I

I. Introduction

The CRBRP Project was established with the principalobjectives of demonstrating that an LMFBR can operatereliably and safely in a utility environment, demonstratingfast breeder reactor technology and serving as a successfultransition from R&D efforts to large-scale commercial LMFBRplants.

The LMFBR has long been recognized as the most viablealternative reactor type to the LWR and the uranium fuelcycle. The U.S. and other industrial countries have madeextensive national commitments toward developing LMFBRswhich include the successful operation of several liquidmetal test and power reactors.

II. Design

A. Major Design Objectives

The unique design objective of LMFBRs is to breed fuel.A parameter defined to quantify the efficiency of thisdesign objective is the "breeding ratio." The breedingratio is defined as the amount of fissile materialproduced divided by the amount of fissile materialconsumed in a specified operating period. CommercialLWRs are called thermal converter reactors since thisratio is less than one. LMFBRs are designed to havebreeding ratios greater than one.

In order to attain a high breeding ratio, the neutronflux of a LMFBR must be a "fast flux" (>100 Kev) ratherthan a "thermal flux" as is present in LWRS (0.025 ev atroom temperature)

This is primarily due jýa the large reduction in captureto fission ratio in Pu'"J with increasing energy. Dueto the relatively small fission cross-sections offissile nuclei at higher energies, the fuel of a LMFBRmust have a higher enrichment than a LWR to becomecritical. To ensure that the high energy neutronsutilized in a fast breeder reactor are not slowed downor absorbed, a coolant with a low absorption andscattering cross-section is required. This is indirect contrast to a LWR where a good moderator isessential.

B. Reactor Design

The differences in the LWR and LMFBR reactor coredesigns are due primarily to the requirement on theLMFBR to breed fuel. LMFBR reactor cores containregions of fertile material (depleted U-238) commonlyreferred to as "blanket regions" or "blanket

assemblies" where the majority of breeding occurs. Acore arrangement in which the fertile blanket regionremains entirely outside of the active core region(where an initial loading of fissile fuel is located)is termed, "homogeneous." This term is used because ofthe relatively uniform or homogeneous distribution offissile fuel in the active core region. A corearrangement that includes fertile blanket assemblieswithin as well as around the active core region istermed "heterogeneous." Heterogeneous core designs aredesirable because of their higher breeding ratios andtheir reduced positive sodium void coefficients ofreactivity. Figure 1 illustrates a typical homogeneousand heterogeneous core arrangement. Early U.S.prototype LMFBRs and all foreign LMFBRs use thehomogeneous core design. Demonstration of theheterogeneous core design in the Clinch River BreederReactor is expected to provide a significant advance inLMFBR technology. Radial shield assemblies areprovided around the periphery core to reduce theneutron fluence experienced by the reactor vessel andsurrounding structures.

The size of a LMFBR core is not influenced by the needto optimize the fuel to moderator ratio as in a LWR.Instead, a very "tight" design is desired to offset therelatively low fission cross section for high energyneutrons. Thus, the volume fraction of coolant andstructural materials in an LMFBR core is reducedrelative to a LWR. The high fuel enrichment and highthermal conductivity of the coolant makes it desirableand possible to design a core with a power density muchhigher than that of a LWR. As a result of this, aLMFBR core is smaller than a LWR core of the samethermal power. To maximize the fuel volume fraction atriangular fuel lattice and hexagonal assemblystructure is utilized in an LMFBR in place of therectangular lattice and assemblies used in LWRs.

Figure 2 is a simplified schematic of a typical LMFBRfuel assembly. There are 217 fuel rods in each CRBRPfuel assembly, spaced in a triangular array by spiralwire wraps. Each rod contains- a 36-inch-long stack offuel pellets and two 14-inch-long stacks of blanketpellets (located above and below the fuel pellets).Blanket assemblies are similar, but contain largerpins; 61 per assembly; loaded entirely with depleteduranium oxide pellets. (See Figure 3).

C. Heat Transport System

LMFBRs require a coolant with low absorption cross-section, low neutron thermalization characteristics andhigh heat transfer coefficients. Sodium has been

3

generally chosen as the coolant for fast.breederreactors because of its relatively low cost,availability, acceptable nuclear properties andexcellent heat transfer properties. One of the mostimportant properties of sodium is high thermalconductivity: approximately 30 Btu/hr-ft- 0 F vs. 0.3Btu/hr-ft-°F for water.

Furthermore, sodium has a boiling point of about 1620 F(A'880°C) at atmospheric pressure. This allows LMFBRsto operate at essentially atmospheric pressure sincecoolant operating temperatures are typically on theorderof 600-700 F below the boiling point. This isin contrast to LWRs which operate at pressure around1050 psia for BWRs and 2100 psia for PWRs.

