<PAGE>


                                                              Exhibit 10 (aa)


                                 EMPLOYMENT AGREEMENT

    This Agreement is made effective as of the first day of October, 1997, by
and between Starmet Corporation, a Massachusetts corporation with its principal
office in Concord, Massachusetts (the "Corporation"), and William T. Nachtrab
(the "Employee") of Maynard, Massachusetts.

    WHEREAS, the Corporation is engaged in the business of developing and
marketing metallurgical and other products; and

    WHEREAS, the Employee is employed by the Corporation as its Vice President,
Technology; and

    WHEREAS, the parties hereto wish to describe the terms under which the
employment relationship presently existing between Corporation and Employee will
continue in the future;

    NOW, THEREFORE, in consideration of the foregoing promises and the mutual
agreements herein contained, the parties hereto agree as follows:

    1.   Employment

    The parties agree to continued employment of the Employee by the
Corporation during the Term of this Agreement and according to the terms and
conditions hereof.

    2.   Service by the Employee

    Except as specifically provided herein, the Employee shall continue to
devote his best efforts and his entire working time during the usual business
hours of the Corporation, and at such other times as is necessary and
appropriate to advancing the best interests of the Corporation.  Without
limiting the generality of the foregoing, such activities shall include 

<PAGE>

serving as an executive employee of the Corporation.

    3.   Compensation

         3.1  Initial Compensation

         The Employee's compensation shall be at the minimum annual rate of
    $136,000 for the Term of this Agreement.

         3.2  Payments

         Such compensation shall be paid to the Employee on a weekly, bi-weekly
    or monthly basis, consistent with the Corporation's normal practice in
    paying compensation to its other executive employees.

         3.3  Increases

         The Board of Directors or President may, from time to time during the
    Term of this Agreement, review compensation and such compensation may be
    increased (but not decreased below the levels provided in this Agreement)
    from time to time on a basis consistent with the performance of Employee
    and of the Corporation and the salary levels prevailing generally for
    positions of comparable responsibility.

         3.4  Bonuses

         In addition to the foregoing initial compensation, the Corporation may
    pay the Employee a bonus in any year in such amount as may be determined by
    the Board of Directors of the Corporation.

    4.   Employee Benefits

    In addition to the other employee benefits which may be made available to
the Employee from time to time, the Corporation shall provide the following to
the Employee:

                                         -2-
<PAGE>


         4.1  Benefit Plans

         The Corporation shall include the Employee in all of its benefit
    plans, including stock option plans, vacation, pension benefits and major
    medical, accident and health insurance, while he remains an active employee
    of the Corporation during the Term of this Agreement, on the same basis as
    the coverage provided to other executive employees of the Corporation.

         4.2  No Reduction of Benefits

         The Corporation shall not eliminate or reduce benefits in effect on
    the date hereof.

    5.   Termination

         5.1  Termination Payments

         The Board of Directors of the Corporation acting on behalf of the
    Corporation and after notice and an opportunity to be heard, may terminate
    the Employee's employment hereunder if a majority of the whole number of
    Directors then in office determines, in good faith, that:

         (a)  the Employee has willfully refused to perform substantially 
    all of the services required of him hereunder; or

         (b)  the Employee has been convicted of a crime of moral turpitude.
    In such event, or in the event of voluntary termination by the Employee
    without "good reason," as herein defined, the Employee's right to all
    future payments set forth in Article 3 hereof shall cease; or

         (c)  prior to a "change of control," as herein defined, by either
    party on twelve months' written notice to the other.  "Change of control"
    shall mean (i) a change (other 


                                         -3-
<PAGE>

    than a change resulting from death, retirement or disability) in the
    identity of a majority of the Board of Directors of the Corporation or in
    the identity of both the President and Chairman of the Board of the
    Corporation, or (ii) the occurrence of a transaction or event which a
    majority of directors of the Corporation in office immediately prior to
    such transaction deem to involve a change of control.

         5.2  Death

         In the event of the death of the Employee, payments set forth in
    subsection 3.1 shall be made to the executor or administrator or other
    personal representative of the decedent and shall be continued until the
    earlier of one year from the date of death of the Employee or the end of
    the Term of this Agreement.

