<PAGE>
 
                                                                    EXHIBIT 10.6


                             SHAREHOLDERS AGREEMENT


SHAREHOLDERS AGREEMENT, dated December 18, 1993 ("Agreement"), by and among:

(1)  Morrison Knudsen BV, a company incorporated in the Netherlands, whose
     registered office is at Amsterdam ("MK");

(2)  LAMBIQUE BEHEER BV, a company incorporated in the Netherlands, whose
     registered office is at Amsterdam ("NRG"); and

(3)  ERGON OVERSEAS HOLDINGS LIMITED, a company incorporated in England, whose
     registered office is at London ("PowerGen").

RECITALS

A.   MK, NRG and PowerGen each own one third (1/3) of all the issued shares of
MIBRAG BV, a company incorporated in the Netherlands (herein referred to as
either "MIBRAG BV" or the "Company"), which has agreed, pursuant to the Sale and
Purchase Agreement of even date herewith (Urkundennummer ______/1993 of the
notary Axel Rodert, Cologne) among the Company, the Treuhandanstalt ("THA"),
Mitteldeutsche Braunkohlengesellschaft mbH, a company incorporated under the
laws of the Federal Republic of Germany, whose registered office will be in
Bitterfeld ("MIBRAG GmbH"), and Mitteldeutsche Bergbau-Verwaltungsgesellschaft
mbH ("MBV") to acquire 99% of the shares of MIBRAG GmbH.

B.   MK, NRG and PowerGen are entering into this Agreement in order to set out,
among other things, the terms governing their relationship as shareholders in
the Company, as well as the Company's exercise of its rights as a shareholder to
MIBRAG GmbH.

IT IS AGREED, therefore, as follows:
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INTERPRETATION

DEFINITIONS

1.1  In this Agreement, the following expressions shall (unless the context
requires otherwise) have the following meanings:

Articles of Association:  the articles of association of the Company;

Audited Accounts:  the audited consolidated balance sheet and consolidated
profit and loss account in respect of a particular Fiscal Year;

Aufsichtsrat:  the Aufsichtsrat (or supervisory board) of MIBRAG GmbH;

Budget:  the capital and operating budget for the MIBRAG Group and each of its
undertakings for a particular Fiscal Year as determined by the Geschaftsfuhrung
and agreed hereunder;

Business Day:  a day (other than a Saturday) on which banks generally are open
in Amsterdam, Berlin, London and New York for a full range of business;

Business Plan:  the rolling business plan for the MIBRAG Group for a particular
Fiscal Year and the four (4) succeeding Fiscal Years as determined from time to
time by the Geschaftsfuhrung of MIBRAG GmbH and agreed hereunder;

Business:  the business to be carried on by the MIBRAG Group, as defined in
clause 4.1;

Company:  MIBRAG BV and its successors and assigns;

Event of Default:  the events set forth in clause 8.1;
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Fiscal Year:  the financial period of the Company commencing (other than in the
case of the initial financial period) on January 1 and ending on December 31;

Geschaftsfuhrer:  The Geschaftsfuhrer/managing director of MIBRAG GmbH;

Geschaftsfuhrung:  the Geschaftsfuhrung (or management board) of MIBRAG GmbH;

Group:  at the relevant time in relation to a Party, that Party and its
Subsidiaries, any undertaking (Holding Undertaking) of which that Party is a
Subsidiary (whether directly or indirectly) and any undertaking which is a
Subsidiary of such Holding Undertaking;

Joint Aufsichtsrat Members:  the members of the Aufsichtsrat designated jointly
by the Parties;

Lippendorf Coal Contract(s):  the two coal supply agreements, dated March 8,
1993, having identical terms, one among MIBRAG AG, Bayernwerk AG,
Energieversorgung Schwaben AG and Badenwerk AG and the second one between MIBRAG
AG and Vereinigte Energiewerke AG;

MIBRAG AG:  MIBRAG Vereinigte Mitteldeutsche Braunkohlenwerke
Aktiengesellschaft, a stock corporation incorporated under the laws of the
Federal Republic of Germany;

MIBRAG Group:  The Company and any undertakings in which the Company has an
interest after the Spaltungsplan has become effective;

Mining Services Agreement:  the Agreement for the Provision of Mining
Consultancy Services to be executed between MIBRAG GmbH and MK GmbH for certain
services with respect to the operation and maintenance of the mining business of
the MIBRAG Group;
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MK Aufsichtsrat Member:  The Aufsichtsrat Member designed by MK;

MK GmbH:  Morrison Knudsen Deutschland GmbH, a company incorporated under the
laws of the Federal Republic of Germany, whose registered office is in
Frankfurt, Germany;

NRG Aufsichtsrat Member:  The Aufsichtsrat Member designated by NRG;

Parties:  Morrison Knudsen BV, Lambique Beheer BV and Ergon Overseas Holdings
Limited (and Party shall be construed accordingly);

Power Services Agreement:  the Agreement for the Provision of Consultancy
Services to be executed between MIBRAG GmbH and SES for certain services with
respect to the operation and maintenance of the power stations of the MIBRAG
Group;

