CA2738302C

CA2738302C - Reactor for medical isotope production

Info

Publication number: CA2738302C
Application number: CA2738302A
Authority: CA
Inventors: John Wells Hilborn; David J. Winfield; Geoffrey Raymond Dimmick; Richard Thomas Jones
Original assignee: Individual
Current assignee: Individual
Priority date: 2010-04-29
Filing date: 2011-04-28
Publication date: 2018-12-18
Anticipated expiration: 2031-04-28
Also published as: CA2738302A1

Abstract

A nuclear reactor core for production of one or more medical isotopes, the reactor core having a negative power coefficient of reactivity, and comprising: a)an inner core having multiple inner lattice sites, each lattice site containing a multiplicity of fuel/target elements, with every fuel/target element containing highly-enriched uranium (HEU) or low-enriched uranium (LEU); b)an outer core having multiple outer lattice sites containing beryllium blocks, the outer core serving as a primary reflector; c) at least three absorber rods within the outer core; and d) at least two independent and diverse shutdown systems; wherein the reactor core is surrounded by a secondary reflector; and the reactor core and secondary reflector are placed in a water- filled pool. The medical isotope is preferably Molybdenum-99.

Description

REACTOR FOR MEDICAL ISOTOPE PRODUCTION
TECHNICAL FIELD
The present disclosure relates to the field of nuclear reactor design. More specifically, the present disclosure relates to improvements in reactor design for the commercial production of medical isotopes.
BACKGROUND
Radioisotopes figure prominently in a number of medical applications, allowing physicians to explore bodily structures and functions in the human body, with a minimum of invasion to the patient. Radioisotopes are also used in radiotherapy to treat some cancers and other medical conditions that require destruction of harmful cells.
Currently, about 80% of nuclear medicine procedures rely on just one isotope, molybdenum-99, which is a parent radioisotope to the daughter radioisotope technetium-99, which is used in many medical procedures. Some of these procedures are performed using medical isotopes that have left the reactor only some 40-50 hours earlier.
There have been concerns about the long-term, secure supply of molybdenum-99.
Canada's ageing NRU reactor at Chalk River, Ont., had supplied a third of the world's medical isotopes until it was shut down in the spring of 2009 to repair a radioactive water leak. The three nuclear reactors that produce the radioactive substance molybdenum-99 ¨ needed to generate technetium-99 ¨ were shut down for much-needed maintenance and refueling. As a result, thousands of patients worldwide were not able to get the diagnostic procedures they need.
The aging NRU reactor was to have been replaced by one or more Multipurpose Applied Physics Lattice Experiment (MAPLE) reactors, dedicated exclusively to medical isotope production (principally molybdenum-99, a fission product of U-235). However, prolonged delays, along with safety concerns regarding a positive power coefficient of reactivity (PCR), have resulted in cancellation of the MAPLE project by Atomic Energy of Canada Limited.
US Patent No. 4,111,747 (Eck et al.) discloses a neutron nuclear reactor including a core and a plurality of vertically oriented neutron shield assemblies surrounding the core. Each assembly includes closely packed cylindrical rods within a polygonal metallic duct. The shield assemblies are cooled by liquid coolant flow through interstices among the rods and duct.
US Patent No. 3,202,619 (Le Baron et al.) discloses a neutron reflector that incorporates beryllium metal or beryllium in the form of a beryllium compound into purified raw uncalcined petroleum coke or binder pitch which is originally made sufficiently pure for reactor purposes or into mixtures of such purified carbonaceous materials; and binder pitches. Beryllium metal or-a beryllium compound is uniformly dispersed or distributed through certain raw materials used in the manufacture of reactor grade carbon or graphite.
There is thus a pressing need for a safe, reliable nuclear reactor for the production of medical isotopes.
SUMMARY
The present invention relates to whole-core production of Mo-99 from U-235 fuel, rather than the driver-target approach used in the prior art NRU and MAPLE reactors. In addition, the reactor of the present disclosure operates in the range of 3-6 MW, instead of the 10 MW
rating of the present MAPLE reactors.
The reactor of the present invention uses the same core geometry and dimensions as the MAPLE
reactor, while replacing the MAPLE core with a much simpler design that operates at a lower power than MAPLE; provides for the specific production of Mo-99 and other medical isotopes, and conforms to most of the safety, control and operating protocols already established for MAPLE.

