<PAGE>

    THE SECURITIES REPRESENTED BY THIS DEBENTURE HAVE NOT BEEN REGISTERED UNDER
THE SECURITIES ACT OF 1933, AS AMENDED ("ACT") AND MAY NOT BE OFFERED OR SOLD,
TRANSFERRED, PLEDGED, HYPOTHECATED OR OTHERWISE DISPOSED OF EXCEPT PURSUANT TO
(i) AN EFFECTIVE REGISTRATION STATEMENT UNDER THE ACT, (ii) TO THE EXTENT
APPLICABLE, RULE 144 UNDER THE ACT (OR ANY SIMILAR RULE UNDER SUCH ACT RELATING
TO THE DISPOSITION OF SECURITIES), OR (iii) AN OPINION OF COUNSEL, IF SUCH
OPINION SHALL BE REASONABLY SATISFACTORY TO COUNSEL TO THE ISSUER, THAT AN
EXEMPTION FROM REGISTRATION UNDER SUCH ACT IS AVAILABLE.

                                 STARMET CORPORATION
                        10% Convertible Subordinated Debenture
                                Due December 31, 1999


($500,000.00)                                    December 23, 1997



    Starmet Corporation., a Massachusetts corporation (the "Company"), for
value received, hereby promises to pay to WIAF Investors Co. or registered
assigns (the "Holder"), the principal sum of Five Hundred Thousand Dollars
($500,000.00) on December 31, 1999 (the "Maturity Date") with interest from the
date hereof (computed on the basis of a 365-day year) at the rate per annum of
ten percent (10%) until paid in full.  The Company will pay interest in arrears
semi-annually on the last day of each June and of each December (each an
"Interest Payment Date") to the Holder of record.  Interest will accrue from the
most recent date to which interest has been paid or if no interest has been
paid, from the date of issuance of this debenture. 

    1.   General.  This  debenture (the "Debenture") is transferable only by
surrender thereof at the principal office of the Company located at 2229 Main
Street, Concord, Massachusetts  01742, duly endorsed by, or accompanied by a
written instrument of transfer duly executed by, the registered Holder of this
Debenture or his attorney duly authorized in writing and a completed "investor
questionnaire" duly executed by the transferee reasonably satisfactory in form
and substance to the Company.

    2.   Events of Default.  An "Event of Default" occurs if:

         (a)  the Company defaults in the payment of interest on this Debenture
    when the same becomes due and payable and the default continues for ten
    (10) days after notice thereof is given to the Company;

                                         -1-
<PAGE>

         (b)  the Company defaults in the payment of the principal of this
    Debenture when the same becomes due and payable and the default continues
    for ten (10) days after notice thereof;

         (c)  the Company, pursuant to or within the meaning of any Bankruptcy
    Law (A) admits in writing its inability to pay its debts generally as they
    become due, (B) commences a voluntary case or proceeding under any
    Bankruptcy Law with respect to itself, (C) consents to the entry of a
    judgment, decree or order for relief against it in an involuntary case or
    proceeding under any Bankruptcy Law, (D) consents to the appointment of a
    bankruptcy trustee (a "Bankruptcy Trustee") of its or for any part of its
    property, (E) consents to or acquiesces in the institution of bankruptcy or
    insolvency proceedings against it, (F) applies for, consents to or
    acquiesces in the appointment of a Bankruptcy Trustee, (G) makes a general
    assignment for the benefit of its creditors, or (H) takes any corporate
    action for any of the foregoing purposes; or

         (d)  a court of competent jurisdiction enters a judgment, decree or
    order for relief in respect of the Company in an involuntary case or
    proceeding under any Bankruptcy Law which shall (A) approve as properly
    filed a petition seeking reorganization, arrangement, adjustment or
    composition in respect of the Company, (B) appoint a Bankruptcy Trustee of
    the Company or for any part of its property, or (C) order the winding-up or
    liquidation of its affairs; and such judgment, decree or order shall remain
    unstayed and in effect for a period of 60 consecutive days; or (D) any
    bankruptcy or insolvency petition or application is filed, or any
    bankruptcy or insolvency proceeding is commenced against the Company and
    such petition, application or proceeding is not dismissed within 60 days.

    The term "Bankruptcy Law" means Title 11, U.S. Code or any similar federal
or state law for the relief of debtors.  