The exit ( 1000°F) temperature of sodium leaving thereactor in an LMFBR is considerably higher than that ofa LWR ( 600 F). The temperature differential betweenthe hot and cold leg of an LMFBR ( 3000F) is largerthan for a LWR ( 100°F).

All nuclear reactor designs must include heat transportsystems which are capable of providing a heat sinkduring operation in addition to removing the decay heatproduced in the reactor following shutdown. BWRsutilize a direct cycle where heat is transferreddirectly from the reactor core to the turbinegenerator. PWRs on the other hand utilize an indirectcycle where the heat produced in the reactor core istransferred to the steam generators in order togenerate steam for driving the turbine generator.

LMFBR heat transport systems consist of a primarysodium system, an intermediate sodium system, and afeedwater/steam system. The intermediate sodiumsystem, unique to LMFBR, is used to provide anadditional barrier between the radioactive primarysodium and the water in the steam generators. Theintermediate heat exhanger (IHX), between the primaryand intermediate sodium coolant, is unique to LMFBRs.The radioactive primary coolant system is enclosedwithin inerted primary equipment cells insidecontainment. Only the intermediate (non-radioactivesodium) is circulated outside containment to the steamgenerators. This design approach affords protection ofplant personnel from the primary sodium radiationduring operation and also functions to mitigateaccidental sodium leakage from the primary system.

iNormal full power operating pressures of about 170 psig areattributed to pump and static head pressures.

The two types of primary heat transport systemarrangements that are currently being used in LMFBRsare the pool and loop system. In the pool system thereactor, intermediate heat exhangers and primary sodiumpumps are all located in a tank full of sodium. Thispool design was first used in the U.S. experimentalbreeder reactor EBR-rl but has since been adopted byFrance, the United Kingdom, and the Soviet Union. Inthe loop system the primary coolant is circulatedthrough a piping configuration which extends outside ofthe reactor vessel to the pumps and intermediate heatexchanger. The loop design is currently being used byGermany, Japan, the United States, and the SovietUnion.

The CRBRP heat transport system, which is• a typicalloop design, is illustrated in Figure 4. The CRBRPheat transport system features three redundant andphysically independent loops for normal and decay heatremoval. Both the primary and intermediate heattransport systems are fully welded coolant boundaries.Coolant leakage is essentially zero during normaloperation. This contrasts with LWRs which use flangesat some locations in the coolant boundary and acceptsome coolant leakage.

D. Auxiliary Systems

An LMFBR design includes several auxiliary systems thatare not found in LWRs and vice-versa. Auxiliarysystems typical of those used in LMFBRs are discussedbelow.

Refueling operations in LMFBRs are performea'withoutremoving the reactor vessel head. Figure 5 shows thereactor refueling arrangement for CRBRP. The CRBRPreactor refueling system utilizes major features of therefueling systems in earlier liquid-metal cooledreactors in the United States - the Fast Flux TestFacility (FFTF), Hallam, Sodium Reactor Experiment,Fermi, and EBR-lI reactors - as well as features fromthe German, French, and British LMFBR programs.

Core assemblies are removed from the reactor undersodium cover by a straight-pull in-vessel transfermachine (IVTM). The IVTM is sealed and mounted withinthe smallest of three eccentric rotating plugs on thereactor closure head. The IVTM grapple lifts the coreassembly above the core. The core plugs are rotatedand the core assembly is placed in a core component pot

waiting in the reactor vessel, but outside the coreregion. The ex-vessel transfer machine (EVTM) removesthe core component p6t filled with sodium and the coreassembly from the reactor vessel and transfers it tothe ex-vessel storage tank.

The sodium-filled ex-vessel storage tank (EVST) is usedfor storing spent fuel assemblies. This facility isfunctionally similar to the LWR fuel storage pool butincorporates features necessary to accommodate sodium.

An auxiliary liquid metal system is utilized in LMFBRs.The auxiliary liquid metal system performs thefollowing functions:

o Receives liquid metal and transfers it to storagevessels in the plant..

o Purification (cold trapping) and storage of sodiumlimiting the oxygen and hydrogen concentration ofthe sodium.

" Filling and draining the reactor and EVST coolingssystems.

These systems perform functions analagous to LWR waterclean up systems. Liquid metals can be uniquelycleaned by using cold traps.

