t- - NucleGAir Cleaning by Dr. Ronald R. Bellamy, NRC Presented at Filtration 94 Conference and Exposition (Reference 1)

Transcription

1 t- - NucleGAir Cleaning by Dr. Ronald R. Bellamy, NRC Presented at Filtration 94 Conference and Exposition (Reference 1)

2 DISCLAIMER Portions of this document may be illegible in electronic image products. Images are produced from the best available original document.,

3 NUCLEAR AIR CLEANING Or. Ronald R. Bellamy US Nucl ear Regul atory Commission King of Prussia, Pa INTRODUCTION The systems designed for air cleaning are a vital part of every nuclear install at ion. Whether the instal 1 ation is an el ectrical-producing generating unit, a fuel fabrication unit, a defense unit, a research and development unit, a manufacturing unit, or a waste disposal unit, the control of particulates is a primary concern. High efficiency particulate air (HEPA) filters are the obvious removal component of choice, and are extensively used in the nuclear industry. The history of use has shown these fragile components to provide margins of protection for the worker and the members of the public, yet questions remain about their place in future applications. REGULATORY GUIDANCE The necessity of protecting workers and the public from the potential effects of radiation associated with particulate material has long been recognized. There is no shortage of regulatory guidance to provide the industry with applicable criteria. The use of filtration media is considered to be of such importance that requirements for such components are explicitly stated in the laws governing nuclear installations. Title 10 of the Code of Federal Regulations is a part of the voluminous laws of the United States. Entitled "Energy", it contains those regulations governing nuclear power installations. Numerous parts of this Title are of applicability. Part 50 "Domestic Licensing of Production and Utilization Facilities", requires filters for specific areas of nuclear installations. For worker protection, air filtration is required on control rooms. For public protection after an incident, air filtration is required on containment ventilation systems, on fuel hand1 ing area ventilation systems, and on waste storage systems. For pub1 ic protection during normal operations, l icensees are required to satisfy Appendix I to 10 CFR Part 50. This appendix requires filtration systems such that the maximally-exposed individual in an unrestricted area receives less than 15 millirems per year due to particulate releases. Once this level of exposure is achieved, a costbenefit analysis is required to consider whether.additiona1 HEPA filtration can reduce the dose further for less than $1000 per personrem. Thus, redundant banks of HEPA filters may be required because of this regulation. Additionally, Part 20 of Title 10, "Standards for Protection Against Radiation", contains appendices that list specific numerical values for effluent releascs that must be maintained. HEPA filters are the removal component of choice to ensure that these effluent release limits are maintained for particulate material that is avai 1 ab1 e for re1 ease. 37 2

4 Siting of nuclear power reactors i s governed by Part 100 t o T i t l e 10. This part, "Reactor S i t e C r i t e r i a ", s p e c i f i e s t h a t an exclusion area must be provided of such s i z e t h a t an individual located a t i t s boundary f o r two hours immediately a f t e r an accidental design basis f i s s i o n product release would not receive a t o t a l r a d i a t i o n exposure i n excess of 25 rem, or a thyroid exposure i n excess of 300 rem. Further, a low population zone must be provided of such size tha; an individual located a t i t s boundary f o r the duration o f the accident would n o t receive a t o t a l radiation exposure i n excess of 25 rem, o r a thyroid exposure i n excess of 300 rem. HEPA f i l t e r s a r e used so t h a t the size of the exclusion area and low population zone a r e kept t o a minimum. A1 though these regulations provide a regulatory framework, they a r e not considered t o be s p e c i f i c enough f o r implementation purposes. Specific guidelines are p u t forth i n regulatory guides. Regulatory guides a r e documents issued t o describe and make available methods acceptable f o r implementing the regulations. A1 though they a r e not substitutes f o r regulations and compliance w i t h them i s not required, they provide useful information t o the industry on regulatory philosophy. The primary regulatory guide f o r a i r cleaning systems is Regulatory Guide 1.52, "Design, Testing and Maintenance C r i t e r i a f o r Post-Accident Engineered Safety Feature Atmosphere Cleanup System Air F i l t r a t i o n and Adsorption Units O f Light-Water Cooled Nucl e a r Power P1 ants. 'I T h i s regulatory guide has undergone a number o f revisions, yet each version provided s p e c i f i c c r i t e r i a for HEPA f i l t r a t i o n u n i t s. HEPA f i l t e r s are t o be designed, constructed, and t e s t e d i n accordance w i t h applicable standards of t h e American National Standards Institute. Since the regulatory guide has not been revised f o r a number of years, the guide s t i l l r e f e r s t o ANSI-509, "Nuclear Power Plant Air Cleaning Units and Components," although t h e l a t e s t industry guidance is contained i n a j u irit ANSI/American Society o f Mechanical Engineers document. ASME/ANSI AG-1 i s an evolving code, e n t i t l e d "Code on Nuclear Air and Gas Treat men t It. The section o f the Code applicable t o HEPA f i l t e r s i s I'FC". I t is applicable t o extended-media, dry type f i l t e r s f o r use in a i r and gas streams w i t h a 250 F maximum continuous temperature. Three types of HEPAs are addressed: 1. Type A f i l t e r s w i t h folded f i l t e r media and corrugated separator/supports, 2. Type B f i l t e r s w i t h minipleat medium and g l a s s fiber o r noncombustible thread sedarators. 3. Type C f i l t e r s w i t h kontinuohs corrugated f i l t e r media folded w i t h o u t separators. According t o t h i s code section, case materials shall be of s t e e l sheet o r s t a i n l e s s steel sheet conforming t o American Society of Testing and Materials specifications. Elastomers conforming t o ASTM specifications 38

