Smp Plutonium Grain Size

TABLE III.

MOX FUEL ASSEMBLY DESIGN FEATURES.

  1. MOX fuel rod rating - 82w/cm2
  2. Peak Assembly burn-up - 25,000 MWd/Te
  3. Local peaking factor - 1.172
  4. Axial peaking factor - 1.273
  5. Peak pellet burn-up - 37,200 MWd/Te

TABLE IV

EXPERIMENTAL IRRADIATIONS OF MOX FUEL (BWR) TYPE CLUSTERS IN CIRUS

Fuel Composition : U02 - 4% PuOz

Sr. Designation Max.linear Rating Burn-up No. W/cm MWD/Te

Remarks

1.

AC-2

414

16,265

2.

AC-3

490

16,000

3.

AC-4

490

Standard

Limited burn-up planned. Design variables studied include fuel clad gap, annular pellets, LTS, grain size and Pu cluster size.

A manufacturing plant for MOX fuel was planned and constructed at Tarapur. The plant has 2 fabrication lines, in parallel, and die initial work has commenced on one of these. To start with, work has been taken up on 10 Kg batch scale for the mechanical milling of starting powders - U02 and Pu02 using an attritor. The attritor mill gives excellent homogeneity in short times and is very convenient for operation in glove-boxes. Sintering is done on 30 Kg scale in a batch sintering furnace. Although we have equipped the plant with continuous sintering systems, we have found the demands on services of batch sintering furnace much easier to meet and for the initial work, we have utilised the latter. The centreless grinder used for the sintered pellet diameter control is fitted with a composite diamond grinding wheel, which has a fairly long life, and the cutting is achieved in one pass. The coolant recirculation line has a hydroclone for the separation of the sludge and clarification of coolant. TIG welding is practiced for welding the second end-plug to tubes loaded with pellets and internal components, helium gas being filled in the annulus to a pressure of 2.5 bars. The rods are checked for contamination followed by quality control checks and are configured into fuel assemblies.

Based on satisfactory irradiation performance of the initial fuel assemblies from the plant, scaling up of production operations is in hand to recycle Pu within the allowed limits of loading in the reactors.

QUALITY CONTROL DURING MOX FUEL MANUFACTURE

Prior to the manufacturing campaign of the BWR-MOX, a detailed quality control(Q.C) plan was formulated to meet the stringent specifications of MOX fuel. The Q.C plan was finalised taking into account various aspects like sample size, and accuracy required. Quality control facilities were set up to carry out the various checks. The Q.C actions are subjected to Quality Surveillance (Q.S) by an independent agency which performs this function for all nuclear fuel manufacturing activities in India.

The Zircaloy hardware and U02 powder are supplied by the Nuclear Fuel Complex (NFC), Hyderabad which manufactures NU/LEU fuel for all Indian power reactors. Pu02 powder prepared by die oxalate route is received from the Power Reactor Fuel Reprocessing Plant (PREFRE) at Tarapur. The documents received are scrutinised for compliance with specifications for starting materials prior to start of manufacture.

During the MOX fuel fabrication programme, two techniques of Q.C were developed and adopted. A quick check of the blends for Pu content has been carried out using a Neutron Well Coincidence Counter (NWCC). This technique has the advantages of being fast and representative of bulk material compared to the X-ray Fluorescence (XRF) Technique originally planned to be employed. During this campaign, a large number of samples were analysed for Pu content and a good control correlation was obtained with the chemical analyses carried out.

Use has been made of a passive gamma scanner to detect faulty mix-up of pellets of different enrichments in the three types of fuel rods. Further, a gamma-auto-radiography technique (GAR) has been utilised to detect mix-up of pellets of different enrichments and the detection of Pu02 agglomerates present in the peripheral zone of the cylindrical surfaces of pellets. The standard Q.C check for ensuring plutonium homogeneity in the pellets is alpha-autoradiography of a sample pellet from every blended batch. However, GAR enables detection of Pu02 agglomerates lying at the periphery of all the pellets in the finished rod, which is vital informaltion as Pu clusters close to the clad inner surface can be deleterious to the fuel rod performance. The GAR technique has been useful in changing some of the manufacturing procedures to improve the quality of die fuel. Figs.4 (a) & (b) are gamma-auto-radiographs showing Pu02 agglomerates and presence of U02 pellet in a MOX fuel rod. Microdensitometric analysis of the gamma-auto-radiographs would yield quantitative information on the plutonium enrichment in the clusters. Both these techniques have been adopted as regular Q.C checks in the manufacturing activity.