2 =
The two MAPLE reactors at Chalk River have operated at power levels up to 8MW
for short periods of time.
In one aspect of the present invention, there is provided a nuclear reactor core for production of one or more medical isotopes, the reactor core having a negative power coefficient of reactivity, and comprising: a) an inner core having multiple inner lattice sites, each lattice site containing a multiplicity of fuel/target elements, with every fuel/target element containing highly-enriched uranium (I-IEU) or low-enriched uranium (LEU); b) an outer core having multiple outer lattice sites containing beryllium blocks, the outer core serving as a primary reflector; c) at least three control absorber rods within the outer core; and d) at least two independent and diverse shutdown systems;
wherein the reactor core is surrounded by a secondary reflector, and the reactor core and secondary reflector are placed in a water-filled pool. The medical isotope is preferably Molybdenum-99.
Preferably, The I-IEU has a concentration of at least about 93% U-235, and is in the form of an alloy with aluminum, with an aluminum cladding completely enclosing each element.
Alternatively, where LEU is used, it preferably has a concentration of at most about 20% U-235, in the form of LEU oxide, with the element having zircaloy cladding.
The number of inner lattice sites is preferably seven, with each inner lattice site having a hexagonal cross-section, and containing 36 fuel/target elements. The number of outer lattice sites is preferably twelve, with the twelve outer lattice sites preferably consisting of six circular outer lattice sites and six hexagonal outer lattice sites. In a preferential arrangement, three of the six circular outer lattice sites each contain a control absorber rod and the remaining three circular outer lattice sites each contain a shut-down absorber rod. Each control absorber rod and each shut-down absorber rod may form an annulus that surrounds the circular beryllium block. An alternate arrangement is to have the twelve outer lattice sites consist of three circular outer lattice sites and nine hexagonal outer lattice sites. In this case, the three circular outer lattice sites each have a control absorber rod.
Preferably, each fuel/target element has a height of from about 400 mm to about 600 mm, and each beryllium block has a height of about 600 mm. In addition, the water-filled pool may serve as a

3 radiation shield and a heat sink. The reactor core preferably has an operation power range from about 3 MW to about 6 MW. In addition, the secondary reflector may contain heavy water, beryllium metal, beryllium oxide, graphite, or any combination thereof. The shutdown systems are preferably selected from the group consisting of: a gravity drop of at least three absorber rods; a gravity dump of the heavy water reflector; and a liquid absorber injection into the water-filled pool.
In another aspect of the present invention, there is provided a nuclear reactor core for production of Molybdenum-99, the reactor core having a negative power coefficient of reactivity, and comprising: a)an inner core having seven inner lattice sites, each lattice site containing thirty-six fuel/target elements, with every fuel/target element containing either highly-enriched uranium (HEU) or low-enriched uranium (LEU); b) an outer core having twelve outer lattice sites containing beryllium blocks, the outer core serving as a primary reflector; c) at least three control absorber rods within the outer core; and d) at least two independent and diverse shutdown systems;
wherein the reactor core is surrounded by a secondary reflector, the secondary reflector contains heavy water, beryllium metal, beryllium oxide, graphite, or any combination thereof, and the reactor core and secondary reflector placed in a water-filled pool.
In yet a further aspect of the present invention, there is provided a method for producing one or medical isotopes from inadiation of U-235 in a nuclear reactor core, the nuclear reactor core having a negative power coefficient of reactivity and comprising: a) inner core having seven inner lattice sites, each lattice site containing thirty-six fuel/target elements, with every fuel/target element containing either highly-enriched uranium (HEU) or low-enriched uranium (LEU);
b)an outer core having twelve outer lattice sites containing beryllium blocks, the outer core serving as a primary reflector; c) three control absorber rods within the six circular outer lattice sites; and d) at least three control absorber rods within the outer core; and at least two independent and diverse shutdown systems;
wherein the reactor core is surrounded by a secondary reflector, with the secondary reflector containing heavy water, beryllium metal, beryllium oxide, graphite, or any combination thereof;
and the reactor core and secondary reflector are placed in a water-filled pool.