5 shall seal the gasket t o the f i l t e r frame. The media i s considered t o be of such importance t h a t the requirements a r e contained i n a mandatory appendix, which i s derived from the Military Specification MIL-F-51079D f o r Fire-Resistant, High Efficiency F i l t e r Media. C r i t e r i a a r e specified f o r form, s i z e, splices, airflow resistance, t e n s i l e strength, elongation, a c i d i t y, thickness, combustibility, and flexing. A test aerosol penetration of less than 0.03% using 0.3 micron DOP p a r t i c l e s i s required. Other aspects of t h e HEPA f i l t e r s t h a t a r e discussed i n t h i s code section include separators, faceguards, nominal sizes and ratings, inspection data, packaging, shipping, storage, and nameplates. INSTALLATION Nuclear i n s t a l l a t i o n s use HEPA f i l t e r s i n various v e n t i l a t i o n and a i r cleaning systems. The f i l t e r s are generally r a t e d a t 1000 cfm. For nuclear power plants, there are approximately 8-10 systems per plant, w i t h a t o t a l capacity ranging from 250,000 t o 300,000 cfm per plant. Since there a r e usually two f i l t e r banks of HEPA f i l t e r s per v e n t i l a t i o n system, this r e s u l t s i n active HEPAs per plant. In today's environment, uti1 i t i e s are known for keeping spare replacements on-site, which would imply a use of about 1000 HEPAs per plant. US nuclear power planss t o t a l 120. Yet this i s only one use of HEPAs. Research and t e s t r e a c t o r s employ ventilation systems, b u t on a much smaller scale. Research and development corporations operate w i t h HEPA f i l t r a t i o n systems, as do hospitals. US Department of Energy and Department of Defense i n s t a l l a t i o n s u t i l i z e HEPAs on a much l a r g e r s c a l e than nuclear power plants. I t is estimated t h a t commercial power plants use only about 10% o f the HEPAs employed i n nuclear applications i n this country. Roughly, t h i s t r a n s l a t e s t o over 1 million a c t i v e HEPAs being used i n nuclear applications, and this is simply f o r the United S t a t e s. Once i n s t a l l e d, HEPA f i l t e r banks must be tested inplace t o ensure t h a t t h e i n s t a l l a t i o n i s leak-tight. Guidance f o r such t e s t s is contained in ASME N510, "Testing o f Nuclear Air Treatment Systems". A DOP challenge i s injected i n t o the airstream upstream of the HEPA f i l t e r bank, w i t h concentrations determined upstream and downstream of the f i l t e r bank. Percent penetration i s determined from the r a t i o o f the downstream t o upstream concentration. T h i s guidance will a l s o be found i n Section TA t o t h e ASME/ANSI Code on Nuclear Air and Gas Treatment already discussed. I t is of utmost importance t o perform t h i s inplace leak t e s t a t various times. A t i n i t i a l i n s t a l l a t i o n and upon replacement of any components, the t e s t will verify proper i n s t a l l a t i o n of the HEPA f i l t e r s, and their leak-tightness a t t h a t point i n time. However, t h e r e can be changes in the performance of the system over time, and t h u s t h e r e a r e s t r i n g e n t c r i t e r i a f o r periodic inplace t e s t i n g. The Regulatory Guide 1.52 previously discussed r e f e r s t o ASME N510 f o r t e s t i n g c r i t e r i a, and recommends t h a t HEPA f i l t e r sections be t e s t e d 39 4

