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Cold neutron source at the Budapest reactor
Physica B 180 & 181 (1992) North-Holland
PWSICA Ll
932-934
Cold neutron
source at the Budapest
reactor
T. Grosz, L. Cser, L. Rosta, M. Szalok and G. Zsigmond Central Research institute for Physics, P. 0. B. 49, H-1525 Budapest, Hungary
The installation of a liquid hydrogen cold and utilizing the thermosyphon principle is a nearly tangential horizontal channel with inside the Be-reflector. The cold neutrons
neutron source assembly with a single closed circuit feed by two cryogenerators in progress at the reconstructed Budapest reactor. The end of the in-pile part is a moderator cell of 250 cm3 volume made of aluminium alloy located in a hole will be directed to the user positions by three mirror guide tubes.
1. Introduction The
reconstructed
and
10MW
upgraded
WWR-SM
is going to be started iti the near future [l]. Part of the planned instrumentation, installed in a new guide hall where the neutrons are supplied by three mirror guides, is a small angle spectrometer, a triple axis spectrometer, a neutron spin echo spectrometer and a polarized neutron reflectometer. The operation of this equipment is mainly based on extensive use of cold neutrons, commonly defined as neutrons having energies less than 5 meV, or wavelengths longer than 4 A. The thermal neutron spectrum of a steady-state research reactor is very nearly the Maxwell distribution corresponding to the moderator temperature. Our reactor with water moderator temperature about 300 K provides a neutron distribution with maximum around 1.3 A, but at longer wavelengths the flux drops off very rapidly. The flux of 4 A neutrons is a factor of 16 less than the maximum and the overall intensity of cold neutrons is less than 2% of the total neutron flux in the thermal distribution. The obvious method to enhance the number of cold neutrons in the spectrum is to re-thermalize them in a small amount of good neutron moderator placed at the in-pile end of the beam tubes having a temperature (20-100 K) corresponding to that of the wavelength of the cold neutrons. The efficiency of the cold source depends on the moderating material, the shape, size and temperature of moderator and its relative position to the core. A number of cold neutron sources have been installed in nuclear reactors at research centres throughout the world and deep knowledge of the design criteria and a lot of operating experience have been accumulated since the preliminary experiments and the first cold moderator was installed in the reactor BEPO at Harwell in 1956, reported by Butterworth et al. [2]. Essentially two types of cold neutron sources are installed using either liquid or supercritical gaseous hydrogen or deuterium operating in the L-30 K temreactor
at Budapest
0921.4526192/$05.00
0
1992 - Elsevier
Science
Publishers
WAVELENGTH i
Fig.
1. Gain
of cold neutron
flux.
perature range or liquid hydrocarbon (mainly methane) maintained around 100 K. The use of solid methane is limited for low flux reactors due to the problems of heat removal and radiation damage. The gain in cold neutron flux does not approach the theoretically possible one for complete thermal equilibrium at low temperature due to the above mentioned limiting factors. In fig. 1 the gain factor as a function of wavelength is shown in thermal equilibrium for hydrogen sources (the region between the 15 and 30 K lines) and for methane sources (100 K line) as well as in the case of a real liquid hydrogen cold moderator (EL3 reactor, Saclay [3]). 2. Plan of the assembly The water moderated Budapest research reactor will be operated at 10 MW (later this may be increased to over 20 MW). The cold moderator cell will be placed in a nearly tangential horizontal channel in a hole formed in the Be-reflector surrounding the core (see fig. 2). The expected thermal flux at this point is about
B.V. All rights
reserved
T. Grosz et al. I Cold neutron source at the Budapest reactor
Be M
V
Fig. 2. Arrangement of cold moderator at Budapest reactor. C, reactor core; Be, berilium reflector; W, water moderator; T, reactor tank; A, air gap; B, biological shield (steel and concrete); M, moderator cell; V. vacuum case.
1OL4n/cm* s. Figure 3 shows a schematic diagram of the liquid hydrogen cold source assembly to be installed. The adopted system is not only very similar to that which was used in the EL3 reactor at Saclay [3]
Fig. 3. Schematic layout of the cold neutron source. moderator cell, 2; vacuum case, 3; hydrogen transfer lines, 4; criogenerators, 5; joint box, 6; water cooling, 7; helium cylinders, 8; hydrogen cylinder, 9; hydrogen reservoir, 10; vent chimney, 11; vacuum units, 12; vacuum valves, 13; gas analyzer, 14; burst discs, 15; biological shield, 16; wall of reactor hall.