    3.   Remedies.  If an Event of Default (other than an Event of Default
specified in Section 2(c) or 2(d) in respect of the Company) occurs and is
continuing, the Holder may, subject to Section 4 hereof, by notice of the
Company, declare all unpaid principal and accrued interest to the date of
acceleration on the Debenture then outstanding (if not then due and payable) to
be due and payable and, upon any such declaration, the same shall become and be
immediately due and payable.  If an Event of Default specified in Section 2(c)
or 2(d) in respect of the Company occurs, all unpaid principal and accrued
interest on the Debenture then outstanding shall ipso facto become and be
immediately due and payable without any declaration or other act on the part of
any Holder.  The Holder by notice to the Company may rescind an acceleration and
its consequences if (i) all existing Events of Default, other than the
non-payment of the principal of the Debenture which has become due solely by
such declaration of acceleration, have been cured or waived, (ii) to the extent
the payment of such interest is lawful, interest on overdue installments of
interest and overdue principal, which has become due otherwise than by such
declaration of acceleration, has been paid, and (iii) the rescission would not
conflict with any judgment or decree of a court of competent jurisdiction.

                                         -2-
<PAGE>

    4.   Subordination.  The Company covenants and agrees, for itself and its
successor and assigns, and each Holder hereof, for itself and for any subsequent
holder of this Debenture, by its acceptance hereof, covenants and agrees that
the indebtedness evidenced hereby is hereby expressly subordinated, to the
extent and in the manner hereinafter set forth, in right of payment to the prior
payment for satisfaction in full in cash of all Senior Indebtedness, as
hereinafter defined.

    Until all Senior Indebtedness shall have been performed or paid in full in
cash, (a) the Company shall not, directly or indirectly, and the holder of this
Debenture shall not be permitted to receive any payment on account of this
Debenture and the holder of this Debenture shall not demand or accept from the
Company or any person any such payment or otherwise discharge any part of the
obligations of the Company hereunder and such holder shall not take any action
prejudicial to, or inconsistent with, the priority position of the Senior
Indebtedness over the holder hereof, including, without limitation, declaring
all unpaid principal and accrued interest under this Debenture due and payable,
commencing any action or proceeding against the Company to enforce to collect
this Debenture, or any portion hereof or commencing or joining any "Proceeding"
(as defined below) against the Company, and (b) the holder of this Debenture
shall have no right of subrogation to receive payments or distributions of
assets of the Company applicable to the Senior Indebtedness.  Notwithstanding
the foregoing, so long as no default exists under any agreement or instrument
evidencing any of the Senior Indebtedness, interest and principal may be paid on
this Debenture in accordance with the terms hereof.

    At any meeting of creditors of the Company or in the event of any
proceeding ("Proceeding"), voluntary or involuntary, for the distribution,
division or application of all or part of the assets of the Company, whether
such proceeding be for the liquidation, dissolution or winding up of the Company
or its business, or proceeding for relief under any bankruptcy, reorganization
or insolvency law or any other law relating to relief of debtors, readjustment
of indebtedness, reorganization, arrangement, composition or extension or
otherwise, the holder of the Senior Indebtedness is hereby irrevocably
authorized by the holder of this Debenture at any such meeting or in any such
proceeding:  (a) to prove any claim (by the filing of proof(s) of claim(s) or
otherwise) on account of this Debenture either in its own name or in the name of
the holder hereof; (b) to collect any assets of the Company distributed, divided
or applied by way of dividend or payment, or any securities issued, on account
of this Debenture and apply the same to the Senior Indebtedness; (c) to vote
claims on account of this Debenture to accept or reject any plan for
liquidation, reorganization, arrangement, composition or extension; and (d) to
take generally any action in connection with any such meeting or proceeding
which the holder of this Debenture might otherwise take.

    Upon the occurrence of any default under any agreement or instrument
evidencing any of the Senior Indebtedness, should any payment be received by the
holder of this Debenture, such payment shall be delivered forthwith to the
holder of the Senior Indebtedness for application to the Senior Indebtedness. 
The holder of the Senior Indebtedness is irrevocably authorized to supply any
required endorsement or assignment.  Until so delivered, any such payment shall
be 

                                         -3-
<PAGE>

held by the holder of this Debenture in trust for the holder of the Senior
Indebtedness and shall not be commingled with other funds or property of the
holder of this Debenture.

    Without the necessity of any reservation of rights against or any notice to
or further assent by the holder of this Debenture, any demand for payment of any
Senior Indebtedness made by the holder of the Senior Indebtedness may be
rescinded in whole or in part by such holder, the holder of the Senior
Indebtedness may exercise or refrain from exercising any rights and remedies
against the Company and others, the Senior Indebtedness or any collateral
security or guaranty therefor or right of offset with respect thereto, may be
executed, or released by the holder of Senior Indebtedness, and any agreement or
instrument evidencing, securing or otherwise relating to Senior Indebtedness may
be amended or modified, all without impairing, abridging, releasing or affecting
the subordination provided for herein.  The holder of this Debenture waives any
and all notice of the creation or modification of any Senior Indebtedness and
notice of or proof of reliance by the holder of the Senior Indebtedness upon the
subordination provided for herein.