5.1 Fuel Characteristics

Inspection of the pellets for density and dimensions was carried out as per MIL-STD 1Q5D special level IV. All the pellets were also examined for physical integrity. The Pu02 cluster size of sample pellets, examined by alpha-auto radiography, did not exceed 100 microns. Microstructural evaluation of pellets was done in a separate metallography line. The chemical characteristics like Pu and heavy metal contents, O/M ratio, impurities and hydrogen content have been achieved well within the specifications and histograms of some of characteristics are presented in Figs. 5(a) to (d). Dissolution test is performed on all pellet batches to ensure compliance of reprocessing requirements.

For the fabrication of the fuel rods, qualification of the TIG welding machine and its operation were carried out as per the QC plan. The welded fuel rods were subjected to contamination check, helium leak testing, visual examination, metrology, X-radiography, gamma scanning and gamma auto-radiography. The fuel assemblies were constituted from the accepted rods and subjected to various QC checks prior to shipping to TAPS.

6. CONCLUSIONS

The work carried out on MOX fuel development so far has been encouraging and it is planned to exploit the potential of plutonium utilisation initially in the BWRs, to be followed by the PHWRs until

U, O/M, IMPURITIES , MOISTURE

WEIGHING WEIGHING

Pu, ISOTOPICS, IMPURITIES, MOISTURE

MIXING/MILLING

NW CC

GRANULATION

BOTTOM END PLUG WELDED ZIRCALOY TUBES,PLUGS I PLENUM SPRING

VACUUM DEGREASING

STORAGE

STACK'NG

STORAGE

STACK'NG

INSPECTION

FIG. 3. Flow Sheet for Fabrication ofMOX Fuel with QC Points

FIG. 4a Gamma Autoradiograph Showing surface Pu02 Agglomerate

FIG. 4b Presence of U02 Pellet in a MOX Rod
Mox Fuel Pellet

cd r-10

io cm

MEDIUM

to ro in cm ro io

N cvj lO cm fi

HIGH

FIG. 5a Percentage ofPu02 in MOX Pellets the larger requirements of FBRs are to be met. Progressive enhancement of plutonium quantities in these two reactor types has been planned. These developments would run parallel to the fast breeder development programme and the fuel manufacturing technological innovations realised would greatly facilitate the breeder fuel manufacturing work.

SPECIFICATION : (10.19- 10.52) gm/cc AVERAGE : 10.35

Plutonium Pellets

o ro

o ro

in to 6

ro o

FIG. 5b

Density ofMOX (BWR) Fuel Pellets

Plutonium Pellets

FIG. 5c

Equivalent Boron Content

FIG. 5c

Equivalent Boron Content

Plutonium Recycling
HYDROGEN CONTENT (ppm)

FIG. 5d Hydrogen Content in MOX Pellets

REFERENCES

  • 1] "Plutonium Recycling in RAPS" - BARC Internal Document.
  • 2] "MOX Fuel for TAPS" - BARC Internal Report.
  • 3] KAMATH, H.S, PURUSHOTHAM, D.S.C., SAH, D.N. AND ROY, P.R , "Some Aspects of Plutonium Recycling in Thermal Reactors', IAEA Specialists Meeting on Pu Utilisation and Improved Performance of Fuel for Light Water Reactors, CEN, MOL, May (1984).