         5.3  Other Terminations

         In the event of termination by the Corporation other than pursuant to
    subsection 5.1 or of termination by the Employee for good reason, the
    amounts payable pursuant to said subsection 3.1 for the balance of the Term
    of this Agreement shall be due and payable upon termination.

         5.4  Definition

         "Good reason", within the meaning of this Agreement, means "after a
    change of control," which, in the reasonable judgment of Employee,
    constitute a material reduction in the Employee's job title, perquisites,
    duties, authority, amenities, benefits or responsibilities; or (b) require
    the Employee regularly to perform services at a location more than
    twenty-five (25) miles distant from the present location of Corporation in
    Concord, Massachusetts, without his consent.

                                         -4-
<PAGE>

         5.5  Survival.  The provisions of Sections 8, 9 and 10 shall survive 
the termination of this Agreement.

    6.   Payments After Disability

    The Corporation shall pay to the Employee, or to his legal representative,
the amounts payable pursuant to subsection 3.1 during any period of partial or
total disability hereunder until the first to occur of (a) one year after the
date of the Employee's death, or (b) the end of the term of this Agreement.  In
the event of partial disability, Employee shall exert reasonable effort to
perform such services thereunder as his disability will permit.  The payments
made for such disability hereunder shall be reduced by the amount of any wage
continuation insurance payments payable to the Employee or his representative
during such period on the basis of any disability insurance coverage paid for by
the Corporation.  

    8.   Non-Compete.

         8.1  The Employee agrees to devote his entire business time, attention
and energies to the business and interests of the Corporation during the
Employment Period.

         8.2  (i)   During the period of employment by the Corporation and for
    a period of eighteen (18) months after any termination of employment, the
    Employee agrees that he will not, directly or indirectly, alone or as a
    partner, officer, director, consultant, employee or direct or indirect
    owner (except a less than 2% stockholder of a public company) of any
    company or business organization, engage in any business activity which is
    directly or indirectly competitive with the products or services being
    developed, manufactured or sold by the Corporation, or any subsidiary or
    affiliate of the Corporation.  The term "directly or indirectly
    competitive" shall include the performance 

                                         -5-
<PAGE>

    of any services as an employee, consultant or otherwise in connection with
    any business activity which is competitive with or adverse to the products
    and services sold by the Corporation. Such limitations shall apply only
    within the continental United States, and in such foreign jurisdictions as
    to which there is a distributor or other representative of the Corporation
    or a subsidiary, division, licensee or affiliate of the Corporation in
    existence on the date of termination. 

              (ii)  During the period of employment by the Corporation and for
    a period of eighteen months after any termination thereof, the Employee
    agrees that he will not directly or indirectly, either for himself or for
    any other person, business, partnership,  association, firm, company or
    corporation, call upon, solicit, divert or take away or attempt to solicit,
    divert or take away, any of the customers, business or prospective
    customers of the Corporation.  The Employee also agrees that during his
    employment with the Corporation and for a period of eighteen months
    thereafter, he will not solicit or discuss with any employee of the
    Corporation (a "Corporation Employee") the employment of such Corporation
    Employee by any business, firm, company, partnership, association,
    corporation or any other entity other than the Corporation.

         8.3  If any restriction set forth in this Section 8 is found by any
court of competent jurisdiction to be unenforceable because it extends for too
long a period of time or over too great a range of activities or in too broad a
geographic area, it shall be interpreted to extend only over the maximum period
of time, range of activities or geographic area as to which it may be
enforceable.

         8.4  The restrictions contained in this Section 8 are necessary for
the protection 

                                         -6-
<PAGE>

of the business and goodwill of the Corporation and are considered by the
Employee to be reasonable for such purpose.  The Employee agrees that any breach
of this Section 8 will cause the Corporation substantial and irrevocable damage
and therefore, in the event of any such breach, in addition to such other
remedies which may be available, the Corporation shall have the right to seek
specific performance and injunctive relief.

    9.   Nondisclosure.

         9.1  The Employee agrees that he will not at any time, whether during
or after the termination of his employment, reveal to any person, entity,
association or company any Proprietary or Confidential Information of the
Corporation, except as may be required in the ordinary course of performing his
duties as an employee of the Corporation (and then with due regard for continued
confidentiality) or as may be required by law, and he shall keep secret all
matters of such nature entrusted to him and shall not use or attempt to use any
such information in any manner for his own benefit or the benefit of others, or
as may injure or cause loss or damage to the Corporation.

