ML20204C478
| ML20204C478 | |
| Person / Time | |
|---|---|
| Issue date: | 10/13/1988 |
| From: | Kerr W Advisory Committee on Reactor Safeguards |
| To: | Zech L NRC COMMISSION (OCM) |
| Shared Package | |
| ML20204C413 | List: |
| References | |
| ACRS-R-1323, ACRS-R-1324, NUDOCS 8810200478 | |
| Download: ML20204C478 (8) | |
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ol ADVISORY COMMITTEE ON REACTOR SAFEOUARDS wAswmaTo% o. c.aosee October 13, 1988 The Honorable Lando 11. Zech, Jr.
Chairman U.S. Nuclear Regulatory Comission Washington, D.C.
20555
Dear Chairman Zech:
SUBJECT:
PREAPPLICATION SAFETY EVALVATION REPORT FOR THE MODULAR HIGH TEftPERATURE CAS COOLED REACTOR Introduction During the 342nd meeting of the Advisory Comittee on Reactor Safe-guards, October 6-7, 1988, and in previcus meetings of the Comittee and our Subcomittee on Advanced Reactor Design =, we reviewed a draf t of the subject Safety Evaluation Report (SER).
During these meetings, we had j.
the berefit of discussions with representatives of the NRC staff and its consultants,withrepresentativesoftheDesartmentofEnergy(DOE),and representatives of General Atomics, the chief design contractor for the Ptodular High Temperature Gas Cooled Reactor (MHTGR).
We also had the benefit of the documents referenced.
1 The MHTGR concept is a product of a joint DOE / industry program to develop a design for a nuclear power plant using HTGR technology and having important inherently safe characteristics.
The NRC staff is reviewing the concept under the advanced reactor policy to alp assure that the final desigt vill develop along lines acceptable to the NRC.
The draft SER indicates that the staff believes the conceptual design is gercrally satisfactory and that work directed tcward eventual certifica-tion shculd continue.
The staff has provided a number of conditions along with this endorsement and also believes that a continuing program of research and development will be necessary to support final design and eventual licensing.
We are in general agreement that design and development should continue along the lines eutlined by the NRC staff.
We can agree to moving forward, however, only because we understand that an NRC endorsement at
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this time does not imply a final comitment either to the general design or to its details. We believe that ongoing research and development can resolve important safety issues before licensing.
We have a nurter of coments discussed below about the design.
8810200478 881011 32 PDC
l The Honorable Lando W. Zech, Jr.
-2 October 13, 1988 Key Features of the MHTGR The MHTGR differs in important ways from existing light water reactor (LWR) plants and from previous gas cooled reactor plants, including several new safety characteristics.
The goal of the designers is that I
the improved safety features will more than make up for the absence of others (e.g., containment).
They believe the MHTGR design will provide a plant that is safer than LWRs.
Safety of the RHTGR is keyed to properties of its unique fuel particles.
l Millions of these microspheres of enriched uranium oxycarbide, each the size of a grain of sand, are in the reactor core.
Each fuel particle is i
coated with four successive protective shells that includes a buffer layer of a porous carbon and then bonded with others into a fuel rod i
which is, in turn, sealed in vertical holes in graphite blocks.
These graphite blocks provide neutron moderation and are the chief structural material in the core.
The maximum fuel particle temperature in normal operation will be about I
j 1150'C.
An expected very small fraction of defective particles will cause a measurable, but acceptably low, level of chronic fission-product activity in the coolant and reactor systems.
So long as the particles are naintaineel below 1600'C, fuel, transur.
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anics, and fission products will be retained by the particle coatings, with very high efficiercy. At temperatures abLve about 2000'C, failures of particle coating will become significant, and above about 2300'C the t
coatings will fail completely. All other safety features nf the reactor systems are designed to assure that particles will remain below 1600'C i
over a wide range of challenges and circumstances.
u, It is expected that temperatures can be maintained below 1600'C, in any conceivable reactor transient, because of two favorable characteristics i.,f 'he reactor cere:
(1) Strong negative reactivity changes with increased temperatures in fuel or moderator and (2) Large thermal l
inertia of the core and fuel structure.
It is also expected that temperatures will be maintained below 1600's even with loss of normal cecay heat removal because of the following irportant features:
(1) The same strong temperature-reactivity effects will assure a very low equilibrium pcwer even with failure of reactivity control and l
shutdown systems.
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(2) At these low or decay power levels, if normal heat transfer systems fail, all heat can be removed from the reactor by a passive heat 4
transfer system that permits atmospiseric air to flow by natural l
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O The Honorable Lando W. Ze.h, Jr. October 13, 1988 convection through a cavity surrcunding the reactor vessel.
Under these conditiens, the reactor core and the vessel will attain temperatures only slightly above their normal operating values.
(3)
If this passive heat renoval system should become unavailable (e.g., by blockage of air flow), heat at low power or at decay heat levels would be transferred from the reactor cavity by conduction directly to the earth surrounding the reactor building.