Since the melting point of sodium is about 210 F,electrical trace heaters are provided around theoutside of LMFBR piping and components to preheatsystems prior to sodium fill. Trace heaters need onlyoperate -on the reactor vessel and heat transport systempiping when insufficient heat is available from poweroperation or decay heat to maintain the sodium in aliquid state. Many drain lines and other equipmentassociated in the auxiliary liquid metal system requirecontinuous operation of the trace heater system whenthose features are being used. The extent of thissystem is unique to the LMFBR but is functionallysimilar to heating water pipes to avoid freezing.

The reactor vessel, guard vessel, and primary heattransport system piping and components of LMFBR designsare usually placed in cells having atmosphereschemically inert to sodium. Nitrogen is most oftenused as the inert atmosphere for these cells because itis readily available and inexpensive. A small amountof oxygen (approximately 1%) is maintained in cellnitrogen to prevent nitriding. The inert atmospheresignificantly reduces fire consequences as the resultof a postulated sodium spill. Because pure nitrogencan not be used in steel enclosures at higher

temperatures (>750 0 F) due to nitriding, and even 1% 02would not co-exist with liquid sodium, argon has beenselected as the cover gas for use within the reactorvessel and all other major HTS piping and components.

An inert gas receiving and processing system istypically included in the LMFBR designs to providenitrogen to inerted cells and to provide Argon covergas to all free liquid metal surfaces and to componentand reactor head seals. Subsystems are provided forprocessing contaminated (radioactive) Argon andnitrogen.

IIIo Safety

A. Fundamental Safety Considerations

A major effort is made to assure LMFBR's, like LWR's,are designed to operate reliably within normal plantparameters so that safety systems are not challengedfrequently. However, it is recognized that safetyfeatures must be provided and carefully assessed toassure protection of public health and safety for evenlow likelihood events.

The three fundamental safety considerations for an LMFBRare no different than the safety considerations for aLWR.

First, if any significant off normal event occurs, thereactor must be shutdown in a timely manner to avoidexceeding fuel design limits. The CRBRP ReactorShutdown System (RSS) consists of two redundant anddiverse shutdown systems: the Primary RSS and theSecondary RSS. Each one of these reactor shutdownsystems independently brings the reactor to a safeshutdown condition by inserting control rods into thereactor core.

Second, the reactor must be adequately cooled followingreactor shutdown. The CRBRP design meets this objectiveby providing four separate paths for decay heat removal.

Guard vessels that surround major heat transport systemcomponents to protect against the potential loss ofsodium are features found in most LMFBR designs. Theguard vessels are located and sized to accommodate pipe leakswithout lowering the sodium level in the reactor vesselbelow the minimum level required for cooling the reactorvessel core. In the CRBRP design, guard vessels areprovided for the reactor vessel, primary pumps,intermediate heat exchangers, and all primary systempiping at an elevation below the top of the guardvessels. The guard vessel-elevated piping concept is

illustrated in Figure 6. This passive approach toensuring a sufficient inventory of primary coolantperforms a function similar to that of the activeEmergency Core Cooling System (ECCS) of a LWR.

The high thermal conductivity of sodium and thesubstantial margin to boiling allow adequate decay heatto be removed from the core at low flow rates. Thesodium expansion through the core and compressionthrough the intermediate heat exchanger (IHX) allows anatural thermal driving head to be established. Figure7 illustrates how the location of component thermalcenters enhances natural circulation. This naturalcirculation process can adequately remove decay heatfrom the core even if all other pumping power for theprimary sodium is lost.

Third, any significant leakage of radioactive materialfrom the reactor or coolant system must be contained tomitigate offsite dose consequences. An LMFBR reactorcontainment, like a LWR reactor containment, is designedto accommodate, without exceeding design leak rates, thepressure and temperature conditions of Design BasisAccidents (DBAs). The CRBRP containment building shownin Figure 8 is surrounded by a confinement building. Anegative pressure differential (-1/4 inch w.g. withrespect to the atmosphere) is maintained in the annulusbetween the' containment and confinement building toensure that any potential leak would be into theannulus. All leakage into the annulus is filteredbefore being vented to the atmosphere. Thus, thecontainment/confinement structure provides a multiplebarrier to a postulated release of radionuclides.

B. Use of Sodium as a Coolant

The, advantages of sodium coolant thermal characteristicsduring a decay heat removal operation were brieflydiscussed in the last section. A key advantage of usingsodium as a coolant is its high boiling point. Asdiscussed in Section•.C, the large margin to boilingprovided by sodium coolant allows LMFBRs to operate atatmospheric pressure. Thus, the potential for a suddendecrease in the saturation temperature due to rapiddepressurization, a major concern in LWR safety, is nota concern in a LMFBR.

The primary disadvantage of using sodium as a coolant isits "chemical incompatibility" with water and air.