    For purposes of this Debenture, "Senior Indebtedness" shall include any and
all indebtedness, liabilities, duties, undertakings, warranties, covenants and
agreements (including those of payment or performance) of the Company or any of
its wholly-owned subsidiaries to State Street Bank and Trust Company and its
respective successors or assigns (the "Lender"), of every kind, nature and
description including, without limiting the foregoing, that arising pursuant to
the terms of the Forbearance and Amendment Agreement ("Amendment Agreement")
dated as of January 11, 1996 between the Company, its wholly-owned subsidiary,
Carolina Metals, Inc. ("CMI," now Starmet CMI, Inc.), and the Lender, as it has
been and may be amended, modified, supplemented and/or restated from time to
time, the Loan Documents and Bond Documents, as defined in the Amendment
Agreement, as such Documents may be amended, modified, supplemented and/or
restated from time to time, or otherwise, whether or not the same are: now
existing or hereafter arising; imposed by agreement or by operation of law; due
or not due; absolute or contingent (including any reimbursement obligations
relating to any letter of credit issued for the account of the Company or CMI);
liquidated or unliquidated; voluntary or involuntary, evidenced by a writing;
presently contemplated by the parties; direct or indirect; liabilities or
undertakings of the Company or any of its subsidiaries as surety, guarantor or
endorser with respect to obligations of one or more other parties specifically
described as secured or unsecured, hereafter acquired by the Lender or any of
them by assignment, other transfer or operation of law.  Senior Indebtedness
also shall include any refinancings thereof or replacement financing therefor.

    5.   Conversion Right.  Subject to and upon compliance with the provisions
of this Debenture, the Holder of this Debenture has the right at any time, at
his option, to convert the principal of this Debenture into such number (the
"Debenture Shares") of fully paid and non-assessable shares of common stock,
$.01 par value (the "Common Stock") of the Company as shall equal the principal
amount of this Debenture divided by the Conversion Price (as hereinafter
defined).  A conversion shall only occur upon surrender of this Debenture to the
Company  at its principal place of business, duly endorsed by, or accompanied by
instruments of transfer (in 

                                         -4-
<PAGE>

form reasonably satisfactory to the Company) duly executed by the Holder or his
attorney duly authorized in writing, together with a conversion notice and
transfer tax stamps or funds therefor, if required. If this Debenture is
surrendered for conversion  prior to an Interest Payment Date, the Holder shall
be entitled to receive interest accrued prior to conversion on the relevant
Interest Payment Date.

    6.   Manner of Exercising Conversion Rights.  In order to exercise the
conversion privilege, the Holder shall surrender this Debenture to the Company
at any time during usual business hours at its offices in Concord,
Massachusetts, and shall give written notice to the Company at such office that
the Holder elects to convert this Debenture.  Such notice shall also state the
name or names (with address or addresses) in which the certificate or
certificates for shares of Common Stock which shall be deliverable upon such
conversion shall be registered.  As promptly as practicable after the receipt of
such notice and the surrender of this Debenture as aforesaid, the Company shall
then cancel this Debenture and shall issue and deliver at such office to such
Holder, or on his written order, a certificate or certificates for the number of
full shares of Common Stock deliverable on such conversion of this Debenture in
accordance with the provisions of this Debenture and cash as provided in the
following Section, in respect of any fraction of a share of Common Stock
otherwise deliverable upon such conversion.  Such conversion shall be deemed to
have been effected immediately prior to the close of business on the date
(herein called the "Date of Conversion") on which such notice shall have been
received by the Company and this Debenture shall have been surrendered as
aforesaid, and the person or persons in whose name or names any certificate or
certificates for shares of Common Stock shall be issuable upon such conversion
shall be deemed to have become on the Date of Conversion the holder or holders
of record of the shares represented thereby; provided, however, that any such
surrender on any date when the stock transfer books of the Company shall be
closed shall be deemed effected at the opening of business on the next
succeeding day on which such stock transfer books are open, and the person or
persons in whose name or names the certificate or certificates for such shares,
are to be issued shall be deemed to have become the record holder or holders
thereof for all purposes as of such date.

