the melt escalation and the formation of a large COMeCkd molten fuel pool. By this the possible ramp rates at prompt critical states can be strongly mitigated. This less ambitious goal takes into account that the proof of effectiveness under accident conditions must hold for a large number of accident conditions and that experimental testing of these measures/devices has not yet been started. IMPORTANT CRITERIA FOR THE CHOICE OF PREVENTIVE/MITIGATIVE MEASURES An excellent and comprehensive review of criteria for preventive and mitigative measures against core disruptive accidents is given in /7/. Some criteria are referred here to underline their importance and some new criteria are added. As example, the measures and devices should possess the following ‘qualities’: ?? ?? ?? ?? ??

They They They They They

should should should should should

not interfere with normal operating conditions not interfere with the safety systems and the third shutdown level work entirely passive under accident conditions act safety directed (no massive positive reactivity insertion) work for different accident initiators (and scenarios)

These are conditions which may not all be met in a satisfactory way. Concerning the effectiveness of action of these devices / measures under accident conditions, the important criteria are the timing and time scales for fuel discharge and the masses of ejected fuel (negative reactivity ramp-rates and reactivity levels). ASSESSMENT OFTHE ACCIDENT BEHAVIOR OFTHE DEDICATED DEVICE’; In our investigations we concentrated on the fuel release potential of the diluent system of the core. In Fig.3 some typical proposals are given for devices with the potential for enhanced fuel discharge. Investigations on the other fire1 removal paths through the hollow pm and the subassembly subchannel system are underway. As type of accident for our analyses the unprotected loss of flow (ULOF) is chosen because it reflects the most important accident phenomena during a core melt-down accident. val DeyIc& In Fig.3 one design proposal allows for the introduction of additional absorber material into the core. Absorber balls should fall into the core under core-melt conditions. The proof of proper action for this design variant is diicult. This is related to the behavior of the absorbing materials. In the case of B4C bails / pellets one must account for BqC / steel eutectica formation which leads to total liquefaction at 1500 K. Another difficulty seen in transient calculations is, that the dropping boron balls do not effectively penetrate fuel pools or are separated from the fuel melt easily because of the density ratio. This could result in a non-safety directed action with reactivity addition. The use of Eu20 balls would lead to a eutectica with fuel /8/, but also in this case the distribution of me1 and Eu2O3 is difficult to predict.

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CORE LEVEL

Fig.2

Capra core layout

., _ 0.0

Fig.3

”_ , ””

,

,

0.5

1.0

,

,

,

,

,

,

1.5

2.0

2.5

3.0

3.5

4.0

Diluent design variants

)

,

4.5

5.0

Time [s]

Fig.4

Fuel release scenario

simulated with the SIMMER-III

code

Burner cores

64.5

The utilization of diluents for controlled material removal has the advantage that the structure: of the fuel bundle must nor be changed. Additionally, in a fuel bundle the pressurized gas reservoirs dedicated for the fuel release could increase the voiding potential in the case of early failure. However, as for the ‘diluentCMR’the hexcan structures of the fuel bundle and the diluent have to be destroyed before the fuel removal paths can be accessed, this type of Ch4R works on a longer timescale. val Tw The diluent subassembly system of the CAPRA core offers a large potential for fuel disctarge under accident conditions, As displayed in Fig.2 the CAPRA core contains 52 diluent subassemblies where 22 of them are partly filled with llB4C as moderator to improve void and Doppler values. The diluents can become activated for fuel removal alter local fuel pool formation and melt-through of the subassembly walls. The ‘light’diluents as presently foreseen in CAPRA, contain 37 hollow steel pins, in the ‘heavy’ diluents 15% of the pins are filled with * lB4C. The sodium volume fraction directly available for fuel as flow path is low with - 27%. This empty pin structures can however be eroded after fuel enters the bundle and very large hydraulic diameters of -0.15 m become available for the me1 flow. One should note that in the CAPRA design 6 fuel subassemblies are grouped around one diluent. The fuel removal through the diluent system was investigated using the advanced SIMMER-111 code /9/. This code is developed by PNC under cooperation with FZK and CEA. In the geometric core model, the central diluent subassembly filled with stagnant sodium has been used as possible fuel release path. It is surrounded by a fire1 pool confined in blockages. The pool conditions cover temperature ranges from melting, up to 3600 K and power levels up to 10 times steady-state, reflecting conditions which also cover mild recriticalities. Different diluent designs have been tested for their fuel removal potential. The necessary time frame on which the fuel removal must take place can be deduced from accident calculations on whole core pool formation /lo/ and from the propagation velocities found in the SCARABEE-N tests /l l/. Accident calculations show that the melt-down phase (transition ph.lse) after a primary excursion until formation of a whole core pool (if conditions allow to achieve this state) lasts between 5-20 sec. /IO/ The SCARABEE-N experimental information and extrapolation to reactor conditions /I l/ give time scales in the same range. To be effective, the fuel discharge must therefore take place within a period of -5 set under the conditions given in /lo/. The process of fuel removal as simulated with SIMMER-III is displayed in Fig.4. After hexcan failure fuel penetrates into the diluent with accompanied local FCIs and sodium ejection. In the example displayed, the pool is monotonously discharged. 1. The calculations show that efficient fuel release on the right time scale can only be expec.ted with a sufficient temperature/pressure level in the pools (e.g. after a mild recriticality). The rele,ase of fuel simultaneously reduces the driving force for the release. A massive removal of a fuel particle melt with high viscosity cannot be expected. Under the conditions of a low temperature melt with a large particle fraction the failure location and the size of failure of the hexcan is a significant parameter. For a better de,scription of the fuel pools experimental information on CAPRA pools with inert materials is urgently needed. 2. The discharge rates dM/dt in a diluent subassembly could be as high as -100-200 kg/s for temperature/pressure levels characteristic for a mild power excursion, This would guarantee the necessary fuel removal in the right time frame of 2-3 set if -50 % of the available bundler; would be activated. These are typical numbers for conditions of a neutronically active core which would lead to pool propagation and finally to the formation of a whole core pool, if no fuel release would take place. Experimental confirmation is needed on these release rates. The diluents might be optimized for maximum fuel release reducing the internal steel structures.