6 1. Initially 2. At least once per 18 months thereafter, and 3. following painting, fire or chemical release in any ventilation zone communicating with the system confirm a penetration of less than 0.05%. frequencies for testing, to include to ASME N510 also recommends 1. Acceptance testing after initial installation 3. At least once each operating cycle 2. After each HEPA filter replacement, and with acceptance testing a1 so being recommended after any major system modification or repair, For nuclear power plants, part of the license issued to the station contains technical specification that govern periodic inplace testing. The technical specifications are therefore binding regulatory requirements on each licensee. They contain two parts: limiting conditions for operation, and surveillance requirements. The limiting conditions tell what needs t o be operating (i-e., two control room ventilation systems), when the systems need to be operating (i.e., during all power modes for the nuclear power unit), and what is required i f two systems are not operable (i.e., if one system is inoperable, return to operable status within 7 days or the reactor unit must be shut down). It is very desirable not to shut a reactor down, for economic and safety reasons, so one can see the importance of keeping air cleaning systems in an operable mode. To be sure that there i s no misunderstanding on what i s defined as operable, these criteria are contained in the surveillance requirements. It is required that the inplace leak test on the HEPA filter banks be done 1. At least once per 18 months 2. After any structural maintenance on the HEPA filter or charcoal adsorber housings, or Following painting, fire, o r chemical release in any ventilation zone communicating with the system. 3. The inplace leak test must verify that the bank does not exhibit a penetration o f greater than 0.05%. The technical specifications also required that a check be performed every month to verify that the pressure drop has not increased by more than a factor of two for the filter bank over the clean pressure drop. CHANGES IN CRITERIA Although the design criteria for HEPA filters has not undergone substantial change, there has been modifications in the testing criteria, specifically what HEPAs are required to be tested by a USDOE quality Assurance Filter Test Station. The DOE operates a number o f 40

7 f i l t e r t e s t i n g s t a t i o n s t h a t v i s u a l l y inspect and DOP t e s t every HEPA f i l t e r t o be installed in a DOE f a c i l i t y. These t e s t s duplicate the t e s t s required t o be performed by the manufacturer. Once a l s o required for each HEPA t o be i n s t a l l e d in a nuclear power f a c i l i t y, this i s no longer applicable. Studies have shown t h a t i t i s n o t cost-beneficial t o require each f i l t e r t o be sent t o a d i f f e r e n t location, subject the f i l t e r t o a very much higher frequency of transportation damage, and r u n duplicative t e s t s. F i l t e r f a i l u r e s a t the Quality Assurance Stations are generally due t o factors other than performance, t o include labelling, poor gasket s e a l s, and obvious physical damage. One would expect t h a t inplace testing and visual examination would identify any performance deficiencies in the f i l t e r s, without r e l i a n c e on a governmental testing operation. This change i n philosophy occurred i n ~ t h e r e has been no measurable decrease i n t h e the early ~and performance of HEPA f i l t e r s used in nuclear power s t a t i o n s. FUTURE SOURCE TERM REVISIONS Work has been ongoing f o r the past several years t o refine the source term used i n accident calculations (the amount and d i s t r i b u t i o n o f radioactive material avail able f o r r e l e a s e t o t h e environment). The source term starts w i t h an evaluation o f the amount o f fission products available f o r release t o the containment, and considers w h a t factors might change the amount and d i s t r i b u t i o n of t h e radioactive materials. Recent developments have resulted in more r e a l i s t i c c r e d i t being given t o removal mechanisms, including pl ateout and sprays removal mechanisms. Changes i n the i n i t i a l amount available f o r r e l e a s e show an increase i n the p a r t i c u l a t e fraction, b u t u s i n g t h e more r e a l i s t i c removal c r e d i t r e s u l t s in a decrease in the amount of p a r t i c u l a t e s actually reaching HEPA f i l t e r s. Regulatory guidance now c a l l s f o r a second redundant bank of HEPAs, and t h e r e may be s u f f i c i e n t j u s t i f i c a t i o n t o eliminate the need f o r t h i s second bank of f i l t e r s, and p o t e n t i a l l y both banks in certain accident scenarios. RECENT DEVELOPMENTAL ACTIVITIES I t i s worthwhile t o discuss two developmental a c t i v i t i e s with particulate f i l t r a t i o n. The f i r s t concerns e x i s t i n g HEPA f i l t e r s, and the second concerns a new generation of f i l t e r s. To date, HEPA f i l t e r s have been considered t o be a disposable commodity, without s i g n i f i c a n t consideration f o r aging c h a r a c t e r i s t i c s and shelf 1 i f e. Extreme d i f f i c u l t i e s with disposal (technical and economic) make the prediction of useful l i f e of HEPA f i l t e r s of major importance, w i t h useful l i f e being of concern from b o t h a service standpoint and a shelf l i f e standpoint prior t o i n s t a l l a t i o n. Predicting service l i f e is d i f f i c u l t, since i t i s obviously s o much dependent on the s p e c i f i c s of the application. Nonetheless, as discussed above f o r inplace t e s t i n g c r i t e r i a, a service l i f e of 18 months i s considered appropriate f o r the time between inpl ace t e s t s t o verify adequate leak-tightness. 41 6