BNFL SUPPLY OF MOX FUEL ASSEMBLIES TO THE BEZNAU 1 PWR OF NOK

  1. EDWARDS, R.D. GRIMOLDBY, S.J. MARSHALL British Nuclear Fuels pic, Sellafield, United Kingdom
  2. W. STRATTON

Nordostschweizerische Kraftwerke AG,

Baden, Switzerland

Abstract

The Swiss utility NOK has been using MOX fuel in its Beznau I and 2 reactors since 1978, and since that time it has irradiated more than ISO MOX fuel assemblies produced by Westinghouse, Belgonucleaire (COMMOX) and Siemens. In 1991 NOK decided to support BNFL on the provision of the MOX Demonstration Facility (MDF) at Sellafield by becoming their first customer. Following the successful commissioning of the plant, BNFL, under a sub-contract to Westinghouse, produced their first LWR MOX fuel assemblies in mid-1994 for loading into the Beznau 1 reactor. This paper considers the recycle philosophy adopted by NOK and their approach to utilising MOX fuel made by a new supplier in their reactors. The paper considers the fabrication process used by BNFL in the MDF with particular attention being given to the Short Binderless Route used to prepare the MOX press feed material. It also reviews the quality characteristics of the pelleted fuel made for the NOK fuel assemblies and provides data on the predicted performance of the fuel in reactor. After reviewing the irradiation conditions in the reactor, details are provided of a planned Post Irradiation Examination programme that is scheduled to take place in 1997-98.

1. INTRODUCTION

Nordostschweizerische Kraftwerke (NOK) is a Swiss utility supplying the north east corner of Switzerland with electric power from a mixture of hydroelectric and nuclear sources. It owns and has operated two Westinghouse PWRs of 350 MW(e) at the Beznau site on the river Aare since 1969 with very high reliability. Since the beginning Westinghouse has supplied Beznau I (KKB-1) with uranium fuel assemblies and in 1978 supplied the first four MOX assemblies to be loaded by NOK.

Since 1978 NOK has continually built up its usage of MOX based on its reprocessing contracts with COGEMA and BNFL. Initially due to the slow build up of its own Pu coming from La Hague, NOK began by using Pu loaned from other parties, for return when NOK's own material became available. In this way more flexible management of the recycle process has been possible which has allowed NOK lo build up a wealth of experience in the use of MOX assemblies. The approach also enabled the Swiss licensing authorities to gain early experience in the licensing and handling aspects of MOX assemblies and as a result generate confidence in the use of MOX in the Beznau plants, allowing for example up to 48 (from a total of 121, ie 40%) MOX assemblies to be in the core at any one time. Later, based on the experience in KKB-1, MOX was also introduced to Beznau 2 (KKB-2) under the same conditions; then as now, KKB-2 fuel was supplied by Siemens.

In 1989 BNFL embarked upon a strategy aimed at becoming one of the world's leading MOX fuel suppliers and thereby provide services which give its customers the opportunity to close the fuel cycle. The strategy included constructing at Sellafield a small-scale demonstration fabrication plant - the MOX Demonstration Facility (MDF) and later to construct a larger-scale fabrication plant - the Sellafield MOX Plant (SMP). In 1991 NOK decided to support BNFL in developing this strategy by becoming their first customer for fuel from the MDF. Following the successful commissioning of the MDF in 1993, BNFL, under a sub-contract to Westinghouse, produced their first LWR MOX fuel assemblies which were loaded into KKB-1 reactor in mid 1994.

This paper considers the recycle philosophy adopted by NOK and the approach to utilising MOX fuel produced by a new supplier in their reactors. The paper also considers the fabrication process used by BNFL and l:he quality characteristics of MOX fuel made in the MDF. Finally the paper reviews the expected performance of the fuel and summarises plans for a Postlrradiation Examination Programme which will be undertaken to support validation of the fuel performance codes and provide the first data on the performance of BNFL's MOX fuel in a power reactor.

2. NOK'S RECYCLE PHILOSOPHY AND THE APPROACH TO USING A NEW SUPPLIER

Due to its early entry into the recycle business NOK adopted a philosophy of collaboration with its various fuel vendors. Figure 1 shows how a number of vendors have been involved in some or all of the stages of MOX fuel fabrication for the utility. Following the initial supply of four MOX fuel assemblies by Westinghouse (with the pellets and rods supplied from Cheswick, USA and the assemblies being manufactured in FBFC, Belgium) NOK moved to Siemens (Hanau) as a MOX supplier, then Belgonucleaire with Westinghouse and even later COMMOX involving both Westinghouse and ABB. These sometimes complex supply arrangements resulted in the delivery and irradiation of 152 MOX fuel assemblies to Beznau to date with about two thirds of the total being used in KKB- 1.

