Safety analysis of the fer fuel circulating system

Fusion Engineering and Design 10 (1989) 373-383 North-Holland, Amsterdam

SAFETY

ANALYSIS

373

OF THE FER FUEL CIRCULATING

Y a s u s h i S E K I 1, H i r o m a s a I I D A 1, S h i g e t a d a K O B A Y A S H I T s u t o m u H O N D A 2, a n d I s a o I S H I K A W A 3

SYSTEM 2, H i r o s h i O H M U R A

2,

t Naka Fusion Research Establishment, JAERI, Naka-machi, Naka-gun, Ibaraki, 311-02 Japan : Fusion Technology Development Office, Toshiba Corporation, 1-6, Uchisaiwaicho l-chome, Chiyoda-ku, Tokyo, 100 Japan s Advanced Technology Development Dept, Ishikawajima-Harima Heavy Industries Co, Ltd, Tokyo-Chuo BIdg, 6-2, Marunouchi l-chome, Chiyoda-ku, Tokyo 100 Japan

This paper describes the results of an evaluation on the safety concerns of the fuel-gas purification system (FPS), the fuel-gas isotope separation system (ISS), and the fuel-gas storage system (FSS) for the Fusion Experimental Reactor (FER). The results of this evaluation are presented as a probability-consequence plot. The primary function of the FPS is to remove impurilies from the vacuum pumping system in the fuel stream. In this system, a palladium diffuser removes all of the impurities, and then the purified hydrogen isotopes are transferred to the ISS. The ISS separates the stream of mixed hydrogen isotopes into three high purity streams of D 2, T 2, and DT. The ISS makes the required separation by means of four interlinked cryogenic distillation columns. The FSS stores the purified fuel gas by means of metal bed absorbers. This evaluation was performed by using Probabilistic Risk Assessment (PRA). The first step of this assessment is to determine the accident initiators, which was to perform a failure mode and effects analysis (FMEA). The second step is to develop an event tree for each of the important categories of accident initiators identified; event tree analysis (ETA). The third step is fault tree analysis (FTA). Fault trees are required to determine the branching probabilities in the event trees. The fourth step is the determination of the release magnitudes. The fifth step is to assign the consequences to the accident sequences and to derive probability-consequence curves for risk comparisons.

1. Introduction

This p a p e r starts by describing the process schemes of these systems, a n d then the P R A procedure.

This paper presents the probabilistic risk assessment ( P R A ) for the fuel circulation system of the Fusion Experimental Reactor (FER). The results of this evaluation are presented as a probability-consequence plot. The F E R fuel circulation system (FCS) consists of a fuel-gas purification system (FPS), a fuel-gas isotope separation system (ISS), a fuel storage system (FSS), a vacuum p u m p i n g system (VPS), a n d a fuel injection system (FIS). In this paper, an assessment of the FCS, the ISS, and the FSS is described. The first step of this assessment was to determine the accident initiators which was accomplished by performing a failure mode and effects analysis (FMEA). The second step was to develop an event tree for each of the i m p o r t a n t categories of accident initiators identified; that is, an event tree analysis (ETA). The third step was fault tree analysis (FTA). Fault trees are required to determine the b r a n c h i n g probabilities in the event trees. T h e fourth step was to assign consequences to the accident sequences, and to derive probability-consequence curves for risk comparisons. 0920-3796/89/$03.50

2. System description T h e main tritium system of the F E R are s h o w n in fig. 1. T h e fusion reactor's core is a torus vessel, which is kept at a b o u t 10 - 4 to 10 -7 Pa, into which the d e u t e r i u m - t r i t i u m fuel is fed by the fuel injection system (FIS). T h e b u r n i n g ratio is 10% at most. Thus, helium, which is a fusion product, a n d a small a m o u n t of the tritium isotope's oxide, nitride, or carbide are mixed with the fuel r e m a i n i n g in the torus vessel. This mixture is p u m p e d out of the vessel by the v a c u u m p u m p i n g system (VPS), a n d transferred to the fuel purification system (FPS). In this system, all impurities except p r o t i u m are removed, a n d the mixed hydrogen isotopes are transferred to the isotope separation system (ISS). T h e ISS separates these mixtures individually, a n d each isotope is stored in the fuel storage system (FSS), except H 2. These stored isotopes are reused as fuel a n d transferred to the ISS.