Although containment pressurization due to coolantflashing is not a concern for an LMFBR, a large sodiumfire or similar chemical reaction can cause thecontainment to be pressurized. However, when inerted

cells are used to minimize the effects of postulatedpipe leaks, the largest credible sodium fire postulatedto occur in the reactor containment building of an LMFBRwould typically result in pressures much lower (on theorder of one psig) than the pressures in an LWRcontainment following a major loss of coolant accident(normally in excess of several tens of psig). Inaddition, a sodium fire would occur on a much longertime scale (>50 hours) than the time for primary coolantblowdown following an LWR LOCA accident (on the order ofseveral seconds).

Sodium/water reactions resulting from leaks in steamgenerator tubes are also safety concerns associated withLMFBRs. Considerable loadings could be applied to theIHTS and the intermediate heat exchanger followingpropagation of the sodium water reaction pressure front.Design features which preclude and/or mitigate theconsequences of a sodium/water reaction in the steamgenerator system are included in the CRBRP design.

A steam/water to sodium leak detection system isprovided to ensure that a sodium/water reaction could bedetected and the necessary operator corrective actionstaken, before extensive intermediate system and steamgenerator system damage could occur.

In addition, the CRBRP design includes a passivesodium/water reaction pressure relief subsystem (SWRPRS)that provides overpressure protection for the IHTS andIHX. The SWRPRS is illustrated in Figure 9.

C. Hypothetical Core Disruptive Accident

In an LWR.the fuel, coolant, and structural material ofthe reactor core are arranged in a manner that maximizesK•. Any change in the core configuration due to fuelrsThcation or coolant loss- tends to shut the reactordown neutronically.

An LMFBR reactor core, however, does not require amoderator, operates on a hardened neutron spectrum andis not in its most reactive configuration. Thus, if afuel melting incident should occur, material motionscould result in a net increase in reactivity.

Such an event is called a Hypothetical Core DisruptiveAccident. HCDA progression may involve sodium boiling,cladding melting and fluel melting with relocation of thematerials under gravitational and/or hydraulic forces.

The design approach for CRBRP is to exclude HCDAs fromthe DBA spectrum by providing features to prevent theirinitiation. The major features incorporated in the

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CRBRP to prevent occurrence of HCDA initiatingconditions include: 1) the redundant and diverseshutdown systems, 2) the redundant shutdown heat removalsystems, 3), the means to prevent a double-ended ruptureof the reactor vessel inlet pipe, and 4) the means tomaintain individual subassembly heat generation andremoval balance.

Even though HCDAs are beyond the design basis, featuresare included in the CRBRP design to provide additionalmargin for mitigation of these hypothetical accidents.Evaluation -of HCDAs has shown that these features ensurethat the residual risk is low.

Evaluation of HCDA energetics involves consideration ofthree accident phases: the initiating phase; a meltoutphase which is entered if the damaged core is not stableand coolable at the end of the initiating phase; and alarge-scale pool phase which may occur in thepessimistic case in which permanent subcriticality isnot achieved in the earlier phases. A fourth accidentphase, hydrodynamic disassembly, would occur if asustained, super-prompt-critical excursion were to occurin any of the other three phases.

Realistic assessments of HCDA sequences, including bestestimate analysis and consideration of uncertainties,have been performed and predict a non-energetic outcome(that is, there is no early mechanical challenge toprimary system integrity). This is due primarily to theinherent dispersive nature of the fuel during theinitiation phase of an HCDA. Fission gas and fuel vaporpressures provide a mechanism for enchanced fuel removaland thus, a greater potential for permanent subcriti-cality following initiation of an HCDA. Furtheranalyses involving significant deviations frombest-estimate understanding of accident physics havebeen performed. Pessimistic assumptions, well beyond

,those appr~opriate for a realistic assessment, must beinvoked to predict energetics. The structural marginbeyond the (SMBDB) design basis provided in CRBRP isadequate to contain the energetics predicted in theseanalysis.

HCDAs that involve whole core melting could thermallychallenge structures, including the reactor vessel andguard vessel. If the reactor vessel and guard vesseleventually fail, material would be released into thereactor cavity cell. The release of material into thereactor cavity could result in increasing pressure andtemperature in containment from chemical reactions andheat input.

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These potential thermal challenges from HCDAs areaccommodated by designing to meet the Thermal MarginBeyond the Design Basis (TMBDB) Requirements. To meetthe Thermal Margin Beyond the Design Base Requirements,design features have been added, including a ventbetween the reactor cavity cell and the containment, asystem to vent and purge the containment through acleanup system, a system to cool the containment vesseland the containment building, and containmentinstrumentation to permit the operator to follow thecourse of the accident.