4 BRIEF DESCRIPTION OF DRAWINGS
FIGURE 1 illustrates the prior art MAPLE core layout.
FIGURE 2 illustrates an embodiment of a reactor core of the present invention.
DETAILED DESCRIPTION
Where characteristics are attributed to one or another variant of the invention, unless otherwise indicated, such characteristics are intended to apply to all other variants of the invention where such characteristics are appropriate or compatible with such other variants.
The following is given by way of illustration only and is not to be considered limitative. Many apparent variations are possible without departing from the scope thereof.
Prior art MAPLE reactors are designed for the sole purpose of producing medical isotopes. These are open pool-type reactors, each with a thermal power of 10 MW. The reactors are fuelled with Low-Enriched Uranium (LEU ¨20% U-235) silicide fuel dispersed in an aluminum matrix. The reactors are licensed to irradiate Highly Enriched Uranium (HEU ¨ 93% U-235) targets in their core to produce the following medical isotopes as fission products of U-235: Mo-99, Iodine-131 and Xenon-133. The fuel and targets are removed manually from the top of the reactor pool, when the reactor is shut down.
The reactor core (1), shown in FIG. 1, measures about 400 mm in diameter and about 600 mm in height. The core is near the bottom of a pool, which is itself about 10 m deep. As shown in FIG. 1, the hexagonal core has nineteen lattice sites (10, 20, 30). Nine sites (10) contain 36-element hexagonal LEU fuel bundles, six sites (20) contain 18-element circular LEU
fuel bundles, and the remaining four sites (30) each contain 12 HEU targets for Mo-99 production.
The 36-element (10) and 18-element LEU fuel bundles (20) are designated driver fuel. A heavy water reflector (40) surrounds the core and contains irradiation sites for the production of iodine-125.

5 Each MAPLE reactor core contains about 5 kg of U-235, contained in the LEU
driver fuel (10, 20) and HEU targets (30). The LEU fuel is a composite material, containing 61 wt %
uranium suicide dispersed in aluminum. Each fuel element contains about 12 g of U-235. They are about 6 mm in diameter by 600 mm in length, aluminum clad with eight cooling fins at 45 intervals.
The HEU targets (30) each contain about 18.5 g of U-235; and the target material is a thin annulus of uranium dioxide crush-compressed between two concentric zircaloy tubes. The nominal dimensions of the targets are 15 mm OD, 13 mm ID , and 486 mm length. The average linear power is about 32 kW/m for the fuel elements and 140 kW/m for the targets.
The reactor core is cooled by forced water flow which is circulated by a primary cooling pump. It enters an inlet plenum and flows vertically upward through the flow tubes that contain fuel or targets. The water exits from the flow tubes into a chimney, where it mixes with a descending flow of pool water. The combined flow leaves the chimney through two outlets, and then returns to the suction side of the pump. The pump directs the flow through a heat exchanger, where heat is rejected to the process water system. The total water flow through the reactor core is about 320 kg/s, and the maximum outlet pump pressure is about 705 kPa (g). The core inlet water temperature is about 30 C, and the core outlet temperature is about 37 C.
The MAPLE reactor has two independent and diverse safety systems:
1) System #1, which has three hydraulically-actuated shut-off rods.
2) System #2, which has three electromagnet actuated control absorber rods and a hydraulically actuated reflector dump system.
The reactor can be put in a stable sub-critical state by dropping any two of the three shutoff rods into the reactor core. The same objective can be achieved by dropping any two of the three control absorber rods into the reactor core. Finally, the reflector dump system will also place the reactor in a