PowerGen Aufsichtsrat Member:  The Aufsichtsrat Member designated by PowerGen;

Sale and Purchase Agreement:  the agreement ("Kaufvertrag") of even date
herewith (Urkundennummer _____/1993 of the notary Axel Rodert, Cologne) among
the Company, THA, MIBRAG GmbH and MBV relating to the acquisition by the Company
of 99 % of the shares in MIBRAG GmbH and certain mining proprietary rights;

Satzung:  the Satzung (or constitutional documents) of MIBRAG GmbH;

SES:  Saale Energie GmbH Services i.G., a company incorporated under the laws of
the Federal Republic of Germany, whose registered office will be in Schkopau, an
indirect joint undertaking of the NRG Group and the PowerGen Group;
<PAGE>
 
Shareholders:  such members of the MK Group, the NRG Group and the PowerGen
Group as are at the relevant time holders of shares of the Company (and
Shareholder shall be construed accordingly);

Shareholder Aufsichtsrat Members:  the members of the Aufsichtsrat nominated by
the Shareholders, namely the MK Aufsichtsrat Member, the NRG Aufsichtsrat
Member, the PowerGen Aufsichtsrat Member and the Joint Aufsichtsrat Members;

Spaltungsplan:  as such term is defined in the Sale and Purchase Agreement;

Subsidiary:  in relation to a Party, any other undertaking in which the Party
(or persons acting on its or their behalf) for the time being directly or
indirectly holds or controls either:
(a)  a majority of the voting rights exercisable at general meetings of the
     members of that undertaking on all, or substantially all, matters; or
(b)  the right to appoint or remove directors having a majority of the voting
     rights exercisable at meetings of the board of directors of that
     undertaking on all, or substantially all, matters,

and any undertaking which is a Subsidiary of another undertaking shall also be a
Subsidiary of any further undertaking of which that other is a Subsidiary;

undertaking:  a body corporate or partnership or an unincorporated association
carrying on trade or a business with or without a view to profit;

Transition Agreement:  the Transition Agreement dated December 18, 1993 between
the Company and THA relating to the period from the execution of the Sale and
Purchase Agreement to transfer of title to 99 % of the shares in MIBRAG GmbH to
the Company;

Vorsitzender:  The Vorsitzender (or Chairman) of the Aufsichtsrat.
<PAGE>
 
INTERPRETATION

1.2  For the purposes of this Agreement:
(a)  clause and paragraph headings in this Agreement and the Appendices are
     inserted for convenience only and shall not affect the construction of this
     Agreement;

(b)  any reference to an amount in Dutch Guilders or Deutsche Marks shall
     include an equivalent amount at the relevant time in any other currency or
     combination of currencies;

(c)  any reference to an agreed draft is to the form of the relevant document as
     agreed between the Parties and signed on their behalf for the purpose of
     identification (with such amendments, if any, as may subsequently be agreed
     in writing between the Parties).


EXERCISE OF POWERS OF CONTROL

1.3  Where any obligation in this Agreement is expressed to be undertaking or
assumed by any Party, such obligation shall be construed as requiring that Party
to exercise all rights and powers of control over the affairs of any other
person which that Party is able to exercise (whether directly or indirectly) in
order to secure performance of such obligation.

1.4  Each Party undertakes to exercise its rights and powers as a shareholder
and member of the management board of the Company, in furtherance of the
Company's objectives in accordance with the Articles of Association and the
provisions of Dutch law, to cause the Company to exercise all its rights and
powers available under the Satzung and the laws of the Federal Republic of
Germany (with respect to MIBRAG GmbH) to procure that to the greatest extent
possible effect is given to the matters agreed herein, in the Sale and Purchase
Agreement, the Transition Agreement, the Mining Services Agreement and the Power
Services Agreement.
<PAGE>
 
SCOPE OF BUSINESS AND MANAGEMENT OF THE COMPANY

BUSINESS OF THE COMPANY

2.1  Unless Unanimously agreed otherwise by the Parties, the sold business of
the Company shall be the holding of the shares in MIBRAG GmbH (and the exercise
of the related shareholder rights), the exercise of any rights or claims under
the Sale and Purchase Agreement accruing to the Company as the purchase
thereunder and the fulfillment of the Company's commitments and obligations
thereunder.  The acquisition or disposition by the Company of any shares of, or
any other participation in, any undertakings shall require the unanimous consent
of the Parties.

CAPITALIZATION OF THE COMPANY

2.2  On or prior to the Legal Transfer Date (as defined in the Sale and Purchase
Agreement), each of the Parties shall contribute a minimum of DM 16, 700,000 to
the stated capital of the Company.

2.3  In addition, each Party shall ensure that, on or prior to the Legal
Transfer Date, a letter of credit or other guarantee in the amount of at least
DM 16,700,000 will be provided in favor of the Company to meet the requirements
under Article 30.1 of the Sale and Purchase Agreement.  Commencing January 31,
1997, the amount of each of these guarantees shall be increased to DM 26,700,000
(Article 10.6 of the Sale and Purchase Agreement).