    7.   No Fractional Shares.  No fractions of shares of Common Stock or scrip
representing fractions of shares of Common Stock shall be issued upon conversion
of this Debenture.  If more than one Debenture shall be surrendered for
conversion at one time by the same Holder, the number of full shares of Common
Stock which shall be deliverable upon conversion thereof shall be computed on
the basis of the aggregate principal amount of the Debentures so surrendered. 
If any fraction of a share of Common Stock would, except for the provisions of
this Section, be deliverable on the conversion of this Debenture or Debentures
(or specified portions thereof), the Company shall make payment in lieu thereof
in an amount of cash equal to the value of such fraction multiplied by the
Conversion Price.

    8.   Conversion Price Adjustments.  (a) For purposes of this Debenture:

                                         -5-
<PAGE>

    "Additional Shares of Common Stock" shall mean all shares of Common Stock
issued (or, pursuant to this Debenture, deemed to be issued) by the Company,
other than shares of Common Stock issued or issuable:

         (i)  upon conversion of the Debentures; or

         (ii) in lieu of interest on the Debentures.

    "Conversion Price" shall mean the price at which shares of the Common Stock
shall be deliverable upon conversion of this Debenture as adjusted from time to
time as herein provided.  The initial Conversion Price is $21.50.  The
Conversion Price for this Debenture shall be subject to adjustment as herein
provided.

    "Recapitalization Events" shall mean stock splits, stock dividends,
recapitalizations, reclassifications and similar events.

         A.   Adjustment for Stock Splits and Combinations.  If the Company at
    any time or from time to time after the Issuance Date effects a subdivision
    of the outstanding Common Stock, the Conversion Price then in effect
    immediately before that subdivision shall be proportionately decreased, and
    conversely, if the Company at any time or from time to time after the
    Issuance Date combines the outstanding shares of Common Stock, the
    Conversion Price then in effect immediately before the combination shall be
    proportionately increased.  Any adjustment under this subsection shall
    become effective at the close of business on the date the subdivision or
    combination becomes effective.

         B.   Adjustment for Certain Dividends and Distributions.  In the event
    the Company at any time, or from time to time after the Issuance Date
    makes, or fixes a record date for the determination of holders of Common
    Stock entitled to receive, a dividend or other distribution payable in
    Additional Shares of Common Stock, then and in each such event the
    Conversion Price then in effect shall be decreased as of the time of such
    issuance or, in the event such a record date is fixed, as of the close of
    business on such record date, by multiplying the Conversion Price then in
    effect by a fraction (a) the numerator of which is the total number of
    shares of Common Stock issued and outstanding immediately prior to the time
    of such issuance or the close of business on such record date, and (b) the
    denominator of which shall be the total number of shares of Common Stock
    issued and outstanding immediately prior to the time of such issuance or
    the close of business on such record date plus the number of shares of
    Common Stock issuable in payment of such dividend or distribution;
    provided, however, that if such record date is fixed and such dividend is
    not fully paid or if such distribution is not fully made on the date fixed
    thereof, the Conversion Price shall be recomputed accordingly as of the
    close of business on such record date and thereafter the Conversion Price
    shall be adjusted pursuant to this Section 8B as of the time of actual
    payment of such dividends or distributions.

                                         -6-
<PAGE>

         C.   Adjustments for Other Dividends and Distributions.  In the event
    the Company at any time or from time to time after the Issuance Date makes,
    or fixes a record date for the determination of holders of Common Stock
    entitled to receive a dividend or other distribution payable in securities
    of the Company other than shares of Common Stock, then in each such event
    provision shall be made so that the Holder shall receive upon conversion of
    this Debenture, in addition to the number of shares of Common Stock
    receivable thereupon, the amount of securities of the Company which they
    would have received had this Debenture been converted into Common Stock on
    the date of such event and had the Holder thereafter, during the period
    from the date of such event to and including the date of conversion,
    retained such securities receivable by them as aforesaid during such
    period, subject to all other adjustments called for during such period
    under this Section 8 with respect to the rights of the holders of this
    Debenture.

         D.   Adjustment for Reclassification, Exchange and Substitution.  If
    the Common Stock issuable upon the conversion of this Debenture is changed
    into the same or a different number of shares of any class or classes of
    stock, whether by recapitalization, reclassification or otherwise (other
    than a subdivision or combination of shares of stock dividend or a
    reorganization, provided for elsewhere in this Section 8, then and in any
    such event the Holder of this Debenture shall have the right thereafter to
    convert such stock into the kind and amount of stock and other securities
    and property receivable upon such reorganization, reclassification or other
    change, by holders of the number of shares of Common Stock into which such
    a Debenture  might have been converted immediately prior to such
    reorganization, reclassification or change, all subject to further
    adjustments provided herein.

