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A team of electric utility representatives (Utility Review
Team) conducted an in-depth independent evaluation of the
Modular High Temperature Gas Reactor (MHTGR) design. The
emphasis of the review was on the fuel design with respect
to safety objectives, the licensability of the proposed
containment concept, refueling operations and equipment,
spent fuel storage capacity, staffing projections, and the
economic competitiveness compared to contemporaneous
designs. This review, at the request of Electric Power
Research Institute (EPRI), addressed the main issues raised
in an earlier report (August, 1989) titled Utility Industry
Evaluation of the Modular High Temperature Gas Cooled
Reactor (EPRI Report NP 6676). In each of the assigned
topics, the Utility Review Team considered the MHTGR's
strengths and weaknesses, the prospects for successful
licensing, the prospects for utility acceptance and
deployment, and the current research and development
programs in support of MHTGR design and licensing
activities. This section provides a summary of the main
conclusions and recommendations that are explained in more
detail in the subsequent sections.

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The unique MHTGR radionuclide control concept takes full
advantage of many inherent, passive safety features of the
MHTGR fuel and core design. A very large fraction of
fission products are retained by several barrier mechanisms.
The primary barrier to fission product release is the fuel
particle with its thermally resistant and impermeable
pyrolytic carbon and silicon carbide coatings. A
non-defective fuel particle will contain nearly all of the
fission products where they are generated. The second
barrier consists of the fuel matrix and the graphite
moderator blocks through which the fission products must
migrate. The third barrier is the closed primary coolant
system boundary consisting of the reactor vessel, crossduct
and steam generator. The fourth barrier is the reactor
building. Although this is a low-pressure, non-leak tight
structure, some credit is taken for radionuclide holdup and
plateout for certain postulated accidents to meet the
top-level requirements summarized in Table 2.2.1. The MHTGR
Participants postulate that a very small fraction of fission
products will be released into the primary coolant system
during normal operation provided the fuel temperatures are
maintained below 1330 °C, and therefore, it is likely that
the primary coolant system will have minimal contamination

The pure helium coolant of the primary system is inert so that no chemical reaction with the fuel or coatings is possible; provided air, steam, and other impurities are monitored and maintained to very small quantities to prevent their chemical attack on the fuel and coatings. Pure helium does not produce any significant reactivity effects. Flashing or boiling of the pure helium coolant is not possible. Coolant level measurements are not required. The annular core configuration and other core physics characteristics provide a negative temperature coefficient under all operating conditions. In addition, the annular core combined with the low core power density and the uninsulated steel vessel, allow for passive decay heat removal by conduction, convection, and radiation through the Reactor Cavity Cooling System to the ultimate heat sink during limiting events.

In summary, the above described passive safety features are intrinsic to the MHTGR design. They require neither operator action nor reliance on backup AC power to accomplish the safety functions. The same features offer the prospects of simplified plant operations, and a slow, stable response for the reactor to mi smatches in heat generation and removal. The lack of dependence on active safety equipment together with fewer safety related components offers the potential for easier satisfaction of regulatory operational and maintenance requirements compared to today's Light Water Reactor (LWR) plants. In addition, the MHTGR design is radiologically cleaner with the potential for lower plant contamination levels, shorter outages due to reduced radiation levels, easier plant maintenance, lower personnel exposures, lower radwaste volumes to be disposed, and ultimately, an easier decommissioning. These features are considered to be very valuable and significant advantages from a utility perspective.

The MHTGR safety assessments conducted to date have evaluated a broad spectrum of events ranging from anticipated operational occurrences to postulated low probability accidents. The objective in each evaluation was to show that any release of radionuclides is controlled within the top level regulatory offsite dose limits. . Assessment results to date support the MHTGR Participants' position that no public sheltering or evacuation plans are required beyond the plant exclusion area boundary for design basis events and events beyond the design basis. These results contain uncertainties that must be resolved by future development work if this position is to prevail.

While the unique MHTGR radionuclide control concept takes full advantage of many inherent passive safety features of the fuel and core design, the concept places a heavy burden

on establishing and protecting the fission product retention capabilities of the fuel under all normal, off normal, and accident conditions. Secondary barriers provide some additional radionuclide retention, but these barriers are not specifically designed to carry out this function. Very stringent MHTGR fuel quality and performance requirements must be met by billions of microspheres of enriched uranium oxycarbide fuel particles to protect the public, plant personnel, and components throughout the plant lifetime. The viability of these high quality fuel and performance requirements on a commercial scale rests upon much technology that is currently not well established. Increased emphasis is placed upon the acccuracy, thoroughness and validation of the complex phenomena represented in the safety analysis codes, especially the fuel performance at high temperatures and fission product transport through all barriers. Emphasis is also placed on plant systems and components that must protect the fuel during all accident conditions. These systems and components must maintain fuel particle temperatures below 1700°C by controlling heat generation and heat removal from the fuel. The systems and components must also limit chemical attack from air, steam, water, or other impurities on the fuel for all initial and transient conditions addressed in the design safety analyses.