8 There i s l i t t l e technical information on which t o establish a shelf l i f e. The different components of the f i l t e r are subject t o aging a t d i f f e r e n t r a t e s, compounding the problem. This applies t o the organic binder t h a t h o l d s the glass fibers together in a sheet, the organic sealant t h a t glues the f i l t e r pack into the frame, and a l s o forms a s e a l, organic gaskets and the glass f i b e r s. In preliminary r e s u l t s presented by Gilbert, e t. a l., a t the 23rd DOE/NRC Nuclear Air Cleaning and Treatment Conference, a very small sample o f f i l t e r s was analyzed. The two t e s t s of n o t e performed on the f i l t e r s i n an attempt t o predict shelf l i f e were the overpressure test and the heated a i r evaluation. In the overpressure t e s t, a f i l t e r i s subjected t o a 60-minute flow of humidified a i r w i t h steam droplets. Flow i s a t an elevated resistance of 10.0 inches, water gauge. Acceptance i s defined as l e s s t h a n 0.03% penetration a t 20% a i r flow. F o r heated a i r, a f i l t e r i s heated t o 700 F f o r f i v e minutes. Acceptance i s defined as l e s s than 3% penetration a t 100% a i r flow. The preliminary r e s u l t s form these qualification t e s t s showed t h a t f i l t e r age may n o t be the primary f a c t o r affecting f i l t e r performance. Rather, i t may be t h a t the type o f materials and construction d e s i g n are the important parameters. Insufficient binder i n the medium is proposed as the reason f o r structural weaknesses. Combustible sealants are also prone t o b u r n i n g through the f i l t e r depth and causing a loose f i l t e r pack i n the f i l t e r frame. Nonetheless, aging o f HEPAs i s an area f o r future analysis by the manufacturers, users and regulatory a u t h o r i t i e s. Studies need t o be done t o include the sealants, gasket and frame, and tests should simulate tornado pulses and smoke plugging e f f e c t s. Future uses and generations o f HEPA f i l t e r s will be seen i n l i g h t of d i f f e r e n t applications. Particularly in the gas processing area, adequate a i r cleaning will n o t be achieved by simply i n s t a l l i n g HEPAs a t the end of a p i p e. Waste storage f a c i l i t i e s will have d i f f e r e n t c h a r a c t e r i s t i c operating temperatures, re1 a t i v e humidities, and pressures. New f i l t e r s will need t o be made of d i f f e r e n t media, such as sintered micron s i z e s t a i n l e s s steel f i b e r s. The sintered f i b e r s can be bonded between s t a i n l e s s steel fine mesh screens. Media can be pleated in a cylindrical shape t o form a f i l t e r element, elements welded together t o form long tubes, and tubes assembled i n a vessel t o make a f i l t e r u n i t. The f i l t e r units can be advantageously designed for custom applications. These types of f i l t e r s can withstand high temperatures, wet conditions, s t r e s s due t o shock, excess a i r flow, and p a r t i c l e loading. They have high streng.th, are welded into housings w i t h o u t gaskets and adhesive s e a l s, and can be cleaned and re-used. Although they have a high pressure drop, a single stage f i l t e r provides f i l t r a t i o n e f f i c i e n c i e s equal t o two stages o f conventional HEPA f i l t e r s. Their use is conceptually beneficial for waste processing applications and other h i g h temperature/high moisture content environments.

9 SUMMARY The conventional extended media, dry-type HEPA filter has proven exceptionally beneficial for nuclear appl ications. They have prevented the release o f radioactive particulate matter, and also non-radioactive particulate matter (which eliminates a public perception issue). They are relatively inexpensive, versatile, and easily tested. The regulatory guidance is abundant for design and testing. Nonetheless, it i s necessary t o look forward. New applications for H E P A s in various g a s processing modes will required filters able t o operate in high temperature/high humidity environments, with novel chemical components that may be corrosive in nature. The HEPA industry has come a long way, and the conventional nuclear applications remain, but there are many new avenues to explore for future uses o f HEPA filters in the nuclear industry. 43 8