Under these conditions, fuel would remain below 1600*C, but the reactor vessel would eventually heat te well beyond its normal operating tenperature.
Whether the reactor could be returned to nomal operation af ter exposure of the vessel to such overtenperature is problenatic at the present tire.
But, the vessel would remain sufficiently intact for the safe removal of decay heat.
The passive heat trensfer functions in items (2) and (3) above require that the reactor core and vessel be snall enough so that heat transfer can be accerplished without core terperatures becoming excessive.
This dictates the reactor size and leads to the modular design and the long, small-diareter core.
The reactor core is norrally cooled by inert helium gas circulated thrcugh the core at high
)ressure.
Certain irprobable failures of the reactor vessel ceuld pem'it air to enter the core.
However, air flow through the core by natural convection would be at a very Icw rate.
With this restricted supply of oxygen, oxidation of graphite would be so slow that af ter mery hours only a small fraction of the graphite would be consured and the core would remain structurally intact.
Even if the graphite should burn, through sore undetemined mechanism, the indica-l tions are that the graphite temperature would be well belew the 1600'C critical temperature for the fuel particles. The combination of nuclear decay and conbustion heat would not be expected to increase core tem-perature to greater than 1600'C.
The Safety Issues The challence in assuring that the key safety characteristics claimed for the MHTGR design are realized in an actual plant is, in simplest tems, in assuring that the follcwing issues are adequately addressed:
(1) Fuel particles rust have the retention capabilities attributed to them and this must be assured with recognition of inevitable variability and irperfection in the fuel particles and their compaction process. This will req; ire a higher level of quality in manufacture than has been achieved and must be experirentally j
verified.
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The Honorable Lando W Zech, Jr.
-4 October 13, 1988 (2) The reactivity and temperature-reactivity characteristics used in safety analyses are based on limited data. Further verification of these characteristics as a function of fuel burnup, core shuffling, and a v6riety of operational transients is needed, i
(3)
Inadvertent ingrees of water or steam into the core must be pre-eluded with high reliability. Water or steam could cau4e corrosion and mechanical damage to the graphite and would also add a positive i
reactivity contribution.
This seems to be a possible cceplication of, for example, steam generator tube failures that is not present in LWRs.
Internal flooding of the underground reactor cavity cculd
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1ead te similar problems.
l (4) There must be assurance that decay and low-power heat transfer can l
be accomplished without causing excessively high core temperatures.
Performance of the passive atmospheric cooling system and the I
ability te ccnduct hcat to the surrcunding earth must be demon-l strated.
(5) The structural properties of the graphite must be demonstrated and i
j assured.
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(6) Scre of the irrectant safety benefits of the design (e.g., passive decay heat removal and resistance to graphite burning) depend upon 4
the core gecretry renaining unperturbed.
Questions of seismic
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resistance, effects of aging, and the possible cascading effects of l
certain reactor accidents remain to be fully answered.
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A major issue is whether a conventional contair. cent structure or some l
cther nitigation systen or process should be required.
Neither the designers, the FPC staff, nor the members of the ACR5 have been able to postulate accident scenarios of reasonable credibility, for which an t
additional physical barrier to release of fission prcducts is required
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in order to provide adequate protection to the public.
This does not i
3 mean that e conventional containment shculd not be provided or required
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l as further defense in depth against unforeseen and unforeseeable events.
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However, it does reen that the design basis for a containment would have to be arbitrary, not altogether unlike what was done in the early days f
for LWRs.
We believe that the decision to require a containment will have to be made on the basis of technical judgment, with apprcpriate f
l censideration of the effects on other technically based safety features now a part of the design.
In addition, there nay be safety and economic tradeoffs between provision for containment and provision for passive
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l decay heat reroval.
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The Honorable Lando W. Zech, Jr. October 13, 1988 Recomenda tions A substantial program of research and development must be continued to support the final design for the MHTGR, This program should concentrate on providing assurances relative to the safety issues we have discussed above.
General Atemics has generated extensive data on fuel performance, but a comprehensive program on the reference fuel appears to be needed.
This would include testing of irradiated fuel, fuel from large-scale man-ufacturing, and fuel exposed to a variety of environmental conditions and temperatures such as might be encountered in possible accidents.
A hot critical experiment may be necessary.
The core is of an unusual gecretry and has nuclear characteristics different from those in previ-ous HTGRs.
Assuring that the safety response of the plant is as pre-dicted will require comprehensive inferration on the reactivity charac-teristics of the core over a broad range of nortnal and accident con-ditions.
More extensive analysis is needed of the response of the plant to acciderts that might change the core geometry. Certain accident scenar-ios can be hypothesized thai; would affect core geometry and influence coolant distribution and reactivity characteristics.
A prototype should be built and appropriately tested before design certification.
Concepts for a containment or another sort of physical mitigation system require further study.
Finally, there are two issues identified in our letter to you dated July 20, 1988, "Report on Key Licensing Issues Associated With DOE Sponsored Reactor Desigrs " that we believe should be given early consideration as the ded gn of this plant progresses. These issues are related to design for (1) resistance to sabotage end (2) operation and staffing.