2.4  Each Party shall severally maintain the stated capital, guarantees and
other funding of the Company as required under the terms of the Sale and
Purchase Agreement in proportion to its shareholdings in the Company.
<PAGE>
 
SCHKOPAU INDEMNIFICATION

2.5  NRG and PowerGen shall severally indemnify the Company for any deduction
made from the full (100%) compensation payable by Treuhandanstalt pursuant to
clause 16.2.2 of the Sale and Purchase Agreement.

INDEMNIFICATION FOR REGULATORY COMPLIANCE

2.6  NRG shall indemnify MIBRAG GmbH for any and all costs and expenses which
MIBRAG GmbH may incur solely to comply with the provisions of the Public Utility
Holding Company Act (PUHCA).

MANAGEMENT OF THE COMPANY

2.7  The Company will be managed by a management board, which shall consist of
all of the Parties.  The rights and obligations of the Parties as shareholders
and members of the management board of the Company are set forth in the Articles
of Association and in this Agreement.

2.8  The Parties agree that the Company shall not exercise any of its rights nor
take any other action regarding MIBRAG GmbH (including the exercise of its
voting rights or other rights, including those rights exercised through the
Shareholder Aufsichtsrat Members) unless such action shall have previously been
approved by the management board of the Company.

MEETINGS OF THE MANAGEMENT BOARD

2.9  The Parties shall appoint a Chairman and a Vice Chairman of the management
board.  The Vice Chairman shall replace the Chairman in the event the Chairman
is prevented from performing his duties. The appointment of Chairman and Vice
Chairman shall be for a term of one year unless otherwise revoked by a decision
of the management board.
<PAGE>
 
2.10  Meetings of the management board shall be called by the Chairman of the
management board by giving seven days written notice (unless an immediate action
of the management board is required, in which case a meeting shall be called
upon 48 hours written notice setting forth the matter of urgency) to all Parties
and shall be held at the registered office of the Company or at such other place
outside of the Federal Republic of Germany as may be agreed between the Parties.
Upon request of a Party, the Chairman is obliged to call a meeting of the
management board.

2.11  The management board cannot take any decisions or other actions unless all
the Parties are represented, whether in person, by telephone or by proxy.  The
Chairman shall ascertain whether a quorum is present.  In the event that no
quorum is present, the Chairman shall call a new meeting concerning the same
agenda within 48 hours with a notice period of at least four days to and
exclusive of the new date of the meeting.  The meeting of the management board
convened thereupon constitutes a quorum irrespective of the Parties represented.

2.12  The Chairman shall lead the meeting of the management board. There shall
be minutes of each meeting, which shall be kept at the registered office of the
Company.  The minutes shall be signed by the Chairman and shall be immediately
passed on to the other members of the management board for approval.



<PAGE>
CHANGE IN CONTROL OVER A PARTY

OPTION TO PURCHASE
 
3.1  In the event of a change in control over any Party, each of the other
Parties shall be entitled to purchase any or all of the shares of such Party in
the Company (the "Option") proportionate to their shareholdings at a price to be
agreed or, if no agreement can be reached, at a fair market value price
established by an independent expert, who shall be designated by the appointed
auditor of the Company.

CHANGE IN CONTROL

3.2  A change in control shall be deemed to have occurred if 50% or more of any
kind of shareholder's rights with respect to any Party or any undertaking
directly or indirectly controlling the Party within the meaning of Article 24
(a) of Book 2 of the Dutch Civil Code shall transfer to a third party.

NOTIFICATION AND METHOD OF EXERCISE

3.3  Promptly, but in no case later than 10 days after any change in control,
each Party shall inform the other Parties in writing about such change in
control.  Within a period of 60 days after receipt of such written notice or the
date on which the other Parties obtain knowledge of such change in control,
whichever is later, the other Parties can exercise their option by written
notice to the relevant Party.  In the event that no all of the other Parties
exercise their option to purchase in full, all shares of the relevant Party may
be acquired proportionate to the shares held by the Parties exercising their
rights.


<PAGE>
THE COMPANY AS SHAREHOLDER OF MIBRAG GMBH

BUSINESS OF THE MIBRAG GROUP

 
4.1  The Parties agree that the business of the MIBRAG Group shall be to carry
on and develop the business of brown coal mining and electrical and thermal
energy production (as further described in the Satzung) on sound, commercial,
profit-making principles in accordance with the general principles of the
Business Plan and in accordance with the Budget.  The Business Plan and the
Budget shall be  reviewed by the  Parties at  the end of each  quarter (or such
other period as the Parties may agree) and shall be updated annually.

GOOD FAITH

4.2  Each Party shall act in good faith towards the Company, the other Parties
and MIBRAG GmbH in order to promote the success of the Company and the MIBRAG
Group, and, to the extent it is within its power, it shall procure that the
other members of its Group do likewise.

SERVICES AGREEMENTS

4.3  The Parties shall use best efforts to procure that MIBRAG GmbH will (i)
execute, as soon as practicable after the execution of this Agreement, the
Mining Services Agreement in the form attached as EXHIBIT A hereto and the Power
Services Agreement in the form attached as EXHIBIT B hereto, (ii) make such
amendments as may be required pursuant to Article 29.2.3 of the Sale and
Purchase Agreement and (iii) continue to perform its obligations under these
Services Agreements.  In the event any of these Services Agreements is invalid,
terminated (for reasons not set forth in such Services Agreement) or contested
pursuant to the Sale and Purchase Agreement, the Company shall use best efforts
to cause MIBRAG GmbH to execute as soon as possible an agreement with terms as
near as possible to those contemplated by such Services Agreement.