The present refueling plan for the MHTGR is to replace one-half of the core (approximately 330 hexagonal fuel elements) and one-sixth of the replaceable reflector elements (approximately 270 elements) every 20 months. A single refueling outage involves removing and replacing 1064 elements. This process must be accomplished without misloading or damaging the fuel. When compared to present LWR designs, this process appears significantly more complicated to accomplish while maintaining accurate inventories of the fuel blocks. This comparison is even more significant when comparing the quantity of fuel handling maneuvers of two MHTGR plants (with four modules each) to a single 1100 MWe LWR of today's vintage. Likewise, fuel management appears to be more complex than current LWR designs and can be further complicated by the potential of asynchronous operation of two MHTGR reactor modules paired to one turbine.

The MHTGR modular design (single turbine with two reactor modules) offers several potential advantages such as shorter lead times per module pair, shorter construction periods, incremental capital investments with earlier revenues, and incremental additions of system capacity better matched to system growth needs. Utilities will compare these modular advantages beyond the planning and construction stages to operation and maintenance considerations of increased refueling operations, more complicated fuel-management of multiple units, the overall management of multiple units,

the potential cost (materials, resources, and lost revenues) of retrofits for multiple units due to regulatory or efficiency reasons, and the relative small output of power per module for capital cost investment when replacing the larger capacity units of today's vintage.


Prospects for Licensing, and Utility Acceptance

The stated goals of the MHTGR Program are to provide safe,
economical power that complies with top level utility/user
and regulatory requirements as organized by the Gas Cooled
Reactor Associates (GCRA) and the Department of Energy
(DOE). Utilizing years of development, testing, and
operation of the HTGR Programs, the MHTGR Participants have
developed an innovative modular design taking full advantage
of inherent, passive safety features associated with ceramic
coated fuel particles, graphite core structures, and helium
coolant. While taking full advantage of the passive safety
characteristics, the MHTGR radionuclide control concept
places a heavy burden on establishing and protecting the
fission product capabilities of the fuel. Very stringent
MHTGR quality and performance requirements must be met by
billions of microspheres of enriched uranium oxycarbide fuel
particles to accomplish this objective. Fuel manufacturing
and quality control must maintain fuel quality throughout
the life of the plant without a single programmatic
breakdown. The viability of these high quality fuel and
performance requirements on a commercial scale rests upon
much technology that is not currently well established.
This especially includes the analytical models and
supporting data to license the fuel and the radionuclide
control concept. The fuel and fission product technology
development plan is comprehensive, but appears to be overly
optimistic. Suitable fallback positions should be
identified in the event a key program element yields an
unexpected result. Many key technology development elements
that are interrelated, or related to the preliminary design
process are scheduled to run in parallel; and therefore
changes in the preliminary design may invalidate parallel or
supporting technology development. The current Technology
Development Needs (TDNs) must be completed prior to NRC
certification of the current design to successfully
demonstrate the MHTGR concept and to assure utility
acceptance. The scope of these TDNs includes (a)
prototypical fuel performance data, (b) successful
validation of fuel performance and fission product transfer
models, and (c) establishment of measurable safety margins
between the calculated peak fuel temperature and the fuel
particle failure temperature limit.

The MHTGR safety assessments conducted to date support the MHTGR Program Participants' position that no public sheltering or evacuation plans are required beyond the plant exclusion boundary for design basis events and events beyond the design basis. These assessments provide the technical basis for the MHTGR Program Participants' position that no low leakage containment is required around the primary coolant boundary. These assessments contain uncertainties that must be resolved by future development work.

The NRC Staff has concluded in SECY-88-203 that with proper engineering analysis, judgement, and demonstrated performance, it is possible for advanced reactor designs (including the MHTGR) to achieve a level of safety equivalent to that of current generation LWRs without utilizing a non-mechanistic source term, conventional containment building, or traditional offsite emergency planning.

In reviewing the NRC Staff's draft Preapplication Safety Evaluation Report of the MHTGR design, the NRC's Advisory Committee on Reactor Safeguards (ACRS) reached essentially the same conclusion: (a) whether or not a conventional containment structure should be required remains as a major unresolved licensing issue, (b) no additional scenarios of reasonable credibility could be postulated for which an additional physical barrier would be required for public protection, and (c) any requirements to add a containment as further defense in depth against unforeseen and unforeseeable events would be arbitrary. This discussion is similar to what occurred in the early days of LWR development. The ACRS position was not unanimous. Three ACRS members stated that experience has shown that new reactor designs have technical unknowns, and therefore, did not recommend the MHTGR reactors for design certification without a more extensive external barrier. They indicated this barrier should consist of either a conventional containment structure or other appropriate mitigation systems.

The current MHTGR design is responsive to the NRC's Policy Statement on Advanced Reactor Concepts. It offers many passive safety features including decay heat removal, long operator reaction times, and insensitivity to incorrect operator actions. Whether or not a conventional containment or a confinement-type structure is required will continue to be a primary licensing issue with much debate between and utilization of resources by the regulators, MHTGR Participants, the industry, and the public. This issue currently appears to be at a stalemate. The resolution of the containment issue could have a significant impact on the capital and O&M costs; and therefore, should be a priority for the MHTGR Participants to have ample opportunity to consider alternate designs. Acceptance of the MHTGR by utilities will depend, to a great extent, on expected capital and O&M costs, as well as resolution of licensing issues such as the containment.

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