© E l s e v i e r S c i e n c e P u b l i s h e r s B.V.

374

Y. Seki et aL / Safety analysis of FER fuel circulating system

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2.1. Fuel gas purification system (FPS)

1 Sstem

The primary function of the FPS is to remove impurities from the VPS fuel stream in preparation for the ISS. The FPS process flow diagram is shown in Fig. 2

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[]1. First of all the VPS fuel stream is temporarily stored in surge tank I. In this tank, not only is the VPS fuel stream introduced, but also the mixed hydrogen isotopes, which are also a product of this system. The gaseous mixture in surge tank I consists of the hydrogen isotopes and other impurities, that is helium, which is a fusion product, and a small amount of the hydrogen isotopes' oxide, nitride, carbide etc. The stored gas is transferred by a transfer pump to the palladium diffuser, which removes all impurities. The hydrogen isotopes pass through a cooler and then pumped out through surge tank II to the ISS by a vacuum pump. The removed impurities are temporarily stored in surge tank III in which oxygen gas and helium gas are added to the impurities' mixture, providing oxidation and dilution of them to prevent explosion or combustion. Then the gaseous mixture is introduced to the catalytic oxidizer by a transfer pump, and is changed into oxide of hydrogen isotopes and other chemical species without hydrogen isotopes. It passes through a cooler and heater, and is then transferred to the freezer, where the oxides of hydrogen isotopes are frozen.

Fig. 1. Main tritium system of the FER.

When the fusion facility has a tritium breeding blanket, a bred tritium recovery system is required. These gaseous tritium processing systems are placed in glove boxes in case tritium gas permeates the system's boundary and thus diffuse into the room. For the purpose of tritium decontamination in the glove boxes or contaminated rooms, atmospheric tritium clean-up systems are required. In addition, a heating, ventilating, and air conditioning system, a radioactive waste disposal system, and a primary cooling system are needed for tritium processing. In this section, a description of these three systems, that is the FPS, ISS, and FSS, is

given.

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Y. Seki et al. / Safety analysis ofFER fuel circulatingsystem

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Other chemical species which pass through the freezer are released to the environment from a stack after confirming that the tritium concentration is low enough for release. The trapped oxides of the hydrogen isotopes are regenerated by heating, passed through a heater, and introduced into the ceramic electrolysis cell, where the oxide is electrolyzed to a mixture of hydrogen isotopes and an oxygen component. The oxygen component is released to the environment from a stack after confirming that the tritium concentration is low enough. The mixed hydrogen isotopes pass through a cooler and then return to surge tank I. The FPS has metal getter beds

for emergencies, where tritiated gaseous mixtures are introduced by a transfer pump.

2.2. Isotope separation system (1SS) The ISS separates the stream of mixed hydrogen isotopes, after the non-hydrogen species have been removed in the FPS, into three high purity streams of 02, T2, and DT. The ISS makes the required separation by means of four interlinked cryogenic fractional distillation columns. The ISS process flow diagram is shown in Fig. 3 [1].

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(

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376

Y. Seki et al. / Safety analysis of FER fuel circulating system

The stream prepared in the FPS is transferred to Column 1, where the first separation is performed. In the top product stream, the tritium component is reduced, and the protium and deuterium components are enriched. This top product stream is heated by passing it through a heatgr and isotope equilibrator, then transfe.rred to Column 2 where the second separation is performed. In the top product stream of Column 2, the tritium component is reduced even more than the top stream of Column 1. This is then released to the environment from a stack, after confirming that the tritium concentration is low enough. Most of the bottom product stream gas from Column 2 is deuterium, which is transferred to Column 3, where a third separation is performed. The top product stream gas of Column 3 is high-purity deuterium which is transferred to the FSS through a heater. In the bottom product stream gas of Column 1, the protium component is negligible. This stream is transferred to Column 4 where it is separated into D T and

T~. The bottom product stream gas of Column 3 and the top product stream gas of Column 4 are high-purity D T component gases which are transferred to the FSS through a heater. The bottom product stream gas of Column 4 is high-purity tritium gas which is transferred to the FSS. The columns are thermally insulated in a common vacuum jacket which also serves as a secondary containment vessel.