The appropriate excerpts from that letter are attached.
Additional coments by ACRS Mer.bers Forrest J. Remick and Charles J.
Wylie, ar.d William Kerr are presented below.
Sincerely.
William Kerr Chairman v
The Honorable Lando W. Zech, Jr. October 13, 1988 Additional Coments by ACRS Menbers Forrest J.
Remick and Charles J.
Wylie In general, we agree with our colleagues in the above letter.
However.
ve cannot in good conscience recommend a design of a nuclear power plant for design certification which does not have a conventional containment or other mitigation system which would serve as a more robust external barrier than is currently proposed to protect the public from radio-logical releases, i
The designers of the NHTGR deserve much credit for their effort to I
incorporate inherent and passive safety features in the design concept.
However, even though we believe that the prcposed design has a good potential for providing enhanced safety, experience has shown that new reactor designs have technica! unknowns.
Because of the possible technical unknowns, the known uncertainties associated with the pos-i tulated inherent and passive safety features and the lack of experience 3
with operation of a reacter of this new design, we do not recomend these reactors for design certification without a nore extensive ex-ternal barrier consisting either of a conventional containment structure or other appropriate mitigation systen, We think it inportsnt that the ACRS and the Comission make this techni-cal judgrent et this tir.e in order that the designers of this promising j
reactor concept hava ampic opportunity to thoroughly consider alternate
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Additional Ccerents by ACRS Member William Kerr l
I remind the Comission of the coments on containment included in the l
Comittee's letter of July 20, 1988, namely i
"We are not prepared at the present tire to accept these J
approaches to defense in depth as being completely adequate.
further, we are not prepared at 1.:ii s time to accept the arguments that increased prevention of core melt or increased retention capacity of th9 fuel provide adequate defense in depth to justify the elimination of the need for conventional containment structures.
This is not to say that we could not decide otherwise in the future, in response to an unusually persuasive argument."
That is still my position on the containment issue.
I would add only that I have not yet heard the "persuasive argument."
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0 The Honorable Lando W. Zech, Jr. October 13, 1988
References:
1.
Office of Nuclear Regulatory Research, "Pre-Application Safety Evaluatien Report for the Medular High Ter'perature Gas Cooled Reactor," dated August 1988 (Predecisicnal Draft) 2.
Stone Webster Engineering Corporation (DOE Contract),
HTGR-06-024 "HTGR Preliminary Safety Information Document for the Standard itHTGR," Volumes 1-5, 1986 3.
GA Technelegies, Inc.
(DOE Contract),
DOE-HTGR-86-011 "HTGR Probabilistic Risk Assessment for the Standard Modular High Temperature Gas-Cooled Reactor," Volunes 1-2, January 1987
Attachment:
Excerpts frem July 20, 1968 ACRS Letter, "Report on Key Licensing issues Asscciated With DOE Sponsored Reactor Designs"
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s' ATTACHMENT TO ACRS LETTER ON HODULAR il!GH TEMPERATURE GAS COOLED REACTOR Excerpt tron July 20, 1988 ACRS Letter, "Report on Key Licensinc issues
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Associated With DOE 5ponsored Reactor Designs" i
Design for resistance to sabotace It is often stated that significant protection against sabotage can be inexpensively incorporated into a plant if it is done early in the design process. Unfortunately, this has not been done consistently because the NRC has developed no guidance or requirements specific for plant design features, and there seems to have been no systematic attempt by the industry to fill the resulting vacuum. We believe the NRC can and should develop some guidance for designers of advanced
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reactors.
It is probably unwise and counterproductive to specify t
highly detailed requirements, as those for present physical security systc>ms, but an attempt should be made to develop some general guidance.
Operation and staffing Little is said in the staff paper about requirements for operation ano i
staffing of advanced reactors. We find this to be a serious over-l sight.
Experience with LKRs has shown that issues of operation and staffing are probably more important in protecting public health and l
safety than are issues of design and construction. The designers of the three reactor proposals seem to be claiming that the designs are so inherently stable and error-resistant that the questions of opera-tion ard staffing, so important for LWRs, are unimportant for the l
advanced reactors. And that, in fact, the advanced plants can be l
operated with only a very small staff. We believe these claims are l
unproven and that nere evidence is required before they can be ac-l cepted.
The two major accidents that have been experienced in nuclear power, those at THI-2 and Chernobyl 4, were caused, in large measure, by human error.
These were not simple "operator errors" but instead were caused by deliberate, but wrong, actions. There are some indirations that the advanced reactor designs being considered have certain characteristics tending to nate them less vulnerable to such mal-operation. But, this has not been demonstrated in any systematic way.
The traditional methods of PRA are not capable of such analyses; but, we believe a systematic evaluation should be made. There seems little merit in making claims for the improved safety of new reactor designs if they have not been evaluated against the actual causes of the most important reactor accidents in our experience.