In keeping with its philosophy of collaboration and its strategy of having as wide a source of supply as possible for its fuel cycle products and services, NOK supported BNFL in the commissioning and operation of the MOX Demonstration Facility. Under carefully defined contractual terms and conditions, which included some flexibility in the delivery of the first fuel assemblies to allow for proper commissioning and qualification of the plant and process, BNFL worked closely and successfully with NOK, Westinghouse and NOK's expert consultant to develop a MOX fuel design and produce four fuel assemblies which were subsequently loaded into KKB- 1.

Because of NOK's MOX fuel utilisation, few special measures were required to license the BNFL fuel in KKB-1. All handling procedures were in place and fully proven. On arrival at the reactor site die assemblies are stored in the standard dry fuel store for uranium assemblies in individual locked and IAEA sealed storage channels. No special precautions for radiation protection are taken, over and above those imposed by routine radiation measurements on receipt and during handling. Operator proximity to the assemblies and handling times are adjusted accordingly. However the frequency of IAEA inspections is raised from three monthly to monthly during the time the MOX assemblies are in the fuel store. Movement to the transfer pool takes place at the time of reactor reloading. Unlike uranium fuel, MOX fuel assemblies are loaded at the next available fuel reload and do not form part of the strategic store of (uranium) fuel held at the plant.

NOK's policy is to operate the MOX assemblies under similar conditions of power and burn-up as the uranium assemblies loaded in the reactor. Core loading evaluation needs to take into account the possible increase in assembly peaking, slower reactivity burn out and slightly lower control rod worths although experience has shown fliat these can be accommodated within the normal reload planning without special operational or equipment modifications. In some more extreme loading patterns, the available neutronics codes are not able to accurately predict control rod worths and this has to be accommodated in the design. Unlike uranium assemblies the decay of M1Pu and consequent build up of americium needs to be taken into consideration when defining the reactivity on the date the fuel is loaded into reactor.

To prove that the fuel provided by BNFL could meet the design performance limits it was necessary to evaluate and predict the likely performance of the MOX fuel which was to be produced by a new fabrication process and thus had unproven characteristics. To obtain the appropriate information on the fuel rod behaviour the BNFL fuel performance code ENIGMA was used. After completing an extensive joint BNFL and NOK programme which involved NOK in employing an experienced consultant, the BNFL MOX assemblies were cleared to reach a peak bum-up of 50 GWd/t on five annual irradiation cycles.

Plant

Beznau I

Beznau I

Beznau II

Beznau !

Beznau II

Recycling

1978 - 1981

1988 - 1997

1984 - 1995

1994 - 1999

1996 - 2005

Matrix-material

Natural Uranium

Tails Uranium

Natural Uranium

Tails Uranium

Tails Uranium

MOX-F.Assy. Manufacturer

WH (BN, FBFC)

COMMOX (WH, ABB, FBFC)

SIEMENS

WH (BNFL)

SIEMENS (BNFL)

Max. Assy Bu MWD/T

30 000

43 000

36 000

38 000**

42 000**

Enrichment of surrounding U-F Assay.

3.30%

3.25%

3.40% - 4.00%

3.25% - 4.00%

FIG. 1. Overview of using MOX Fuel in the Beznau Reactors

  1. THE MOX FUEL ASSEMBLY CHARACTERISTICS AND APPROACH TO LICENSING THE DESIGN
  2. 1 FUEL ROD DESIGN

The MOX fuel rod has been designed using a MOX version of the ENIGMA fuel performance code[l]. The ENIGMA code was developed jointly by Nuclear Electric and BNFL during the 1980's for the assessment of U02 fuel performance. The code contains all the submodels necessary for such analyses and in sufficient detail for design and licensing evaluations. It has been independently verified and extensively validated against a data base of around 350 U02 fuel rods from a range of international projects and commercial irradiations. In recent years BNFL have extended the code's capability to analyse MOX and gadolinia-doped and niobia-doped uranium fuels. The MOX version of the code contains modifications to just 3 sub-models; these are: fuel thermal conductivity, fuel creep and radial power depression in the pellet. In addition a new model has been included to account for the enhanced helium release in MOX fuel. The MOX version has been validated against data from the PRIMO 1 international project and Halden projects.