2.3. Fuel-gas storage system (FSS) The FSS stores the three high-purity products, that is D 2, T 2, and DT, as additional fuels for supply. The FSS makes the required storage by means of metal getter beds. The FSS process flow diagram is shown in fig. 4 [2]. Each high-purity product is individually transferred to a metal getter bed from the ISS by a transfer pump. The metal getter beds are kept at room temperature by cooling water. When the stored gas is transferred to the next system for use, the bed is heated. The composition of the gaseo.us mixture is regulated in the mixer, and then the regulated gaseous mixture is transferred to the FIS.

3. Probabilistic risk assessment procedure 13] The FPS, ISS, and FSS are important systems which handle a large amount of tritium, therefore, system

trouble might result in an undesired tritium release to the environment. This paper mainly assesses tritium release accidents in the glove box and in the room. The PRA procedure used is described in the following. (1) The accident initiators were determined by means of an FMEA. (2) The accident sequences for the accident initiators were depicted by means of an event tree. (3) An FTA was performed to determine the branching probabilities in the event tree. (4) The tritium release magnitudes were determined for each accident sequence. (5) The probability-consequence curves for risk comparisons were derived.

3.1. Accident sequences and initiators [3] The accident sequences for each system are shown in fig. 5. For these sequences the accident initiators, which result in tritium release in the glove box or in the room, are shown in the following. (1) FPS (1) Failure of the palladium diffuser gas temperature controller (2) Large leak in surge tank I (3) Failure of the helium dilution supply system (4) Overcooling of the freezer tube (5) Failure of the helium freezer gas pressure controller (6) Failure of the electrolysis cell gas temperature controller (2) ISS (7) Gas feed pipe ruptures in glove box (8) Cryogenic distillation column damaged (3) FSS (9) Loss of cooling water in the metal getter bed (lo) Failure of the metal getter bed heater

3.2. Event tree analysis (ETA) [3] For this report, five initiator event trees were developed by considering the magnitude of the tritium release accident. (1) Large leak in surge tank I of the FPS. This initiator was selected because surge tank I has the largest tritium inventory in the FPS. Though surge tank II has almost the same tritium inventory, surge tank I is selected as typification. (2) Failure of the helium dilution supply system of the FPS. This initiator was selected because of the potential of a methane explosion in the catalytic oxidizer.

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Y. Seki et a L / Safety analysis of FER fuel circulating system

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(3) Damage to the cryogenic distillation column of the ISS. This initiator was selected because of the overpressure potential by vaporization of the liquid tritium isotope. (4) Loss of cooling water in the metal getter bed of the FSS. This initiator was selected because the metal getter bed contains the largest tritium inventory of these three systems. (5) Failure of the metal getter bed heater of the FSS.

This initiator was selected because the metal getter bed contains the largest tritium inventory of these three systems. The event trees for these sequences are shown in fig. 6.

3.3 Fault tree analysis (FTA) [3] Initiator frequencies and failure rates for the c o l u m n headings of the event trees are mainly based o n FTA.

Y. Seki et a L / Safety analysis of FER fuel circulating system

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10 -4 i 0 -s

\

t~

10"6

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i0 -13

10"1.

10"2

I 0 "~ 100

I0'

102

I0 z

104

IOs

106

I0 r I0 e

Consequence ( Trilum releose) ( C i / E v e n ! l

Fig. 7. (a) Risk plot - tritium release to glove box; (b) risk plot - tritium release to room.

The data are based on the IEEE Std 500-1984 [4], or the WASH-1400 [5]. The system is assumed to operate on a 720 h cycle and an overhaul performed once a month when there are some redundancies, and on an 8 640 h cycle when there are no redundancies.

