MACCS

Dear MACCS Code Users and Colleagues,

Please join us for the 2026 International MACCS User Group (IMUG) Meeting from June 2 to 5, 2026. Registration is in the link below.

This year, 2026 IMUG will be held concurrently with the "Cooperative Severe Accident Research Program (CSARP)" and "MELCOR Code Assessment (MCAP)" meetings. International MACCS users from CSARP member organizations are welcome to also attend CSARP/MCAP, which has a separate registration.

This year’s IMUG meeting and workshop will be in person at the "Residence Inn by Marriott Bethesda Downtown" in Bethesda, Maryland, with an option for virtual attendance. Links to the Microsoft Teams meetings will be sent at a later date prior to the beginning of the meetings. The current dates are as follows:

• Tue., June 2: IMUG Session 1
• Wed., June 3: IMUG Session 2
• Thu., June 4: MACCS Workshop 1
• Fri., June 5: MACCS Workshop 2

The first two IMUG Sessions will start at 8:30 AM US EDT (UTC-4). The Workshops will start at 9:00 AM US EDT. A preliminary agenda for the meeting is forthcoming.

Workshop

After the IMUG meeting is complete, we will hold a MACCS Workshop. This will include a mixture of code demonstrations (including MACCS-UI), instructor-led presentations, and breakout periods for in-room exercises.

• In-person participants: You should bring your laptop with MACCS-UI installed if you wish to participate in the in-room exercises.
• Virtual attendees: You can benefit from the code demonstrations and the instructor-led presentations during those portions of the workshop. Sadly, we will not be able to give you individual attention during the in-room exercises. (Users can always submit their questions to maccs-questions@sandia.gov.)

What are CSARP, MCAP, and IMUG?

• CSARP is an international program on severe accident phenomenological research and code development activities organized by the NRC. The MACCS code is part of the CSARP code-sharing program, and our international MACCS users are members of CSARP.
• MCAP is a MELCOR user group meeting. The meeting provides forum for exchanging information and research experience among users of the MELCOR code. Many of our MACCS users are also MELCOR users.
• IMUG is a MACCS user group meeting. The meeting provides forum for exchanging information and research experience among users of the MACCS code and others interested in radiological consequence analysis.

The IMUG meetings are open to all MACCS users and colleagues and will begin Tuesday, June 2, 2026. IMUG will provide a MACCS overview and recent updates, presentations from the MACCS community, and code demonstrations with the most recently released version of MACCS.

Before IMUG, the CSARP meeting will begin on Monday, June 1, 2026, with a day of CSARP presentations discussing the status of severe accident research programs from around the world.

Virtual or In-Person?

We encourage all users to come in-person, subject to space limitations. The meetings will have an internet broadcast to allow virtual participants to attend using the Microsoft Teams platform.

Registration

The IMUG meetings and the CSARP/MCAP meetings have separate registration forms. All IMUG participants should register using the IMUG form.

• 2026 IMUG Registration Form

CSARP/MCAP registration is limited to individuals from CSARP member organizations. If you are an individual from a CSARP member organization, you should reach out to your CSARP representative to inquire how you can register for CSARP/MCAP. Alternatively, you can reach out to Shawn.Campbell@nrc.gov for more information.

Software Availability

The MACCS Code Suite is available to registered MACCS users on the MACCS download site:

• Existing code users can access their approved codes straight away. (Sandia may need to manually enable your downloads if you are logging into your account for the first time.)
• New code users should create a web account and follow the instructions.

MACCS-UI v5.2 is planned for release on or about May 22, 2026 (the week before IMUG). If you are not already registered as a MACCS user, please do so at your earliest convenience to ensure access to the code in time for IMUG. Registration for new international code users can take several weeks to complete.

Lodging

If you would like a room at the Residence Inn Bethesda Hotel, a block of rooms at a special conference rate ($227/night plus applicable taxes) has been set aside. Reservations can be made by calling 1-301-718-0200 (hotel directly) or 1-800-228-9290 (toll-free) using a credit card. Guests should identify themselves as part of the CSARP Meeting Room Block. Reservations may be canceled without penalty up to 48 hours before the first night of stay. If you have any issues or questions regarding reserving a hotel room, please contact Sonya Mahler at smahler@sandia.gov.

To make reservations online please go to: Residence Inn by Marriott Bethesda Downtown.

We greatly look forward to your participation!