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The reactivity reducing effects of absorber balls above the core level which should drop into the diluent could not be fully confirmed. Installing absorber structures at the end of the diluents will however keep the relocated fuel in locally subcritical conditions. Further investigations on absorber behavior in pools are necessary.

4. Fuel removal under ULOF conditions was monotonous and within the diluent no recompaction at midplane could be observed. One could however observe the triggering of sloshing motions in the pool by sodium evaporation. This underlines the importance of the heterogeneous core structure. 5, While the first three criteria in Chapter 3 seem to be met, more investigations are necessary to show that fuel release leads to a continuous reduction of reactivity (no reversal and fuel compaction). This must also be shown for different initiators (e.g. under TOP conditions). CONCLUSIONS The mitigative potential of the diluent system on the recriticality potential of CAPRA burner cores has been investigated. Firstly, these diluents might prevent the build-up of any sloshing motion with associated coherent fuel compaction and associated reactivity ramp rates and energetics. Secondly, these diluents could provide a well defined and reliable fuel release path in the case of a core-melt accident. The diluents could be optimized for a maximum of fuel release. The results show that under accident conditions with molten, mobile fuel a suflicient percentage of the inventory could be discharged from the core within the proper timescale to prevent the formation of a neutronically active whole core pool. The fuel release would drastically reduce any recriticality and energetics potential. The results of these exploratory investigations are promising that an additional line of defense might be introduced into CAPRA cores to cope with the recriticality risk. An experimental demonstration for the operability of these tie1 release paths is urgently needed. REFERENCES Ill

121 131 141 /5/ 161 171 181 191 /lOi

Ill/

A. Languille, et al. (1995). CAPRA Core Studies - the Oxide Reference Option. GLOBAL ‘95, Paris. France. W. Maschek et al. (1996). Core Disruptive Accident Analyses for Advanced CAPRA Cores. ICONE-4. New Orleans. USA. G. Heusener et al. (1994). EFR Safety Concept. ARS’94. U. Imke et al. (1994). Status of the SAS4 Code Development for Consequence Analyses of Core Disruptive Accidents. FRS’94. Obninsk. Russia. H. N. Abramson (1963). Dynamic Behavior of Liauid in Moving Container. Applied Mechanics Reviews, Vol. 16, No 7, 501. W. Maschek et al. (1992). Investigations of Sloshing Fluid Motions in Pools Related to Recriticalities in Liquid-Metal Fast Breeder Reactors. Nucl. Techn. 98(l), 27 (1992) Y. Ieda et al. (1994). Assessment of Proposed Passive Prevention and Mitigation Measures for Future Fast Breeder Reactors. ARS’94. Pittsburgh. USA. W. Maschek (1995). A Preventive/Mitigative Measure Avoiding Recriticahties in Liquid Metal Reactors. TOPSAFE’. Budaoest. Hunaarv. Sa. Kondo et al. (1992). SIMMER-III: An Advanced Computer Program for LMFBR Severe Accident Analysis. ANP’92. W. Maschek et al. (1994). Energetics Potentials of CDAs in Fast Reactors with Transmutation / Burning Capabilities. ARS’94. G. Kayser et al (1994). Potential Lessons form SCARABEE for the Transition Phase. IWGRF/89 IAEA Mtg.. Oarai. Japan.