         9.2  The Employee further agrees that during his employment he shall
not make, use, or permit to be used any notes, memoranda, drawings,
specifications, programs, data or other materials relating to any matter within
the scope of the business of the Corporation or concerning any of its dealings
or affairs otherwise than for the benefit of the Corporation.

         9.3   The Employee further agrees
              (i)   that he shall not, after the termination of his employment,
    use or permit to be used any such notes, memoranda, drawings,
    specifications, programs, data or other materials, it being agreed that any
    of the foregoing shall be and remain the sole 

                                         -7-
<PAGE>

    and exclusive property of the Corporation and that immediately upon
    termination of his employment he shall deliver all of the foregoing, and
    all copies thereof, to the Corporation;

              (ii)  that all files, letters, memoranda, reports, records,
    experimental and other data, sketches, drawings, molds, models, patterns,
    samples, laboratory notebooks, computer disks or other storage devices for
    information, any computer, program listings, or other written,
    photographic, or other tangible material containing or incorporating
    Proprietary Information, whether created by Employee or others, which shall
    come into Employee's custody or possession, shall be and are the exclusive
    property of the Corporation to be used only in the performance of
    Employee's duties for the Corporation;

              (iii) that all such records or copies thereof and all
    tangible property of the Corporation in Employee's custody or possession
    shall be delivered to the Corporation, upon the earlier of a request by the
    Corporation or any termination of  employment; and

              (iv)  that after such delivery, Employee shall not retain any such
    records or copies thereof or any such tangible property, and Employee
    agrees to destroy any Proprietary Information that may be in his possession
    but stored on media (such as computer disks) that themselves are not
    property of the Corporation.

         9.4  Employee agrees that his obligation not to disclose or to use
information and records of the types set forth above (including knowhow), and
his obligation to return records, copies and tangible property set forth in
paragraph (b) above, also extends to such knowhow and other information,
records, copies and tangible property of customers of the 

                                         -8-
<PAGE>

Corporation or suppliers to the Corporation or other third parties who may have
disclosed or entrusted the same to the Corporation or to Employee in the course
of the Corporation's business.

         9.5  Proprietary or Confidential Information of the Corporation
includes any process, document, software, technique or other information
possessed or used by the Corporation and not generally known including, without
limiting the foregoing, trade secrets, processes, techniques, data, software,
code, algorithms, marketing plans, unpublished financial statements or
projections, budgets, sales records, licenses, prices, costs, and information as
to existing and prospective customers, employees and suppliers.

    10.  Assignment of Inventions.  The Employee agrees as follows:

         10.1 To disclose to the Corporation all inventions, patentable or
unpatentable ideas, and all technical or business innovations developed,
conceived, prepared or created by him solely or in combination with others
during the period of his employment ("Inventions");

         10.2 To assign, and Employee hereby does assign, to the Corporation,
as its exclusive property, all Inventions (i) that are in any way within the
scope of or related to the business of the Corporation, or as to which the
Employee may receive information due to his employment, or (ii) that result from
or are suggested by any work which the Employee may do for the Corporation, or
(iii) that otherwise are made through the use of Corporation time, facilities or
materials;

         10.3 That any such Invention so required to be assigned shall be
considered a work made for hire for the Corporation as such term is defined in
Section 101 of the United States Copyright Act of 1976, as amended;


                                         -9-
<PAGE>

         10.4 To execute all necessary documents and otherwise provide proper
assistance during and subsequent to his employment to enable the Corporation to
obtain for itself or its nominees, copyrights, patents or other legal protection
for such Inventions or innovations;

         10.5 To make and maintain for the Corporation adequate and current
written records of all such Inventions or innovations, which records shall at
all times remain on the Corporation's premises.

         10.6 Employee hereby represents that, except as he has heretofore
disclosed in writing to the Corporation, he is not bound by the terms of any
agreement with any previous employer or other party to refrain from using or
disclosing any trade secret or Confidential or Proprietary Information in the
course of his employment with the Corporation, or to refrain from competing,
directly or indirectly, with the business of any such previous employer or any
other party.  Employee further represents that his performance of all the terms
of this Agreement and as an employee of the Corporation does not and will not
breach any agreement to keep in confidence Proprietary Information, knowledge or
data acquired by him in confidence or in trust prior to his employment with the
Corporation; and Employee will not disclose to the Corporation or induce the
Corporation to use any Confidential or Proprietary Information or material
belonging to any previous employer or others.