3.4. Tritium release magnitude [3] The tritium release magnitude for each accident sequence is assessed in the following. (1) Large leak in surge tank I of the FPS. The tritium inventory of surge tank I is 23.36 g. If the metal getter beds are not available, all of the tritium inventory, 2.24 × 105 Ci, is released from the system (sequence numbers A-3, A-5, A-6, and A-7.) If metal getter beds are available, 2.05 × 105 Ci tritium is released, because 8.5% of the surge tank I inventory is assumed to be absorbed by the metal getter.beds (sequence A-l). (2) Failure of the helium dilution supply system of the FPS. When the inlet valve of the catalytic oxidizer is successfully closed, the tritium release amount is relatively small, 84 Ci, which is the inventory of the catalytic oxidizer, 8.7 × 10 -3 g, even though the catalytic oxidizer is damaged (sequencies B-6, B-7, B-15, and B-16). When the inlet valve of the catalytic oxidizer cannot be closed, the tritium release

amount is much larger than the previous case, 2.59 × 103 Ci, which is the inventory of both the catalytic oxidizer and surge tank III, 0.26 g. In this case, a methane explosion may be considered to happen (sequencies B-9 and B-18). When an operator cannot identify the accident,the result is the same as in the previous case (sequence B-19). (3) Damage to the cryogenic distillation column of the ISS. In this case, the total tritium release amount is considered to be 4.28 × 105Ci, which is the sum of the Column 4 inventory, 22.8 g, and the surge tank II inventory, 21.82 g (sequences C-9, C-12, and C-15). (4) Loss of cooling water in the metal getter bed of the FSS. In every case, the tritium release amount is 1.2 × 106 Ci, which corresponds to the metal getter bed tritium inventory (sequences D-3, D-9, D-14, D-5, D-11, and D-16). (5) Failure of the metal getter bed heater of the FSS. In this case, the tritium release amount is the same as in the previous case (Sequencies E-3, E-9, E-14, E-5, E-11 and E-16).

3.5 Risk assessment (3) The probability-consequence curves are shown in Fig. 7. In this figure, [Risk] = [probability] × [tritium release amount].

Y. Seki et al. / Safety analysis of FER fuel circulating system 4. Conclusions In this paper, accidents relating to five accidental initiators have been discussed. The accident initiated by a large leak in surge tank I has both a large probability and a large risk. For the tritium release accidents in the glove box, the large leak accident of surge tank I (A-l, A-3, and A-6) has both a large probability and a large risk. The probability of sequence A-1 is comparable to the initiator frequency, but it is possible to reduce the tritium release amount. Tritium release in the room is dependent on whether the glove box is intact or not. The probability is below 10-6/Reactor year in any case. The following dicussions are necessary for future system designing: (1) the reduction of the tritium release amount in the glove box, in case of a large leak accident in surge tank I; (2) counterplan in the cold trap failure and the heater structure design; (3) discussion of operating and control methods in the failure of the helium dilution supply system or oxygen supply system of the FPS; (4) counterplan in the case of the blockage of the adsorbent bed or equilibrator; (5) discussion of the release valve of the ISS vacuum jacket;

383

(6) discussion of the release valve of the metal getter bed and the overheating preservation plan. Note that in 1983, Bruske and Holland [6] reported on the risk assessment of a fusion-reactor-fuel processing system. However, their report is different from this paper because of different processing systems and scope.

References [1] H. Iida et al., Design study of a plant system for the fusion experimental reactor (FER), JAERI-M 86-149 (November 1986). [2] R. Saito et al., Conceptual design study of the fusion experimental reactor (Y86 FER): plant system design, JAERI-M 87-091 (August 1987). [3] Y. Seld, H. Iida and T. Honda, Conceptual design study of the fusion experimental reactor 0(86 FER): safety, JAERIM 87-111 (August 1987). [4] IEEE Guide to the Collection and Presentation of Electrical, Electronic, Sensing Component, and Mechanical Equipment Reliability Data for Nuclear-Power Generating Stations; IEEE Std 500-1984. [5] Reactor Safety Study-An Assessment of the Accident Risks in U.S. Commercial Nuclear Power Plants, WASH-1400, NUREG-75/014 (October 1985). [6] S.Z. Bruske and D.F. Holland, Risk Assessment of a Fusion-Reactor Fuel-Processing System, ECK3-2266 (July 1983).