CAPITALIZATION OF MIBRAG GMBH

4.4  The Parties agree that the Company shall contribute to MIBRAG GmbH such
additional capital as is required under Article 29 of the Sale and Purchase
Agreement.
<PAGE>
 
ACTIONS OF THE COMPANY AS SHAREHOLDER AND PURCHASER OF MIBRAG GMBH

4.5  The following decisions and actions of the Company shall require a majority
vote of the management board unless otherwise specified herein or in the
Articles of Association:

     (a) all decisions and actions of the Company as a shareholder of MIBRAG
         GmbH; and

     (b) the taking of any action of the Company permitted or required under the
         Sale and Purchase Agreement (other than the actions set forth in clause
         4.6 hereof) after the Legal Transfer Date, including, without
         limitation, the rights and options under Articles 8, 21.6, 30.2 and
         40.3 of the Share and Purchase Agreement.

UNANIMOUS CONSENT

4.6  In addition to the decisions and actions of the Company set forth in the
Articles of Association and in clauses 2.1, 5.1, 6.1 and 6.2 of this Agreement,
the following decisions and actions shall require the prior unanimous consent of
al Parties represented on the management board:

     (a) the taking of any action of the Company permitted or required under
         the Sale and Purchase Agreement prior to the Legal Transfer Date and
         under the Transition Agreement, including the waiver of any conditions
         set forth in Articles 38 and 39 of the Sale and Purchase Agreement;
<PAGE>
 
     (b)  the exercise of the Company's option (the "Lippendorf Option") to
          require the repurchase by the THA of the Company's shares in MIBRAG
          GmbH in the event that construction of units 1 and 2 of the Lippendorf
          power plant has not begun by 1 January 1998 pursuant to Article 40.1
          of the Sale and Purchase Agreement;

     (c)  the exercise of the Company's right (the "EC Rescission Right") to
          rescind the Sale and Purchase Agreement or to exercise the right to
          waive rescission pursuant to Article 38.3 thereof;

     (d)  the taking of any action of the Company permitted or required under
          the Sale and Purchase Agreement after the Legal Transfer Date pursuant
          to Articles 1.3 (Old Wahlitz), 4.1 (future coal fields), 5.1 (future
          power plant sites) and 40.2 (MUEG) of the Sale and Purchase Agreement;

     (e)  the approval of, and any subsequent changes to, the business plan and
          the budget of the Company; and

     (f)  the approval of, and any subsequent changes to, the Business Plan and
          Budget of MIBRAG Group.

LIPPENDORF PUT

4.7  In the event that the management board does not decide that the Company
shall exercise the Lippendorf Option (within 3 months after the Lippendorf
Option has become exercisable), any Party that favored the exercise of the
Lippendorf Option, shall have 
<PAGE>
 
the right to sell its shares in the Company to the other Party or Parties who
did not favor the exercise of the Lippendorf Option at a price which equals the
amount such Party would have received had the Company exercised the Lippendorf
Option and distributed the net proceeds to the Parties.

EC PUT

4.8  In the event that the management board does not decide that the Company
shall exercise the EC Rescission Right (within 3 months after the EC Rescission
Right has become exercisable), any Party that favored the exercise of the EC
Rescission Right, shall have the right to sell its shares in the Company to the
other Party or Parties who did not favor the exercise of the EC Rescission Right
at a price which equals the amount such Party would have received had the
Company exercised the EC Rescission Right and distributed the net proceeds to
the Parties.

OTHER ACTIONS OF THE COMPANY

4.9  The Parties shall cause the Company to use its rights and powers to
procure, so far as are legally possible (including by the exercise of its voting
rights and through the Shareholder Aufsichtsrat Members), that none of the
actions specified below shall be taken (whether such action is to be taken by
the Aufsichtsrat, the Geschatsfuhrung, MIBRAG GmbH or any of the officers or
managers of MIBRAG GmbH) unless the Company shall previously have approved such
action:

     (a)  the appointment and removal of the Vorsitzender of the Aufsichtsrat;

     (b)  the appointment and removal of the independent member 

          of the Aufsichtsrat; and

     (c)  the amendment, termination or any change in the terms and conditions
          of the Mining Services Agreement or the Power Services Agreement.


<PAGE>

BUSINESS PLANS AND BUDGET

BUSINESS PLAN AND BUDGET OF THE MIBRAG GROUP

5.1  The Parties shall cause the Company to use its rights and powers to
procure, so far as legally possible, that the Geschaftsfuhrung of MIBRAG GmbH
shall submit to the Company by October 1 of each year for review, revision (if
necessary) and approval, a draft of the Business Plan and Budget for the next
Fiscal Year. The Parties shall use their best efforts to unanimously agree on
the draft Business Plan and Budget by December 15 in each year, to instruct the
Company to adopt the same at a shareholders' meeting as the Business Plan and
Budget of the Company and to procure that the MIBRAG Group shall conduct the
Business in accordance with the Business Plan and Budget. Any provisions in a
Business Plan or a Budget providing for a mandatory increase of the stated
capital of the Company or any other additional funding by the Parties shall
require the unanimous consent of all Parties as set forth in clause 6.2.