6 stable sub-critical state. While the reactor is in operation, the three shut-off rods are fully withdrawn above the core.
The reactor power is controlled by the control absorber rods. When the reactor needs to be shut down (e.g. for fuelling), the control absorber rods are driven into the core, and the shut-off rods remain fully withdrawn above the core.
One embodiment of a reactor core of the present invention is shown in FIG. 2.
Here, the inner core is composed of seven hexagonal lattice sites (50), each site containing 36 HEU
cylindrical fuel/target elements. They have a dual purpose; a) provide fuel to keep the reactor critical, and b) provide targets to produce Mo-99 as a fission product. The dimensions of each hexagonal lattice site (50) and each fuel/target element are identical to the corresponding dimensions of a MAPLE
lattice site and LEU fuel element with 8 cooling fins. However, the HEU
fuel/target elements (50) shown in Fig. 2 preferably contain uranium/aluminum alloy, instead of LEU
uranium suicide;
and they preferably have aluminum cladding.
In another embodiment (not shown), the HEU fuel/target elements (50) can be replaced by LEU
fuel/target elements. The LEU is in the form of a uranium oxide ceramic pellets similar to those used in pressurized heavy water reactors such as Canada Deuterium Uranium (CANDU) reactors.
Surrounding the seven core sites are twelve primary reflector sites (60, 70);
six containing hexagonal (60) beryllium blocks and six containing cylindrical (70) beryllium blocks. In each case, the prior art MAPLE fuel bundle is replaced with a solid beryllium block; and each cylindrical block (70) is surrounded by an annulus which serves either as a control absorber rod (CAR), or a shutdown absorber rod (SAR). Three of the six cylindrical (70) beryllium blocks contain control rods, while the three remaining cylindrical blocks contain shutdown rods. The rods (both shutdown and control) are each preferably thin moveable annuli surrounding the solid beryllium block. The CARs and SARs preferably alternate around the hexagonal core, as shown in FIG.
2.

7 In another embodiment of the invention (not shown), the outer core may consist of three circular beryllium blocks, and nine hexagonal beryllium blocks. In this case, each of the three circular beryllium blocks contains a control absorber rod.
Conversion of the twelve outer lattice sites from fuel to reflector, reduces the fission power and uranium consumption of the reactor without any reduction in the output of Mo-99.
The cylindrical (70) and hexagonal (60) beryllium blocks are cooled preferably with water. The central lattice site along the axis may be used as a fuel/target site, beryllium site, or control/safety site, depending on the requirements for isotope production, safety and control.
Chemical extraction of Mo-99 can be achieved by replacing one of the seven fuel/target bundles (50) every one or two days, depending on demand.
The reactor core of the present invention addresses the power coefficient of reactivity problem of the prior art MAPLE reactor. While the specific activity of the fuel/target material (Curies Mo-99/gram U-235) is somewhat less than that of the NRU targets, a costly redesign of the core is avoided by eliminating driver fuel and using the existing hexagonal core structure and heavy water reflector as is.
By replacing the fuel bundles in the twelve outer lattice sites with beryllium blocks, and replacing the fuel bundles and target assemblies in the seven inner sites with HEU
aluminum (or LEU oxide) fuel/target elements, the reactor of the present disclosure requires about half of the total U-235 and half of the power needed for a prior art MAPLE reactor. The HEU requirement is about 2 kg, with the reactor operating at about 3 MW. Replacement of one fuel/target assembly can occur every two days. The weekly production of Mo-99 is approximately the same as that from NRU.

8 A secondary reflector (80) surrounds the reactor core, with the beryllium blocks (60, 70) serving as the primary reflector. The function of the two reflectors is to reflect escaping neutrons back into the core, which enhances the efficiency of the reactor. The secondary reflector (80) contains heavy water, beryllium metal, beryllium oxide, graphite, or any combination thereof.
.The secondary reflector (80) may be in the form of eighteen hexagonal beryllium blocks or a solid beryllium annulus.
At least two independent and diverse shutdown systems are required in order to shutdown the reactor and satisfy safety regulations. As an example, a liquid absorber injection may be used in addition to a gravity drop of the control absorber rods and the heavy water dump.
CONCLUSION
Although embodiments of the invention have been described above, it is limited thereto and it will be apparent to those skilled in the art that numerous modifications form part of the present invention insofar as they do not depart from the scope of the claimed and described invention.

9

Claims

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE PROPERTY
OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A nuclear reactor core for production of one or more medical isotopes, the reactor core having a negative power coefficient of reactivity, and comprising:
a) an inner core having multiple inner lattice sites, each lattice site containing a multiplicity of fuel/target elements, with every fuel/target element containing highly-enriched uranium (HEU) or low-enriched uranium (LEU);
b) an outer core having multiple outer lattice sites containing beryllium blocks, the outer core serving as a primary reflector;
c) at least three control absorber rods within the outer core; and d) at least two independent and diverse shutdown systems;
wherein the reactor core is surrounded by a secondary reflector; and the reactor core and secondary reflector are placed in a water-filled pool.