The design and licensing methodology adopted was broadly in line with standard USNRC approved reload methodology. Thus the design and licensing criteria applied in the evaluation of MOX fuel were the same as those applied in the evaluation of U02 fuel. However because at the time of this first demonstration BNFL had no substantive manufacturing experience with MOX fuel, the fuel parameters were allowed to vary over the full range of manufacturing tolerances given on the drawings or in the product specifications in uncertainty analyses. Also because of the small data base available to BNFL on MOX fuel compared with that available on U02 fuel, the calculational uncertainties applied in the evaluations were larger than can be justified for U02 fuel. Nevertheless BNFL were able to satisfy the traditional approach to licensing by showing that all the design and licensing criteria were met for the anticipated fuel duty at Beznau Unit 1 when uncertainties were included at the 95% probability level.

Because the above approach inevitably lead to reduced margins to design and licensing criteria for MOX fuel relative to U02 fuel, and therefore a theoretical increase in failure probability for MOX fuel relative to U02 fuel, NOK requested a demonstration that the failure rate of BNFL MOX fuel would be no higher than the current experience in PWR's world-wide of approximately 1 failure in 100,000 fuel rods. BNFL therefore agreed to carry out additional calculations similar to those described above but including uncertainties at the 99.999% level rather than the 95% level. A joint NOK/BNFL review of all design and licensing criteria highlighted that the criterion most likely to be challenged by this treatment of uncertainties was that of rod internal pressure. Thus further work concentrated on rod internal pressure and successfully demonstrated that the existing criterion, of no fuel/clad gap re-opening, was satisfied even when uncertainties were included at the 99.999% level.

3.2 NUCLEAR DESIGN

With regard to the nuclear design of the MOX assemblies, the principle design issue has been in allowing for isotopic variations in the plutonium feed material. Such variations can affect two important performance characteristics of a fuel assembly - the reactivity of assemblies over their life-time in reactor (the life-time averaged reactivity or LAR) and the within-assembly power peaking factors. By recourse to the principles of reactivity equivalence, however, BNFL has been able to account for both of these effects [2]. Reactivity equivalence, expressed through a simple formula, relates variations in isotopic composition to plutonium concentration changes such that the LAR of MOX assemblies made is kept constant.

Using an equivalence formula as described in Reference 2, the assembly average plutonium concentration was fixed so as to give an LAR equivalent to an existing 3.25 w/o enriched U02 assembly in the KKB-1 core. Low, medium and high plutonium concentration zones within the assemblies were then fixed so as to ensure acceptable power peaking. Additional peaking issues caused by isotopic variability in the plutonium in each zone were accommodated within the engineering hot channel uncertainty factor

  1. The licensing limit for FQE was increased from 3% to 4% in order to accommodate the additional uncertainties arising from the use of MOX fuel.
  2. FABRICATION PROCESS USED BY BNFL

The flowsheet adopted by BNFL in the MDF, summarised in Figure 2, is basically the same as that used by other MOX fuel manufacturers. The exception to this is the use of the Short Binderless Route for blending and conditioning of the MOX powder and preparation of the press feed.

The MDF consist of four main areas of plant:

Fuel pellet production

Fuel rod production

Fuel rod inspection

Fuel assembly manufacture and inspection

The plant is co-located with other MOX support facilities including a comprehensive ceramography and metallography facility for carrying out quality control of fuel pellets and fuel rod weld samples, and development facilities for plant trouble-shooting and process optimisation.

The heart of the process is the pelleting plant where U02 and Pu02 feed materials are weighed out in the correct proportions and processed to press feed by the Short Binderless Route. This Short Binderless Route process uses a high energy attritor mill to blend the feed powders and a spheroidiser to condition the powder before it is used as a feed to a pelleting press. At the milling stage zinc stearate lubricant and Conpor pore former are added; the latter is used to control the pellet density and ensure that the characteristics of the MOX pellets are similar to those of U02 pellets produced by BNFL from 1DR-U02 powder. The milling stage is undertaken for a period of up to 60 minutes and spheroidising is completed within a similar time frame. Using this process 25 kg batches of press feed of consistent quality can be manufactured during a period of about two hours. In MDF the press feed granules are pressed to green pellets which are transferred by a cushion transfer conveyor to the furnace boat load station. Here the pellets are carefully loaded by a 'pick and place' machine into sinter furnace boats which are then charged to the furnace. Pellets are then sintered at 1650°C over a cycle time of 24h before being discharged from the furnace for dry grinding and subsequent inspection. After inspection pellets of acceptable quality are loaded into an in-line pellet store pending loading into fuel rods.