5.2  Each Party may propose at any time that the Business Plan and/or Budget be
amended (which amendment may take the form of a supplement), submit such
amendment to the Parties for approval and request that all corporate action of
the Company be initiated to implement such amendment.

5.3  If by December 15 no agreement has been reached by the Parties on the
Business Plan for the following Fiscal Year, the Parties shall cause the Company
to permit the MIBRAG Group to proceed with the matters agreed in respect of that
particular Fiscal Year in the then current Business Plan.
<PAGE>
 
5.4  If by December 31 in a relevant year no agreement has been reached on the
Budget, the Parties shall:

     (a) cause the Company to permit the MIBRAG Group to proceed with those
         matters and incur those costs and expenditures in respect of which
         agreement had been reached;

     (b) cause the Company to permit the MIBRAG Group to discharge liabilities
         and obligations the non-performance of which would render the MIBRAG
         Group or any member of it liable to court action (including, without
         limitation, the payment of any rent or any other periodic payment);

     (c) use all reasonable endeavors to reach agreement on the relevant Budget
         as soon as possible thereafter.

5.5  If it appears to any Party that an undertaking of the MIBRAG Group may
exceed its budget, or the budgeted amount for any specific item with a budgeted
value in excess of DM 1,000,000, by more that 10 %, such Party may notify the
other Parties and require that a management meeting of the Company be convened
to consider the matter.  Pending such meeting and until the budget for the
relevant item is increased (if it is increased), the Parties shall cause the
Company to use its reasonable endeavors to procure that, except for liabilities
and obligations referred to in paragraph (b) of clause 5.4, the MIBRAG Group
does not exceed the total budget or the amount provided in the budget for such
item, as the case may be, by more than 10 %.

DIVIDEND POLICY

5.6  Subject to the Articles of Association and the provisions of the Sale and
Purchase Agreement, the Parties shall, unless otherwise agreed between them
pursuant to Article 21 of the Articles of Association, cause the Company in
relation to any Fiscal Year and subject to (i) the restrictions in the Sale and
Purchase Agreement, if any, and (ii) the retention of sufficient funds to meet
the working capital needs of the Company (as determined by the Business Plan for
the relevant Fiscal Year), to take all steps to procure that there shall be
distributed in respect to each Fiscal Year all of the distributable profits of
the Company as shown by the Audited Accounts of the Company for that Fiscal
Year.
<PAGE>
 
BUSINESS PLAN, BUDGET AND DIVIDEND POLICY OF THE COMPANY

5.7  The business plan, budget and dividend policy of the Company shall be
discussed and implemented together with the Business Plan, the Budget and the
dividend policy of MIBRAG GmbH.


CAPITAL AND ADDITIONAL FUNDING

ISSUANCE OF NEW SHARES

6.1  The issued share capital of the Company may from time to time be increased
by such amount as shall be unanimously agreed between the Parties
proportionately to the number of shares they hold immediately prior to such
increase.

FUNDING SUPPORT BY THE PARTIES

6.2  It is the intention of the Parties that the Company and MIBRAG GmbH should
be self-financing and should obtain additional funds from third parties without
recourse to their respective shareholders. However, the Parties recognize the
need to support the Company in accordance with the Business Plan and Budget from
time to time and to provide additional funding as unanimously agreed by the
Parties to ensure the adequate funding of the Company and the MIBRAG Group and
to support their short and long term objectives. The Parties shall not
unreasonably withhold their consent to any additional funding required to
perform the obligations of the Company under the Sale and Purchase Agreement.
<PAGE>
 
AUFSICHTSRAT OF MIBRAG GMBH

MEMBERS

7.1  For as long as the Aufsichtsrat of MIBRAG GmbH shall comprise a maximum of
eleven (11) members, each Party shall:

     (a) be entitled to designate one (1) Member of the Aufsichtsrat: and

     (b) be entitled to designate jointly with the other Parties two (2) Members
         to the Aufsichtsrat (the Joint Aufsichtsrat Members).

ELECTION AND REMOVAL

7.2  Each of the Parties agrees to cause the Company to vote in favor of the
     election to the Aufsichtsrat of the persons designated by the Parties under
     clause 7.1 above and in favor of any person designated pursuant to clause
     7.1 to replace any such person.

7.3  Each of the Parties agrees to cause the Company to remove and replace the
     Joint Aufsichtsrat Members in accordance with the provisions of the Satzung
     and this Agreement.

7.4  The Parties shall cause the Company to procure that their respective
     Aufsichtsrat Members and the Joint Aufsichtsrat Members vote jointly in the
     election of the Vorsitzender.