2. The nuclear reactor core of claim 1, wherein the HEU has a concentration of at least 93% U-235, and the LEU has a concentration of at most 20% U-235.

3. The nuclear reactor core of claim 1 or 2, wherein the fuel/target element is made of either:
i) an alloy of HEU and aluminum, the element having aluminum cladding; or ii) LEU oxide, the element having zircaloy cladding.

4. The nuclear reactor core of any one of claims 1 to 3, wherein the number of inner lattice sites is seven, and the number of outer lattice sites is twelve.

5. The nuclear reactor core of any one of claims 1 to 4, wherein each inner lattice site is hexagonal and contains 36 fuel/target elements.

6. The nuclear reactor core of claim 4, wherein the twelve outer lattice sites consist of six circular outer lattice sites and six hexagonal outer lattice sites.

7. The nuclear reactor core of claim 6, wherein three of the six circular outer lattice sites each contain a control absorber rod and the remaining three circular outer lattice sites each contain a shut-down absorber rod.

8. The nuclear reactor core of claim 7, wherein each control absorber rod and each shut-down absorber rod form an annulus that surrounds the circular beryllium block.

9. The nuclear reactor core of claim 4, wherein the twelve outer lattice sites consist of three circular outer lattice sites and nine hexagonal outer lattice sites.

10. The nuclear reactor core of claim 9, wherein the three circular outer lattice sites each contain a control absorber rod.

11. The nuclear reactor core of any one of claims 1 to 10, wherein the water-filled pool serves as a radiation shield and a heat sink.

12. The nuclear reactor core of any one of claims 1 to 11, wherein each fuel/target element has a height of from 400 mm to 600 mm, and each beryllium block has a height of 600 mm.

13. The nuclear reactor core of any one of claims 1 to 12, having a reactor operation power range from 3 MW to 6 MW.

14. The nuclear reflector of any one of claims 1 to 13, wherein the secondary reflector contains heavy water, beryllium metal, beryllium oxide, graphite, or any combination thereof.

15. The nuclear reactor core of claim 14, wherein the at least two independent and diverse shutdown systems are selected from the group consisting of: a gravity drop of at least three absorber rods; a gravity dump of the heavy water reflector; and a liquid absorber injection into the water-filled pool.

16. The nuclear reactor core of any one of claims 1 to 15, wherein the medical isotope is Molybdenum-99.

17. A nuclear reactor core for production of Molybdenum-99, the reactor core having a negative power coefficient of reactivity, and comprising:
a) an inner core having seven inner lattice sites, each lattice site containing thirty-six fuel/target elements, with every fuel/target element containing either highly-enriched uranium (HEU) or low-enriched uranium (LEU);
b) an outer core having twelve outer lattice sites containing beryllium blocks, the outer core serving as a primary reflector;
c) at least three control absorber rods within the outer core; and d) at least two independent and diverse shutdown systems;
wherein the reactor core is surrounded by a secondary reflector, the secondary reflector contains heavy water, beryllium metal, beryllium oxide, graphite, or any combination thereof; and the reactor core and secondary reflector are placed in a water-filled pool.

18. The nuclear reactor core of claim 17, wherein the HEU has a concentration of at least 93%
U-235, and the LEU has a concentration of at most 20% U-235.

19. The nuclear reactor core of claim 17 or 18, wherein the fuel/target element is made of either:
i) an alloy of HEU and aluminum, with the element having aluminum cladding; or ii) LEU oxide, with the element having zircaloy cladding.

20. The nuclear reactor core of claim 19, wherein each fuel/target element is made of an alloy of HEU and aluminum, and the element has aluminum cladding.

21. The nuclear reactor core of claim 19, wherein each fuel/target element is made of LEU oxide, and the element has zircalloy cladding.

22. The nuclear reactor core of any one of claims 17 to 21, wherein the twelve outer lattice sites consist of six circular outer lattice sites and six hexagonal lattice sites.