Fully inspected pellets are manually loaded, in lm long sub-stacks into pre-dried fuel cans in which the bottom end plug has been welded and inspected. Each pellet sub-stack is weighed and when the complete fuel column has been loaded into the rod it is processed through various work stations to have the spring and top end plug inserted and welded before final pressurisation with helium and eventual seal welding. The surface of the rod is then checked for contamination before it is transferred to the rod inspection area. Each fuel rod is uniquely identified with a bar code which is entered into a computer based traceability system at each work station in the rod production and inspection areas.

The fuel rod inspection area contains shielded work stations to inspect and check that the rods meet the specification requirements for:

Leak tightness.

Conformance with the weld acceptance criteria.

Dimensions and straightness.

Rod enrichment.

Surface finish.

Certified fuel rods are transferred to the rod store where they are loaded into magazines in the same location as in the finished fuel assembly; rejected rods are passed back to the rod fabrication plant for reworking or breakdown.

SHORT BINDERLESS ROUTE

The MDF Flowsheet

The MDF fuel assembly and inspection area has been designed for PWR fuel assemblies although it could be modified to accommodate BWR designs. In this part of the plant certified magazines are removed from the rod store and brought adjacent with a rod loading machine which transfers the rods into the fuel assembly skeleton. After fitting the top and bottom nozzles assemblies are then inspected to check they meet the specification requirements for:

Dimensional envelope Channel spacings Cleanliness

Control rod withdrawal force Surface finish

In this area of die plant all fabrication processes and inspection work is carried out inside heavily shielded enclosures to minimise dose uptake to plant operations personnel.

5. QUALITY CHARACTERISTICS OF BNFL MOX FUEL PELLETS

Following the commissioning of the MDF, plutonium was introduced into the plant in October 1993. The plant and processes were then qualified to meet NOK and Westinghouse acceptance criteria. The following paragraphs summarise the data obtained on the key quality characteristics during the fabrication of several tonnes of MOX fuel pellets in the plant. The MOX pellets were produced against a BNFL MOX pellet specification developed in conjunction with NOK, Westinghouse and NOK's experienced consultant. The details of this specification are confidential but the following comments are made about the pellet quality.

  • i) No difficulties have been experienced controlling the pellet dimensions, the density, surface finish or ihermal stability of the fuels made in MDF. The standard deviation on pellet diameter is 0.003 mm and on geometric density is 0.026 g/cc. The surface roughness of pellets produced in the plant averages 0.58 /¿Ra with a standard deviation of 0.092 /xRa. The thermal stability of the MOX pellets is well within the specification limits set by BNFL for U02 fuel.
  • ii) The key chemical characteristics of the fuel pellets are summarised in Table 1. The hydrogen content of the pellets produced was low and tests showed that pellets produced do not pick up hydrogen or moisture when stored in air. This behaviour is similar to that of IDR-U02 pellets produced by BNFL Fuel Division at Springfields. The oxygen to metal ratio (O/M ratio) of the fuel produced was consistently close to 2.000.

No difficulties were experienced controlling the fissile material content of the fuel within the specified enrichment tolerances. This is an important observation as BNFL is the only MOX producer who directly produces fuel of the correct enrichment without going through a master blend process, and thus supports the application of the process in the larger-scale Sellafield MOX Plant.

(iii) The microstructural properties of the pellets produced are excellent and consistent. The grain size of the fuel is consistent throughout the pellets and averages 7.4 fim with a standard deviation of 0.6 /¿m. The pore size distribution is frequently assessed as part of the specification requirements and shows that it is consistent throughout the pellets and between the lots examined. For pores with a diameter greater than 5 fim the median pore size has never exceeded 15.4 fim during the production to date.

The homogeneity of the fuel pellets produced by the Short Binderless Route is extremely good when measured by colour autoradiography. To substantiate this claim the Pu/(U+Pu) ratio has been assessed using Electron Probe Micro Analysis. The data available, summarised in Figure 3, show that in pellets with a mean Pu/(U+Pu) ratio of 5.5% the highest plutonium rich region found in the pellets had a Pu/(U+Pu) ratio of 32%.

Was this article helpful?

0 0

Post a comment