DEFAULT
EVENTS OF DEFAULT

8.1  It shall be an Event of Default in relation to a Party if:
<PAGE>
 
     (a) proceedings shall have been commenced or a resolution passed to wind up
         a Party (other than for the purpose only of solvent amalgamation or
         solvent reconstruction previously approved in writing by the other
         Parties);

     (b) any step is taken and not withdrawn within 90 (ninety) days to appoint
         a manager, receiver in bankruptcy (curator), administrator,
         administrative receiver, or any similar officer in respect of any
         assets of that Party or in respect of the shares in that Party or any
         undertaking of which that Party is a Subsidiary;

     (c) any distress, execution, sequestration or other process is levied or
         enforced (and not discharged within 7 (seven) days) against any
         substantial part of the assets of that Party;

     (d) that Party shall become insolvent, enter into suspension of payments
         (surceance van betaling) or otherwise be unable to pay its debts as and
         when they fall due as defined pursuant to Dutch or English law, as it
         is applicable to that Party;

     (e) that Party shall cease or threaten to cease to carry on a substantial
         part of its business (other than for the purpose only of solvent
         amalgamation or solvent reconstruction previously approved in writing
         by the other Parties) and such cessation would adversely affect to a
         substantial degree its ability to discharge its obligations under this
         Agreement from time to time;
<PAGE>
 
     (f) that Party convenes a meeting of its creditors or makes or proposes any
         arrangement or composition with, or any assignment for the benefit of,
         its creditors;
         
     (g) the equivalent of any of (a) to (f) above occurs in respect of any
         Party under the laws of England, the Netherlands or the United States;

     (h) that Party fails to pay within 30 (thirty) Business Days of the due
         date any amount payable by it under clause 6.2 of this Agreement in the
         manner in which it is expressed to be payable in this Agreement or the
         resolution authorizing such payment; or 

     (i) that Party commits a material breach of any provision of this Agreement
         and fails to remedy the same within 30 (thirty) days of notice to do so
         being given by the other Parties (and in which the other Parties
         express their intention to exercise their rights under this clause);

and for the purposes of this clause 8, any reference to Party shall include any
member of that Party's Group.
<PAGE>
 
ACTIONS FOLLOWING AN EVENT OF DEFAULT

8.2  If an Event of Default applies in relation to a Party (the "Defaulting
Party"), then, without prejudice to any provision of Dutch law or contained in
the Articles of Association, until such Event of Default is cured or waived in
writing by the other Parties (the "Non-Defaulting Parties"):

     (a) the Non-Defaulting Parties shall be entitled by notice in writing to
         the Defaulting Party to require that any consent or approval required
         of a Party pursuant to this Agreement shall not hereafter be required
         of the Defaulting Party or any of its appointed representatives;

     (b) the Defaulting Party shall be deemed to have nominated one of the Non-
         Defaulting Parties' Aufsichtsrat Members as the alternate to the
         Defaulting Party's Aufsichtsrat Member, to attend and vote meetings of
         the Aufsichtsrat in his place (such alternate to act pursuant only to
         the joint instructions of the Non-Defaulting Parties); and

     (c) The Non-Defaulting Parties shall have the right to exercise an option
         to purchase any or all of the shares of the Defaulting Party
         proportionate to the ratio of their shareholdings at a fair market
         value price (less adjustments for any failure of the Defaulting Party
         to make payments duly authorized pursuant to clause 6.2) as established
         by an independent expert who shall be designated by the appointed
         auditor of the Company. The Non-Defaulting Parties must exercise their
         option to purchase the shares of the Defaulting Party by written notice
         to the Defaulting Party. Such notice shall include the price to be paid
         for the shares. In the event that not all of the Non-Defaulting Parties
         exercise their right to acquire in full, all shares of the Defaulting
         Party may be acquired proportionately to the ratio of the shares held
         by the Non-Defaulting Parties exercising their rights.


NON-COMPETITION

MIBRAG CUSTOMERS

9.1  Each of the Parties undertakes that, for so long as it or any member of its
Group is beneficially interested in any share in the capital of the Company and
for a period of one year thereafter, it shall procure (to the extent it is
legally able to do so) that neither it, nor any member of its Group (whether
alone or jointly with others, or whether as principal agent, shareholder or
otherwise and whether for its own benefit or that of others) entices away or
endeavors to entice away any material customer of the MIBRAG Group.
<PAGE>
 
CONFIDENTIALITY

CONFIDENCE INFORMATION

10.1  Each of the Parties undertakes that for so long as it or any member of its
Group is beneficially interested in any shares in the capital of the Company and
for a period of one year thereafter, it shall use (and to the extent it is
legally able shall ensure that each of the members of its Group shall use) all
reasonable endeavors to keep confidential (and to ensure that its and their
officers, employees, agents and professional and other advisers keep
confidential) any information:

     (a) which it may have or acquire (whether before or after the date of this
         Agreement) in relation to the customers, business, assets or affairs of
         any member of the MIBRAG Group (including, without limitation, any
         information provided pursuant to the Business Plan and Budget); or

     (b) which, in consequence of the negotiations relating to this Agreement or
         of being a shareholder of the Company or having appointees on the
         Aufsichtsrat of MIBRAG GmbH or the exercise of its rights or
         performance of its obligations hereunder, it may have or acquire
         (whether before or after the date of this Agreement) in relations to
         the customers, business, assets or affairs of another Party or any
         member of another Party's Group; or

     (c) which relates to the contents of this Agreement, the Mining Services
         Agreement, the Power Services Agreement, or any agreement or
         arrangement entered into pursuant to this Agreement.