933
but some key parts dismounted from that assembly will be reinstalled (two PPH 110 Philips cryogenerators of 100 W cooling power each, the hydrogen reservoir, the control system, etc.). ‘A single closed hydrogen circuit is used, the liquefier feed by the cryogenerators filling the moderator cell directly through vacuum insulated pipelines. The stream of liquid hydrogen is driven through the cell by a small differential pressure maintained by a height distance of approximately 2 m between the liquefier and the cell according to the thermosyphon principle. In order to minimize the nuclear heating a small size rectangular shape moderator cell is used which has a volume of approximately 250 cm3 (60 x 120 mm* surface, 40 mm thick) and is made of aluminium alloy. The wall thickness of the moderator cell is 1.5 mm, the internal pressure is 1.3 bar according to about 21 K liquid hydrogen temperature. According to computer calculation [4] the nuclear heating at 10 MW reactor power is expected to be 40 W. The additional estimated heat loss due to convection and radiation is about 30 W. Having a total cooling power of 200 W we have a large safety margin and probably stationary operation can be assured running only one 100 W cryogenerator. In the first part of the in-pile assembly the moderator cell is surrounded by a thick double walled water cooled vacuum case of AlMgSi alloy (see fig. 4). For safety consideration, i.e. to avoid HZ-O, reaction in case of leakage, as well as to provide helium backfilling cooling in the in-pile vacuum space, there is a helium blanket between the two walls of the vacuum case. In the outer part of the in-pile assembly three Nis8 coated glass neutron guides are positioned in bulky steel holders providing gamma and fast neutron shielding. This neutron guide part is also evacuated, but with a rotary pump to only 10m3 Torr, while the lo-’ Torr high vacuum inside the vacuum case separated by a diaphragm is maintained by a rotary and a diffusion pump.
de
v
W
Fig. 4. Front section of the in-pile part. M, moderator cell;
V, vacuum case; He, helium blanket; W, water cooling.
934
T. Grosz et al. i Cold neutron source at the Budapest reactor
transfer lines and in case of sudden pressure rise in the system the hydrogen vapour is vent to a vent line and helium purge is initiated simultaneously.
3. Safety
A number of safety measures are taken to ensure safe exploitation of the cold source and to avoid any damage of the reactor in case of eventual accident. The safe operation is achieved through the following means: -the hydrogen circuit is entirely encased by a vacuum casing and surrounded by a helium blanket around the cell; - the vacuum case is explosion proof; -the vacuum and the moderator cell temperature is monitored and exceeding pre-set limits the operation is stopped and the system is evacuated; -burst discs and safety valves are mounted in the
References [l]
L. Rosta, Workshop on Neutron Physics, Budapest (1986) p. 1. [2] I. Butterworth, P.A. Egelstaff, H. London and F.J. Webb, Philos. Mag. 2 (1957) 917. [3] G. der Nigohossian, Liquid Hydrogen Moderator Channel H’ EL3, CEA-N-1571, Saclay (1972). [4] J. Vegh, Nuclear Heating Calculation for the Design Criteria of the Cold Neutron Source to be Installed at the WWR-SM Reactor, KFKI Internal Report 11-1986, Budapest (in Hungarian).
PWSICA Ll
932-934
Cold neutron
source at the Budapest
reactor
T. Grosz, L. Cser, L. Rosta, M. Szalok and G. Zsigmond Central Research institute for Physics, P. 0. B. 49, H-1525 Budapest, Hungary
The installation of a liquid hydrogen cold and utilizing the thermosyphon principle is a nearly tangential horizontal channel with inside the Be-reflector. The cold neutrons
neutron source assembly with a single closed circuit feed by two cryogenerators in progress at the reconstructed Budapest reactor. The end of the in-pile part is a moderator cell of 250 cm3 volume made of aluminium alloy located in a hole will be directed to the user positions by three mirror guide tubes.
1. Introduction The
reconstructed
and
10MW
upgraded
WWR-SM
is going to be started iti the near future [l]. Part of the planned instrumentation, installed in a new guide hall where the neutrons are supplied by three mirror guides, is a small angle spectrometer, a triple axis spectrometer, a neutron spin echo spectrometer and a polarized neutron reflectometer. The operation of this equipment is mainly based on extensive use of cold neutrons, commonly defined as neutrons having energies less than 5 meV, or wavelengths longer than 4 A. The thermal neutron spectrum of a steady-state research reactor is very nearly the Maxwell distribution corresponding to the moderator temperature. Our reactor with water moderator temperature about 300 K provides a neutron distribution with maximum around 1.3 A, but at longer wavelengths the flux drops off very rapidly. The flux of 4 A neutrons is a factor of 16 less than the maximum and the overall intensity of cold neutrons is less than 2% of the total neutron flux in the thermal distribution. The obvious method to enhance the number of cold neutrons in the spectrum is to re-thermalize them in a small amount of good neutron moderator placed at the in-pile end of the beam tubes having a temperature (20-100 K) corresponding to that of the wavelength of the cold neutrons. The efficiency of the cold source depends on the moderating material, the shape, size and temperature of moderator and its relative position to the core. A number of cold neutron sources have been installed in nuclear reactors at research centres throughout the world and deep knowledge of the design criteria and a lot of operating experience have been accumulated since the preliminary experiments and the first cold moderator was installed in the reactor BEPO at Harwell in 1956, reported by Butterworth et al. [2]. Essentially two types of cold neutron sources are installed using either liquid or supercritical gaseous hydrogen or deuterium operating in the L-30 K temreactor
at Budapest
0921.4526192/$05.00
0
1992 - Elsevier
Science
Publishers
WAVELENGTH i
Fig.