23. The nuclear reactor core of claim 22, wherein three of the six circular outer lattice sites each contain a control absorber rod and the remaining three circular outer lattice sites each contain a shut-down absorber rod.

24. The nuclear reactor core of claim 23, wherein each control absorber rod and each shut-down absorber rod form an annulus that surrounds the circular beryllium block.

25. The nuclear reactor core of any one of claims claim 17 to 21, wherein the twelve outer lattice sites consist of three circular outer lattice sites and nine hexagonal outer lattice sites.

26. The nuclear reactor core of claim 25, wherein the three circular outer lattice sites each contain a control absorber rod.

27. The nuclear reactor core of any one of claims 17 to 26, wherein the water-filled pool serves as a radiation shield and a heat sink.

28. The nuclear reactor core of any one of claims 17 to 27, wherein each fuel/target element has a height of from 400 mm to 600 mm, and each beryllium block has a height of 600 mm.

29. The nuclear reactor core of any one of claims 17 to 27, wherein the secondary reflector contains heavy water, beryllium metal, beryllium oxide, graphite, or any combination thereof.

30. The nuclear reactor core of claim 29, wherein the at least two independent and diverse shutdown systems are selected from the group consisting of: a gravity drop of at least three absorber rods; a gravity dump of the heavy water reflector; and a liquid absorber injection into the water-filled pool.

31. The nuclear reactor core of any one of claims 17 to 30, having a reactor operation power range from 3 MW to 6 MW.

32. A method for producing one or more medical isotopes from irradiation of U-235 in a nuclear reactor core, the nuclear reactor core has a negative power coefficient of reactivity and comprises:
a) an inner core having seven inner lattice sites, each lattice site containing thirty-six fuel/target elements, with every fuel/target element containing either highly-enriched uranium (HEU) or low-enriched uranium (LEU);
b) an outer core having twelve outer lattice sites containing beryllium blocks, the outer core serving as a primary reflector;
c) at least three control absorber rods within the outer core; and d) at least two independent and diverse shutdown systems;
wherein the reactor core is surrounded by a secondary reflector; and the reactor core and secondary reflector are placed in a water-filled pool.

33. The method of claim 32, wherein the HEU has a concentration of at least 93% U-235, and the LEU has a concentration of at most 20% U-235.

34. The method of claim 32 or 33, wherein the fuel/target element is made of either:
i) an alloy of HEU and aluminum, with the element having aluminum cladding; or ii) LEU oxide, with the element having zircaloy cladding.

35. The method of claim 34, wherein each fuel/target element is made of an alloy of HEU and aluminum, the element has aluminum cladding, and the method further comprises extraction of Molybdenum-99 from the alloy.

36. The method of any one of claims 32 to 35, wherein the twelve outer lattice sites consist of six circular outer lattice sites and six hexagonal lattice sites.

37. The method of claim 36, wherein three of the six circular outer lattice sites each contain a control absorber rod and the remaining three circular outer lattice sites each contain a shut-down absorber rod.

38. The method of claim 37, wherein each control absorber rod and each shut-down absorber rod form an annulus that surrounds the circular beryllium block.

39. The method of any one of claims claim 32 to 35, wherein the twelve outer lattice sites consist of three circular outer lattice sites and nine hexagonal outer lattice sites.

40. The method of any one of claims 32 to 39, wherein the water-filled pool serves as a radiation shield and a heat sink.

41. The method of any one of claims 32 to 40, wherein each fuel/target element has a height of from 400 mm to 600 mm, and each beryllium block has a height of 600 mm.

42. The method of any one of claims 32 to 41, wherein the secondary reflector contains heavy water, beryllium metal, beryllium oxide, graphite, or any combination thereof.

43. The method of any one of claims 32 to 42, wherein the at least two independent and diverse shutdown systems are selected from the group consisting of: a gravity drop of at least three absorber rods; a gravity dump of the heavy water reflector; and a liquid absorber injection into the water-filled pool.

44. The method of any one of claims 32 to 43, having a reactor operation power range from 3 MW to 6 MW.

45. The method of any one of claims 32 to 44, wherein the medical isotope is Molybdenum-99.