None of the Parties shall use for its own business purposes or disclose to any
third party any such information (collectively, Confidential Information)
without the consent of the other Parties.
<PAGE>
 
EXCEPTIONS FROM CONFIDENTIALITY OBLIGATION

10.2  The obligation under clause 10.1 shall not apply to:

     (a) the disclosure of information on a "need to know" basis to a company
         which is another member of a Party's Group where such disclosure is for
         a purpose reasonably incidental to this Agreement;

     (b) information which is independently developed by the relevant Party or
         acquired from a third party to the extent that it is acquired with the
         right to disclose the same;

     (c) the disclosure of information to the extent required to be disclosed by
         law, any stock exchange regulation or any binding judgement, order or
         requirement of any court or other competent authority;

     (d) the disclosure of information to any tax authority to the extent
         required by law for the purposes of the tax affairs of the Party
         concerned or any member of its Group;

     (e) the disclosure in confidence to a Party's professional advisers;
 
     (f) information which is or becomes within the public domain (otherwise
         than as a result of a breach of this clause 10); or

     (g) any announcement made in accordance with the terms of clause 18.

10.3  For the purposes of this clause 10, any reference to Party shall include
any member of that Party's Group.
<PAGE>
 
EFFECTIVE DATE AND TERM

11.1  This Agreement shall have effect from the date of execution and shall
continue until (i) all but one (1) of the Parties have transferred all their
shares in the Company outside their Group or (ii) the Company has sold all its
shares in MIBRAG GmbH, whichever event occurs first.

11.2  The termination of this Agreement for any reason shall not:

     (a) relieve any Party from any liability or obligation in respect of any
         matters, undertaking or conditions which have not been done, observed
         or performed by that Party prior to such determination; or

     (b) affect the terms of any Agreement entered into between the Parties, or
         any successor of them holding shares of the Company, to replace this
         Agreement; or

     (c) affect the terms of clause 10 of this Agreement.

NON-ASSIGNMENT

12.1  Each of the Parties may assign any or all of its shares in the Company in
accordance with the provisions set forth in the Articles of Association and in
this Agreement.  A Party desiring to transfer its shares in the Company
undertakes to procure that any new shareholder shall agree in writing to be
bound by the terms of this Agreement before such transfer becomes effective.

12.2  The Parties agree that, in the event a Party desires to transfer its
shares in the Company and its rights hereunder within its group, the other
Parties shall not exercise any right of first refusal under the Articles of
Association or this Agreement.
<PAGE>
 
WAIVER OF RIGHTS

13.  No waiver by a Party of a failure by another Party to perform any provision
of this Agreement shall operate or be construed as a waiver in respect of any
other failure whether of a like or different character.

AMENDMENTS

14.  This Agreement may be amended only by an instrument in writing signed by
duly authorized representatives of each of the Parties.

INVALIDITY

15.  If any of the provisions of this Agreement is or becomes invalid, illegal
or unenforceable, (a) the validity, legality or enforceability of the remaining
provisions of this Agreement shall not in any way be affected or impaired, and
(b) the Parties shall negotiate in good faith in order to agree on terms of a
mutually satisfactory provision, achieving as closely as possible the same
commercial effect, to be substituted for the provision found to be void, illegal
or unenforceable.

NO PARTNERSHIP OR AGENCY

16.1  Nothing in this Agreement (or any of the arrangements contemplated hereby)
shall be deemed to constitute a partnership between the Parties for any matters
other than those set forth in this Agreement, no, save as may be expressly set
out herein, constitute any Party the agent of any other Party or Parties for any
purpose.
<PAGE>
 
16.2  In addition, unless otherwise agreed in writing between the Parties, none
of them shall enter into any contracts with third parties as agent for any
member of the MIBRAG Group or for any other Party no shall any Party describe
itself as such an agent or in any way hold itself out as being such an agent.

ARTICLES OF ASSOCIATION OF THE COMPANY AND OF MIBRAG GMBH

17.1  The Parties agree that as soon as practicable after the date of execution
of this Agreement, they will procure that (i) the Articles of Association of the
Company are amended in accordance with EXHIBIT C to this Agreement, and (ii) the
articles of association (Gesellschaftsvertrag) or MIBRAG GmbH are amended to
read substantially as set forth in EXHIBIT D to this Agreement.

17.2  In the event of any conflict between the provision of this Agreement and
the Articles of Association of the Company, the provision of the Articles of
Association shall prevail.  The Parties shall exercise all voting and other
rights and powers available to them so as to give effect to the provisions of
this Agreement and shall procure to the extent within their power that any
amendment is made to the Articles of Association of the Company as may be
necessary for this purpose.