Realistic assessments were performed considering sodium-concrete interactions, sodium-water reactions, decayheat, sodium burning, and hydrogen burning. Theseassessments show that the features provided wouldprevent uncontrolled release of radioactivity to theenvironment. These features maintain temperature,pressure, and hydrogen concentration in the ReactorContainment Building at acceptable levels in concertwith the venting of the containment through cleanupsystems. Best-estimate analyses show that theinitiation of venting would not be required for morethan a day after initiation of the HCDA. These analysesshow that the radiological consequences of ventingthrough the cleanup system would be acceptably low.

Sensitivity analyses have been performed relating theradiological consequences of HCDAs to the time ofventing and show that the consequences would remainacceptably low for substantially earlier vent times thanpredicted in the best estimate analyses. Additionalsensitivity analyses have shown that containmenttemperature and pressure margins would accommodate awide range of material releases to containment in excessof those predicted from realistic assessment.Therefore, the design provides additionalmargins beyondthose discussed in the preceding paragraphs.

CRBRP HOMOGENEOUSCORE DESIGN

CRBRP HETEROGENEOUSCORE DESIGN

4]• FUELINNER COREOUTER CORE

o BLANKET

C RADIAL SHIELD

o PRIMARY CONTROL

* SECONDARY CONTROL

108 0900

150 )324

154 0

CYCLESODD EVEN156 162208 202

FUELBLANKETRADIAL SHIELDPRIMARY CONTROL

SECONDARY CONTROL

ALTERNATE FUEL/BLANKET

3129

6

6

Figure 11-92-1296-18-8

CRBRP FUEL ASSEMBLY

TOP LOAD PADDUCT

ABOVE-CORELOAD PAD

PISTON RINGSINCONEL 718

HANDLING SOCKETOUTLET NOZZLE

SA 316 SS

FUEL ROD BUNDLE(217 RODS)

DISCFN INLET SLOTS

RIMINATORPOST SHIELD BLOCK

SA 316 SS

INLET NOZZLE ASSEMBLY

FLAT TO FLAT OF DUCT 4.575 IN.LENGTH 168 IN.

ORIFICE PLATEASSEMBLY

RODATTACHMEN

ASSEMBLY

Figure 2

10-82-2573-2-1

CRBRP FUEL ROD

Dished Fuel Pellet0.1935" Dia.91.3% T.D.

Bottom End Cap

Blanket Pellet0.190" Dia.

96% T.D. 20% CW 316SS

Top End Cap

g Gas Capsule

302SS Spring

14" U02 Blanket Pellet Stack

36" Mixed Oxide PuO2 U02Fuel Pellet Stack

Blanket Pellet Stack

Pull Through WireAttachment Cladding - 20%

CW 316SS

6SS Length 114.4"

Cladding0.230" Dia.0.015" Wall

6115-1

Figure 3

') rwe.. 4

CRBRP HEAT TRANSPORT ANDPOWER GENERATION

ONE OFTHREE SETS U U uOF SYSTEMS AND FEEDWATER

I COMPONENTS SHOWN HEATERS6DIUMS(

STEAM/WATER

Figure 4

2-82-335-84-4

RSB PlugUnloading Station

Hatch Cover(Stored Position)-

Mezzanine (IVTM andReactor Closure

Head Control Lqcation) -

Auxiliary HandlingMachine (AHM) in Storage Pit

RSB Crane

Reactor ServiceBuilding (RSB)

Refueling

Storage

IVTMStorage Pit Spent-Fuel Shipping Cask

Ex-Vessel StorageTank

Cask Shaft

Fuel Handling Cell

Primary and SecondaryControl Rod Driveline

Storage, AHM Floor Valve

NSpent-Fuel Shipping CaskEVTM Gantry and Transporter

Ex-Vessel TransferMachine (EVTM)

EVTM Floor Valve andFuel Transfer Port AdapterIVTM Port Plug Storage, AHM Floor

Valve

Machine (IVTM)

IVTM Drive Storage IVTM Port Adapter Storage

Figure 5

t,

1(

IHX

Reactor VesselFlow--•.

Figure 6

THE ELEVATION DIFFERENCES IN MAJORCOMPONENTS -PROVIDE A NATURAL

CIRCULATION CAPABILITY FOR CRBRP

.PROTECTEDAIR COOLEDCONDENSER

REACTORCONTAINMENT

FLOOR "STEAM CENTERS

DRUM

EVAPORATORSREACTOR-

COREMIDPLANE

REACTOR CONTAINMENTBUILDING

STEAM GENERATORBUILDING

10-81-2597-15

Figure 7