    11.  Remedies upon Breach.  The Employee agrees that any breach of the
provisions of Sections 8, 9 and 10 of this Agreement by him could cause
irreparable damage, and that, in the event of such breach, the Corporation shall
have, in addition to any and all remedies of law, the right to an injunction,
specific performance or other equitable relief to prevent the violation of the
Employee's obligations hereunder.

                                         -10-
<PAGE>

    12.  Term of Agreement

    This Agreement shall continue in force for an initial period of three (3)
years from the date hereof, unless terminated by either party in accordance
herewith.  Annually, the Board of Directors, in its discretion, may extend the
term of this Agreement for an additional year.  Reference herein to the Term of
this Agreement shall include the initial period and any period thereafter for
which this Agreement continues in effect pursuant to the provisions of this
Article 12.

    13.  Miscellaneous

         13.1 Binding Nature of Agreement

         This Agreement shall be binding upon the parties hereto, and their
    heirs, legal representatives, successors and assigns.  The Corporation
    shall require any successor to a majority of its business activities to
    assume the obligations of the Corporation herein, however, the Corporation
    shall continue to be liable to Employee for all payments due pursuant to
    this Agreement.

         13.2 No Assignment

         The parties agree that the nature of the Employee's services are
    personal, and that the Employee may not assign any of his rights and duties
    hereunder.

         13.3 Notices

         All notices required under this Agreement shall be sufficient if made
    by certified or registered mail, return receipt requested, delivered to the
    then residence of the Employee and to the Corporation at its then principal
    office.

                                         -11-
<PAGE>


         13.4 Amendments and Waivers

         This Agreement represents the exclusive statement of the entire
    agreement between the parties concerning the subject matter hereof and
    supersedes all prior written agreements, but not including any stock
    purchase or stock option agreement between Corporation and Employee.  This
    Agreement may not be amended, modified or revoked in whole or in part
    except by written agreement of the parties hereto.  Any waiver of any
    provision of this Agreement, to be enforceable by the non-waiving party,
    must be in writing and signed by the party to be charged therewith; and a
    waiver of any such occasion shall not be construed as a bar to or waiver of
    any such right or remedy on any future occasion, unless the waiver
    specifically provides otherwise.

         13.5 Disputes

         Any controversy or claim arising out of or relating to this Agreement,
    or the breach thereof, shall be settled by arbitration in the City of
    Boston in accordance with the rules then in effect of the American
    Arbitration Association, and judgment upon the award rendered by the
    arbitrator may be entered in any court having jurisdiction thereof.

         13.6 Counterparts

         This Agreement may be executed in several counterparts, and all so
    executed shall constitute one agreement, binding upon each party,
    notwithstanding that either party did not sign the same counterpart as the
    other party.

         13.7 Construction

         The headings and subheadings of this Agreement have been inserted for
    convenience only and are to be ignored in any construction of the
    provisions hereof.  The 

                                         -12-
<PAGE>

    use of the singular shall be deemed to include the plural and vice versa,
    and the use of the masculine and neuter gender shall be deemed to include
    the feminine, masculine and neuter gender unless the context otherwise
    requires.  No conclusion may be drawn from any difference between this
    Agreement as finally signed and any prior agreement or draft of all or any
    part of any prior agreement.

         13.8 Law Governing

         This Agreement shall be governed by and construed in accordance with
    the laws of the Commonwealth of Massachusetts.

         IN WITNESS WHEREOF, the Employee has signed and sealed this Agreement
and the Corporation has caused this Agreement to be executed and its corporate
seal to be affixed hereto effective the day and year first written above.

                                       STARMET CORPORATION

Attest: /s/ Thomas A. Wooters          By:  /s/ Robert E. Quinn
                                            ----------------------------
                                       Date: 10/1/97
                                            ----------------------------



{Corporate Seal}



Attest: /s/ Thomas A. Wooters                 /s/ William T. Nachtrab
        --------------------------          ----------------------------
                                               (employee)

                                       Date: 10/1/97
                                             ----------------------------

                                         -13-