ANNOUNCEMENTS

18.  No announcement in connection with the subject matter of this Agreement
shall be made or issued by or on behalf of nay Party (or any member of its
Group) without the prior written approval of all other Parties (such approval
not to be unreasonably withheld or delayed) except for any announcement required
by law, any stock exchange regulation or by any binding judgment, order or
requirement of any court or other competent authority.  The relevant Party
shall, to the extent reasonable practicable, give the other Parties an
opportunity to comment on any such announcement before it is made or issued.
<PAGE>
 
COSTS

19.  Each of the Parties shall pay its own costs, charges and expenses incurred
in connection with the preparation and implementation of this Agreement and the
transactions contemplated by it.

ENTIRE AGREEMENT

20.1  This Agreement together with the Articles of Association, the Sale and
Purchase Agreement (including exhibits), the Transition Agreement, the Mining
Services Agreement and the Power Services Agreement constitute the entire
agreement and understanding of the Parties with respect to the subject matter
hereof and none of the Parties has entered into this Agreement in reliance upon
any representation, warranty or undertaking by or on behalf of any Party which
is not expressly set out herein.

20.2  Without prejudice to the generality of clause 20.1, the Parties agree that
this Agreement superseded any or all prior agreements, understandings,
arrangements, promises, representations, warranties and/or contracts of any form
or nature whatsoever (whether oral or in writing and whether explicit or
implicit) which may have been entered into prior to the date hereof between the
Parties or on their behalf as to the subject matter of this Agreement including,
without limitation, the provisions of the Memorandum of Understanding dated 7/21
October 1992 between NRG Energy, Inc. and PowerGen plc., and the letter from
Morrison Knudsen Corporation to NRG Energy, Inc. of 4 June 1993 as amended on 11
June 1993, provided, however, that the execution of this Agreement shall not
           --------                                                         
relieve any Party from any claims of another Party existing at the time of the
execution.
<PAGE>
 
NOTICES

ADDRESS FOR NOTICES

21.1  Any notice or other communication to be given under this Agreement shall
be sent by telefax or courier mail as follows:

          if to MK:           Morrison Knudsen B.V.
                              Weena 70
                              3012CM Rotterdam
                              Attention:
                              Robert A. Tinstman

          if to NRG:          Lambique Beheer B.V.
                              Vijzelstreet 32
                              1017 H1 Amsterdam
                              Attention:
                              David H. Peterson

          if to Powergen:     Ergon Overseas Holdings Limited
                              53 New Broad Street
                              London EC2M 1JJ
                              Attention:
                              David J. Jackson

CHANGES

21.2  A Party may change the address, fax number of the name of the person for
whose attention notices are to be addressed by serving a notice on the other in
accordance with the clause 21.

DEEMED SERVICE

21.3  All notices given in accordance with clause 21.1 shall be deemed to have
been served as follows:
<PAGE>
 
     (a) if delivered by courier, at the time of receiving confirmation of
         delivery by the courier; and

     (b) if communicated by telefax, at the time of receiving confirmation by
         the recipient.

SETTLEMENT OF DISPUTES

LEGAL DISPUTES

22.1  If any dispute arises between the Parties in connection with this
Agreement or any associated agreement entered into pursuant to this Agreement (a
Dispute), they shall use all reasonable endeavors to resolve the matter on an
amicable basis.  If one Party serves formal written notice on the other that a
material Dispute of such a description has arisen and the Parties are unable to
resolve the Dispute within a period of thirty (30) days from the service of such
notice, then the Dispute shall be referred to the respective senior officers of
the Group of each Party.  No recourse to arbitration under clause 22.2 by one
Party against the other under this Agreement shall take place unless and until
such procedure has been followed.

ARBITRATION

22.2  If the senior officers of the Group of each Party shall have been unable
to resolve a Dispute within 30 days after it has been referred to them under
clause 22.1, such Dispute shall be referred to and finally settled by
arbitration under and in accordance with the Rules of Conciliation and
Arbitration of the International Chamber of Commerce by three arbitrators
appointed in accordance with those Rules.  The place of arbitration shall be
Amsterdam. The language of the arbitration proceedings shall be English.
Reasonable expenses of the arbitration shall be payable by the Parties in such
proportions as shall be determined by the arbitrators.
<PAGE>
 
GOVERNING LAW

23.  This Agreement shall be governed by and construed in accordance with the
laws of the Netherlands.


AS WITNESS this Agreement has been signed by the duly authorized representatives
of the Parties on the day and year first before written.

MORRISON KNUDSEN BV                 ERGON OVERSEAS HOLDINGS LIMITED

     
By:  /s/ Terry K. Eller          By:     /s/ Roy Adair
   ----------------------           ------------------------
     Terry K. Eller                      Roy Adair
     (under a power-of-attorney          (under a power-of attorney
     attached as Exhibit E)              attached as Exhibit F)

LAMBIQUE BEHEER BV

By:  /s/ Carl A. Carreca
   ----------------------
     Carl A. Carreca,
     Managing Director
     (bestuurder)
<PAGE>
 
                             SHAREHOLDERS AGREEMENT
                                    EXHIBITS

     [The Registrant agrees to provide the Securities and Exchange Commission,
     upon request, with copies of the Exhibits hereto.]