         E.   Reclassification, Reorganization, Etc.  If any of the following
    shall occur, namely: (a) any reclassification or change of outstanding
    shares of Common Stock issuable upon conversion of this Debenture (other
    than a change in par value, or from par value to no par value, or from par
    value to no par value, or from no par value to par value, or as a result of
    a subdivision or combination), (b) any consolidation or merger to which the
    Company is a party other than a merger in which the Company is the
    continuing corporation and which does not result in any reclassification
    of, or change (other than a change in name, or par value, or from par value
    to no par value, or from no par value to par value, or as a result of a
    subdivision or combination) in, outstanding shares of Common Stock or (c)
    any sale or conveyance of all or substantially all of the property or
    business of the Company as an entirety, then the Company, or such successor
    or purchasing corporation, as the case may be, shall, as a condition
    precedent to such reclassification, change, consolidation, merger, sale or
    conveyance, execute and  deliver a certificate providing that the Holder of
    this Debenture then outstanding shall have the right to convert such
    Debenture into the kind and amount of shares of stock and other securities
    and property (including cash) receivable upon such reclassification,
    change, consolidation, merger, sale or conveyance by a holder of the number
    of shares of Common Stock deliverable upon conversion of this Debenture
    immediately prior to such reclassification, change, consolidation, merger,
    sale or conveyance.  If, in the case of any 

                                         -7-
<PAGE>

    such consolidation, merger, sale or conveyance, the stock or other
    securities and property (including cash) receivable thereupon by a holder
    of Common Stock includes shares of stock or other securities and property
    of a corporation other than the successor or purchasing corporation, as the
    case may be, in such consolidation, merger, sale or conveyance, then such
    certificate shall also be executed by such other corporation and shall
    contain such additional provision to protect the interests of the Holders
    of the Debentures as the Board of Directors of the Company shall reasonably
    consider necessary by reason of the foregoing.  The provisions of this
    Section shall similarly apply to successive consolidation, mergers, sales
    or conveyances.

    9.   Interest Limitation.  If a law, which applies to this Debenture and
which sets maximum loan charges, is finally interpreted so that the interest or
other loan charges collected or to be collected in connection with this
Debenture exceed the permitted limits, then:  (i) any such loan charge shall be
reduced by the amount necessary to reduce the charge to the permitted limit; and
(ii) any sums already collected from the Company which exceeded permitted limits
will be refunded to the Company.  The Holder may choose to make this refund by
reducing the principal owed under this Debenture or by making a direct payment
to the Company.

    10.  Consent to Jurisdiction.  The Holder hereby irrevocably agrees that
any legal action or proceedings with respect to this Debenture against the
Company may be brought only in the courts of the United States of America or The
Commonwealth of Massachusetts. By acceptance of this Debenture, the Holder
hereby (i) accepts the exclusive jurisdiction of the aforesaid courts; (ii)
irrevocably agrees to be bound by any judgment of any such court with respect to
this Debenture; and (iii) irrevocably waives, to the fullest extent permitted by
law, any objection which it may now or hereafter have to the laying of venue of
any suit, action or proceedings with respect to this Debenture brought in any
court of the United States of America or The Commonwealth of Massachusetts, and
further irrevocably waives any claim that any such suit, action or proceeding
brought in any such court has been brought in an inconvenient forum.

    11.  Miscellaneous. 
    
         (a)  THIS DEBENTURE SHALL BE GOVERNED BY THE LAWS OF THE COMMONWEALTH
    OF MASSACHUSETTS WITHOUT REFERENCE TO CONFLICT OF LAWS RULES OR PRINCIPLES.

         (b)  The Company and all endorsers of this Debenture hereby waive
    presentment, demand, protest or notice of any kind in connection with the
    delivery, acceptance, performance or enforcement of this Debenture.

                                         -8-
<PAGE>

    (c)  No provision thereof shall alter or impair the obligation of the
    Company which is absolute and unconditional, to pay the principal and 
    interest on this Debenture as herein prescribed.

                                  STARMET CORPORATION.



                                  By: /s/ James M. Spiezio
                                      --------------------



                              SCHEDULE TO EXHIBIT 10(JJ)
                                           

Other similar 10% Convertible Subordinated Debentures of the Company dated
December 23, 1997:

Holder                       Amount

Melvin B. Chrein             $150,000
Joshua M. Feibusch           $150,000
Marshall J. Chrein           $50,000
George J. Matthews           $50,000





                                         -9-