1. Gain
of cold neutron
flux.
perature range or liquid hydrocarbon (mainly methane) maintained around 100 K. The use of solid methane is limited for low flux reactors due to the problems of heat removal and radiation damage. The gain in cold neutron flux does not approach the theoretically possible one for complete thermal equilibrium at low temperature due to the above mentioned limiting factors. In fig. 1 the gain factor as a function of wavelength is shown in thermal equilibrium for hydrogen sources (the region between the 15 and 30 K lines) and for methane sources (100 K line) as well as in the case of a real liquid hydrogen cold moderator (EL3 reactor, Saclay [3]). 2. Plan of the assembly The water moderated Budapest research reactor will be operated at 10 MW (later this may be increased to over 20 MW). The cold moderator cell will be placed in a nearly tangential horizontal channel in a hole formed in the Be-reflector surrounding the core (see fig. 2). The expected thermal flux at this point is about
B.V. All rights
reserved
T. Grosz et al. I Cold neutron source at the Budapest reactor
Be M
V
Fig. 2. Arrangement of cold moderator at Budapest reactor. C, reactor core; Be, berilium reflector; W, water moderator; T, reactor tank; A, air gap; B, biological shield (steel and concrete); M, moderator cell; V. vacuum case.
1OL4n/cm* s. Figure 3 shows a schematic diagram of the liquid hydrogen cold source assembly to be installed. The adopted system is not only very similar to that which was used in the EL3 reactor at Saclay [3]
Fig. 3. Schematic layout of the cold neutron source. moderator cell, 2; vacuum case, 3; hydrogen transfer lines, 4; criogenerators, 5; joint box, 6; water cooling, 7; helium cylinders, 8; hydrogen cylinder, 9; hydrogen reservoir, 10; vent chimney, 11; vacuum units, 12; vacuum valves, 13; gas analyzer, 14; burst discs, 15; biological shield, 16; wall of reactor hall.
933
but some key parts dismounted from that assembly will be reinstalled (two PPH 110 Philips cryogenerators of 100 W cooling power each, the hydrogen reservoir, the control system, etc.). ‘A single closed hydrogen circuit is used, the liquefier feed by the cryogenerators filling the moderator cell directly through vacuum insulated pipelines. The stream of liquid hydrogen is driven through the cell by a small differential pressure maintained by a height distance of approximately 2 m between the liquefier and the cell according to the thermosyphon principle. In order to minimize the nuclear heating a small size rectangular shape moderator cell is used which has a volume of approximately 250 cm3 (60 x 120 mm* surface, 40 mm thick) and is made of aluminium alloy. The wall thickness of the moderator cell is 1.5 mm, the internal pressure is 1.3 bar according to about 21 K liquid hydrogen temperature. According to computer calculation [4] the nuclear heating at 10 MW reactor power is expected to be 40 W. The additional estimated heat loss due to convection and radiation is about 30 W. Having a total cooling power of 200 W we have a large safety margin and probably stationary operation can be assured running only one 100 W cryogenerator. In the first part of the in-pile assembly the moderator cell is surrounded by a thick double walled water cooled vacuum case of AlMgSi alloy (see fig. 4). For safety consideration, i.e. to avoid HZ-O, reaction in case of leakage, as well as to provide helium backfilling cooling in the in-pile vacuum space, there is a helium blanket between the two walls of the vacuum case. In the outer part of the in-pile assembly three Nis8 coated glass neutron guides are positioned in bulky steel holders providing gamma and fast neutron shielding. This neutron guide part is also evacuated, but with a rotary pump to only 10m3 Torr, while the lo-’ Torr high vacuum inside the vacuum case separated by a diaphragm is maintained by a rotary and a diffusion pump.
de
v
W
Fig. 4. Front section of the in-pile part. M, moderator cell;
V, vacuum case; He, helium blanket; W, water cooling.
934
T. Grosz et al. i Cold neutron source at the Budapest reactor
transfer lines and in case of sudden pressure rise in the system the hydrogen vapour is vent to a vent line and helium purge is initiated simultaneously.
3. Safety
A number of safety measures are taken to ensure safe exploitation of the cold source and to avoid any damage of the reactor in case of eventual accident. The safe operation is achieved through the following means: -the hydrogen circuit is entirely encased by a vacuum casing and surrounded by a helium blanket around the cell; - the vacuum case is explosion proof; -the vacuum and the moderator cell temperature is monitored and exceeding pre-set limits the operation is stopped and the system is evacuated; -burst discs and safety valves are mounted in the
References [l]
L. Rosta, Workshop on Neutron Physics, Budapest (1986) p. 1. [2] I. Butterworth, P.A. Egelstaff, H. London and F.J. Webb, Philos. Mag. 2 (1957) 917. [3] G. der Nigohossian, Liquid Hydrogen Moderator Channel H’ EL3, CEA-N-1571, Saclay (1972). [4] J. Vegh, Nuclear Heating Calculation for the Design Criteria of the Cold Neutron Source to be Installed at the WWR-SM Reactor, KFKI Internal Report 11-1986, Budapest (in Hungarian).